What causes this machining mark pattern on an Atlas/Craftsman lathe?

[ARC-170]

I was using the power feed on my Atlas/Craftsman 101.07403 lathe at its slowest setting to cut this steel rod to diameter. I was taking very light cuts: 0.005"-0.010". Spindle speed was about 266 RPM. I got a "scallop pattern as shown in the picture. The "scallops" shown vary in height about 0.001"-0.002" and are every 1/8" or so.



There is some play (maybe 0.005"; I need to measure it) on the cross feed; I think the screw (#10F-36) and/or the nut (#10F-19) are worn. Could the cross feed gibs not be tight enough? I had to hold the cross feed crank and watch the DRO to make sure the cutter didn't back out. I suspect one or both of these has something to do with it.

Any idea what could cause this?

 

[mickri]

I am guessing that there is wear in the half nuts on the lead screw so that as the lead screw rotates in the half nuts the carriage is forced in and out in a consistent pattern. This is just a guess on my part.

 

[wa5cab]

Another possibility is that the lead screw is bent. The thread pitch is 1/8".

 

[Tozguy]

The same thing happens to me. Very consistent patterns along the full length of the cut. It seems like it might be caused by harmonics in the gear train. Have you tried different feed rates to see how it affects the pattern?

 

[markba633csi]

Since this is a change gear lathe, check carefully all the zamak gears for binding and/or foreign material stuck in the teeth. Also a problem with the leadscrew or halfnuts is a a possibility as Robert mentioned.
Another thing to check is the big spindle step pulley. The bronze bushings can develop a lot of wear which could impart a harmonic to the spindle as it rotates
-Mark

 

[JPMacG]

• Lock the gibs on the cross slide for your final cut.
• Make the carriage gibs as snug as possible while still allowing the carriage to move without binding.

Other than that, yes, it may be a vibration problem - interaction of two frequencies to produce a lower frequency that gives the pattern (this is intermodulation, not harmoics). I spent about a year chasing down vibrations on my Craftsman lathe. It was an adventure.

 

[ARC-170]

I seemed to have solved it. The gib screws were not tight at all! Duh.
I put a dial indicator on the machine to see if I could check the lead screw, and as I was cranking the carriage, I noticed the dial moved as I rotated each time. Then I noticed the carriage was really loose; I could rotate it slightly by hand. That led me to the gib screws. I tightened them up and the "scallops" went away.

 

[wa5cab]

Overly loose gib screws shouldn't cause the pattern but if the lead screw run-out is minor, tightening them could "fix" it. If the run-out isn't bad enough to be obvious to the naked eye, it probably isn't bad enough to worry about.

 

[ARC-170]

How would I go about checking if the lead screw is bent? I tried putting a dial indicator on the carriage and moving the carriage with the tip of the DI on the threaded part, but any rocking/rotating movement of the carriage gives a false reading. Having the measuring tip on the lead screw as it rotates (the carriage is not moving) doesn't really work because the thread moves and the DI tip dips into and out of the thread.

 

[mickri]

Take the lead screw off and roll it on a flat surface. Look for gaps between the surface and the teeth on the lead screw. The bed of the lathe should work for the flat surface.