[How do I?] Parting Blade Help

[higgite]

The OP's image shows a 250-xxx holder which is a BXA if I'm not mistaken. Should be plenty rigid enough, not sure how it fits on a 6" lathe though...

FWIW, the OP's QCTP is a size 0XA, 2 sizes down from a BXA. Aimed at mini lathes. Mine started out on a 7x14 and is now on a 8.5x20. Should be plenty stout for a 6X.

Tom

 

[Dan_S]

I stand corrected. for some reason I though his was thicker.
The principles are the same regardless.
He is parting 1/4 the diameter with a tool 1/3 the thickness. If I had a small lathe, I would probably also being using a super thin tool on 1/2" stock. The thinner the to0l, the less HP and effort parting requires.

That's only half the equation, a thinner tool, needs to have a lot less stick-out, because it's a lot less rigid. The bottom of the op's blade is jewelers slitting saw thin. Personally I would not let a tool such as that project more than 3/8" from the tool holder.

 

[John Hasler]

The bottom of the op's blade is jewelers slitting saw thin.

And I don't understand why. I find that when I grind parting tools I need only a few thous of side relief. Anything more just weakens the tool.

 

[Dan_S]

And I don't understand why. I find that when I grind parting tools I need only a few thous of side relief. Anything more just weakens the tool.

I checked some blades I have:
Import P-1 : .062" cutting width, base is 0.32"
Cleveland Mo-max P-2: .093" cutting width, base is 0.64"

.015" of relief per side does seem excessive.

 

[Tozguy]

For a given size wouldn't a parting blade with tapered flanks be inherently more rigid than one with a T shaped profile? Since proper set up and operation for parting yields heavy chips on top of the tool, what purpose does the extra clearance of a T blade serve?

Also, I find that dropping/brushing heavy oil onto the outer edge of the kerf might not lube the cut well beyond a certain depth. The oil is slung away before reaching the bottom of the groove. Slow speed and a squirt bottle would be more effective in getting oil on the cut.

 

[joshua43214]

I just stumbled on this video and though I would link it here.
The guy is using a very small and lightweight lathe to machine a crank shaft for a steam engine. The stock looks to be about 1" in diameter, and he is parting it off with a P-1-N parting blade. Parting starts at about the 7 min mark. He uses a 1/16" thick parting blade to machine one of the cam lobes, but he parts with a the thinner blade. He does the part in little nibbles, and you can actually see the whole QCTP flex a bunch during the part. He gets through it with no broken blades and a pretty clean surface even though the set up is so far from rigid it might as well be made from wood.

 

[zimm0who0net]

Thanks for that video. Wow, it looks like he's only going about 60-100RPM for parting.

 

[atunguyd]

Running any tool upside down on a small manual machine is questionable. The cutting forces, will be trying to pick the carriage up off the bed, instead of pushing it down against. You might be able to get away with it in softer materials if you run all the gibs super tight.

Machines that where designed to do it, usually had special carriages & beds.

This is exactly why mounting the tool upside down works well in a small lathe. The cutting forces act to push the blade away from the work so any flex in the machine actually relieves the cutting forces and it becomes almost self relating.

On the other hand, a upright tool will flex into the work and act like positive feedback so a small amount of flex causes the tool to dig which puts more force on the machine so it flexes more which causes more force etc... Till something gives.

Sent from my SM-N920C using Tapatalk

 

[4GSR]

Off topic a little, when I was about 12 years old and had about two years behind my belt running dad's 9" SBL, I spent almost two days parting off four quarter size slugs on dad's lathe. Decided real quick, this wasn't a money making proposition!

 

[Bill Gruby]

Be careful when expressing opinions, they can and sometimes do come on a bit strong. Different opinions are good, but can become bad in a hurry. Express yours as if it were being expressed to you.

"Billy G"