Making Rotary Broach
The other thread got me thinking... has anyone tried to make their own rotary broach tool holder? I saw Frank Ford's website where he makes his own 3/8" hexagonal rotary broach, but I was wondering if anyone made the actual tool holder.
Seems like it's fairly simple in principle but might be difficult to build one that stands up to the pressures involved. I reckon the big sticking point would be getting a good (i.e. expensive) bearing. No $5 chineese bearing... That and the taper if you weren't familiar with cutting tapers.
I've got plenty of projects on my plate, but I was sort of thinking about drawing up some plans in my new Autodesk Inventor 2009 and putting them in a folder for later...
<edit> oops - that should read "making a rotary broach" ... me speek good anglish
I had the same thoughts after seeing the other thread. It looks like an interesting process. I would not want to spend the $1k plus on a head though. From what I gather via google, the head holds the tool at a 1° angle, and is free to rotate w/ the work causing a sort of wobbling action. Seeing how it works, and understanding how to make it are two different things though
Hopefully one of our genius types will chime in.
Last edited by jacampb2; 12-18-2008 at 04:34 PM.
I think ther was a write-up in one of the two mags about it. I will look tonight and see if I can find it.
They had an article in the Sept./Oct. 2002 issue of HSM on doing just that. The article was a little light on the construction details, but I think it would give you a good starting point.
Edit: I added the year to the issue. That would have helped initially.
Last edited by hornluv; 12-18-2008 at 04:46 PM.
Stuart de Haro
Dang - I get Machinist Workshop, not HSM... Thanks for the encouragment though! At least it doesn't sound too ridiculous to build.
What exactly is rotary broaching anyway? Will it let me make hex blind holes?
I frankly find it a bit inexplicable that the tools are so insanely expensive. I do not know of any other sort of tooling that is that size and precison and yet costs so much.
Edit: wait which issue? I do not remember ANYthing on rotobroaching this year.
rotary broach bearings
My suspicion is that were I to set out to build a rotary broach, I might be better off trying to NOT build a rotary broach and using cheap bearings to boot. I'd have that 1 degree wobble for sure I wonder if that's how the inventor stumbled onto the design in the first place. The cutting action of a rotary broach seems not all that intuitive in the first place.
Here is an interesting article on the math behind the early precursor to what is being done now. It appears, according to this article, that one of the early inventors was Harry Watts, who deigned bits to drill square, hex and octagonal holes all the way back in 1914. The method is different, as seen in the article, but a neat read, none the less.
Doh! Edit for link: Clicky
Sorry, it was the Sept/Oct 2002 issue of HSM. I got distracted and forgot to put the year down. It's a blue cover with the lead article "Gear Repair by Bronze Surfacing".
Stuart de Haro
I built an ORB tool using two 7R-10 bearings (5/8" bore) that were sitting around waiting to be used. The spindle was 0.625" through the bearings and extended 1/2" beyond the bearings at 3/4" diameter to accommodate a set screw to retain the cutting tool which has a 0.250" diameter. The hole for the tool is drilled and reamed into the spindle at a 1 deg angle. This differs from the commercial units: Slater and Somma build the 1 deg angle into the mounting plate which also has adjustment screws to center the tip of the cutter. This detail is important: the tip of the cutter is centered with a dial indicator so that it aligns with the hole in the workpiece. My simpler unit did not incorporate this: I centered the cutter by putting a spacer behind it. There is a lot of thrust while cutting and the set screw will not retain the tool axially.
The hex sides of the tool were ground at 3 deg clearance. The cutting face of the tool was ground with some "back rake" with a ball burr in a die grinder mounted to the tool and cutter grinder wheelhead.
This tool made 3 special socket head cap screws in brass at .220" across the flats to fit a 7/32" Allen wrench. A later attempt to make a 9/64" hex in O2 tool steel failed: not enough thrust to make the cut. I do not know if making the tool with the Slater/Somma type of offset mounting would have been successful. Someday when I run out of other projects I might make that version.