Offseason Project Help

Again I seek the never ending support of the CD community,
I am working on a project for my Robotics class in school(Implemented by my team!) and I need to use the lathe to make a nice surface finish. I have used the lathe before but at the incorrect feed rates. It is a normal lathe, not CNC enabled, and I have carbide inserts for tooling. The way the lathe is set up it bases its table for feed rates based off of the size of the chip it creates. How do I know what to set it to! Math is our friend and any formulas would help tremendously.
If you are wondering im making the casing for a Tesla Turbine and need it to be as round as possible!

This guy has some really good videos about turning, and I know that in one of them he talks about SFM (surface-feet per minute) for cutting tools.

I can’t remember which video exactly, but I think it might be one of these two (don’t have time to watch through them at work…)

Essentially, for a given tool + material combo (i.e. HSS + steel, HSS + Aluminum, Carbide + steel, etc.) you have a target SFM that you want at the cutting tool, which you can calculate based on the diameter of the part you’re cutting (C = pi * diameter).

There are some more equations here:

What material are you cutting? Some materials won’t give a nice surface finish no matter what. Feed rate is less important than cutting speed when trying to make a pretty finish. For example, you can feed a polished, big-nose-radius, “up-sharp” carbide insert (see Korloy or Walter in the tool catalogs) meant for aluminum turning, at rates so high that you see the feed pattern like a fine screw thread, but the finish will be shiny.

The trick with relatively gummy material like 6061-T6 aluminum (or stainless steel) is to keep the chips from hitting the freshly-cut surface. The chip wants to weld to the surface, and that ruins the finish. This can be a challenge at times, and WD-40 on aluminum helps, but one way to avoid it is to take a finish pass feeding in the “backward” direction, which often directs the chip away from the freshly-cut surface.

The trick with soft steel is to get the surface speed above 300 ft/min (at least) with carbide, and take a healthy cut, like >0.015" on the diameter for a normal robot-shop lathe. If you end up needing to take a 0.001" finish pass, be prepared for the surface finish to suffer. When facing, the surface speed goes to zero approaching the center, so the finish often suffers inevitably near the center.

The cutting edge needs to be in good shape. Even a tiny wear flat on the nose of a carbide insert can kill surface finish.

On a manual lathe, I typically cut by feel and appearance based on experience, rather than by the numbers. That is the wrong approach on a CNC machine. So, play around with speeds and feeds and experiment while the part is oversize (or use scrap) to see what happens, and find the sweet spot.

By the way, roundness is not surface finish, except on the micro scale. Roundness is more likely to be affected by squeezing the part too hard in the chuck, or warpage from residual stresses in the material. Are you looking for “pretty” or “geometrically correct?”

Pretty helps but geometrically correct matters more to me.

There’s also a lot that goes into “geometrically correct.”

Circularity, cylindricity, and concentricity are probably the 3 main “geometrically correct” terms that you’re looking for, and it depends on what you need to happen to get those right on the lathe.

Circularity pretty much happens by default on a lathe (as long as everything is tightened down and doesn’t flex too much. This is just how close any “cross-section” of the surface your cutting is to a perfect circle.

Cylindricity will come when you move the tool down the part and is mostly determined by how parallel your whole set-up is (i.e. does the tool move parallel to the axis of the spindle, or are you cutting a slight taper?)

The fun one is Concentricity. If you’re making two different cuts in one setup (i.e. cutting ID and OD while leaving the tool still in the chuck), there’s a really good chance that those two diameters are going to be very concentric. If you take the part out of the chuck and re-insert it (like if you need to turn it around), then you need to make sure that it is properly aligned. Any error in this alignment will lose you concentricity