Author Topic: Accuracy or lack off - what to expect  (Read 4502 times)

Offline Admiral_dk

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Accuracy or lack off - what to expect
« on: December 28, 2020, 10:43:39 PM »
Hi All

I'm frustrated with my tools .... My lathe has a Z-Axis accuracy that I'm happy with + it can be locked, but the Y-Axis has me very frustrated. If I put a mike on the Y-Axis and try to push or pull as hard as I can with my bare hands - I can move the Y-Axis some 0.4mm according to the DRO  :hammerbash:

As this is in Diameter mode, it equals to a movement off 0.2mm. ~ 0.08" and it really explains why I can do deep cuts and have a very nice finish and then try to take a very thin finishing cut that is all over the shop => in a horrible finish ....  :rant:

I tried a number of things and reduced the problem some - but I have a Acme-Thread on the Y-Axis where the nut can move 0.09mm ~ 0.035" on the axle (that can't be removed).

I had a look at some suppliers of Acme and Ball-Bearing Spindles and they will not promise any better than 0.05mm ~ 0.02" accuracy on an Acme Spindle .... But as we all know - Accuracy and Backlash isn't the same. If I had zero backlash (I wish), I could be rather sure that the diameter I set the Tool on the DRO, would result in a diameter on the subject that will be extremely close to the diameter I want (if I haven't made any mistakes).

What kind of Backlash do you find acceptable / expect ?

I can see that my new Mill have Acme-Nuts that are sawn 2/3 through and two small screws to adjust the backlash more or less away - works very well on X, but not so well on Y, and Z is so heavy I do not expect to see any here. I haven't tried to adjust Y on it yet, as it is a pain in the proverbial to get access to do so.

Is a similar thing a kind of cure for the lathe or what do you think ?

Per

Offline Charles Lamont

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Re: Accuracy or lack off - what to expect
« Reply #1 on: December 29, 2020, 12:06:59 AM »
Firstly, there seems to be something wrong with your decimals. 0.2mm is about 0.008", not 0.08".

Secondly, that is not an unreasonable amount of backlash for a machine in reasonable order. Feedscrews wear in the most regularly used portion. My elderly mill has about that much on both x and y, and although it has anti-backlash adjustable nuts, that is about the best I can do without making the screws too tight at the extremes of travel.

Thirdly, given that with normal screws and slides, some backlash is inevitable, learn to live with it. Make sure the slides are well adjusted. Feed in one direction, and if you have to go the other way, wind back well beyond the backlash and then approach again. Lock the stationary slides when taking a cut.

If you need extra precision, use a dial gauge (and slip gauges, AKA Jo-blocks) to check the actual movement.     

Offline steamer

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Re: Accuracy or lack off - what to expect
« Reply #2 on: December 29, 2020, 04:22:19 AM »
I agree with Charles.    Remember that there is a very old saying...." Everything is made of rubber"....  A machine tool will deflect under load, and as such you have to work with that deflection.

Instead of taking a bunch of heavy cuts, and then a light one....try taking even steps  to your diameter.

So lets say you want to remove 3mm from the diameter of a nice stout work piece...you could take 2.5mm, and then sneak up on the last 0.5....OK...but during the first cut you put some spring tension in, and as you try to take the " light" cuts, some of that tension is released as you cut....making the cut deeper than you intend.   This often results in overshooting the dimension, and is frustrating.

Try the following
with a 25 mm or so piece of bar in the lathe, ( use a free cutting steel) and a sharp suitably sharpened lathe tool  ( positive top rake)

on a 3mm cut......
Cut 2mm on diameter.
measure the result in several places, and write down your results
Go back to your start and take a dead pass.....meaning do not dial in any more cut....just leave it where it is
Observe, and measure the cuts again

That is a "dead pass" or a spring pass, and is an indication of of how much deflection is in the system during the cut.

Now take a 0.5 mm cut, and a dead pass

How much spring did it have with that DOC?....

Now ...how much is left?....( should be something like 0.5 mm...maybe a little less....)

Dial in the difference, and take the cut.....measure where you are.

Now we know, from the previous pass, about how much we will take with a dead pass....you should be near the difference between that cut and the dead pass.

The point is ...to take equal depth cuts   once you start getting close...not to take smaller and smaller cuts each pass....as with that method the amount of spring in the system is constantly changing.  So it ends up doing something you don't expect.

Now....if I'm roughing....
I may take a .125" DOC per pass until I get to within 0.125" of the size.. I'll let it cool for a half hour or so.

Then I'll start taking 0.025" cuts, in equal steps, from there with some dead passes.....Then when I go for the last pass, I have some confidence that the system will behave like the  previous 4 or 5 passes.    Just divide the remainder up into equal depths of cut....That way when you get to the last past, you can dial in the remainder, and you'll probably nail it.

If you're running coolant, you can attack all the way down to size...taking equal cuts...until you get to size.  Same thing but usually you can be far more aggressive.

A smaller tip radius on the tool, and more top rake will usually result in less cut pressure, and less spring....on light lathes, tool geometry is very important, and usually, our light lathes will not tolerate negative rake carbide tooling very well.    HSS works well there.....horses for courses...

