Parting off PVC rod - tool sharpening?

Something is wrong but I can't tell what without examining all the details of your setup, many things can cause problems.

When my 1/16" wide HSS cutoff blade stops behaving well, usually when parting stainless, I hollow-grind the end to the curve of the wheel and stone the cutting edge to feel sharp to my finger. My Multifix tool holder gives the blade a 4 degree back rake angle. I stopped grinding a chip breaker in the top surface because I can't make it as smooth as the factory finish.

As for speed, if the blade chatters I reduce it until the cut quiets down, typically at 300 RPM.

It cuts PVC water pipe very smoothly, enough to use it for turning to final diameter.

-jsw

Thanks Jim.

As usual after posting asking for advice I tried again with more success.

I ground the front face square with a little bit of rake.

The top face is flat & square I dressed it with a flat diamond knife sharpener. I increased the speed to 850RPM.

I had to grind the top face down about 4mm as the tool holder isn't the correct one for my lathe.

It's not perfect but its much better than the 'rubbed & melted" cut I was getting before.

I suspect that your cutoff bit may not be perfectly square to the spindle axis.

When I reinstall the toolpost I turn the compound to 28 or 29 degrees for threading, roughly align the toolpost and tighten its bolt, then loosen the compound swivel screws and press the cutoff holder against the spindle end to perfectly square it while retightening the swivel screws.

If I tighten the toolpost bolt last instead I unavoidably shift things a little from the wrench torque.

-jsw

Thanks Jim, I've been squaring it by eye. On my machine it might be easiest to try squaring the side of the parting off blade against the face of the chuck body. I'll try it out.

PVC and poor finish says "heat" to me. Try water as a coolant.

The photos appear to show that the bit deflected when the decreasing rigidity of the parted-off end could no longer force it straight.

Maybe, but in the 2nd picture, the tool appears to be aligned properly. Too much "in", if anything.

And the twisted off nub looks gummy, as if over heated.

Of all the possible problems, heat is the easiest one to check for: do a cut while spritzing it with water.

I'll try some coolant.

Maybe methylated spirits / denatured alcohol ? (Less clean up issues)

I'd be careful with anything flammable around powered machinery. Usually I machine dry unless a bit of cutting oil quiets chatter or improves the finish. Parting off is the exception, then I drip oil for steel or kerosine for aluminum in the cut with a needle oiler, which is tedious but effective. PVC machines very well if the machine is set up right and adequately supports the work without deflecting it.

-jsw