Parting tool drift

I have a T-shaped parting tool in an Aloris BXA7 holder that won't cut straight. It drifts to one side or the other as the cut progresses. The bit is .063" at the bottom, .094 at the top, .686 high. As far as I can tell the cutting edge is ground square (parallel to the lathe axis), the bit is vertical in the holder to within a thou or two, and the bit was square with the workpiece at center height.

I have a thinner tapered parting bit that does cut straight. It's .0625 at the top, .043 at the bottom, .687 high.

The lathe is a 15x50 gearhead, no problem with stiffness or rigidity.

I'm not exactly a newbie with a lathe, but this one has me puzzled.

Any suggestions about what's going on here, please?

Thanks!

Don Foreman

Hey Don:

The only part you didn't mention was overhang - or how far the parting tool is sticking out. This happens occasionally to me and I will pull back on the depth of the tool to get it started.

Don, Sometimes, when this happens to me, the parting blade will be dull on the side(s) just back from the cutting edge. So, instead of having two sharp corners there will be a slight radius along one edge. This is the edge where the top meets the side. Then, grinding the tool back fixes the problem. ERS

Hi, John! Yeah, there was quite a bit of overhang.

Did you move? I enjoyed your zinc-casting site, saw mention of a barn. I don't remember seeing a barn in SLP!

Hmm ... one possibility is that either the compound or the toolpost are slipping under the greater cutting forces with the wider parting tool. This will change the angle of the tool, and will cause it to drift. (Obviously, if it is higher than center, you have extra force on the end of the tool, but I presum that you have eliminated that as a cause.)

One trick which may prevent the toolpost from rotating under pressure is to take some thin cardboard or thick paper, cut it to match the footprint of the toolpost, with a hole to pass the clamp-down bolt. This will deform into any small imperfections in the finish of the top of the compound and the bottom of the toolpost, and make it harder for it to turn in place. If it is the compound turning, instead, I will leave it to you to figure out how to prevent it from turning.

You *do* have the carriage locked to the bed while you're doing this, don't you?

Now I will sit back and see what others say.

Good Luck, DoN.

Don Foreman wrote in news: snipped-for-privacy@4ax.com:

Don, If there is any perceptable difference in the edge radii on the tool, it will pull. You may not *think* you can flex the tool holder...but you can. Cutting tools, especially grooving type tools, will follow physics and take the least resistive path. If one corner is machining more freely than the other, the tool will drift to that side.

Toolholder is not flexing or turning, nor is the compound. The parting tool flexes in the cut, is still square after the cut is made.

The edge radii explanation makes sense. It's new, but that doesn't mean it's well-ground. Import, from India. The thinner bits I have no problem with (other than breaking if push them too hard) are from Mo-Max in Cleveland.

Don.

Between Anthony and Eric, your problem is explained.

Cutoff and deep grooving is one area where carbide inserts really shine compared to HSS bits. I went with Manchester Separator style. (there's also a Kenametal copy) This is sooo much better, you'll never use HSS again.

Karl

You usually don't want to grind the top especially if this is the long blade type. Grind the front face back. Also, a machinest aquaintance told me to experiment with the angle of the front face (i.e. front clearance). There is an optimum for best cutting versus chatter. I found this to be true.

Ted

I never have ground the top of a parting tool -- but then I've never had problems with them either. I'll try a little grinding before I pitch it and pick up a new one.

Karl sure likes his carbide tool. Maybe I'll try >D>

Thanks, Eric and Anthony! I ground the top of the bit a little and ground a bit of relief on the top of the T as well. It runs much better now. I think I got bit by bad import quality on this one. The Mo-Max Cleveland bits cost twice as much, but they sure work well!

On Thu, 29 Jul 2004 00:19:16 -0500, Don Foreman wrote: Glad I could help Don. About those Cleveland bits. Worth more than twice what you paid. They really work so much better than the imported ones. Especially ones from India. I bought, by mistake, some of the Indian ones. Ended up throwing them out. Good for absolutely nothing. And will make you think you have a problem with your setup or technique. In the future, you will probably find it works best to just grind the face of the parting tool. Grind back far enough to restore the original tool profile. Then, if you need to part off a large diameter, you won't need to grind more on the top or sides. Eric

It seems the Cleveland tools were actually less expensive.

michael