Dave
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Offline steamer

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Re: Accuracy or lack off - what to expect
« Reply #3 on: December 29, 2020, 04:36:26 AM »
Oh and my newly rebuilt SB....which is 8 years old now, has about .020" backlash in the cross slide....I suspect some of that is the screw, as the nut is " new"...er ah 8 years old, while the cross slide leadscrew is original and from 1952....but no matter.....I can hit .0005" on diameter pretty easily.....

but I'll tell you ....put a dial indicator on the spindle and tension the belt....and wow!....it moves a lot!!!

Big lathes just dont move as much...because they are so much more heavily built....

Dave
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Offline Jasonb

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Re: Accuracy or lack off - what to expect
« Reply #4 on: December 29, 2020, 07:14:48 AM »
What lathe is it?

What cutting tools are you using, an insert may work well on a reasonably deep cut but not so well if only very shallow in which case try to make the final cut of reasonable depth.

I have a lot more backlash than that and get an acceptable finish and can work to 10ths of a thou if needed.

Offline Jo

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Re: Accuracy or lack off - what to expect
« Reply #5 on: December 29, 2020, 07:55:14 AM »
The old trick with worn nuts is to only cut in one way. Adding a Lock on the saddle is simple enough to do.

But I get the idea you are saying the nut itself is moving??? Is that correct? If so you would need to push it to one end of the movement and then find a means of keeping it there  :thinking: Could you provide photos of the problem?


What kind of Backlash do you find acceptable / expect ?

I have DROs to my two larger lathes I have not noticed any Backlash on either :) But some people can make do with unacceptable levels of backlash that cannot be gotten rid of and still knock out working models - they have learnt to work around the failings of their machines 1  .

Jo


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Offline Charles Lamont

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Re: Accuracy or lack off - what to expect
« Reply #6 on: December 29, 2020, 08:57:27 AM »
Looking at this again, your poor finish on finishing cuts is probably nothing to do with backlash. There is backlash of several thou  on the cross-slide of my Myford, but I can take a final sizing pass 1/4 thou deep (~ 0.005mm) or less (without locking the slide). But I only use carbide tools for cast iron. For practically everything else I use HSS tools, and have learned how grind and hone them, and set them accurately on centre height. Cutting lubricant is another thing to look at.   

Offline Ramon Wilson

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Re: Accuracy or lack off - what to expect
« Reply #7 on: December 29, 2020, 09:29:10 AM »
Hi Per,

Backlash as such is something that all standard leadscrew and nut machines suffer with.

Forgive me stating the obvious but this is usually due to wear either on the screw or within the nut - usually the latter but mostly a combination of the two. If the leadscrew can move laterally to any great degree in it's bracket or the nut within it's mounting however that is a somewhat different situation. There has to be a degree of clearance in the leadscrew bracket but this can usually be adjusted to a minimum condusive to the friction of overtightening. The nut should not move in it's housing however. Insufficiently tightened gib strips will exacerbate the problem too.

Backlash due to wear can always be taken out by making sure the slide is always being pushed but if the wear is in the mounting of nut and/or leadscrew bracket you have no way of ensuring the tool tip is where you think it might be. With a heavy cut the forces involved will assist in pushing back against such potential movement but on a light cut the 'slack' of the system could affect the tool position on a light cut and hence variation in results

First off for me would be to check those areas first and, if possible, reduce lateral movement to a minimum. Any backlash in the screw thread itself being taken care of by the usual method

Making sure the gibs are adjusted to give a firm hold and using a lock (though if there is slack in the system a lock can sometimes have varying results in it's clamping effect) are also factors to consider though I rarely use the lock on the cross-slide whilst actually turning. Possibly a typo on Jo's part but a lock on the saddle will have no effect on the Y axis.

Without checking I'm not sure what the exact backlash is on my Myford but at 40 plus years old its not anything that causes problems - never really think about it and no, I don't have DRO on the lathe. As I sit here I would guess about 8-10 thou, possibly less

Hope that adds a little to previous comments - good luck with it :ThumbsUp:

Our regards for a better New Year to you and John

Tug

PS I would agree with Charles on using HSS particularly where finish is concerned but then I'm a bit long in the tooth to change spots now  :)
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Offline Admiral_dk

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Re: Accuracy or lack off - what to expect
« Reply #8 on: December 29, 2020, 11:08:35 AM »
Ups - yes I wrote the wrong decimal points on the Imperial measurements.

What really annoys me, is those cuts where I leave / do not touch the depth of the cut. It might start out very well and just make a very nice surface for a while and then it starts to cut deeper and I can see this on the DRO too. I have tried to adjust the gibs to the point where I need to put force into turning the handle (not violence) and the same thing happens if just a smitten better.

The reason I know how well it (most of the time) does roughing cuts, is that I have a good amount off 32mm aluminium I got for free years ago, and that I often need a part that is 10-20mm. So I do a few where I remove 2.5-3mm. per cut and they are usually with a nice surface. Getting closer I have tried to remove 0.6mm. per pass according to the DRO only turning the handle in one direction and then I see the tool moving on the DRO and a horrible surface on the part. I have tried to do smaller depth per cut, but the amount of change when it decides to misbehave is not constant with same depth and it might do most of the cut at same depth (making me thinking I got it right this time) and increase the depth by anything between 0.002-0.2mm. DRO readout  :hammerbash:  This is without me touching anything but having a finger close to the Z-Feed half nut lever.

