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Simple broaching problem

Peter Colman

Stainless
Joined
Sep 22, 2004
Location
Rugeley UK
I have to broach a 10 mm keyway in a 32 mm hole in a mild steel component 60 mm thick. I made a bush for the broach that was a nice sliding fit on the body of the broach and a close push fit in the hole, the flange on the end of the bush was 10 mm thick and it finished flush with the the underside of the part ie it was 60 mm under the head.
The bush was made so the broach would pass about one third of the way through the hole before it started to cut.
I have a well made broaching press with a geared ram and three support pillars, so the pressure is exerted perpendicular to the workpiece.
I set the job up on the base of the press and stared to broach the hole, the broach dug into the work and pulled into the keyway so getting out of line.
I turned everything over and the problem was the same when working from the other side.
How do I stop this happening?
Thanks
Peter
 
Sounds like the broach is not ground correctly for the material, To much rake or clerance for that rake on the teeth and they will pull in like you have found, especially in something as soft and gummy as M/S
 
In my opinion, it is because you have set it up to pass a third into the hole before cutting. The lead on the front of the broach shoild be a good fit at the top, this helps to align and keep everything upright from the go. Without it the broach has the ability to cut itself away from its supporting guide. Make a shim, or new guide to facilitate this.

Jules.

Sent from my D6603 using Tapatalk
 
The Dumont literature says to stone the broach

The best way that I have used is to pause and back up look for movement in the broach

If there is movement than tilt the blank or lean the broach and blank assembly prior to the ram making contact.
 
I agree with English Jules make the slot a bit deeper in the guide bush and an extra shim ,once you have taken the first pass things should go better ,basically you want less of the broach sticking up in the first place to prevent it buckling over.
 
Sorry sable, that's not quite what I meant.

You want almost the full broach sticking out at the start. At the very bottom of the broach should be a plain part without teeth about 3/4 inch long or so. This part should be a snug fit into the top of the part hole / guide slot. This will prevent the broach from pulling itself into the work, as long as you press carefully downwards without tipping the broach.

If you have the dumont set with pre made guides / shims, please inspect the stock guides and the engagement supplied and hopefully you should see this.

Jules.

Sent from my D6603 using Tapatalk
 
You do realize his part is almost 2.5" thick. Taking a full depth cut with that bore length may well pack the gullets of the broach. Pretty sure that's why he is not taking a full cut. Just speculation on my part though.
 
If the gullets fill too much, which I think he will be OK on. Then just feed half way, withdraw, clear gullets and go again. I am merely offering up the solution to the problem at hand.

Edit; besides, the gullets will still fill from being a third in, just not the lower third!

Jules.

Sent from my D6603 using Tapatalk
 
You do realize his part is almost 2.5" thick. Taking a full depth cut with that bore length may well pack the gullets of the broach. Pretty sure that's why he is not taking a full cut. Just speculation on my part though.

Thinking about that ,even if only one tooth cuts to its' full depth it could be too much if the depth of cut per tooth is too great.
 
A small diameter workpiece may not be stable enough to stand squarely on it's end. I sometimes use a small 3 jaw chuck to hold the part so that it will stay vertical.

Nonetheless, I sometimes have this tilting problem with the broach. It is important to have a frame of reference so that you can judge how vertical the broach is. That might be a square or some sort of extension fastened to the press ram that hangs down behind the back of the broach for a few inches so that you can judge the width of the daylight and ascertain that the broach is entering vertically. As soon as the broach comes an inch or so through the bottom of the part, you can pause and take the assembly out of the press and see how the gap is at the bottom. You can straighten it at that time and then continue with the cut and it will probably stay square for the remainder of the pass.
 
Broach should just start in the hole.
Last time that I made a bushing ended up using a lead hammer to lightly tap broach to start it. Either make a new bushing, a different pitch broach, or get a one pass broach.
 
Starting the broach part way down will not help with loading the gullets. Each tooth that cuts will have to pass the full length of the part. The length of the part is what controls the volume the gullet load. The only way to reduce the volume of chip is to remove some material before broaching, say using an end mill to remove some material.
 
Thank you for the advice gentlemen, I always make my guide bushes quite long, in this case it was as long as the component thickness ie 60 mm.
I am sure that I need to make shims to ensure that the broach is a snug fit st the start and to take care with keeping everything square.
I would add that I can't work from both sides until I have made the first pass because of the alignment.
Thanks again
Peter
 
If the gullets fill too much, which I think he will be OK on. Then just feed half way, withdraw, clear gullets and go again. I am merely offering up the solution to the problem at hand.

Edit; besides, the gullets will still fill from being a third in, just not the lower third!

Jules.

Sent from my D6603 using Tapatalk

Not good practice to back out a broach. Similar to backing up a reamer.
 
Not good practice to back out a broach. Similar to backing up a reamer.

One should push the bushing out before removing the broach, if it gets stuck.

I've used C size broaches to cut some pretty long keyways, up to 4" long without any overpacking issue. But if the broach goes crooked then it will overpack and get stuck.
 
Peter Colman: Sorry, Heavey Metal has it right. If you have any doubts, just contact Dumont Broach technical reps.

They will tell you to lay the new broach down on a bench with teeth up. Take a good fine grade India stone and "stone"

the teeth. Yeah,I know it flies in the face of every thing we believe, but it works. About once a year, we go thru this same

argument with the same answer. Somewhere on this forum, a couple of years ago, someone posted the direct response

from Dumont. Do a search and you may find it. We been down this alley before. It does work, I tried it. Sheese.

JH
 
Why does a broach need a relief angle? Each tooth starts cutting at the top of the part and goes straight down. Or at least, that is what we want it to do. There is no need for any individual tooth to penetrate into the cut any deeper than when it starts at the top edge. The next tooth will do that.

I could see having a very small relief angle to decrease the friction. But a half degree or even less would do that. And a zero or very small relief angle would help to prevent the broach from wandering: kind of like the land on the OD of a twist drill. I would think that the manufacturers of broaches would use zero relief angle on the teeth or a very small one.

And besides, the sides of the broach are not relieved in any way. They will rub against the sides of the slot being cut and produce a lot more friction than zero degree relief on the cutting edges would. So you are going to want to use a cutting fluid with good lucrative properties.
 
We have a repeat job, a 4142 acraloy wheel 4" diameter with 25MM hole. Several scrapped blanks has taught us to take great care that the 8MM broach is perpendicular to the upper side of the wheel.
A very good purchase 30 years ago was a set of hardened squares from Poland, these we check, a couple whacks of a copper hammer sets the broach into the bushing, but not so much that it can't be adjusted, then set the square against the back of the broach to make sure it's going down straight.

Simple as that, and no more scrapped parts. We use the black Casteroil cutting oil so the broaches last a long time.
But the advice about honing the front edges what is the actual surface being honed, the top or the face of the teeth? It almost sounds like the intent is to put a slight negative cutting relief on the tops.

When I make a bushing it goes in the scrap bin if the first teeth aren't at least dragging, the second tooth should be making a small chip. Large bushings that are correct are treasures to be cared for, if you have ever priced them.
 
If one is going to be so strict about the broach barely entering, then surely this must be the secret to success in keeping the broach cutting vertically. Otherwise, why bother?
 








 
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