Help me diagnose this parting issue

I have found this can occur when the parting tool is sharp on one corner and dull on the other, or the leading edge has a slight bevel in one direction or the other. Basically it cuts better on the sharp edge or the first edge to make contact.

Look at the chip coming off when you first make contact with the part. If the chip isn't even it indicates a problem.

A magnifier or loupe is helpful for inspecting the parting blade edge.
 
rabler said:
I have found this can occur when the parting tool is sharp on one corner and dull on the other, or the leading edge has a slight bevel in one direction or the other. Basically it cuts better on the sharp edge or the first edge to make contact.

Look at the chip coming off when you first make contact with the part. If the chip isn't even it indicates a problem.

A magnifier or loupe is helpful for inspecting the parting blade edge.
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The edge towards the chuck is sharper than towards the tailstock! Just checked under an 8X magnifier. I will grind the face of the blade back some more to get the cutting edges the same. Hopefully this will help. And indicate the blade...
 
This is where having made the low speed diamond lapping wheel is really handy. It's easy to get a square grind across a parting blade.
 
Could be just an optical effect from your pic, but it looks like your parting blade isn't sitting vertical in the holder.

When not in a holder designed for p type blades, I like to put a shim between the bottom of the blade and holder, to accommodate the difference in thickness between the top and bottom of the blade.

If the blade is leaning too far and results in rubbing on the right side near the bottom, it can deflect while cutting. Also, the non-level top of the blade can result in asymmetric cutting forces.
 
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!/16" is a mighty thin parting tool, I/d try one a bot wider, perhaps 3/32" The bottom picture seems to show a nick on the forward corner of the tool. Also the RH picture seems to show dullness on the leading edge of the tool, if so, it should be ground back to eliminate it or ground on top to eliminate it.
 
Ken226 said:
Could be just an optical effect from your pic, but it looks like your parting blade isn't sitting vertical in the holder.

When not in a holder designed for p type blades, I like to put a shim between the bottom of the blade and holder, to accommodate the difference in thickness between the top and bottom of the blade.

If the blade is leaning too far and results in rubbing on the right side near the bottom, it can deflect while cutting. Also, the non-level top of the blade can result in asymmetric cutting forces.
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It might be. I will have to indicate it, since pictures and lens distortions can be deceiving, especially up close.
 
benmychree said:
!/16" is a mighty thin parting tool, I/d try one a bot wider, perhaps 3/32" The bottom picture seems to show a nick on the forward corner of the tool.
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Don't have one right now, but will order one. Little mini-lathes can't handle 1/8" blades very well, just not enough spindle torque.

I agree with your assessment on the nick. Ground it off, mounted the tool in holder, and it still didn't look right. Turns out the grinder table wasn't square to the table, so neither was the ground blade. Fixed that up, got rid of the nick, and installed in holder. Much better.
 
rabler said:
This is where having made the low speed diamond lapping wheel is really handy. It's easy to get a square grind across a parting blade.
Click to expand...
Do have a carbide grinder, so used a CBN wheel to clean up the parting tool. Does a nice enough job - if the table is trued!

Part of the motivation to get the rotary table was so I could make decent circular slots - so I could make a low speed diamond grinder...
 
WobblyHand said:
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!
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Ok. Problem fixed. It is so much better now.

Measured the blade perpendicularity. Was off by 0.010" in a 0.400" movement. Didn't think it was that far off. Was able to reduce this to 0.002 in 0.500 movement. Making things slightly complicated is the fact the tip of the blade is bent by 0.002". Was able to true the blade surface in the holder to under 0.001 in a couple of inches.

But that's not all. Problem was in the blade as well. Fixed the blade tip by grinding it. Found out the grinder table wasn't true to the wheel. Dope slap. Reground blade after truing table. Stoned off burr on far side of blade. Blade now cuts pretty straight. Not optically straight, but pretty good. Since the blade is bent, I'm going to get another one. Might try to unbend it, once I get a new one, but that's for later.

Not perpendicular to spindle, not ground straight, one side duller than the other and thin blades = recipe for wandering parting cuts.

Thanks for everyone's help! This had been bugging me a while. Really made it difficult to make thin washers.
 
Glad you got it fixed, great job finding all of the bug a boos.
 
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