[How do I?] Parting Blade Help

[zimm0who0net]

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?

Yep. I was thinking the same thing. Especially with the bottom 2/3 of the blade is only 0.012" thick and is not flat against the back of the holder.

 

[stupoty]

For small diametor a parting tool can be ground into a regular hss blank, i've found that more rigid than parting blade holders, maybe just easier to hold.

On my small lathe i saw the sadle lift a bit or the compound slide twist a bit when blades dug in then broke.

Stuart

 

[mikey]

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?

Could feeding too slow really cause my tool to break?

All my P1-N blades are about 0.025" thick below the cutting edge. I don't use Chinese blades but 0.012" seems a bit thin to me.

I agree with Josh - a sharp blade is important. Feed should be positive. By that, I mean you should feel a slight resistance to feed and you will if the set up is correct and you can feed consistently. I've never seen a parting blade snap like that but I would guess that you are digging in suddenly and the stress from that, plus a thin hardened blade held in a crappy tool design, is causing it to break. A dig-in happens in an instant and you will not have time to react. This gets worse as you cut in towards the center because your SFM is dropping but your feed may not slow accordingly - a little to much feed and you'll dig in suddenly and then ... snap.

I have a suggestion you might consider. Mill a slot at the top of the tool slot that is wide enough to accommodate the wide part of the blade. The slot should be deep enough so that the body of the blade, the thin part under the cutting edge, can sit flush against the body of the holder. I've done this and can tell you that it really helps to get the blade vertical and held more solidly. Once you do this, use enough clamping pressure to securely hold the blade in the holder but not enough to bow the blade. Holders of this type are rip offs of the Aloris design and in my opinion it is the single worst design they came up with.

I also agree with Atunguyd - a rear-mounted parting tool will make your parting issues go away. I use a P1-N blade on a Sherline lathe and it will zip through your workpiece at high speed without a problem. Your cross slide does not have T-slots but I would drill and tap a hole to mount a rear-mounted post if I were you. Parting on a little lathe can be problematic from the front but it is a routine non-event from the rear. I haven't had a dig-in for years with this set up.

 

[John Hasler]

For small diametor a parting tool can be ground into a regular hss blank, i've found that more rigid than parting blade holders, maybe just easier to hold.

That's what I do. I rough it out with a cutoff blade in an angle grinder.

On my small lathe i saw the sadle lift a bit or the compound slide twist a bit when blades dug in then broke.

Stuart

That may be an effect rather than a cause.

 

[joshua43214]

That is just how they are held with most aloris style QCTP's.
HSS blades will not buckle, they are far to hard. You are lucky if you can get it tight enough to not slip in an AXA holder.
I can see an insert holder buckling if it was not high quality though since they are not near as hard as HSS.

I snapped the tip off a parting blade yesterday while cutting some hardened dowel rods. I had forgotten to adjust the tip height after sharpening the blade. It made a very loud bang, and I about peed my pants lol.

 

[John Hasler]

Mill a slot at the top of the tool slot that is wide enough to accommodate the wide part of the blade. The slot should be deep enough so that the body of the blade, the thin part under the cutting edge, can sit flush against the body of the holder.

Or put a shim behind the thin part of the blade. That still leaves one side unsupported, though.

 

[JPMacG]

My parting tool is a band saw. With my poor skills I have learned to avoid parting on the lathe.

 

[John Hasler]

That is just how they are held with most aloris style QCTP's.

Well then it's a crap design.

HSS blades will not buckle, they are far to hard.

Anything that tall and slender will buckle under sufficient load. Hardness is not relevant.

 

[genec]

Your setup looks good I'm going to take a guess and say it is your tool holder that is causing the stress .

 

[4GSR]

At the point where you broke off the blade, I would have stopped there and used a hacksaw to finish the cut. I very seldom have success parting off to the center on my 9" SBL using a parting off blade. I used to have a piece of Rex 95 ground to about .060" wide by about 3/8 long that I used to part off with. I left it at 0 deg top rake or neutral for parting with. One thing I notice, the lack of cutting oil on the part and part off tool. Flood it with your favorite cutting oil, don't starve it.