Powerfeed Ripples

Before I get started, I want to thank the incredible community on the Hobby Machinist forum. Here’s the thread discussing this issue. I also owe Precision Matthews an apology. I came out swinging and it had nothing to do with them.

This post is going to mostly be a recap of that forum with some additional commentary. I hope that doesn’t violate anyone’s copyright. I’m also going to assume intermediate understanding of metal lathe operation. It’s just too hard to stop and explain everything. Furthermore, assuming no knowledge gets tedious for the more initiated reader. I’m also going to do a real hack here and link directly to the images from the forum.

So, here I am, new lathe, super excited. Never had a new lathe before and I’d never set up or adjusted a lathe before. Let me turn it over to me back then to explain what happened when I made my first cuts.

When running the power feed, the lathe creates regular ripples on the surface that are measurable high spots. When feeding using the lead screw, the ripples are not present. I’ve tried a wide range of speeds, feeds, DOC, tool geometry, and two materials (12L14, 6061 AL). No matter what, the ripples have persisted.

Here are the images I posted.

As you can clearly see, one had really clear ripples. The rippled were visible, measurable, and could easily be felt. So the difference between high and low spots was deveral thou.

I’ll expand the post later but to make it complete I’ll just to the end of the story. I assumed that the carriage only relied on the top way surfaces. You know what they say about assumptions… The rack that pulls the the carriage along pushes the near side of the carriage up and down a little as it moves. Without anything to counter those forces, the carriage actually lifts slightly with each tooth of the rack. The carriage also has gibs that control the fit against the sliding surfaces of the ways. This gib is on the underside of the far way that prevents this lift. The carriage gib is also extremely inaccessible. To add insult to injury, the exploded diagram that came from the factory did not show the gib.

Bottom line is I wouldn have spun in circles for months if not for extremely helpful people on the Hobby Machinist forum. Experience is everything.