Turning on a lathe -- final pass

[LennyG]

When turning on a lathe, how do you get the last pass accurate?

 

[Biffi]

It depends on part lengh, tolerances and how the part is hold in machine.

 

[LennyG]

When turning on a lathe. I am usually turning ID bushings 2.5" + .020 tolerance. They are 3" to 4" in length. The material is C1045 and somtimes 4340. The tolerances are not critical but I would like to be able to produce them without too much wasted time and be at this ID. I just finished a shaft for a local wood mill. It was 1.5" material made of 4140 precision ground. It was 14" from where I chucked it to the tailstock. It had one step for a collar, this was .001 larger than the rest of the shaft which was 1.245. I managed to do it but it takes a long time and the finish on the shaft is not what I want, even with a new factory bit. Thanks for your help with this and for your previous posts.

 

[funnelguy]

Lenny,
How about we pick one case and go through your concerns on just that case for now. Pick a case and try to supply as much pertinent info as is possible. Material & condition, lathe & condition, tooling used along with speeds and feeds.

The shaft you mentioned sounds like a candidate for a steady or follow rest. Alternatively, try to machine the shaft in such a way a to locate the more critical half towards the headstock to take advantage of the increased rigidity.

How much experience do you have on machine tools?

 

[EdDanzer]

Lenny,
The type of machine, condition of machine, type of chuck, type of center, type of steady rest, and specific material have a profound effect on what you are trying to do. More information would be required to give any real help.
Some rules of thumb;
1.Stick out of part or tool more than 3 times the diameter or cross section will increase the difficulty of machining.

2.The poorer the condition of the machine, and attachments, more time will be required to remove chips, and more difficulty in holding tolerances.

3.The harder the material the more difficult to hold tolerances in a poor machine, but easier in a good machine.

4.Roller steady rests are better than plain, but not as good as a live center, and a live center is not at good as a chuck, so a large through hole head stock lathe is a very good investment.

 

[ubrales]

Ensure that the final pass has enough material to cut and not scrape - allow about 0.010" for this cut - the feed depends upon the finish desired. A common error is to allow 0.005" or less for the final cut - this is too little material for a cut.