Do me a favor.

I am using carbide, CCMT / CCGT in this instance. The diameter is larger, away from the headstock. I did not know about the need for a cut to stabilize.

Yes, depending on the insert type, angle setting, speeds/feeds etc, when the cutter hits the metal, it can either push the cutter away or cause it to dig in at first. I think you're in safe territory and need to learn and explore the machine. Are you measuring the piece with a micrometer or calipers?

What are you speeds/feeds and DoC? Is it 4140 annealed, pre-hard or rusty HR? Are you getting a shiny finish or a matte/fuzzy finish? All these things are factors and/or tell-tale signs of what is happening with the cut. But anyhow, my lathe is known and trusted and I would not lose sleep over 3 tenths on a 1" cut. If it was a 3" long cut on a 3" dia piece of clean/annealed bar-stock with a light DoC at proper F/S and it was getting 3 tenths per inch on a steady taper then, I'd dig-in and fix it.

Ray
 
It is HR, not rusty, and not hardened. Other than that, I'm not certain.
I'm using an Iscar IC907 insert with a 1/64th radius. Cutting speed of about 250 sfm. With DOC above .025 (radius), and feeds above .005, I get a shiny finish. Shallower or slower feed, and it is matte.
This creates another question.. how would I get the shiny finish at a shallower DOC?
 
It is HR, not rusty, and not hardened. Other than that, I'm not certain.
I'm using an Iscar IC907 insert with a 1/64th radius. Cutting speed of about 250 sfm. With DOC above .025 (radius), and feeds above .005, I get a shiny finish. Shallower or slower feed, and it is matte.
This creates another question.. how would I get the shiny finish at a shallower DOC?

IC907 specifies a carbide grade type so I can't tell what your profile or insert size is. What is the CCMT number like 2151, or 321 or 431 etc?

You should be spinning at/around 600-675 RPM for a 1.25 dia piece. Feed at 0.006 ipr (ballpark). Assuming you have a 2151 or a 321 and a typical profile, DoC could be around 0.025 to 0.040".

It's fairly hard to get a shiny finish on cuts under 0.015 with carbide. Need to spin faster.


Ray
 
Is the taper larger away from the chuck , or close to it. How far from the chuck , what diameter are you cutting . Lots of things to consider before worrying the headstocks out. Even the chuck isn't out of being the problem . Only real test is between centers after setting the tailstock to center.
I surely wouldn't worry over that slight amount of taper , it's not a 5,000 lb cnc machine purpose built for tight tolerances . Too many times I read these kinds of worries the only problem I see many times is we expect far more then we should has a hobby . Not putting down any machine or person but ,0003 - .0002 , 1/10th of a thou. Even the micrometer and the person measuring can get that much just measuring.
 
Is the taper larger away from the chuck , or close to it. How far from the chuck , what diameter are you cutting . Lots of things to consider before worrying the headstocks out. Even the chuck isn't out of being the problem . Only real test is between centers after setting the tailstock to center.
I surely wouldn't worry over that slight amount of taper , it's not a 5,000 lb cnc machine purpose built for tight tolerances . Too many times I read these kinds of worries the only problem I see many times is we expect far more then we should has a hobby . Not putting down any machine or person but ,0003 - .0002 , 1/10th of a thou. Even the micrometer and the person measuring can get that much just measuring.

Agree, in many circumstances that amount of error could be attributed to measurement error. I don't think we still know if he is using mics or calipers. On the other hand, the only clue that it isn't measurement error is that he reports no taper on the aluminum test bar.
 
I'm using Starrett & Mitutoyo micrometers. Measurements are repeatable.
The taper is larger away from the chuck.
Turning between centers does not produce any measurable taper, with my micrometers.
Taper occurs when turning items using only the chuck, which is why I refer to such short lengths, as I am trying to eliminate material flex as a cause.
I do realize this is not a serious problem. It's more an issue of wanting to understand the problem. I realize that things are not going to be perfect, but I like to know why.
Thank you for the help.
 
Out of curosity, what size are your tool bit holders?? What tool post??
 
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They are 3/8" IC CCMT, held in a 5/8 SCLCR, on an Aloris BXA QCTP.
 
It has been my experience on my PM-1440GT with tougher materials like steel what you are seeing is tool deflection. Material deflection becomes more of an issue when you get past two diameters of stick out.

The movement can come from the spindle bearings (more so when not at operating temps), carriage to bed, cross slide to carriage, tool post and sometimes the backlash of the cross slide lead screw depending on which direction the cross slide approached the cut dimension. The material, cutter geometry, DOC, feed, cutting speed and tool height all affect how the tool and stock are loaded when cutting.

For tight tolerance work on our lighter duty machines I prefer a well ground and honed HSS tool for cutting steel where I can take a lighter finish pass than with carbide.

Another thing to take into consideration when dealing with sub thousand dimensions is heat generated during the cut.
 
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