turning unwanted tapers

[loose nut]

I was turning a spindle recently using 416 SS. If I made a cut with a good depth of cut (or even as low as .005" doc) and a fairly high speed (carbide inserts) I got a very nice finish but with a slight taper, about .0007 to .0012" over 6". Now if I take another cut at the same depth the taper is machined out but the finish sucks, unwanted lines, dig ins etc. I'm making an assumption that this is due to the tool springing up and down because of lack of loading when the cut is very light, so how do I get rid of this so I can take a light finishing cut.

[lakeside53]

Light cut? how light? Your carbide insert will have a minimum DOC that may exceed your requirements... If so, move to HSS, or take a deeper cut to your final dimension. Are you supporting via the tail stock? If so and you have a taper consider moving the tail stock center. If you get good results at 0.005, make that your final cut and don't attempt a "spring cut" with carbide - it basically rubs, not cuts.

Alternatively... use emery to get rid of the taper. Easy to do and I'm guilty of it all the time

[loose nut]

I was using the tailstock and it is dead on, I don't move it.

Light cuts, 1 or 2 thou, enough to bring something to size, if necessary. Taking the deeper cut is what is giving me the problem. HSS might solve it.

[firbikrhd1]

Stainless has a tendency to work harden when it is machined and that makes shallow cuts difficult. Rigidity of your setup is important for both finish and true turning end to end. If using a tailstock it must be set up properly to assure straight turning as lakeside53 said. If not using one and it is possible to do so that may help with deflection. Even so, the material can flex to some extent while being machined producing taper or smaller diameter at the center point that at the ends in the case of tailstock use, particularly in the case of a relatively dull edge on carbide as compared to HSS. Add the effects of work hardening and you will find it very difficult to remove tiny amounts on a final pass to any degree of accuracy and retain a good finish. When turning stainless a tool bit with a keener edge may help, HSS as suggested by lakeside53 can be sharpened to a keener edge than most carbides. The best method is to deduce how much is actually being removed during initial passes and make your final pass deep enough to take off the work hardened surface and arrive at your finished diameter at the same time.

[lakeside53]

If you are using the tailstock and tool pressure is the culprit then the taper should be "hour glass" shaped. If you have a continous taper, your tail stock is off or it's moving.

In any case, very few carbide inserts will cut at 1-2 thou in 416.

[Bob Fisher]

Very sharp HSS is the way to go on most HSM machines. I have some insert tooling, but seldom use them any longer. I did buy some HSS inserts at a NAMES show, and use those often. But unless you have a lot of power and speed, I think HSS will eliminate your problem. Bob.

[Chuck K]

What the others said. You might try hss ground as a vertical sheer cutting bit. I have taken less than .001 doc with them and had a perfect surface finish. I haven't tried them on stainless though.

Chuck

[darryl]

This might be a case where you could play with the cutter height, and in the case of HSS cutter, you could play with clearance and rake angles. There is probably an optimum here where the work neither springs away nor sucks in towards the cutter. That would let you vary the depth of cut a little without affecting the amount of material that comes off. You could dial in 5 thou and that's what comes off, then you could dial in 1 thou and that's what comes off.

I also like to dispense with the compound and mount the tooling more directly to the crosslide to lessen flex. Depends on the lathe and its parts of course, but it can be a factor.

[oldtiffie]

Use a tool-post grinder with an aluminium oxide wheel - but see that the wheel doesn't "clog up" and keep it well "dressed".

[Jaakko Fagerlund]

Or just forget the HSS and grind one of your carbide inserts to a sharp edge. Have done this many times, just requires a diamond wheel.

I see again people thinking that carbide can't be sharpened and that it is always "dull", but if that was the case there would be no carbide end mills.