My problem is that it's never the same result - if it was - I could learn how to optimise the way I use the lathe.

I'm sure I could be better at grinding HHS and that is probably why I usually get much better results with inserts -> I have recently bought new holders and plates I haven't tried yet as I wanted to solve the backlash / movement problem first.
« Last Edit: December 29, 2020, 11:19:51 AM by Admiral_dk »

Offline Jo

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Re: Accuracy or lack off - what to expect
« Reply #9 on: December 29, 2020, 11:14:16 AM »
Silly question: how much play is there between the saddle and the bed? Is the saddle crabbing?

Jo
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Offline Admiral_dk

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Re: Accuracy or lack off - what to expect
« Reply #10 on: December 29, 2020, 11:33:30 AM »
I believe that this is a valid question Jo - but I must be sure that I understand you right.
You suggest that the saddle isn't moving in a straight line, if I understand you right ?
How would you measure it - both play and "crabbing" ?

The lathe has one "prism" as part of the bed, that should keep the saddle running straight and I can't feel any (not very scientific) movement if I try to move it. I can feel and see the movement of the cross slide.

Offline Jo

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Re: Accuracy or lack off - what to expect
« Reply #11 on: December 29, 2020, 11:41:53 AM »
I believe that this is a valid question Jo - but I must be sure that I understand you right.
You suggest that the saddle isn't moving in a straight line, if I understand you right ?
How would you measure it - both play and "crabbing" ?

Play can be measured with your dial on the front/back of the saddle and you try pulling it left and right.

The crabbing is where the saddle when moving to the left the front of the 'loose' saddle is pushed to the right then moves to the left for no apparent reason and the lathe tool digs in  :toilet_claw: The amount of 'looseness' is not visible but only noticeable due to odd dig-ins of the tool bit  :Doh:

Jo
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Offline Ramon Wilson

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Re: Accuracy or lack off - what to expect
« Reply #12 on: December 29, 2020, 11:42:44 AM »
Per - If the cross slide resistance is as you say - tightish to turn then something else is definitely moving if the tool is being pulled into the work piece as you describe. This could be as Jo suggests a movement in the saddle on the bed or elsewhere - top slide or tool post for example - where exactly though remains to be found. A tool pulling itself into the work piece suggests such freedom or a push off in the first instance followed by another pass which then digs in, pulling the tool in.

There's movement there somewhere but difficult to say from here without seeing it in action :D Video perhaps?

Tug


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Online Laurentic

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Re: Accuracy or lack off - what to expect
« Reply #13 on: December 29, 2020, 11:52:16 AM »
Per - I was thinking along the same lines as Jo and her crabbing saddle, something sounds like it is not quite right with your lathe. 

Normally I know there is backlash or 'lost motion' in my lathe cross slide and ensure it is taken up for the direction I am applying cuts and all is well; to see a tool moving on the DRO and then see a horrible surface finish indicates to me that something else is perhaps not 'tight' and allowing a varying movement, but again strange as you would expect that on all cuts not just light ones, unless heavier cuts force whatever is moving to move itself "back in its box" as we say. 

At the risk of teaching granny to suck eggs, have you been round all aspects of the saddle, cross slide and top slide and ensured all is tight?

Chris

PS - while I was writing it seems Jo and Tug posted along the same lines!

Offline Admiral_dk

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Re: Accuracy or lack off - what to expect
« Reply #14 on: December 29, 2020, 12:23:37 PM »
I have just been trying to measure the saddle with a "clock" with the magnetic holder on the bed, I can lift the back (part furthest away from me) 0.02mm. and the "front" between 0.03-0.08mm.up from the bed depending on where i place the saddle between the chuck and tailstock. The back sounds fine to me, but should I expect the front to be smaller - it is an old lathe.

I think I might have read your reply slightly wrong Jo - or more correct, I started measuring before I saw your reply. I read you as trying to twist the saddle - is this right ?
« Last Edit: December 29, 2020, 12:27:38 PM by Admiral_dk »

Offline Admiral_dk

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Re: Accuracy or lack off - what to expect
« Reply #15 on: December 29, 2020, 12:35:48 PM »
Quote
At the risk of teaching granny to suck eggs, have you been round all aspects of the saddle, cross slide and top slide and ensured all is tight?

That was how I improved it a bit in the first post (without telling how) - the nut wasn't as tight to the slide as it should have been.

Besides with my bad memory I might have forgotten the obvious .... so I'm not offended by any remarks on this subject ....

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Re: Accuracy or lack off - what to expect
« Reply #16 on: December 29, 2020, 12:35:55 PM »
If your lathe has a vee bed then crabbing should not be an issue when the tool is under load. Recognising and overcoming backlash is part of a machinist's general competence. I would suspect slop in the cross slide - i.e. loose gibs, but without a photo or lathe ID your question cannot be answered to anybody's' satisfaction.
John

Offline Admiral_dk

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Re: Accuracy or lack off - what to expect
« Reply #17 on: December 29, 2020, 12:42:13 PM »
Quote
P.S. 1. As you get older and have sufficient finances behind you, you may not want to continue to be one of the "Make do and Mend" generation  ;) .

Oh I wish - but that is not the way my economical situation has been going since I lost the job I had @ Aarhus Universitet in 2014. My current job just about covers my expenses and I will actually be better of on the public pension when I'm old enough to retire @ 67 in a little over 7 years time (I will have finished last payment on the house by then).

