Rotary Broach Holder?

[David Kirtley]

Well, I have been wanting one and they are just too expensive. I decided to make my own. I am taking a bit different approach. Instead of building it all, I picked up a cheap Chinese or Indian live center with changeable tips. This was the only one I could find that had threaded tips. The rest just had tapered stems on the tips.



I turned a holder to match. It is a M12-1.0 thread with a 30 degree taper that fits into an alignment socket on the live center. I had to special order the die as it is an extra fine thread for an M12. I started with some 1.25" diameter 12L14 steel. The thread is a bit longer than the other tips but the threaded hole is deep enough to accommodate.



Then I reversed it in the chuck and drilled it out in steps and reamed to 0.5"



Here it is mounted in the live center.



All it needs is a set screw and a hole for a tommy bar or a notch for a wrench.

But wait! What happened to the 1 degree angled hole for the broach to wobble? Well, I am taking a gamble. I really considered setting up for the 1 degree angle for the holder and even ordered a set of angle setup blocks but then I started thinking about it. Let's look at the odds. For it to not wobble, the following would have to be true:

1. The cheap live center would have to run perfectly true.
2. The holder would have to sit perfectly true in the center.
3. When I reversed the holder in the 3-jaw chuck for drilling, I would have had to get it perfectly centered and running perfectly true and in line with the threaded stud.
4. When I drilled the hole, the drill bit did not wander at all.
4. The hole would have to be perfectly true and within a couple thousandths slip fit with the cutter.
5. My tailstock would have to be perfectly in line with the axis of rotation within the 3-4 inches from the morse taper through the tip of the cutter.
6. The my Harbor Freight lathe spindle and chuck would have to be aligned perfectly to the tailstock taper.

The odds of all those happening at once are pretty slim. I believe it will wobble enough for the rotary broach to work. If not, all I am out is 3" of 12L14 and a some time. If it runs too true, I can always give it a scientifically derived whack with a ball pein hammer. If that doesn't make it wobble enough, all I have to do is make another holder and either try to be sloppier with my methods or more accurate.

Now I have to order a rotary broach from LMS or ebay to give it a try.

 

[HSS]

Nice approach, David. I will be watching this thread, for sure. Might be something there for my toolbox if you can get it worked out.

Patrick

 

[David Kirtley]

Just for entertainment value, I put the set screw in last night after I posted. Well, I did it three times. The first time, I realized that I had done it for a fine thread M5 screw. Ok, I looked around and all I had were coarse thread M5 set screws. Drill out and go to M6. Got it threaded and went to put the set screw in and realized I had picked up the wrong set from my tap and drill set and had actually gotten out the M7. Guess what? No M7 set screws to be found. Well, after looking carefully and verifying stock, I have an M8 set screw in the holder.

 

[David Kirtley]

Well, some progress. I made up a sample cutter from some W1 drill rod. I have a indexing fixture which is just a drilled and reamed piece of hex rod. Put it in the mill vise with a 1 degree angle block. Milled off all 6 sides and used a cutting wheel in the grinder while on the lathe to dish the front. Both are inadequate. Should go up to 2 degree probably and get a smaller radius cutting wheel.



First cutting semi-success material is aluminum. It is a bit over 0.25" (It was an arbitrary size.) The cutter needs to be reshaped and the steel is not hardened (No gas at present). Not perfect but it cuts. The pic is kinda blurry as I don't have a macro on the camera.You can see the shavings in the bottom of the hole if you squint and step back from the monitor.



I might pick up a commercial cutter to play with while I am getting the logistics of the cutter manufacture figured out.

 

[David Kirtley]

Last one for the night. I recut the cutter to 2 degrees. It works much better. This time around, I got off a bit on indexing and my cutter was a bit lopsided. Next up is working towards hitting a certain size on the cutter. I will eventually need to switch to HSS. Also needs liberal lubricant.

Recut on the cutter:



Woo hoo! Chips coming properly:



It would be a better fit it the cutter were not lopsided but it does form the hex:

 

[markknx]

David,
Nice work!! I just finished one for my 4 way tool post. What I did for the center of the cutter to dish it was I drilled it in the lathe before I milled the hex. (just enough to dish it)
I agree about the wobble but since mine is in the tool post it is a quick set up. I also used W1 drill rod harden and tempered it and so far it has worked nice in both AL and steel.
Did you look at this site http://mikesworkshop.weebly.com/rotary-broaching.html If not check it out because you may be able to use his method for cutting the cutters. Mike Made his holder to also serve as a cutting fixture for the broach. and he gave permision to copy.
Your aproach also has me wondering could I do the same thing you did only turn the shaft to fit a collet but 1 deg. out of sqare with center so that it would work in the mill.
Mark.

 

[David Kirtley]

Actually, I had seen that but had forgotten about it. I was loosely following http://www.docsmachine.com/projects/rotarybroach/rb01.html

I don't know why but when I looked at the one on Mikes Workshop, I didn't really understand what he was doing. Now that I have had some time to let it soak in, it is a pretty nice approach. I just have a hard time trusting metal on metal like that for rotation. I get my mind fixed on bearings and it is hard to let go. Thats why I went the live center route. Well, that and I also wanted a live center that I could use with tubing.

I ordered a commercial HSS broach for study and testing. They are kind of pricey but I think of it as taking a class.

I am better prepared now for testing as I finally got in some angle blocks. Looking forward to the weekend to have time to play with stuff.

 

[markknx]

David,
Yeah I looked at that one to didn't have that holder like his and missed that great idea you had. I like the way you went with it. I do not see me using the broach enough to wear it out for a lnog time. It only spins for a shot time for each cut, so the metal on metal is not that big of a factor plus it is well greased and has a single ball thrust bearing at the back of the shaft. But if I was to do it again I think iwould go the way you are!

 

[David Kirtley]

Final update for now. I bought a commercial broach for testing and study. 6MM hex from Polygon Solutions. Not cheap but not that expensive for a quality HSS cutter (about $40). Very nice finish and seems well made. Lots of information sent with it as to the drill sizes to use and such. Well packaged. They also sell on ebay and ship quickly.

My holder is drilled out a bit too shallow for the set screw to fit the flat. I will deal with that later. Yeah, I need better lighting and a better camera for close-ups.



Tested with some aluminum. Predrilled to .248 (this is from memory. Drill size E I think.) Chamfered the edge (a bit too deep but not an issue.) Ran the cutter and sure enough, it cut the socket. My angle may be a bit shallow and it didn't cut as fast as the feed rates suggested in the literature, but also I was running fewer RPMs than suggested. It goes in about the same speed as my fine feed on my feed on the lathe. Doesn't take to long. I did give it a bit of oil for lube.



The radius for the flats is actually quite tiny, I knocked the chips out just because they bothered me. The wrench fits quite nicely. Not too tight, not too much play. Just slides in like it is supposed to.



I will do some experimentation for making my own cutters but for now, I can use the broach and make a socket for an allen wrench. No complaints. If it is too much trouble, I will just pick up another one or two cutters. I don't see that I will need that many sizes. Maybe a 5mm and a 1/4" is plenty. Would also like a 1/4" square broach. Perfect for carriage bolts.

 

[markknx]

Yeah the cuutersare not hard to make and at $40 are worth the time. I used w1 drill rod and it cut nice. I would like to get a chart on hole size and what the cutter size should be. I think I went .001 over and was loose then went exact and it was good. Istill say nice job!