Single tooth hand broacher tool profile?

[great white]

Some of you may have already seen in my recent thread where built a hand powered broacher for my old atlas TH42to do an internal keyway on a project I'm working on:

While it is getting the job done, it's slooooow.

But more importantly: it seems to catch and hang up quite a bit. I'm only advancing the cross slide .001 at a time, but there seems to be just enough play in the "piston" that it sometimes misses doing a full forward cut and you can feel it drag on the return. Then, it will go in hard or grab on the next stroke.

The OCD side of me says "bore a new holder to closer tolerances", but honestly I won't use this much ever again, if at all.

My cutting tool is a what you would expect from teh average DIY'er shop: HSS, leaned back 7 degrees, back rake cut and sides are undercut. Basically, a wedge.

IN all fairness to the tool and lathe, I am cutting a 3/16" keyway in steel, which is probably asking quite a lot of it:



It;s doing it, but it's pretty frustrating overall.

I saw a youtobe video where a gent was talking about "negative rake" on the tool for such a device (it's a point instead of a wedge, like this /\ instead of this |\ ) and that it cuts on both the forward and draw strokes. He was mostly talking about how it cuts twice as fast, but I'm more interested in it cutting on the draw stroke so it is less likely to catch on the next forward stroke. In a nutshell, I'm just looking to make what I've got run a little smoother.

Anyone ever hear of this or think it's work grinding up a blank to try?

 

[great white]

Ah, here was that video:

talks about it at the 4:00 mark.

 

[Nogoingback]

Tubalcain did one as well.

 

[benmychree]

I do not buy into the negative rake idea, a good tool for slotting should have perhaps 3-5 degrees front clearance and perhaps about 10 degrees back rake and perhaps one degree of clearance on the sides; this is what I aim at for slotting cutters for my milling machine slotting attachment and my P&W vertical shaper, some of the cutters that I measured for angles are stock items from a Bridgeport slotting attachment, and they cut freely. Using a good cutting oil or lubricant makes a big difference in slotting, I use TapMagic.
Tubal Cain's slotting tool is within my view of reality and appears to function well; the hand shaper attachment, to me seems to be a solution in search of a problem.

 

[Asm109]

You said you have .003 slop in the ram. Compared to .001 depth of cut. There is your problem.
Figure out a way to bias the ram over in the same direction as the cutting loads. at is will go better.

I once had to make 1/4 inch keyways 4 inches long in Aluminum. That was slow. The bore was 1 inch, I used a 3/4 boring bar with a 1/4 inch HSS bit.
It took a long time. I would dial in a 2or 3 thou cut and run the rack 3 or 4 times before it stopped cutting. Rinse, repeat.

 

[petertha]

But more importantly: it seems to catch and hang up quite a bit. I'm only advancing the cross slide .001 at a time, but there seems to be just enough play in the "piston" that it sometimes misses doing a full forward cut and you can feel it drag on the return. Then, it will go in hard or grab on the next stroke.

I don't have one, so this is just armchair engineering. I can see the potential for the bar flexing though, especially if its a smaller diameter. Think about a single point boring bar of similar diameter. Its common to do 2 or 3 spring passes to remove that last couple thou. But the key way cutter has more edge cutting contact on the tool and it has to plow that material in one short pass vs.a couple hundred revolutions worth of feed by during boring. So it makes sense that the bar is flexing going in & could drag coming back out due to incomplete cut & the bar un-flexing with each pass. So I guess I would make the bar as meaty thick and as short as you possibly can, just enough accommodate the cutter depth

Dale (the YouTube fellow) didn't discuss many aspects I would consider important. How is he locking the spindle? What happens if you don't have a brake? Is the lowest gear sufficient to not allow any rotation to occur during the broaching action? It sure would have nice to see the quality the finished cut rather than all the blah-blah. On another note, maybe I am off base but I would ensure the compound and the cross slide are locked during broaching. Not only is it another place for lash to creep in, but that repetitive, highly leveraged action cannot be very kind to the lead screw & nut assembly as a reaction force. It seems like it would be equivalent (or worse) to plunge knurling where the lead screw takes the brunt of it. Versus kinder to your lathe $ screw assembly scissor by clamp knurling.

 

[great white]

I lock all my slides and gibs when cutting with this tool. The headstock has it's own locking pin. The old Atlas is good that way. There's still some flex, but that's the nature of the beast with this setup.

I grabbed a small piece of HSS and made the "point" cutter he talked about. I'm not going to say it's perfect, but it's a heck of a lot better than the front cutter. It seems to slide easier, although that would make sense with the negative cutting angle. It will hang occasionally, but it's never a hard stop like the front cutter. All I can figure is it doesn't dig in like the front cutter would. It's also only a few strokes to clear the spot where it "hung"and then it's back to reasonably smooth running, advancing 0.001 at a time.

The key fits nice and tight, so that is at least going well.

Still slow as heck, but it seems to go a little better. I've made more (or easier might be a better word) progress with the point cutter than the fornt cutter. Probably just a bit more tolerant of the particulars (IE: slop, flex or whatever you want to point a finger at) of the tool I'm using....

 

[petertha]

I'm sure there are several variations on the lathe slotting theme, but here is another (more substantial IMO) looking design.

 

[BROCKWOOD]

Mr Pete winning with simplicity yet again!

 

[great white]

I've pretty much decided that anything mounted to the QCTP is insufficient for the task, at least on my rig and in anything harder than aluminum.

I designed mine that way so I could have some height adjustment, but the tool post will creep under the force of cutting causing it to "dig in" at some point and require readjustment to keep the cutter parallel to the work.

I eliminated the QCTP by mounting it in the Atlas milling attachment, but that's not an ideal solution either as it has a fair amount of flex in it due to it's relatively tall design.

If I were ever to make another one, it would be fixed to the center height of the spindle and mounted directly to the cross slide carriage. Obviously, much more robust and everything scaled up to handle the loads.

The one I made works, but it's a frustrating process. Something more robust (and precise) would at least make the process less frustrating (IE: grabbing, stalling, etc) even though it would likely not cut any faster.