Help me diagnose this parting issue

[WobblyHand]

So I'm making parts with my mini-lathe to make a gear shaft for my G0752Z. Having trouble with getting square faces parting. The 0.062" HSS blade is pulling towards the headstock. The blade was set up to be perpendicular to the spindle. The carriage is locked. So the parted piece is convex by about 1 mm. For a 2mm thick washer, this is a bit annoying. The remaining piece in the collet is concave. The G0752Z parts very nicely, with the same width blade, in it's AXA holder.

This OXA toolholder holds the blade level. The blade is ground back 7-10 degrees. Haven't measured it, but most likely 7 degrees. I have tried grinding the blade. Still pulls towards the headstock. The block that squeezes the blade into the holder, seems to be 0.25 mm undersize for the slot. I tried putting in a 0.010" brass shim to take up the slop. Blade is still pulling towards headstock. The material is just 12L14, so it cuts/parts easily. Pretty sure I am at center, and little drama, save for the non-straight cut.

Anyone have an idea or two what might make this better? Don't see any obvious wear on the holder. This parting tool used to part ok. The blade tip shows some wear. I could have sworn that I ground the blade back past that point.

 

[higgite]

If you're sure that everything is square and tight, try this. Grind the business end of the blade at an angle such that when you look straight down on it, it is biased toward the tailstock. And/or check it to make sure it's not already biased toward the headstock. It also looks like you could shorten the overhang a little bit, too, for the shallow of a cut. That might help keep it from flexing. Are you parting with the compound or the cross slide? It sort of looks like the blade travel is not perpendicular to the spindle axis.

Tom

 

[Aukai]

I've tried several "tricks" to get the blade square with the work, but I'm most successful with a dial indicator. no over hang on the cross slide, and tighten gibs if needed.

 

[WobblyHand]

If you're sure that everything is square and tight, try this. Grind the business end of the blade at an angle such that when you look straight down on it, it is biased toward the tailstock. And/or check it to make sure it's not already biased toward the headstock. It also looks like you could shorten the overhang a little bit, too, for the shallow of a cut. That might help keep it from flexing. Are you parting with the compound or the cross slide? It sort of looks like the blade travel is not perpendicular to the spindle axis.

Tom

Parting with the cross-slide. Blade is lifting away from the holder. When the cut is made, then the blade springs back to where it should be, causing the parted piece to fly off towards (and past!) the tailstock.

 

[WobblyHand]

I've tried several "tricks" to get the blade square with the work, but I'm most successful with a dial indicator. no over hang on the cross slide, and tighten gibs if needed.

I will try this. Perhaps my eyes were deceiving me. Or the blade is not true to the holder. Your way would ensure the blade is true. Good idea.

 

[jcp]

I It sort of looks like the blade travel is not perpendicular to the spindle axis.

Tom

I agree...

 

[WobblyHand]

If you're sure that everything is square and tight, try this. Grind the business end of the blade at an angle such that when you look straight down on it, it is biased toward the tailstock. And/or check it to make sure it's not already biased toward the headstock. It also looks like you could shorten the overhang a little bit, too, for the shallow of a cut. That might help keep it from flexing. Are you parting with the compound or the cross slide? It sort of looks like the blade travel is not perpendicular to the spindle axis.

Tom

Stick out is ~ 0.42. I am parting a 0.75 rod, so not a lot of excess blade sticking out. Don't like to kiss the tool holder to the workpiece! Going to try @Aukai's suggestion of using a dial indicator.

 

[WobblyHand]

I agree...

Not what I see when I place the blade at the faced end and crank the cross-slide (not cutting, but running parallel to the face). Path of the non-cutting blade appears to be parallel to the face. The blade is seemingly being sucked towards the chuck. Could be the grind on the tip, or maybe it is extremely sensitive to perpendicularity error. But I will measure what I have and report back.

 

[higgite]

I’m sure you know this, but I’ll mention it anyway because it makes me feel smart.

Building on Aukai’s suggestion, sweep the the blade front to back with the indicator to make sure it’s true to the direction of travel, not necessarily true to the tool holder.

Tom

 

[WobblyHand]

I’m sure you know this, but I’ll mention it anyway because it makes me feel smart.

Building on Aukai’s suggestion, sweep the the blade front to back with the indicator to make sure it’s true to the direction of travel, not necessarily true to the tool holder.

Tom

Well you have my permission to feel that way.
Yep, going to sweep the blade in a couple of minutes - only way to answer the question is to actually measure it!