Good Lathe Practice

[Smifffy]

02jcole's recent post has made me think about how I use my lathe.

02jcole has highlighted an issue with his Harrison where his tool cuts into the workpiece when the saddle is backed off from the headstock. Now, I have the same issue with my Boxfod BUD, albeit, it is more of a scratch. However, when ever I back my saddle away from the Headstock I first back off the cross slide. This can be a pain as it does mean that I need to reset the cross slide, which when I am taking very small cuts does lead to accuracy issues.

I was taught how to use a lathe over 20 years ago (with a 20 year gap with no lathe use) and have assumed that what I am doing now is just good practice - am I wrong here ?

02jcole, this is not meant to be questioning your good practice!!

Mark

[brozier]

Hiya Mark,

I'm in the same boat as you - though I didn't really know what I was doing 20 years ago!

I think you get the scratch because the work piece has heated up due to cutting and this causes the work to expand slightly. If it's more than a scratch I guess it could be caused by the saddle rotating due to wear/poor Gib adjustment.

I think it was Harold Hall in his Lathe Course book that suggests setting the top slide at a very acute angle (1:100 rings a bell) so that small finishing cuts can be put on with the top slide. This gives better resolution than just using the cross-slide. I'm guessing this would include backing off when traversing back after a cut has finished.

FWIW the guy demonstrating screw cutting at the last Myford open day was retracting on the traverse back and used chinagraph pencil to mark where the cuts are on the dial....

I guess a George Thomas style retracting tool holder would be equally as good.

Cheers
Bryan

[ausdan]

I thought it was kinda of normal
To apply force and have the tool cutting it will have to dig in and deflect a little, hence scoring when returning the saddle.

Maybe less of issue with HHS bits and smaller cuts

the final cut doesn't matter, because you have can retract the cutter anyway

[steam4ian]

G'day Mark

I came back after 35 years and I recall being taught to retract the tool except for the final cuts, the few thou.
I had done a lot of screw cutting, big bolts in tough steel for concreting into building footings. We didn't have the compound at an angle so you had to advance that with the cross slide. We also had to wind the cross slide out on the return. The bolts had forged heads and were heat treated so we had to centre them first, not a bad job for apprentices and trainees.

My present lathe is an Eastern (for you) effort so unless I retract the tool I get a nice LH thread on the return. To an extent I thought this was"normal". It may also depend on cutting angle, rake and tool height. During the cut lots of rake will pull the tool into the work as will a high tool; on the return, cross slide backlash will relax and allow the tool to ride clear.

Come to think of it I get the LH screw effect more with brass than MS or Ali; my brass tool has no rake.

My few pence worth.

Regards,
Ian

[Smifffy]

...as a matter of course, I normally keep my top slide set at 20deg, and use that to make the very small final cuts. I also now back of the top slide rather than the cross slide. With it set at such an angle there's much less impact if I don't reset it 100% accurately after I've backed off.

I like the suggestion of marking on the dial with a chinagrah pencil.

Thanks for your thoughts guys.

Smifffy

[Nigel Bennett]

Talking of setting the topslide reminds me of when I needed to machine a slight angle (5 degrees) on a buffer head. On the Super 7, the topslide handle fouls the cross-slide, so it doesn't work!

George Thomas came up with a rather nice design for a spur-gear drive within the topslide front plate to cure the problem, but unless you like making things to make things, it's a long and involved solution. I just removed the topslide handle, put in a 2BA hex socket screw and locked it with a 2BA locknut. An Allen key was then used to drive the topslide back and forth. Simple and effective.