Parting Off problems

[MinnesotaHSM]

I have an Atlas 618 MK2 lathe. The other night I was trying to cutoff 1018 steel (for the first time ever) using a 1/16" Cleveland Twist Drill Mo-Max Cobalt Cut-off blade mounted in my rock solid KRF Omni-post QCTP.

I was getting chatter so bad that the bench was bouncing. Then I tried reducing the speed to below 200 rpm which according to the book should be in the right speed zone for 1018 (80 sfm). Still chatter. I re-checked that my tip was on center. Then I tried feeding in really, really, really slow. Chatter was less, but all of sudden it caught and jammed the spindle.

As far as I can tell, nothing broke on the lathe, but it scared me.

This setup slices through 6061 like butter. Any ideas on how I can part-off steel? Am I doing something wrong with the setup?

Thanks.

- Tom

[Thomas Staubo]

"Parting is such sweet sorrow"

From Romeo and Juliet
By William Shakespeare


.

[S_J_H]

200 rpm for parting steel seems pretty high for that light machine and you did not mention the diameter of the part. I would put it in backgear and run it as slow as it will go. Make sure the carriage is locked as well as the compound. The cross slide should not have any slop with well adjusted gibs. I am assuming the parting blade has the correct clearances ground in.
A picture of the setup would help.
Steve

[jimmstruk]

Also, use plenty of good cutting oil. If the part is large diameter back out after cutting in some, move over 1/2 blade width and cut in again, making the cut wider than the cutting blade so as the heat builds up the tool will still have some side clearance. And with a light lathe, throw away the feed &speed book and do what works best. JIM

[HWooldridge]

Are you using cutting oil? That often helps me if I'm having issues with parting off steel. I would also speed up the spindle and reduce the feed so you aren't peeling such a big chip. It is spooky when the bit sticks and it goes bang - I broke a .125x1.00" on the big lathe while parting off a 2" slab but it stuck and went POW! Fortunately, the blade was the only thing harmed.

[beckley23]

In another lifetime, it seems, I had a 6" Atlas that exhibited the same problems that you are experiencing. I changed from a lantern style toolpost to a square toolpost which helped.
When attempting to cut off, I noticed that the cross slide was moving up and down just enough that you could see oil being squeezed out. I took the cross slide and compound rest apart and spotted them on the surface plate. What an eye opener. No consistent spotting, but 2 or 3 corners were spotted, with nothing in between. The members were rocking. I scraped the ways and slides of the cross slide and the compound rest, and the difference was very pronounced.
BTW, the lathe was basically new, it didn't have signs of use when I bought it.
Harry

[oil mac]

I have found that 1/16" toolbits for parting off are excellent only for work up to 3/4" dia, over that dia, i go on to bigger more robust toolbits --1/8" broad I dont seem to have much problems with this the "bane of all turners lifes"! , Much also depends on the condition of your lathe , backlash in your cross-slide screw, loose gibs, gibs poorly adjusted, condition of headstock thrust & bearings, and of course feed speed, and tool angles + centre height of tool & feed rate of tool. The statement that the vibration was going down to the bench, makes me also ask, is your lathe sitting level and in a stable surface?
On parting off on my Myford Super 7, (My pal, has a Myford ML7, poor for parting off , due to smaller & weaker bearings) i restrict this operation to steel of no more than 1.250" dia, and i tend to keep my headstock in direct belt drive, If i use the back gear, i am always nervous of a real smash up, so therefore i dont go down that road, I am lucky to have a big Holbrook which parts off heavy bar for me, Again the dead weight of the lathe leads to stability. Always on parting off, i have a golden rule, part off as near the lathe chuck as humanly possible, and keep the tool working for its living, if you slow your hand feed rate down too slow, you can glaze & cause chatter, and dont have the tool too far out from your toolpost, On a deep workpiece, i tend to feed in, pull the tool back, take the tool to the side & cut on one side of the workpiece,thus stepping the cut, side to side every 1/8" depth, this gives the tool clearance for the cuttings, stops the jam ups
Parting off is like a woman, what works for one man causes the next man problems!

[aostling]

I took the cross slide and compound rest apart and spotted them on the surface plate. What an eye opener.

I also have the Atlas 618 Mk 2. I'd like to know more about this "spotting" procedure. Can you describe it in some detail.

[loose nut]

Light machines just tend to chatter when parting off, it's a bit more like magic than shill to stop it.

[beckley23]

Spotting is part of the scraping process. A marking medium is spread evenly on a surface plate, and the piece to be spotted is laid on the plate and moved back and forth a couple times for a short distance. The high spots are transferred to the piece, evidenced by their showing up as blue, red, whatever color you're using, on the piece. These high spots are then scraped off, and the procedure repeated until there is an even distribution of spots.
This is just the barest of details, for in depth study of the process there Edward Connelly's book "Machine Tool Reconditioning" that descrbes the process in great detail.

To add to my previous remarks, the Atlas lathe, at least the ones I've seen, are not scraped. All of the bearing surfaces are milled, with the exception of the bed which is ground. It is the lack of the final details that contribute greatly to the problems experienced, especially cutting off. Scraping the bearing surfaces will not be the "be all and end all" for the Atlas lathes, but it does help. There are several other issues that can not be overcome, mainly the lack of mass. Oil Mac hit it on the head with his remarks, I've got Monarch lathes, and have never had a problem cutting off, but there is a big difference between 3500 LBS for my lightest one and a couple hundred pounds for the Atlas.
Harry