Offline Admiral_dk

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Re: Accuracy or lack off - what to expect
« Reply #18 on: December 29, 2020, 12:56:31 PM »
I have tried to measure the rise in height off the Cross Slide, over the Prism part of the Bed, when I try to push it to opposite sides in the front and back and I see 0.06mm. on the clock and absolutely zero if I do the same with the clock over the flat rear part of the Bed.

Thanks for the suggestions so far, but I have another appointment for the rest of the day and will be back in the shop tomorrow.

Offline steamer

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Re: Accuracy or lack off - what to expect
« Reply #19 on: December 29, 2020, 01:49:12 PM »
I have tried to measure the rise in height off the Cross Slide, over the Prism part of the Bed, when I try to push it to opposite sides in the front and back and I see 0.06mm. on the clock and absolutely zero if I do the same with the clock over the flat rear part of the Bed.

Thanks for the suggestions so far, but I have another appointment for the rest of the day and will be back in the shop tomorrow.

Show us a good picture of your tool as well....a tool that skips along and digs in sounds dull or of wrong geometry.

Dave
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Offline Jasonb

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Re: Accuracy or lack off - what to expect
« Reply #20 on: December 29, 2020, 03:11:37 PM »
Or above ctr height.

Heavy cuts may be pulling it down to correct height.

Offline Roger B

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Re: Accuracy or lack off - what to expect
« Reply #21 on: December 29, 2020, 03:35:24 PM »
This is the back and forth play in my cross slide, ~0.3mm. I don't seem to have the problems you describe. With my old minilathe most of the difficulties were due to play in the headstock bearings. When I replaced them all was well  :)

<a href="https://www.youtube.com/watch?v=fEcYPNtgwF8" target="_blank">http://www.youtube.com/watch?v=fEcYPNtgwF8</a>

Best regards

Roger

Online Vixen

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Re: Accuracy or lack off - what to expect
« Reply #22 on: December 29, 2020, 03:49:13 PM »
Hello Per,

Do you experience the 'dig ins' during power feed or manual feed? It could be the carriage is lifting when power feed is applied, due to wear or loose gibs or whatever.

A razor sharp tool should give you no problems, if everything it tight and well adjusted. A razor shape tool will provide the minimum of force to deflect the carriage or slides.

There is a simple test to see if the cutting tool is sharp enough. Scrape the cutting tool across your thumb nail in the cutting direction. A 'sharp' cutting tool will cut a small curl of nail swarf. A 'dull' cutting tool will simply slide (skid) over your thumb nail.

Sharp tools give least problems.

Good luck

Mike
« Last Edit: December 29, 2020, 06:31:27 PM by Vixen »
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Offline steam guy willy

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Re: Accuracy or lack off - what to expect
« Reply #23 on: December 29, 2020, 05:07:10 PM »
Hi All chucking in my pennyworth here.. I always thought when I was younger that the "old boys" were successful with their turning because as the lathe started to wear out there increased weight would counteract this as they leant on the slides in the correct fashion ??!! when the new younger chap tried to use the lathe he would not know about this and wonder why his accuracy was all over the place !!!!  There are quite a few tricks from the old days the turners used ..one was to wrap a piece of stout rope around the chuck to stop chatter when parting off ?.

When I made a new crosslide nut for the Southbend lathe that had 3 mm back /front play I made the nut extra long with provision to take out any  future "play", a bit like the Pultra way of doing this. Pics...

willy

Offline john mills

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Re: Accuracy or lack off - what to expect
« Reply #24 on: December 30, 2020, 11:49:56 AM »
i would check that the saddle is contacting on the angular faces and not touching on other places some times with
lot of wear the saddle can be touching parts of the ways its not meant to stopping good contact and guide from the
angular faces that are meant to guide the saddle.  some of the problem sound like the shape of the tools not having sharp cutting edges with enough clearance to cut well.often older machines have lots of clearance in the
nuts and screws and don't give it much thought you just take up the slack traveling in one direction.it is easy on indusial CNC machines with good sized preloaded ball screws but old ones can still have problems after a few
 crashes which resultant damage.most of the time travelling in the one direction once the slack has been taken up
word ok.     
         John   

Offline Mcgyver

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Re: Accuracy or lack off - what to expect
« Reply #25 on: December 30, 2020, 02:45:32 PM »

What kind of Backlash do you find acceptable / expect ?


Hope I don't step on toes with this....but backlash really doesn't matter!

Why?  There has to be some, else the thread wouldn't turn.  Backlash is clearance between the thread flanks and even on a new feedscrew is going to be enough that it will mess you up if the backlash is taken up by say pushing on the crossfeed.  So, sans an antibacklash arrangement, we know no matter what we will have enough BL that exactly how much there is is immaterial.  5 thou or 50, it won't much matter.

Where backlash (imo) gained its status is in inspecting used machines were it can be an indication of overall wear.  Lots of backlash is bad not because it makes a difference in how far the tool is advanced, but because it suggests an well worn machine

Whats going on with yours?  The carriage might be crabbing as suggested, or the overall fit of the dovetail might be loose enough that vibration is moving it.  I was doing some internal grinding the last few weeks and with vibration of the high speed motor everything wanted to move.  Tightening the gib worked for me, but you don't it always really tight or it will wear more quickly.

