[How do I?] Parting Blade Help

[janvanruth]

in parting on a small lathe you want the compound at 90* or 0*, that way the bottom of the qctp will have maximum area of contact with the toolpost
put a screw jack under the front end of the toolholder, it will take the load of the toolpost and will make all the difference in the world

 

[planeflyer21]

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!

That's funny. When it's a kid.

One shop I was in fired a guy from 3rd shift, as he was using the automatic punch press to stamp out quarter slugs by the hundreds.

 

[Bill Gruby]

I had a holder like the one in the beginning of this thread. I have to say it was not the best I have ever used. It had a problem correctly holding the blade no matter what type I used. It would tilt the blade ever so slightly. Try this, put a blade in the holder and look at it from the front end. If the blade is not straight up and down two things could be wrong. #1 --- Wrong style of blade ( mine hated T-Tops. or #2 --- the holder is no good. The OP has been given a lot of info, in my opinion the holder has been mostly overlooked in tracking down the problem. Good luck.

"Billy G"

 

[Bill C.]

I have a small lathe as well, so i'll throw my 2 cents in.

1. It might just be the picture, but the parting blade doesn't look square to the work piece. Mount a dial indicator, and take a reading off the blade, personally Id recommend being square to withing a .001 per inch or better. As other have mentioned the blade grove might not be parallel to the dovetails.
   
2. You need to minimize stick-out, specially on a blade as thin as this one. if you are parting 1/2" stock, don't let the blade stick out more than 5/16" - 3/8". The shorter the stick-out, the more rigid the set-up will be, and the better off you will be.
3. You need to be right on center height. It's hard to tell from your picture, but it looks low.
4. Grind the edge square to the blade. On wider blades you can get away with skewing the tip to leave the nub on one side or the other, but thin blades just don't like it.
5. Cutting fluid/oil is your friend. HSS blades like this are constant width, across the top of the T, so the deeper you plunge the more rubbing you will get. Cutting fluid helps as a lubricant and as a coolant.
6. Speed and feed is important. On my machine 8" I'd be spinning a 1/2" part as 600 rpm or so, and feeding as hard as the machine can take. I usually listen for the motor to load up and then I know I'm in the sweet spot.
7. Use the widest blade you can, it will help maximize the rigidity of the cut. On my machine, I use a P-2, anything narrower gives me grief.

About the tool holder.

1. The Frank Ford T grove modification helps, specially with wider blades.
2. make sure the bottom of the holder has a nice crisp corner as well, I've seen several that a small radius, most likely because the cutter being used to make it was getting old. The radius makes it hard to get the blade nice and vertical.
3. This style holder in PITA to change stick-out, because you have to change too height as well.
4. I really don't think this holder style works well in general, because it's applying all the clamping force at the front of the blade.

That is one thin blade. Parting is a tough setup. Getting the blade seated in the holder and then aligned with face of the part. And don't forget getting the tool length and height correct. Most of the time I was running a lathe once I got the holder setup usually put on the tool storage for the next job.

 

[Bill Gruby]

Also a negative side of that type holder is how it holds. It only captures the front of the blade. The rear part of the blade is free to move. You can literally move it with your fingers when the front is snugged down.It not tightened properly, the blade will move.
and break. Again these are my opinions based on my use of the holder.

"Billy G"

 

[John Hasler]

That's funny. When it's a kid.

One shop I was in fired a guy from 3rd shift, as he was using the automatic punch press to stamp out quarter slugs by the hundreds.

When I was a teenager I hung out at a place that would now be called a sort of an informal "makerspace". We had a soda vending machine with a pitiful excuse for a coin mechanism and one of the guys had the unique ability to cut slugs for it out of sheet metal freehand. It wasn't a problem, though, because the guy who maintained the machine just treated them as IOUs.

 

[Dan_S]

Also a negative side of that type holder is how it holds. It only captures the front of the blade. The rear part of the blade is free to move. You can literally move it with your fingers when the front is snugged down.It not tightened properly, the blade will move.
and break.

In the last video, Joshua posted, the guy is using the type of tool holder I think is best for blades like this. The blade is held parallel to the bed so it has no back rake, and the entire holder body clamps down on the blade.

 

[higgite]

In the last video, Joshua posted, the guy is using the type of tool holder I think is best for blades like this. The blade is held parallel to the bed so it has no back rake, and the entire holder body clamps down on the blade.

On the other hand, in the first video that Joshua posted, the guy uses the same style of tool holder as the OP's, EXCEPT his holds the blade horizontally instead of with back rake. For a mini lathe, especially, I doubt that the difference in the two tool holder clamping mechanisms makes any difference. ymmv

Tom

 

[Ozwelder]

The stick out of your parting off tool is extended way more than it needs to be. Say for instance that s a 1/2 " bar set the stick out at 5/16",just a smidgen over half bar diameter, no more.
The depth (1/2") for the tool is not enough to keep it rdgid. For a tool which is around 1/8" wide I would like it better than 1/2" deep. I believe its flexing there. reducing the stick out will help that.

To part off with out problems you need:

• A properly sharpened tool
  
• Parting tool must be square to the shank to be parted
• Minimum stick out of the tool holder itself-if possible
• The tool post rigidly locked as well as your carriage if there is a carriage lock
• Center height or a touch above
• Constant in feed
• The shape and condition of the swarf produced is a good indicator. If its curling up like a handwritten C that's good.

On stainless a bit of sulphur based lube goop helps a lot.

There can be no possible movement or flexing allowed.Tool holder, tool post everything has to be tight.

 

[kvt]

Ok, I got a QCTP for Xmas and tried it out, What a difference is my parting. It seemed a lot smoother, and everything that my original parting blade holder. Had it set spot on center, and it cut brass and CRS like butter. Don't, know what the diff is because both tools were set to the same. But the new QCTP and parting blade holder really made the difference, wish I had done this sooner.