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Q on broaching 1/4 slot in aluminum

The broach hogging in has been discussed many times. Tipping is the just the symptom of hogging (digging) in. I've talked to a Dumont Broach engineer about this and he said it is due to the back rake being too high for the material. The rake is general purpose and not suited to every material.
So, did he give you any idea which materials require less rake?
 
If you do go with a broach and bushing in your press, be aware that it can be tricky to keep the broach vertical, there seems to be a tendency for it to tip, cutting deeper at the bottom. That was my experience, and I don't think it's uncommon for beginners. I'm sure there are some old threads on here about this, well worth a search (as always!)
It is quite helpful to orient the broach so you can see the profile of the teeth. Makes it much easier to see when the broach starts to dig in. When it does, push the BUSHING out, remove the broach, turn the part over and start again from the other side.
 
You know how a drill can self feed (hog in) in brass, unless you reduce the back rake on it, maybe to zero? This same thing applies to these push broaches, except it's harder to understand because they hog in perpendicular to the way you are pushing them. I can't think of another cutting tool that free-floats in a guide and only cuts on one side, with nothing to keep the tool in contact with the guide except the force perpendicular to the cutting action.

If you imagine a broach with 45* back rake it's easier to see how it might hog in, and pull away from the bushing. Now reduce that back rake and it will hog in less. Reduce it enough and it stops hogging in. Reduce it more, maybe even to the point of being negative, and it will push back into the bushing and never move away from it.

Did you ever notice whether positioning the teeth away from or toward you in the press the broach always digs in the same way? If it's an alignment problem or a crooked problem shouldn't it reverse when you reverse the way it's in the press? But it doesn't, because it's not an alignment problem.

You can flip, reposition, curse, make long bushings that stick out the top, curse some more, worry all day long and the broach will still hog in. Reduce the back rake slightly and everything is smooth sailing!
 
Thank you for the tips/suggestions, all. It sounds like trying a few practice parts is a good idea. Guythatbrews, can you point to a diagram that shows what you mean here? “The solution is to stone a small land to decrease the rake angle a bit. A few thousandths land will do it.”. I’m not sure I follow.
 
“The solution is to stone a small land to decrease the rake angle a bit. A few thousandths land will do it.”. I’m not sure I follow.
Take a sharpening stone and run it across the sharp edge so there's not as much "bite" .... like you unsharpen the leading edge of a drill for brass or bronze or copper.

broaching%2Btool%2Bgeometry.png
 
Emgo beat me to it! The angle you are reducing is the rake angle in his drawing. It's also called hook angle.

Try the stock broach first. Only stone it if it digs in. With a hand operated arbor press it's pretty easy to feel it dig in. The force ramps up quickly to a point where it's obvious something is wrong, and the broach may obviously tilt.

With a hydraulic press you've hardly any force feedback, so how can you tell if the broach is hogging in? Just push it until you have full engagement then back off and see if there is a gap, however small, between the broach and the bushing. It should be absolutely tight. If it's not, get your stone out. It doesn't take much at all.

Don't make the mistake of reducing the "back off" angle, also called clearance or relief angle. While it does help a bit with hogging in, this does ruin the broach. You'll get a rough finish and the required force goes way up. There is a good chance you'll get built up edge on the back off surface and that BUE will make a nasty finish.

Try the stone. It will improve your push broaching experience!
 
I need to cut a spiral (figure 8) grease groove on the I.D. of a bushing and cannot figure out how it is done. Any ideas as to how to do this would be appreciated.
There are weird little tools just for doing that. It's probably not worth buying one for a single part, easier to buy pre-grooved bushings from a bushing supplier.

There's some newer nc lathes that can react fast enough but one of those is going to be even more expensive than the figure-eight grooving tool.
 
If you do go with a broach and bushing in your press, be aware that it can be tricky to keep the broach vertical, there seems to be a tendency for it to tip, cutting deeper at the bottom. That was my experience, and I don't think it's uncommon for beginners. I'm sure there are some old threads on here about this, well worth a search (as always!)
Plus one on this advice. I've seen it done... was quite annoyed with myself.

A small shop might do the job cheaper than the cost of tooling.
 
If you look at the forces when pushing a broach its quite obvious why it hogs in. You are pushing on the back side of the broach, and the cutting action of the teeth puts a moment on the broach that rotates the little end into the part. Make a slipper for the top of the broach that applies the force over the teeth, or even a little past the teeth, away from the back of the broach.
 
I have posted the answer to the problem of a broach digging in too deep on the bottom end a number of times. The solution is a simple one. Take your broach and run a hard stone like an India stone on the cutting edges of the broach to dull it a little bit Just create a flat on the cutting edges of a few thousands and you are good to go. I know this sounds just opposite of what we were always taught, but it works. Dumont used to put this method out when asked, but now they seem reluctant to admit it. I usually just do it to any new Dumont broach when I get it. Saves a lot of aggravation.
JC
 
I have posted the answer to the problem of a broach digging in too deep on the bottom end a number of times. The solution is a simple one. Take your broach and run a hard stone like an India stone on the cutting edges of the broach to dull it a little bit Just create a flat on the cutting edges of a few thousands and you are good to go. I know this sounds just opposite of what we were always taught, but it works. Dumont used to put this method out when asked, but now they seem reluctant to admit it. I usually just do it to any new Dumont broach when I get it. Saves a lot of aggravation.
JC
It's been quite a while back now, but when I pressed the Dumont engineer about stoning the relief surface and how it effectively dulls the broach and leads to poor broach performance and life, he reluctantly said "You are right about that, but it is the easy fix we provide. The real culprit is too much hook angle for your particular situation. Carefully reduce the hook angle by stoning a small land on the rake surface of each tooth. We don't usually mention it because it requires more work and skill."

I think technical support knowledge has decreased across the board at a lot of these places. Youngsters querying knowledge bases and spitting out boilerplate answers. That is, when you get an answer at all.
 
A single point HSS tool bit travel stroking on a lathe or verticle mill would make an easy one-up 1/4" key slot.in aluminum A solid set-up making a key slot about .0002 larger that the tool width.
For one time assembly the steering wheel could be soft pressed with a shim if needed on the shaft to be radially tight.
 
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Thank you, Guys. When you stone the braoch tool, are you doing one tooth at a time, like this? Gbent, I had already ordered a broach tool or would have taken up your offer. Thanks.
 

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I haven't seen it mentioned, be very careful pushing that broach with a hydraulic press! You don't have the feel with hydraulics like you get with an arbor press. Broaches are brittle and will shatter if you apply too much side load, you want to peck it through. Press a few teeth, release the pressure, press a few teeth, release the pressure, rinse and repeat until you're all the way through.
 
Better yet, do get an inexpensive broach. But START the keyway with the file. Let the file take out most of the metal and then use the broach to finish. That will take a lot of the load off the broach but still give you a nice, professional looking keyway. And hopefully you won't need a lot of hand skill for either step.

One tip: When using the broach, watch it carefully to be sure it remains vertical. It needs to go straight in or breakage is likely. Back off and re-align it on the ram as often as needed. Nothing worse than breaking the broach when half way through.

Edit: I just noticed that Firstram already said that, but it bears repeating.



Very true. But a great result is seldom obtained doing something for the FIRST time.
Get your broach tool but have a 1/8" square file as a backup.
 
Thank you guys. Starting off with the file to remove most of the 'meat' sounds like a great idea. I plan to be real careful with the hydraulic press.
 








 
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