You can check for crabbing by taking something known to be round and cylindrical, centreing it in the four jaw (centre in two planes!) to a 10th of a thou.  Then with an indicator on the tool post, sweep the work in a few spots.  If you get different readings, the motion of the carriage is not parallel to the lathe's axis (either crabbing or leveling).  Crabbing should be identifiable to apply a force the carriage by hand and seeing how the needle moves.  This little set up btw is really great for tweaking leveling on fussy work.

When I'm doing really fussy work, like striving for a 10th to fit an expensive spindle bearing say, I get rid of the compound and mount an indicator on the cross slide dovetail.  My indicator mount also has two cylinders with springs to press the crossfeed screw against the correctly flank of the nut.  You have to hold your tongue just so, but I can usually do better than .0002" with this set up.

Its not trivial getting a perfectly working machine...its why the good quality ones cost so much when new and masochist like me spend so much time reconditioning (scraping) them.  So don't feel bad or get frustrated if it takes a bit of chasing.  It is however very when they do perform well.  If none of this helps, the next thing I'd do get it apart and start check fits with some blue.  Some sort of spring arrangement loading the system against the nut might be an experiment to try - see if that fixed it.  If it does, imo its further evidence the crossfeed dovetails need attention.

Here's a couple shots of what I mentioned

everything is solid and scraped to fit, compound removed and replaced with a T slot plate



Cross feed indicator - digital is great for this, micron accuracy and easy to switch between imperial and the french system (haha couldn't resist), err metric



Here the sweep tool.  Its essentially a spindle with very accurate AC bearings.  I mostly use it in conjunction with reconditioning (second photo) but its easily mounted in the tool post for this sort of checking.  btw, in the second picture is a Holbrook B8, marvelous lathe and a current project










Offline steam guy willy

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Re: Accuracy or lack off - what to expect
« Reply #26 on: December 30, 2020, 04:23:55 PM »
one way of taking out the backlash is to tie a piece of string to  the cross slide and with a  heavy weight and dangle it over the back of the lathe ??!!! never tried it but just an idea ??

willy

Offline steamer

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Re: Accuracy or lack off - what to expect
« Reply #27 on: December 30, 2020, 11:55:46 PM »
one way of taking out the backlash is to tie a piece of string to  the cross slide and with a  heavy weight and dangle it over the back of the lathe ??!!! never tried it but just an idea ??

willy

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Offline steamer

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Re: Accuracy or lack off - what to expect
« Reply #28 on: January 01, 2021, 02:52:37 PM »

What kind of Backlash do you find acceptable / expect ?


Hope I don't step on toes with this....but backlash really doesn't matter!

Why?  There has to be some, else the thread wouldn't turn.  Backlash is clearance between the thread flanks and even on a new feedscrew is going to be enough that it will mess you up if the backlash is taken up by say pushing on the crossfeed.  So, sans an antibacklash arrangement, we know no matter what we will have enough BL that exactly how much there is is immaterial.  5 thou or 50, it won't much matter.

Where backlash (imo) gained its status is in inspecting used machines were it can be an indication of overall wear.  Lots of backlash is bad not because it makes a difference in how far the tool is advanced, but because it suggests an well worn machine

Whats going on with yours?  The carriage might be crabbing as suggested, or the overall fit of the dovetail might be loose enough that vibration is moving it.  I was doing some internal grinding the last few weeks and with vibration of the high speed motor everything wanted to move.  Tightening the gib worked for me, but you don't it always really tight or it will wear more quickly.

You can check for crabbing by taking something known to be round and cylindrical, centreing it in the four jaw (centre in two planes!) to a 10th of a thou.  Then with an indicator on the tool post, sweep the work in a few spots.  If you get different readings, the motion of the carriage is not parallel to the lathe's axis (either crabbing or leveling).  Crabbing should be identifiable to apply a force the carriage by hand and seeing how the needle moves.  This little set up btw is really great for tweaking leveling on fussy work.

When I'm doing really fussy work, like striving for a 10th to fit an expensive spindle bearing say, I get rid of the compound and mount an indicator on the cross slide dovetail.  My indicator mount also has two cylinders with springs to press the crossfeed screw against the correctly flank of the nut.  You have to hold your tongue just so, but I can usually do better than .0002" with this set up.

Its not trivial getting a perfectly working machine...its why the good quality ones cost so much when new and masochist like me spend so much time reconditioning (scraping) them.  So don't feel bad or get frustrated if it takes a bit of chasing.  It is however very when they do perform well.  If none of this helps, the next thing I'd do get it apart and start check fits with some blue.  Some sort of spring arrangement loading the system against the nut might be an experiment to try - see if that fixed it.  If it does, imo its further evidence the crossfeed dovetails need attention.

