Q on broaching 1/4 slot in aluminum

[guythatbrews]

When you stone the braoch tool, are you doing one tooth at a time, like this?

IMO this is not the surface you should stone. You are removing clearance from the tool. This does help with the broach hogging in and tilting, but tools need clearance to cut well. You need to stone the cutting face to reduce the "rake angle" in your drawing. Excessive rake for the material is what makes the broach self feed away from the bushing and dig in.
So each tooth must be stoned individually.

Don't just jump right in and stone it before you try it! The majority of the time the broach will work right out of the box. Push it until a few teeth enter the work. Release it and look very closely at the exit side. The back of the broach should be in close contact with the bushing. If it's coming away you should stone it. Push a few more terth and repeat. If you make it all the way through on the first pass, it's very unlikely it will hog in on the subsequent pass. Just go for it.

This all has gotten blown out of proportion. There is no need at all to do any roughing! The broach will EASILY keyway pretty tough steel, and your part is just aluminum. Be sure the bushing is as long as the part so the broach is fully supported. Then just broach the thing.

 

[EPAIII]

I must disagree about the need for clearance. While that is true of most of the cutting tools we use, an individual tooth on a broach cuts straight down, not sideways.

If fact, you do NOT want it to cut sideways. That would increase the depth of cut for that tooth or, in other words, it would pull the broach away from the bushing. You want to avoid that and having a flat on the outer edge of a cutting edge, just like the LANDS on the OD of drill bits, will help prevent it.

Pulling away from the bushing can lead to jamming or even breakage.

IMO this is not the surface you should stone. You are removing clearance from the tool. This does help with the broach hogging in and tilting, but tools need clearance to cut well. You need to stone the cutting face to reduce the "rake angle" in your drawing. Excessive rake for the material is what makes the broach self feed away from the bushing and dig in.
So each tooth must be stoned individually.

Don't just jump right in and stone it before you try it! The majority of the time the broach will work right out of the box. Push it until a few teeth enter the work. Release it and look very closely at the exit side. The back of the broach should be in close contact with the bushing. If it's coming away you should stone it. Push a few more terth and repeat. If you make it all the way through on the first pass, it's very unlikely it will hog in on the subsequent pass. Just go for it.

This all has gotten blown out of proportion. There is no need at all to do any roughing! The broach will EASILY keyway pretty tough steel, and your part is just aluminum. Be sure the bushing is as long as the part so the broach is fully supported. Then just broach the thing.

 

[guythatbrews]

I must disagree about the need for clearance. While that is true of most of the cutting tools we use, an individual tooth on a broach cuts straight down, not sideways.

If fact, you do NOT want it to cut sideways. That would increase the depth of cut for that tooth or, in other words, it would pull the broach away from the bushing. You want to avoid that and having a flat on the outer edge of a cutting edge, just like the LANDS on the OD of drill bits, will help prevent it.

Pulling away from the bushing can lead to jamming or even breakage.

The drill analogy is specious. You are comparing apples to oranges.

The analog of the drill margin is the sides of the broach, which both have no clearance. The drill point makes a hole and the margin fits into that hole, with no clearance. The broach teeth make a keyway and the sides of the broach fit into the keyway, again with no clearance.

The analog of the clearance angle on the end if the drill is the clearance angle for the cutting surface of the broach, opposite the back of the broach. The clearance angles on both allow the cutting edge to penetrate into the work to do it's job. Just as a drill with insufficient clearance requires more force to feed, and machines and inferior hole, so does the broach with insufficient clearance.

Consider another comparison of a drill and broach. If you drill brass with 45* high-spiral drill intended for aluminum, or even a general purpose twist drill in brass, it may self feed and hog into the work. If you use the same drill with a pilot hole that is bigger than the chisel point of the drill, it is very likely to self feed. Nobody that has tried this will dispute this! Nobody! It's a simple fix to reduce the rake to 0 or even negative. Problem solved instantly, and everyone who's done this will agree! The broach is also a general purpose tool, wherein a hook angle compromise has been made. Why is it not the same situation as the drill in brass? Broaches just won't work well with a general purpose hook in all materials. It's a bit harder to see because the self feed is roughly normal to the broach feed, but it is self feeding due to excessive rake for the situation, just like the drill.

 

[Mike C5]

I am going to try a practice part before stoning. The stoning would be like in attached (with stone moving perpendicular to paper)? That would reduce the rake angle...

 

[richard newman]

For the OP, the chart on the link I posted recommends the highest rake for aluminum over the other materials listed. So it makes sense not to alter the broach until you've tried it a few times.

 

[guythatbrews]

I am going to try a practice part before stoning. The stoning would be like in attached (with stone moving perpendicular to paper)? That would reduce the rake angle...

This is the way I think it should be done. Note the way your red stone is tilted, the stoned rake will be negative. You want to reduce the positive rake as supplied, but not so much to make it negative.

it makes sense not to alter the broach until you've tried it a few times.

100% agree. Broaches often work as supplied.

To make a test part that provides meaningful results, it should be the same material and length as the workpiece you need to broach. Maybe the aluminum workpiece is unidentified cast aluminum so an extruded aluminum test piece may be a waste of time. A test piece is really overkill. As I said above start slowly a few teeth at a time and check for a gap on the bottom. If a gap shows stone the rake face. No gap you are good to go a few more teeth. 1st pass ok then the 2nd pass will be fine. Lubing the spine of the broach so it slides easier through the bushing is also helpful.

 

[Mike C5]

Success! Either beginner's luck or aluminum is easy to work with. It took very little force to push the braod through. If fact, I didn't even use the handle to pump the hydraulic. I just did it by hand so I would be sensitive to load changes. I did put the collared bushing 'upsidedown', so more of it was in contact with the broach at the start. Thanks for the help, guys. The last photo shows the whole steering wheel.

 

[guythatbrews]

Glad it was easy. And now you know what to do when it's not so easy.

 

[Mike C5]

Right!