Parting off problem

When parting off with a 1/16" x 5/16" HSS parting tool, I'm left with a concaved surface on the part and a corresponding convex surface on the stock left in the chuck.

Carriage is locked, compound slide is locked and with just enough of the parting blade sticking out to complete the cut on a 12mm dia. piece of brass.

Any idea what's going on?

End of tool is slightly not square, causing it to bend slightly would be my guess

Dave

Or perhaps the parting off blade is not square to the lathe axis or it's mounting is loose.

Rod

Bosun

Have you ground the blade to prevent leaving a pip on the work ?

With a thin blade if the tool is as above it will be pulled to the leading edge of the tool, this is what you describe, as said you need a truly ground blade accurately set at 90 degrees to the lathe centreline.

Emgee

i have always ground a slight angle on the end

Thanks Emgee. Not knowingly, although I will check. The tool is set square using a 1-2-3 block against the chuck face.

If you still get problems after checking out all the suggestions, try facing off a bit of bar with an ordinary tool to make sure the cross slide is square to the spindle axis. Locking the saddle if you can when parting is recommended.

I remain unconvinced that when parting off brass the side forces would be sufficient to bend the tool. This is the tool I use for parting off (mostly steel) on my Britan repetition lathe:

The blade is 1/8" X 5/16" ( a little narrower at the sharp end) and has an angle on the end so no pip is left on the parted off item. Once the item is parted off the blade can be used to remove the remaining conical pip. While I haven't measured any concavity or convexity nothing is noticable.

I suspect something is moving, even though it shouldn't be. How good are locks? What top rake does the tool have? Has the tool ever worked in the past?

Andrew

If it is a tapered section blade make sure it is sitting vertically with clearance on both sides, if not the lower edge of the tool can run and send it sideways.

Thanks for the suggestions.

After a bit of investigation, the parting blade is flexing to the right. I discovered this with a DTI mounted on the cross slide, No movement detected in the QCTP. I also noticed, when the blade clamp is tightened up, the back end of said blade moves away from the holder. (this also moves during the parting operation). The blade holder uses a tapered clamping screw with a shoulder that locates on the blade with a square drive clamping nut.

Although the holder is (allegedly) of German origin, not that that's necessarily a sign of good quality, on close inspection, the overall fit & finish leaves a little to be desired.

I think a better quality holder might be called for. Any suggestions for a 1/16" x 5/16" parallel blade?

I guess it doesn't have to suit a parallel blade, but I can keep the current blade as a spare.