Author Topic: Frustrating Day  (Read 9274 times)

Offline Roger B

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Frustrating Day
« on: December 15, 2013, 07:34:05 PM »
I have been having problems getting good finishes on the CI bits I have been machining recently. I wasn't sure if it was a problem with the tools, the lathe or my skills. I played with different tools, different feeds, and different speeds without success so final decided to install the pair of Japanese taper roller bearings I had bought for my Mini Lathe to replace the Chinese ball races.

I had read David Fenner's Book and the Arc website but was also aware that my lathe was one of the 'other' Chinese ones (Extended lead screw, two clamp screws on the tailstock, gear cover on the apron) so was prepared for the worst. I made two 1.2mm spacers to compensate for the difference in the size of the taper roller bearings against the ball races and started to dismantle.

First problem: The gears would not come off the mainshaft and jammed against the countershaft gears. As these were plastic gears I had to remove the headstock and the counter shaft to get it apart. By this time basically all that was left was the bed.

Second problem: When I put it back together I could not fit the Tumbler gear on the mainshaft as the  plastic spacer was 2mm too long. With a working lathe no problem. With a dismantled lathe big problem  :(  . Luckily it was a press fir over my smallest Rohm chuck and I was able to machine the 2mm off in the drill press using a wood chisel (super bodge  ;) )

Third problem: When I removed the leadscrew to get at the bolts to remove the headstock I found that the length between the bearings was incorrect. This is a problem for another day  :'(

Finally as almost everything else was dismantled I thought I should check and adjust the various slides. The lathe bed is nicely ground, the saddle is very roughly milled  :( Another job for another day.

Maybe my thoughts about buying a German lathe have come at the right time  ;)
« Last Edit: December 16, 2013, 07:39:38 PM by Roger B »
Best regards

Roger

Online sco

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Re: Frustrating Day
« Reply #1 on: December 15, 2013, 09:04:06 PM »
I had the same sort of frustrations with my sx2 mill.

Get a wabeco lathe - i love mine!

Simon.
Ars longa, vita brevis.

Offline ths

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Re: Frustrating Day
« Reply #2 on: December 16, 2013, 05:07:41 AM »
I assume that's an oil way in the bottom of the saddle in the last photo? It does look fairly 'rugged' under there.

Hugh.

Offline Roger B

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Re: Frustrating Day
« Reply #3 on: December 16, 2013, 07:38:54 PM »
It's a bit rough under there. I will see what I can do with a smooth file. I could try to learn scraping (or scrapping  ;) ). I think most of the problem was the two ballraces. If I held the inner races down on the surface plate I could lift the outer races more than 0.1mm, not exactly high precision. I think that there was some preload taking up the clearance. When I was trying to turn a biggish lump of CI I got better results cutting towards the tailstock, the rotating centre probably had better bearings.  :)
Best regards

Roger

Offline steamer

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Re: Frustrating Day
« Reply #4 on: December 17, 2013, 12:59:22 AM »
Don't smooth it.  It's better for it to be a little "rough".  It will hold the oil better...much better than a draw filed polished finish.

Additionally, unless you measure what you're doing, your going to do more harm than good filing it..it is easy to get the saddle out of alignment with the headstock.

...and it's scraping...... 8)

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline Roger B

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Re: Frustrating Day
« Reply #5 on: December 17, 2013, 07:42:41 AM »
I don't intend to do much to it, but from the rubbing marks there looks to be almost point contact at the left hand end of the V slide. I'll blue it and see whats going on then decide what to do.

When I have been using the lathe I have had the feeling that the flat side and the V side are not quite parallel causing the slide to rock or twist. This can be felt especially when milling due to the interupted cutting. I will have go at making it a bit better, remembering that it is a cheap lathe. I could by a new one for the cost of a bed grind as mentioned on the old Myford thread  ::)
« Last Edit: December 17, 2013, 08:26:33 PM by Roger B »
Best regards

Roger

Offline Roger B

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Re: Frustrating Day
« Reply #6 on: December 17, 2013, 08:38:29 PM »
I had a go with the blue and took of the worst high spots with a fine file, put it all back together and spent about 15mins adjusting the slide. It feels a completely different machine  :) . I needed to make a new spacer to replace the bodged one and was able to take cuts (in aluminium) that stalled the motor with no sign of chatter or vibration. Totally different to making the two bearing spacers from the same material before I took it all apart.  :DrinkPint:
Best regards

Roger

Offline smfr

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Re: Frustrating Day
« Reply #7 on: December 18, 2013, 01:23:34 AM »
Nice improvement! That splash of yellow paint on the base cracks me up every time  :ROFL:

Simon

Offline steamer

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Re: Frustrating Day
« Reply #8 on: December 18, 2013, 10:10:25 AM »
Roger,

Glad it's working out for you.   To measure the alignment of the saddle with respect to the spindle is pretty simple to do...even in a home shop.

