Improving lathe saddle feel and turned surface finish

Posted by on June 3, 2014

I kept getting stiction and rough surface finish on my lathe, in a way that I couldn’t pinpoint exactly, until I decided to begin lapping and hand scraping parts on the lathe, to improve it. Prior to a thrust bearing upgrade, among other things. Turns out the saddle clamp (which is held by 3 capscrews on my lathe and it’s simply a machined bar of cast iron with a slight angle to it) had a terrible surface finish, a quick blueing showed it was only making contact on both corners, but no contact anywhere else.

I quickly decided to degrease it, strip the paint and begin the lapping process starting with 80 grit all the way up to 600, with good cleanup in between grits (very important). In the end I had a relatively reflective part with at least 80% of contact area against the flat surface I was working with.

This is great, since we previously established almost no contact by comparison, and a very rough shaper machined surface with deep imperfections.

Once cleaned, dried and oiled, I proceeded to reinstall the part on the lathe, making sure I only adjusted the screws enough so I wouldn’t see any deflection on the dial indicator as I pushed the saddle up with quite a lot of force. Any more torque on the screws would only be detrimental, as it would increase drag with no real added benefit.

In the future I plan to replace this part with a custom machined bronze one, which I believe will perform even better.

Sadly I didn’t take any pictures of the process, but rest assured you will notice a huge difference, specially on the feel of the saddle and the surface finish.

My theory behind the improved surface finish is simply that the previous imperfections were making the saddle tilt ever so slightly, this tilt was amplified by the distance from the toolpost to the pivot, decreasing the quality of the surface finish and of course, in turn, less rigidity was observed previously due to the low contact area.

I stopped lapping at about 80% contact, there is in fact a point where “too much” is “bad” in terms of stiction, also, slight imperfections allow for oil to be retained and carried, without the need of machining oil grooves.

Even better would be to lap the rear bearing surface of the bed, to this now flat part. But, that would take a lot of time and right now I’m happy with the results, so hopefully you’ll try this out and let me know how it went, remember only a small amount of metal is being removed in the process, we’re talking less than a thou typically, unless your part is pretzel shaped…

Given the lower friction and flat surface on the clamp, the saddle now feeds and feels a lot better.

The next “mod” will be to lap and hand scrape the cross-slide, including the gib strip, which is very rough from the factory. I’m doing these improvements slowly, because you can always continue what you were doing later, after you tried it out for a while. Which I feel is the correct way to go, to avoid potentially expensive mistakes.

Good luck and get lappin’!

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