Author Topic: Accuracy or lack off - what to expect  (Read 4503 times)

Online 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
Enjoyment is more important than achievement.

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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Damned ijjit!

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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Damned ijjit!

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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