Taper bore broaching......what am I doing wrong?

[Suzuki4evr]

I have a problem I can't seem to solve and it is costing me money. I have a client I do quite a lot of flanges with a taper bore and 4mm keyway. Now I use to make the keyway on the lathe withe compound and HSS toolbit,but I didn't like to put so much force on the screw so I got a HSS broach on eBay. Since I haven't done taper bore broaching before, I looked up on YouTube for guidance. So what I found is that I had to make a taper plug to guide the broach. I also made a base with the same angle so when you broach,the broach sits upright and not at an angle.


Now my problem is,I broken two broaches already and I don't understand why. The teeth brakes of around the middle of the broach.

At first I just thought ot was poor quality broaches,but when I inspected the bore after the second breakage, I noticed that somehow the broach takes a much bigger bite at the bottom of the bore than the top at the first pass with the back spacers. And that is way too much for the teeth to handle. So what am I doing wrong? The slot in the plug is at the same angle as the taper of the plug,so according to me the material removal should be the same right through, but that is not happening.

Now I had to go back to the old lathe method just to finish the job. I don't want to buy another broach before I can solve the breakage problem. If someone can shine some light,please do. If I left out anything,please ask any questions to make things clear. Sorry I should have taken more pics with the whole assembly, but I had to get the job out and forgot to take pics.

Please any advice will be appreciated.

Michael

 

[macardoso]

I haven't quite grasped your setup yet, but I'll make an initial comment that I have had broaches lift off of the bushing and pull themselves into the cut before. Never broken a broach doing that, but have blown keyways out of tolerance. Also haven't figured out what exactly makes that happen. Mostly seems to happen with aluminum. Something with the broach rake angle and the soft material.

If the breakage occurs on the second pass where you've added the shim, then maybe you need a thinner intermediate shim and take 3 passes. Also verify the angle of your tapered bushing is correct and the bottom of the keyway in the bushing is parallel to the travel of the taper. if not, it will force the broach to take a deep cut.

Also check the material you are using and the cutting fluid.

Last thing, make sure you're driving the broach parallel to the broach path, even if you need to hold the part at some goofy angle with a fixture to do this. Side pressure on the broach from the press (Like if you're pressing top down on the part) can do some weird things and really load the cutter.

 

[macardoso]

NVM, just saw your last sentence that you have an angled fixture to help align the broach. Looks like you did your homework.

 

[Mini Cooper S]

I have had problems occasionally with the broach wanting to tip as it is being pushed through the work piece. This happens when the press has a wobbly ram. I solved the problem by watching carefully as I pumped the ram down and making adjustments to the position of the work under the ram to apply some side pressure in order to keep the broach going straight.

If you have a taper attachment on your lathe you can use the carriage as a shaper. That doesn't put as much strain on the machine parts as using the compound. But if you don't have a taper attachment......well, not much help.

Richard

 

[Suzuki4evr]

I haven't quite grasped your setup yet, but I'll make an initial comment that I have had broaches lift off of the bushing and pull themselves into the cut before. Never broken a broach doing that, but have blown keyways out of tolerance. Also haven't figured out what exactly makes that happen. Mostly seems to happen with aluminum. Something with the broach rake angle and the soft material.

If the breakage occurs on the second pass where you've added the shim, then maybe you need a thinner intermediate shim and take 3 passes. Also verify the angle of your tapered bushing is correct and the bottom of the keyway in the bushing is parallel to the travel of the taper. if not, it will force the broach to take a deep cut.

Also check the material you are using and the cutting fluid.

Last thing, make sure you're driving the broach parallel to the broach path, even if you need to hold the part at some goofy angle with a fixture to do this. Side pressure on the broach from the press (Like if you're pressing top down on the part) can do some weird things and really load the cutter.

I have done everything you have mentioned above. The angle is exactly the same,the broach is parallel to the work,I have made the the base as I mentioned and posted a pic,I have just put it upside down to show the angle. It actually fits nicely into the part so it won't slip when tuned rightway up

 

[talvare]

I noticed that somehow the broach takes a much bigger bite at the bottom of the bore than the top at the first pass with the back spacers.

Not sure if what you wrote is exactly what you're doing, but the shims (back spacers) are not used on the first pass.

Ted

 

[Suzuki4evr]

I have had problems occasionally with the broach wanting to tip as it is being pushed through the work piece. This happens when the press has a wobbly ram. I solved the problem by watching carefully as I pumped the ram down and making adjustments to the position of the work under the ram to apply some side pressure in order to keep the broach going straight.

If you have a taper attachment on your lathe you can use the carriage as a shaper. That doesn't put as much strain on the machine parts as using the compound. But if you don't have a taper attachment......well, not much help.

Richard

No taper attachment, that would have been nice yes

 

[Suzuki4evr]

Not sure if what you wrote is exactly what you're doing, but the shims (back spacers) are not used on the first pass.

Ted

The breakage occurs on the first pass without any spacers. I made the plug so that on the first pass the broach will only start removing material about a quarter to half way up.

 

[Superburban]

The breakage occurs on the first pass without any spacers. I made the plug so that on the first pass the broach will only start removing material about a quarter to half way up.

Are you saying the broach is not cutting parallel to the wall?

 

[Suzuki4evr]

Are you saying the broach is not cutting parallel to the wall?

No,I am saying it is cutting exactly parallel to the wall,that is why I don't understand the the bigger material removal at the bottom causing the break. I have done many many broaching on straight bores before and have never broken teeth or a broach before. I combined what I saw on YouTube and applied it to the job.