Here's a couple shots of what I mentioned

everything is solid and scraped to fit, compound removed and replaced with a T slot plate



Cross feed indicator - digital is great for this, micron accuracy and easy to switch between imperial and the french system (haha couldn't resist), err metric



Here the sweep tool.  Its essentially a spindle with very accurate AC bearings.  I mostly use it in conjunction with reconditioning (second photo) but its easily mounted in the tool post for this sort of checking.  btw, in the second picture is a Holbrook B8, marvelous lathe and a current project





Nice set up!    Are you documenting this rebuild anywhere?....Love to follow along!....Seems we share a similar sickness....LOL

Dave
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Offline steam guy willy

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Re: Accuracy or lack off - what to expect
« Reply #29 on: January 01, 2021, 03:51:30 PM »
To prevent crabbing on flat bed lathes my ford type would it be advisable to remove the middle part of the saddle slides front and back ?? and if so how much would one remove along the length from the middle ??

Willy

Offline Jo

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Re: Accuracy or lack off - what to expect
« Reply #30 on: January 01, 2021, 04:18:17 PM »
 :headscratch:

The problem with early Myford lathes was that the central guide is half the length of the front one. By milling this central vertical face away and using the rear face as the guide it is the same width as the front and is not worn (a shim may be required)  ;)

Jo
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Offline Mcgyver

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Re: Accuracy or lack off - what to expect
« Reply #31 on: January 01, 2021, 06:04:37 PM »
Nice set up!    Are you documenting this rebuild anywhere?....Love to follow along!....Seems we share a similar sickness....LOL

Dave

thanks, I does tend to get its tentacles into you doesn't it?

The alignment device was my take on a Kingway tool and was covered a in build series in Home Shop Machinist starting Mar 2019.

The Holbrook is part of a series i'm working "reconditioning a lathe" which will feature it, a Maximat 10 as well as some of my 10ee and DSG.  Its a followup to  the series did on scraping....couldn't tell you when it'll be done or how long it will be.  Big projects that takes ages with lots diversions and distractions...but 100's of photos are being taken.
« Last Edit: January 01, 2021, 06:07:58 PM by Mcgyver »

Offline Admiral_dk

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Re: Accuracy or lack off - what to expect
« Reply #32 on: January 01, 2021, 10:09:33 PM »
Sorry for being away after my question and thank you for all the nice answers.

I assembled the lathe back together today - no it wasn't that much appart only the subjects in question.
As I wanted to try my "luck" with the great "DCGT 07 02 04 - ALU UK 410" Jason suggested a while back to another member in another thread. I had to mill my new holders first - say what  :headscratch: - Ahem, I could see that my quick-change Tool-Post accepts 16mm. holders and the bigger the fewer vibrations chatter etc. - But did I check that this would mean them seated in the right height ...  :embarassed:

This operation really showed me the truth of one of the first answers to my question, as the holders are made from tool-steel. Using an 10mm. Four Flute HSS End Mill, I tried with a 0.5mm. cut and the mill literally bend and did other not nice movements  :o
It took me some time to know exactly how hard the collect for the Mill Bit needs to be tightened (a lot), but I ended up with a 13mm. high holder and this allows for a cutting edge being 0.5mm. lover that the centre -> enough adjustment.
I made sure I got it up to very close to centre height before testing the setup.

During assembly I also discovered that the DRO is part of the problem - to a degree ....
When I installed it first time on the lathe, it worked fine - BUT it rendered the Tailstock useless  :facepalm:
So I moved it down below the whole Bed - It is out off the swarf as long as I empty the tray before it has more the 5cm. / 2" off swarf - and it can't be filled with what drops down from above.
This means that I had to make a few 'adapters' that are screwed together and onto the Cross-Slide.
I knew that it can be bend a bit, but haven't thought much about this until reassembly, where I noticed that the glass-scale has a certain friction.

This I found to mean that the DRO will decrease the diameter on the display between 0.01-0.02mm. after the lathe begins to vibrate. Starting the lathe before setting the cut depth reduced this to 0.01mm. -> so there is room for improvement with a better adapter.

My first test piece was a Ø 7mm. piece of scrap alu. I aimed for 6.0mm. in my first attempt and got 6.014mm. - not bad. So I tried to let it cut again without touching the Cross-Feed Handle, and I can attest to Jasons claim - it can make swarf so fine it floats in the air, but it removed around 0.003mm. per pass until I got to 5.997mm.
I tried to go for 5.0mm. and got 5.011mm. as first, but this time I ended at 5.003mm. after a number of 'No Cut' passes.
Both test on this piece looked awful to the eye - but my fingers or nails can not detect anything but a smooth surface  :)

OK - this isn't bad - next test with some of the 32mm. alu I have plenty off. I took it to all the whole numbers, 32, 31, 30, 29, 28, 27, 26 as my Micrometer is a 25mm. they were all measured with a caliber and I couldn't detect any difference from the required diameter.
The aimed 25.0mm. measured to 25.032mm. so the 'No Cut' approach wasn't enough, and I had to try a small adjustment and I overshot it a bit - 24.980mm.
Next a try at 24.750mm. - a bit better at 24.740mm.  -  24.500mm. became 24.506mm. in first try and after two 'No Cuts', I ended at 24.497mm.

All the cuts in the 32mm. alu gave a fantastic surface - mirror finish and fingerprints looks like mountains on the surface  :)

Does this means that all my problems (with the lathe) are over - no I do not expect so - but now I'm getting to a point where I should be able to do part I will be happy with  :)

Dave - I like your suggestion about testing with some steel - but all the stock of steel in my stash is scrapobinium - with the exception of some very high tension steel bolts I bought for making crankshafts from - so I'm not sure that I will get the result you ask for.