You will need

A 4 jaw chuck.
A milling machine parallel that is STRAIGHT!...A 6" long one will do.
A dial indicator and fittings so that you can reliably fit it to the cross slide.  A .0005" is good... A .0001" indicator is better.
The lathe powered OFF...and unplugged!

Mount the 4 jaw and invert 2 opposing jaws
Mount the parallel in the chuck as shown


Mount the dial indicator, and crank the cross slide to the back and zero out the indicator on the face of the parallel.

BY HAND, rotate the chuck 180 degrees, and check the reading....bump the parallel until you get the same reading on the indicator regardless of the position of the chuck
Once you get Zero-Zero....the face of the parallel is perpendicular to the axis of the spindle...it can't be anything else.

With me so far?

NOW the real measurement begins!

Rotate the chuck until it's horizontal....like the picture above, and the indicator is zeroed against it.   Now run the cross slide out towards you to the other side...or at least as far as your cross slide travel will allow


Don't adjust your parallel!....What does the indicator say?
The reading on the indicator is TWICE the error in perpendicularity of the cross slide axis with respect to the spindle axis.   You should have a positive number...say about .0004" in 6" of cross slide travel...or .0002" in 3"....that ratio.  Depending on how much cross slide travel you have.

That ratio is twice the error.   so in reality it would be an error of about .0002" in 6" turning concave  "Hollow in the middle"...NEVER convex...or crowned in the middle.

That is why I caution you to go easy with a file in the saddle area of the lathe....that saddle surfaces are very sensitive to metal removal....ie you don't have to move much metal to move the saddle quite a bit.   Especially on the V ways.....it's very easy to turn the saddle.

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline steamer

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Re: Frustrating Day
« Reply #9 on: December 18, 2013, 10:18:27 AM »
You'll notice I used a 3 jaw....it doesn't matter...just as long as you have a hold of the parallel....it'll work.

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline modeng2000

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Re: Frustrating Day
« Reply #10 on: December 18, 2013, 02:32:11 PM »
Very helpful! Now I have something else to worry about 'till I can get round to measuring it.

Many thanks,
John

Offline Roger B

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Re: Frustrating Day
« Reply #11 on: December 18, 2013, 08:32:34 PM »
Dave,

Thank you for that. I don't have any solid parallels, just wavy ones. I found a piece of aluminium bar and checked it on the surface table. It was flat + 0.01mm.

I set it up in the 4 jaw chuck and put my dial gauge in the tool holder. As near to the operator side as I could get the bar was again parallel to a similar tolerance. I then wound it in to the centre, about 50mm and the gauge reading increased around 0.08mm.

It is definitely cutting concave, but perhaps a little more than is nice. I know it has always had tendencies this way. The camshaft housing on my vertical engine has two turned flat faces together, made before I started playing, and as best as I can judge with feeler gauges there is a 3-4 thou gap in the middle of a 2" piece.

After this I put my lump of CI back on the Keats and could actually get a good finish!
Best regards

Roger

Offline steamer

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Re: Frustrating Day
« Reply #12 on: December 18, 2013, 11:33:11 PM »
Good!
Always good to know whats what!

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline Roger B

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Re: Frustrating Day
« Reply #13 on: December 27, 2013, 09:19:11 PM »
I hope everyone had a good Christmas with electricity most of the time  :stir:

While turning the valve blanks for my 25cc engine I found that there was quite a taper developing (can be seen in the pictures, 4.5 -5mm diameter in 50mm ) so I wondered if I had got something else out of true when I rebuilt the headstock.

The first check was the put a length of 10mm silver steel in a collet and check the run out at the far end ~200mm. Better than 0.02mm TIR so at least everything was concentric.

Next check clamp the DTI in the tool post at centre height and run it up and down. Total variation was less than + 0.02mm so the headstock is still parallel to the bed.

Final check. Slide the silver steel back into the collet, centre drill, then bring it back out to 200mm and try it against a centre in the tailstock. Visibly out of line, the bar would deflect as the centre was bought in. When I tried to realign the tail stock there was obviously a problem. My version of the lathe has two clamping screw for the sideways movement and they were obviously fighting against each other. When I took it all apart the critical bearing surface was up to the usual standard with a large chatter mark at one end. That will need some careful attention tomorrow to try and get some sensible adjustment without putting the tailstock barrel out of parallel with the bed.
Best regards

Roger

Offline steamer

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Re: Frustrating Day
« Reply #14 on: December 27, 2013, 10:44:28 PM »
Opposing screws is very common...even on heavy iron

Loose fit on the guiding tongue is not good though.

Tell me that's a good "wring it on" fit...which it should be...if not, we'll have to figure out a way to make it so.

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!