Broaching blind hole

[DPittman]

I'm trying to make a little gear cutter arbor and the cap needs to have a broached recess for the mating key. I've started broaching on the lathe with a piece of 1/4" hss but can already see that this is not going to be successful. I knew it would be difficult but I thought I would give it a try anyhow. I've broached keyways in aluminum before but they were smaller and not blind holes. I milled out the arbor on the lathe successfully but this cap appears to be beyond my capabilities. Any ideas?

 

[PeterT]

Dumb question. If the key is presumably engaged between the cutter & shank to lock the cutter from rotating, why does the cap need the matching key? Could the cap not just clamp the cutter axially? At least that's the principle of my slitting saw arbor but I actually don't have gear cutters yet.

 

[PeterT]

Hmm... maybe because they use a spacer with broach clearance?

 

[DPittman]

Dumb question. If the key is presumably engaged between the cutter & shank to lock the cutter from rotating, why does the cap need the matching key? Could the cap not just clamp the cutter axially? At least that's the principle of my slitting saw arbor but I actually don't have gear cutters yet.

Dumb question???.....NO I'm the dumb one!!!! I think you're exactly right Peter. I obviously don't know what the hell I'm doing LOL! I often get some crazy idea in my head and can't think past those confines. I was making this thing just based on what I "thought" they looked like but how embarrassing to admit I didn't even really know that. Your pictures are most helpful.
Can this whole thread be deleted to save me from continual embarrassment? (I'm sure I will provide myself many more instances for that)

 

[kevin.decelles]

Too late , I've read it

I would have done the same as you, I consistently arrive at a solution the hard way through trial and error



Sent from my iPhone using Tapatalk

 

[Bofobo]

You could possibly mount an hss blank (or ground to key way size) and use it in a manner that emulates a shaper... making incremental plunge cuts. I did this on my 7x12 bench top lathe for a small sprocket it worked well enough that i remembered it as an option and not a total failure.

 

[DPittman]

Too late , I've read it

I would have done the same as you, I consistently arrive at a solution the hard way through trial and error



Sent from my iPhone using Tapatalk

Thanks for the consolation. I have to laugh at myself once in awhile, I watch my son go about things and think "man you are doing things the hard way, STOP and THINK for a moment!" , then I realize how could he be any different with me as his father?! He"s a chip off the old block.

 

[PeterT]

I'm not 100% but I think the purpose of those generic holders is to accommodate a range of cutters, meaning diameter but also likely thicknesses. So they need the spacer washer between the threaded end cap. And therefore the washer needs the keyway slot. Your cap is kind of a combo washer + end cap. I think it would work perfectly fine but ya, I can see blind broach being an extra challenge.

But if you have a limited or more consistent range of cutters, I wonder if the end of the key was just shy of the cutter width, it would have 99% of the retention strength, but then your end cap would be just a simple washer slug with a hole in it clamping on the cutter. You would very likely have to make the keyway slot longer on the inboard side so it has support & only the end is engaged in the cutter. I dunno, pulling rabbits now haha.

 

[historicalarms]

Just kanoodling here, it the problem ever comes up again in a "confide use" case LOL. could you not cut the slot from the outside with a mill and then form a "blank" to set back in the slot that leaves a clearance for the key and then weld the blank in place on the outside? It wouldn't be pretty but should work.

 

[DPittman]

Just kanoodling here, it the problem ever comes up again in a "confide use" case LOL. could you not cut the slot from the outside with a mill and then form a "blank" to set back in the slot that leaves a clearance for the key and then weld the blank in place on the outside? It wouldn't be pretty but should work.

Yes I don't see why that wouldn't likely work. I think maybe even the weld could be machined down to be made to look pretty too!

 

[RobinHood]

Here might be another way to skin that cat: i just drilled a hole for a brass pin as a driver. I looked at the thinnest of my gear cutters and used it as my base for pin diameter. It also works great for slitting saws. The really thin ones do not have a keyed bore, so i just pull the pin out and am ready to go. Has worked very well so far.

 

[DPittman]

Here might be another way to skin that cat: i just drilled a hole for a brass pin as a driver. I looked at the thinnest of my gear cutters and used it as my base for pin diameter. It also works great for slitting saws. The really thin ones do not have a keyed bore, so i just pull the pin out and am ready to go. Has worked very well so far.

View attachment 4886

That sure looks like a nice simple solution. I wonder why that isn't used more often. I suppose it might be a pinch less rigid than a key?

 

[Tom O]

You could possibly mount an hss blank (or ground to key way size) and use it in a manner that emulates a shaper... making incremental plunge cuts. I did this on my 7x12 bench top lathe for a small sprocket it worked well enough that i remembered it as an option and not a total failure.

I’ve done it that way in a blind hole it worked but it doesn’t break off after the shear next time I’ll give it a groove for the cutter to pass into cleaning it up.

 

[RobinHood]

I suppose it might be a pinch less rigid than a key?

Could well be.
I actually like it that way so that if a saw blade was to pinch because of internal stresses in the cut material, it would just shear the pin and not damage the blade. Ditto for a gear cutter if the cut was too deep or something moved in the set-up.
Since the direction of rotation is always the same while cutting, once any slack is taken up, there is constant drive pressure and the pin has worked well.

 

[Janger]

Here might be another way to skin that cat: i just drilled a hole for a brass pin as a driver. I looked at the thinnest of my gear cutters and used it as my base for pin diameter. It also works great for slitting saws. The really thin ones do not have a keyed bore, so i just pull the pin out and am ready to go. Has worked very well so far.

View attachment 4886

Your parts always look so good Robin. Nice. Sand paper and patience?

 

[DPittman]

Ok I did it in the end how I should have done it to begin with. Thank you Peter and all other for suggestions. I'm learning!

 

[RobinHood]

Your parts always look so good Robin. Nice. Sand paper and patience?

Thanks John. Pretty much, and getting better with feeds and speeds.... plus learning when to use carbide vs hss for finishing sice a lot of time i don’t have the correct insert on hand.

 

[RobinHood]

Ok I did it in the end how I should have done it to begin with. Thank you Peter and all other for suggestions. I'm learning!View attachment 4897

Well done. Hope your gears turn out for you as well.