Offline steamer

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Re: Accuracy or lack off - what to expect
« Reply #33 on: January 01, 2021, 11:19:51 PM »
Admiral,

Anything that is not a huge challenge to machine.   Aluminum is "alright"...it tends to grab and cut under....Brass is expensive

If you go to the home center, and grab a piece of round bar....you'll have the steel I was thinking of..finishing this steel is a bit of a challenge ....it tends to tear... you need a sharp tool...and some top rake, and a hint of a radius at the tip of the tool....then stone the tool smooth....The sharpness test McGyver suggested is a good one.

Then.....EXPERIMENT.....  Get to understand the language the lathe is speaking to you...it will tell you what it needs....but you have to learn to listen and watch...

0-1 tool steel
<a href="https://www.youtube.com/watch?v=iuXq5PUB28A" target="_blank">http://www.youtube.com/watch?v=iuXq5PUB28A</a>

Dave
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Offline Admiral_dk

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Re: Accuracy or lack off - what to expect
« Reply #34 on: January 02, 2021, 11:49:23 AM »
That is a very nice deep cut you do there Dave. I looked through the replies a few times but I can't find any sharpness test from McGyver - isn't it the one from Mike, using the cutting edge on a fingernail you refer to ?

My intentions are to get the necessary tool holders and Inserts. This way I can be sure that they are the optimum shape for what I'm working with. I'm aware of the fact that I ought to learn to always do a good grinding of HSS. But with very limited workshoptime I prefer to just make the part and not the tool.

There is no place here in Denmark where you can come in from the street and buy metals as a private person ...!  exception being for house DIY.
With an account, you can as a pro buy metals in full length - 6 meter - and they will not cut it up for you - only delivery on a full size truck.
Thank god that a number of companies in Germany sells metals online @ reasonable prices and cheap shipping to DK.

Offline Hugh Currin

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Re: Accuracy or lack off - what to expect
« Reply #35 on: January 02, 2021, 04:11:02 PM »
I think someone mentioned this above, but I'm not above repeating.

It seems to work well to break the last pass into several equal depth cuts. For example, when trying to cut a finish size of 0.500" rough it down to 0.530". Then take two passes of 0.015". Not large cuts but not tiny. So, take the first cut of 0.015" as if it was the final cut (speed, feed, cutting oil, etc.). Then measure the part. Let's say it measures .513". For the final pass then dial in 0.013", very close to 0.015", and using the same speed, feed, etc. take the final pass. It should hit 0.500". Or for the timid, take three final passes of 0.010".

I started using this method and it works very well. Not intuitive, after sneaking up on dimensions for years, to dial in a "large" final cut though.

Can't claim credit for this, I saw it explained on a UTube video from Stefan Gotteswinter.

Thanks.
Hugh

Offline Admiral_dk

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Re: Accuracy or lack off - what to expect
« Reply #36 on: January 02, 2021, 06:52:49 PM »
As this has been mentioned before - not a problem for me - I try to recap.
Most here prefer to go to a diameter a bit bigger than the requested result.
Measure.
Aim for doing half as a first finish pass.
Measure - see much off this result is from what was dialed in .... compensate for this in the last cut.
Do the last finish cut, and it should hit the mark.

Learn how much bigger to aim for on your own lathe, so you can achieve this every time.

It makes sense to me, as I can see that there are certain small 'increments that are almost impossible to twiddle' - this did happen once in my last test.

I do know about having the compound @ 1 degree from the Z-Axis - so that increasing it 1mm. results in a much finer cut - how much depends on the rest of dimension off the Lathe  + Tools ;)  Peter who told me this, used this technique to adjust the depth cut to 0.001mm. accuracy on the big lathe at his work, when needed.
Unfortunately I can forget all about this titbit if I have to use the Tailstock as the Compound has to be a 90 degrees on my lathe (parallel to Cross Slide) if I want to cut from the stock closest to the Tailstock end and to the Chuck in one go :(

My initial question had to do with two tings.
One being a very poor surface finish on the last cuts compared to rough removal and trying the "perfect Insert + holder" for the material to be cut, solved that one to perfection  :whoohoo:
The other one about the Tool moving as I cut, most likely had something to do with the first + a problem with the mechanical connection of the Glass-scale for the DRO, to the Cross-Slide.Again solving the one above, I'm sure helped a lot here too + I learned a trick that partly solves the second.
Or, at least until I find time to weld up a new reinforcement for the mechanical connection, between the parts mentioned -> so that the DRO follows the Tool in a perfect linear fashion and remove friction and springiness from the equation of the DRO readout.

Mikes tip about the Tool MUST be just as sharp as a wood chisel or other good cutting tools and that it should be able to easily cut your fingernails and produce a smooth shave / cut from said nail - has also told me that most of my old HSS were not good enough (sharp yes, but not enough).
Looking at the tip and shape of the brilliant ALU Inserts I bought also shows me that I where quite a bit away from the perfect shape and they do not look like the Inserts for Steel shape either ....

Again thank you all for helping out.

Best wishes

Per

Offline steamer

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Re: Accuracy or lack off - what to expect
« Reply #37 on: January 02, 2021, 07:41:18 PM »
Also    consider positive top rake tooling...there will be less cutting force.

