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

[wa5cab]

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.

 

[Tozguy]

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.

 

[mickri]

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.

 

[Tozguy]

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

 

[Round in circles]

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 .