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



## ARC-170 (Jun 19, 2020)

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?


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## mickri (Jun 19, 2020)

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.


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## wa5cab (Jun 19, 2020)

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


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## Tozguy (Jun 19, 2020)

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?


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## markba633csi (Jun 19, 2020)

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


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## JPMacG (Jun 19, 2020)

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.


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## ARC-170 (Jun 21, 2020)

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.


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## wa5cab (Jun 22, 2020)

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.


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## ARC-170 (Jun 23, 2020)

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.


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## mickri (Jun 24, 2020)

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.


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## wa5cab (Jun 24, 2020)

Pull the lead screw and roll it on a flat surface while holding the headstock end in contact with the flat surface.  Or if you have a DI and a multi-armed stand, slide the tailstock off of the bed and mount the left end of the Screw in a 3-jaw chuck.  Indicate the small end.


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## Tozguy (Jun 24, 2020)

ARC-170 said:


> 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.


Run the carriage all the way to the tail end.
Mount a dti midpoint on the ways that reaches to the lead screw.
Use a piece of thin shim stock or wide feeler gauge between the dti tip and the threads of the lead screw. The shim stock should bridge at least 3 threads of the screw. Hold the shim stock so it is tangential to the contact point of the dti.
A small amount of run out should not be a problem. 
A bent screw would be a problem but should be visible to the eye when its turning.


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## mickri (Jun 24, 2020)

Whatever is causing the pattern is on a rotating part.  Not a stationary part.  At the same place of rotation something is forcing the carriage into the work.  The work appears to be about 2" long.  Look at the lead screw where it would be rotating in the half nuts when you made the cut.  Look for burrs, chips etc.  Anything that would be a bump.  Also inspect both ends of the lead screw where it rotates in the bearings.  Again looking for a bump.


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## Tozguy (Jun 24, 2020)

Arc, could we see a photo of your set up when turning this pattern, tool overhang, compound extension, etc.


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## Round in circles (Aug 3, 2020)

Nice one Tozguy ,
I thought I'd set the compound  gib's properly cross slide & saddle gibs too . How wrong I was once I started using top speeds and long bars. I had a similar pattern  I  found I hadn't got the best tool height only 5 thou or so to high  and that as a result the whole compound slide was oscillating a minute bit whenever I had the cutting tool over an inch & 1/4  out of the QCTP.

I redid all the gibs as the machine  had been well worked for nearly a year,  having initially  been caked in old oils & hard grease & as I thought well de gunged with WD40 & green Scotchbrite pads .  I thought there was no hidden old crud  on it anywhere , how wrong I was .
Each time I used the lathe it got a spray of WD40 as well as a dose of bed oil & all oil points serviced , at the finish of every session everything was wiped clean and re oiled ..it certainly paid off , the ways are a polished shine these days .

Every surface is now as  clean as a whistle as it should be  .  I must confess to having the lathe on a lockable hard nylon castor'ed girder frame   & I had to move it well away for the walls etc. to do the saddle adjustment then nip round to the apron side and check the differences then go back & make micro adjustments as needed .
That  year of using the lathe ( getting to know it & myself ) gave me the feel of the lathe so I immediately knew I'd succeeded in tightening up the carriage up a smidgeon this time round ,  same with the other gibs .
When I'd first got the lathe I hadn't really much of a clue as to how stiff or slack things should be for optimum performance .

Sorted the tool tip height  by using a big darning needle with the eye cut off and popped it in the three jaw chuck , then set the tool cutting point to the point on the needle whilst eyeballing it along the cutting tool body top , but did it with a magnifying mirror  behind the needle ie the mirror faced me whilst resting on the back of the saddle .

Had a 60 ish yr old skilled engineer / machinist  come & see me yesterday . when he saw the lathe he was immediately interested , was going to suggest some sort of brass gauge sat across the ways to set the tool tip height .. was taken aback when I showed him the needle & mirror way . He said he loved the lathe wouldn't mind  the Atlas /Sphere  10 F himself if one becomes available in the UK .  Said he never knew there was such a quality machine as small as ours .


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