Dave
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Offline steamer

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Re: Accuracy or lack off - what to expect
« Reply #38 on: January 02, 2021, 10:24:07 PM »
John   Klunk

Manufacturing Engineer at Heald Machine....
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Offline Hrcoleman

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Re: Accuracy or lack off - what to expect
« Reply #39 on: January 06, 2021, 05:43:13 AM »
Can I ask what brand and model your machines are?

From what I am reading your issue is not with lead screw/nut back lash.

I had a Seig 7 x 12 that I had issues with early in the piece and was suffering from lack of rigidity.

It was getting so even a 0.5mm cut would chatter.

Turned out the Bearing blocks that held the carriage down to the bed were actually broken and so I made new heavy duty ones from Bronze and hand fitted them for correct clearance along the whole of the bed.

This gave very good results and I was able to take heavy cuts within the capacity of the motor before chatter re-appeared.

Same deal with my little Seig X0 Mill Drill.  Very slack slides as delivered and had to hand fit the gibs in order to attain any sort of accuracy.

My New (Old) 1977 Taiwanese Lathe does not have any issues except that I find my threading lead screw has a great deal of back lash which is only a problem when doing very fine and short internal threads.

My new Seig SX2.7 Mill is just glorious to use (after I trammed the column and head).

Cheers,

Hugh


Offline Hrcoleman

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Re: Accuracy or lack off - what to expect
« Reply #40 on: January 06, 2021, 05:49:08 AM »
When I was a second year apprentice tool maker, I spent a couple of months on an old Repco Power Cylindrical Grinder.

This was a very old machine and would have made a great ship anchor.  You had to switch on the Hydraulics and go and make a coffee before it was warmed up enough to use.

However, once I was familiar with it, I was able to successfully use this clapped out machine and grind diameters accurately to 2 tenths of a thousandth of an inch  (0.005mm).

Probably couldn't do it now...  Too old and my eyes can't cope.

Cheers,

Hugh

Offline Mcgyver

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Re: Accuracy or lack off - what to expect
« Reply #41 on: January 06, 2021, 12:35:39 PM »
I do know about having the compound @ 1 degree from the Z-Axis - so that increasing it 1mm. results in a much finer cut - how much depends on the rest of dimension off the Lathe  + Tools ;)  Peter who told me this, used this technique to adjust the depth cut to 0.001mm. accuracy on the big lathe at his work, when needed.

and

Mikes tip about the Tool MUST be just as sharp as a wood chisel or other good cutting tools and that it should be able to easily cut your fingernails and produce a smooth shave / cut from said nail - has also told me that most of my old HSS were not good enough (sharp yes, but not enough).

The compound is usually slewed over to 5 3/4 degrees (some state this in minutes and seconds which imo gets silly as you won't be able to measure the error vs just eyeballing 5 3/4).  This gives you 10:1, advance the compound .001 and in theory the tool gets .0001 closer to the work. 

It does work to a degree however the spoiler is clearance (and perhaps less than a perfect fit) in the compound.  It can be an improvement but I'm very dubious of claims of turning to a micron with it (or anything else short of lapping) - that exceeds even the lathe's bearing's accuracy.   If you hold your tongue just so, you might be able to work to a 1/4" thou this way.....if the compound ways are in ideal condition.   far better imo to lock compound and rig up an indicator ideally with a bit of backpressure to keep the nut in contact with the feedscrew.

I agree with sharp tools.  It was mentioned the approach of the final two cuts being about the same infeed so the first creates some feedback.  This SOP for machining and does work.  Why it works is that whenever you cut, there's forces and things move - deflection of work and machine, clearance, etc and this is a way compensate for that using the feed back of the first cut.  Its necessary in a lot of cases because the tool geometry simply doesn't work with a "creep up on it" approach - i.e. for a given tool taking .010" off works, taking .0001" does not.  Carbide also needs a decent DOC (the AL positive rake tools might be the exception, but they are AL finishing tools shouldn't be expected to last cutting steel steel)

But it doesn't have to be so.  The key is tool sharpness.  Scraping teaches you that it is very easy to have  a .0001" DOC with either tool steel or carbide, its just a matter of how sharp the cutter is.  In those cases a mirror is put on with a lap and 10 micron diamond paste....and its easily takes a .0001" chip with negative rake.  If its not sharp enough it can't set up the shear plane and bounces across the work. 

Point being, if you get your cutters sharpened properly its doable to reliably take a sub .001" cut and using the slewed over compound might (depending) make things a bit better.  This can be advantageous on a light lathe where reducing forces can really improve things....but if the tool isn't sharp enough a small DOC will be detrimental.

« Last Edit: January 06, 2021, 08:06:19 PM by Mcgyver »

Offline Admiral_dk

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Re: Accuracy or lack off - what to expect
« Reply #42 on: January 06, 2021, 09:58:06 PM »
I'm not sure I can use any lathe to get 0.001mm. accuracy, like my friend Peter (probably very unlikely) - but as NASA had him to make stuff that went to Mars with them in 2004 - I'm sure it can't be far of in his case. He did admit that it isn't easy and that it was only one of all the lathes where he work, that he can do this nine out off ten times with ....

I'm not sure what other Lathe mine is a close cousin to. It was ordered by a Danish company from China and they imported them for a while back in time. I will attach a picture or more.
Model Number is : 60210-BV20-1

 

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