# Fine adjustment



## kiwi2 (Apr 9, 2015)

Hi All,

           I've been doing a project which has required a fair amount of heat shrinking bushings into plates which requires extreme accuracy as regards the diameter of the bushing. For example, a 12.7mm bush going into an aluminium plate heated to 200C needs to have a diameter of 12.728 which I found really hard to achieve with any consistency. The final increments as the target was approached required minute adjustments to the cross feed which were hard to manage accurately.
I got around this problem by setting the tool rest at 5 degrees and using the topslide to advance the tool into the work. 
For every 0.1mm the topslide was advanced the tool was fed into the work by .0087mm. As my lathe is getting on a bit (like me) the other advantage was I could lock the cross slide which removed one more possible source of error.
I'm sure this isn't a new idea, but I thought there may be some people out there who haven't come across it and who would find it useful.

Regards,
Alan C.


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## goldstar31 (Apr 9, 2015)

I recall George Thomas writing this up( in Imperial) in 1973-ish. Now in his  Model Engineer's Workshop Manual in the  Screwcutting chapter. It's only Euclid- which is Greek to me

Thanks for 'metricating it'

Norman


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## bazmak (Apr 9, 2015)

The system is similar to the vernier principle and does work,however your main problem would be the lathe accuracy.Also it is easy to say a dim. to 4 decimal places but difficult to achieve without stating a working tolerance
All precision engineering would have toleranced  dimensions  ie 0.1234 cannot be obtained because as you increase measuring accuracy something has to give
just the same as trying to get an exact no for Pi,cant be done.If you put a tight tolerance on of +/- 0.0001 you have 0.01233 to 0.01235
Which is achievable.Any important dim must have a tolerance.In your case 
i would suggest getting down to to +1 thou  and polish emery the rest
Regards barry


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## digiex-chris (Apr 10, 2015)

It's a good tip and one I try not to forget, even though I use it less now. I find a file to be faster, but sometimes, setting the cross slide over is a better way. I don't have a good method for boring internal holes that accurately, so I tend to ream or bore holes to the thousandth of an inch, then carefully measure with ball hole gauges and make the male feature to fit. Setting the top slide over for this would probably be a good answer if the hole is short enough that my lathe's taper is less of a problem.

I used to set my compound over like that for external work, but I dislike how it messes with my Z measurement. Now I get within .001 or .002" and file it to 0.0005, then polish with emery the rest of the way if I need it really close. Takes lots of measuring to make sure you're not polishing in a taper, but I find it significantly more accurate than my lathe is. My lathe will turn about 0.0005" per inch taper, and it's easy to correct with a file.


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## Swifty (Apr 10, 2015)

digiex-chris said:


> My lathe will turn about 0.0005" per inch taper, and it's easy to correct with a file.



Is there any kind of adjustment on your headstock to correct this taper. I have adjusted larger lathes that have built in adjustment screws. Your taper seems a lot, it must be a real pain on longer bores. 

Another trick to obtain consistent readings on your cross slide is to use a magnetic base with a dial indicator, the mag base would be on your saddle somewhere convienent, have the tip of the indicator against your top slide and watch the movement. The finer the resolution on the indicator, the more accurate you will be able to work.

Paul.


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## digiex-chris (Apr 10, 2015)

I might have remembered that wrong. I might be thinking about over the distance of a test bar. No adjustments but I do know I need to get it bolted down to a sturdy stand and shim the tailstock end level with the headstock end. I'm sure that's where most of it is coming from. Its rare that I bore deeper than 2" on that machine anyway and if its really critical it usually means lapping or reaming after anyway.


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## bazmak (Apr 11, 2015)

If your turning a dia then its your tailstock alignment
If your boring a hole its your headstock/carriage


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## digiex-chris (Apr 11, 2015)

Its a twist in the bed, I need to bolt it down so I can shim it out. Or at least start there, I confirmed its slightly twisted with a machinists level.


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## deverett (Apr 11, 2015)

digiex-chris said:


> I used to set my compound over like that for external work, but I dislike how it messes with my Z measurement. Now I get within .001 or .002" and file it to 0.0005, then polish with emery the rest of the way if I need it really close. Takes lots of measuring to make sure you're not polishing in a taper, but I find it significantly more accurate than my lathe is. My lathe will turn about 0.0005" per inch taper, and it's easy to correct with a file.



If you back your emery cloth with a flat metal piece - say a file - that will help to not get the taper when polishing.

Swifty:  I dislike using magnets in the workshop - but I still do - because all steel bits get magnetised and attracts swarf.

Dave
The Emerald Isle


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## digiex-chris (Apr 11, 2015)

deverett said:


> If you back your emery cloth with a flat metal piece - say a file - that will help to not get the taper when polishing.



It helps, certainly. Similar skill to filing. Its still quite possible to end up with a taper by biasing one side or another, especially when working to a shoulder, or because of the springiness of the job, or the wear on the cloth, etc. The tolerance you're working to dictates how much you need to measure and correct I suppose.


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