Tolerance

[Jerry Levy]

Does anyone know what the tolerances should be on the lathe? In a related question, As I am parting of a piece it seems to get MORE elliptical and tool begins to kick back. Would this improve with more speed?

 

[Brian Gustin]

sharper parting tool would help a lot, but some of it often has to do with the grain density as well. My harbor freight lathe has nasty runout, but I'm generally able to compensate for that and still have things round.. the positive to it is, it also lets me know when it is time to sharpen (tools start to bounce as they dig into side grain and try to lift end grain instead of slicing through it) so first thing I'd do is sharpen the parting tool (and strop the burr off , think of it as a very narrow skew - so it can actually slice fibers as it does the peeling cut) I do think tolerances should be pretty close (less than half a thousandth at worst - beyond that , it'd probably be almost unusable)

 

[Bill Blasic]

None of my lathes run out more than 1 or 2 tenths of a thousandth. When I get a new chuck if it runs out more than 1 to 2 thousandth I send it back. I use a last word indicator. To a point more speed will make the parting cut easier but there is a point that you have cut through the out of round portion and should reach round wood. If it was lathe run out causing it you would have found that out while trying to rough out the piece long before you needed to part off. Wood moves while turning especially wet wood but even dry wood will move as it is turned as you release tension in the wood that will cause movement. When I turn and finish the outside of the bowl my last cut is a finish cut on the tenon as I have removed a lot of wood since I first put the tenon on. This gives me the least amount of run out when I go to rough and finish the inside.

 

[hockenbery]

Does anyone know what the tolerances should be on the lathe? In a related question, As I am parting of a piece it seems to get MORE elliptical and tool begins to kick back. Would this improve with more speed?

99.9% of the time that I use a parting tool it is on spindle work where I use a peeling cut.

Parting tools don’t work well on the end grain/side side grain of bowl oriented wood so you get oval.
Slower feed rate keeps it from happening so much.
A faster lathe speed will slow the feed rate but it is usually better to let the tool do the work and not push so hard.
Peeling cuts will want to catch on the endgrain using the parting tool as a scraper level or tip down slightly works better for me on the end grain side grain.

 

[robo hippy]

Only other thing I might add is that if you have a 1/8 inch parting tool, your kerf/cut needs to be 3/16 wide because the blade can bind in the cut. If you are parting off dry wood, a single kerf will make smoke.

robo hippy

 

[Don Wattenhofer]

Only other thing I might add is that if you have a 1/8 inch parting tool, your kerf/cut needs to be 3/16 wide because the blade can bind in the cut. If you are parting off dry wood, a single kerf will make smoke.

robo hippy

Even with the diamond parting tool those points on the side can wear down near the cutting tip so the kerf is too narrow.

 

[Lou Jacobs]

Just a thought out of left field: are you turning green wood? How long between beginning to turn and parting off? Is it possible the piece has begun to dry and is becoming oval as a result of shrinkage across the grain?

 

[Doug Freeman]

It could be very worn bearings which you could detect by hand. As for spindle or chuck runout (no “play” as in worn bearings) you made the surface “round” when you cut it. It is offcenter to the spindle by 1/2 the runout, but unto itself the piece is centered.

All the comments about tool use and warping are most likely Where your problem is.