[How do I?] Parting Blade Help

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zimm0who0net

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Sep 26, 2013
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I'm new to using a parting blade and I'm clearly doing something wrong but I don't know what. I've got the blade extended just a bit more than the radius of my part. I feed slow (it's a 6" lathe with a broken crank so I can't go that fast). I'm perfectly square with my QCTP to my chuck. I drizzle cutting fluid on the cut as I'm going. And yet, I get a little bit in and the bottom of my parting blade snaps off. What am I doing wrong?

Incidentally, I'm trying to part a 1/2" steel rod with the brand new (now trashed) P1N parting tool from Shars. Snapped it one way, flipped it around, re-checked everything, fed it in and snapped the other side. Ugh... As you can see from the photo, it's just the very thin bottom part of the tool that snaps off.
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Why not start with better tooling like Latrobe? Stay away from made in China.
 
Parting off was one of the things that took me time to get a handle on. Still an issue at times.
I would have as little cutoff blade as required hanging out. I also make sure it is tight! I mean tight.

Will be interested to hear the comments. The blade snapping where it is baffles me unless it is somehow flexing. Is the blade dead flat against the bottom of the tool holder?

I broke my first cutoff blade last week. It is a bit frightening to hear it.
 
Is your blade holder flat where the thin part of blade sets , could be a high spot.
 
There is nothing wrong with that parting tool. I have the exact same one from Shars and it parts steel with no issues at all.
I will lay odds your problem is one of three things, tool is not sharpened, tool is too low, tool is being fed to slowly.

Here are the things that you must do:
Sharpen the tool! They are not sharp from the factory, you must grind the edge. Yours looks like it has not been ground.
Make sure the tool is vertical in the holder. Do not push in flat into the holder, the t-shape will make it lean. Just keep sliding the tool in and out as you slowly tighten the wedge and it will stand up perfectly vertical.
Make sure the tool as perfectly square to the work, not the holder is square to something random like the chuck. If you want to square to the chuck and the holder, you need a top end chuck and a holder. I use a small square or even a fishtail gauge alongside the tool and the work.
Make sure the tip of the tool is perfectly on center height. This is not slightly above or below, it is center height and no other.

On a light machine, lock the carriage and the compound, snug up the cross slide gibs a tad.

Lastly, cram it into the work! most people I see have problems because they are trying to baby the tool. If there is not a good chip rolling up, you are going too slow. On my old lathe I would actually feed so hard it would slow down the lathe. Feed rate is not what is important, it is pressure. You are trying to force the tool into the side of the work, this takes pressure - lots of it.
 
Blade holder seems flat. I flipped the blade upside down and I can't get a 0.001" feeler anywhere along it.

The bottom section of this blade just seems so darn thin. I mic'ed it at 0.012" Now I know that a P1N is going to be small at the bottom, but 12 thousandths just doesn't seem like enough breadth to hold the stresses of the blade. Joshua, what does your blade mic out at?

Joshua, I installed the blade exactly as you mentioned. Seemed odd that there wasn't a step in the holder to accommodate the smaller part of the blade, so I made sure the whole top part was flat against the back (which meant there was a 0.014" or so gap between the bottom of the blade and the back of the holder.

I didn't square it the way you mentioned, but I just went out and checked it with a square to the part it seems to be nearly dead nutz. I didn't sharpen the blade (although it does seem quite sharp from feel), I'm aligned pretty perfect with the center. Originally I started off a few thousandths high and it wouldn't cut at all. Readjusted down a few thousandths to the center and that's when the chips started flying and my blade broke. And I DEFINITELY didn't feed it the way you mention. Could feeding too slow really cause my tool to break?
 
joshua43214 said:
Make sure the tool is vertical in the holder. Do not push in flat into the holder, the t-shape will make it lean.
Click to expand...
Looks kind of like that may be happening here.

I really don't like that clamping arrangement. A thin tool like that should be clamped from the sides to prevent buckling, which may be how it's failing.

[Edit] Is that cam the only thing gripping the tool?
 
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I am surprised no one has mentioned the size of the lathe. Did you say 6" as in 6 inches? That's pretty small. I gave up on parting on my 7x10, the machine was just not rigid enough and always dug in. Since moving to a bigger lathe I am parting just fine with the same tool.

You could try flipping the blade upside down and parting from the back side. A lot of people have claimed success doing this on smaller machines.

Sent from my SM-N920C using Tapatalk
 
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