Rotary broaching

I have just received my little kit from Hemmingway Kits. I do like the well explained instructions and great drawings.

My question is, does anyone use or have made a broaching tool for making an outer shape, like putting a hex on a part instead of trying to set up the mill and cut the flats required on the part?

I remember back in 1982, seeing that the inside of capscrews hex was clearly done with rotary broaching instead of the now forged formed heads that we are used to seeing , and thinking about it, some of the water fittings had been rotary broached for the inner hex as well.

In my case I am going to be just putting hexes inside some brass bits and some 6mm hex into some special screws that I can't buy anymore.

I want to make some ali parts from a material that I can not by is hex form, and want to make the part all on the lathe without having to set up the mill to get the 3/8 and 1/2 inch hex on the parts.

Cheers Neil

I've been using a rotary broach I made for some 30 years now, internal shapes, hexagonal and square mostly. The cutters for the basic internal shapes are straightforward to make, how will you make the external shape cutters?

Easiest way would be to buy? Quite expensive.

If I were to make a rotary broaching tool now I would make sure that I could buy HSS cutters that would fit. I make them from silver steel, okay for softer materials but struggles with small AF's in steels.

John

Neil I think you would be pushing your luck trying to broach those sizes of external hex with the little Hemmingway tool. I have made a couple of the Hemmingway rotary broaches and they work extremely well, a HSS broach 1/16 inch square has broached 1000+ holes without any problems but of course for external broaching your broach will have to have the 2 degree taper internally and that may be problematic, certainly would be for me, depth of form is also a limiting factor on this little tool. I made mine about 5 years ago and I thought how good the instructions and drawings were.

Baz , I was looking at making it larger, with larger bearing etc, and putting a #2MT on the spindle end. Then make it an over cap design for broaches and only have a thin section then have the back end all relieved out at around a 5 deg taper. I will make my initial ones with O1 and if they seem to be working , then will make them from HSS and get them commercially heat treated. While make the current kit will make a bigger unit at the same time for the outer stuff. I won't need larger than 6mm for my internal hexes. I have some hss for making the various broaches that I need. I suppose that carbide is too brittle for broaches.

John, I can get the hexes that I need wire cut from a friend that runs an edm wire cut machine. The radius from the wire will leave a nice corner radius on the parts hopefully. I have access to a cutter grinder to make the broach inserts in hss. .

Out of curiosity, what diameter do you make the broach for say a 4mm hex key? Do you make them 4.02mm or so?

Neil

Looking on the net at rotarybroaching.com they give maximum possible dimensions. One is a difference of no more than 0.040” between material diameter and finished depth of cut. I think that you would need a pretty heavy lathe for it to work comfortably.

Thanks Howard.

That site answers all the questions I had. Yeah I won't be making the external broaching now, not with those sort of forces for an external broach. Milling it will be.

I make the smaller, 2 - 4mm AF, hex broaches 0.05mm bigger than nominal, 5, 6 and 8mm 0.1mm bigger. 10 and 12, 0.15mm bigger. This gives one, maybe two, chances to sharpen. For imperial, the equivalent. From memory I've used similar allowances for square holes. Ultimately it wil depend on how accurate the hole needs to be, for an allen key these allowances are okay. Again, from memory, I think the allowance on the larger sizes could be even bigger to stay within acceptable limits.

For the smaller sizes, if I needed to broach a large number of holes I would buy HSS broaches, I break more silver steel ones than I wear out! For the larger sizes hardened silver steel works well enough, I've managed 6 and 8mm in EN8 steel.

I generally drill the initial round hole a little larger than nominal, I'll have a look at my notes and reply tomorrow.

John

HI Neil,

I made one of the Hemmingway rotary broaching tools a few years ago, not used it too much, but have successfully broached 8mm hex's in 30 off stainless steel fittings (304 grade). It did take a lot of force, enough to knacker the bearings, so i would be very wary of external broaching given the surface areas involved.

Jim

I made a rotary broach some time ago for internal squares and hexagons and engraved the face to tool tip length on the body. This is the null point which is important. Outside rotary broaching is very rare, I had never heard of it before this forum. External shapes are usually produced by milling or with a fly press.

I, too made a rotary broaching tool after the style by Mke Cox although I used a redundant revolving centre as the basis. A few photos in this album. I made the broaching tools out of silver steel, hardened then finish ground using a Stevenson end mill sharpening ER fixture to obtain the angles (only because I had one). A Google for ready-made broaching tools got me to this UK-based supplier with the following page in their catalogue:

So it appears that they are certainly available. I dread to think what the price is, though. It's one of those R-R type catalogues - "If you have to ask the price - you can't afford it".

John

To finish off my previous post I size the drilled pilot hole by multiplying the hex AF size by 1.02 and the square AF size by 1.06. Never an exact size so I round up to the nearest drill size. I arrived at this by trial and error (and a few broken broaches!), when I first made the tool. Some years later I found these factors are about right, maybe a little generous according to some rotary broach suppliers.

I note that the Hemingway version of this tool calls itself compact, I would suggest it is not that compact. That is to say it is overly long. On a lightweight machine overhang needs to be reduced as much as possible. The cutting forces are not truly axial so will try to push the cutter off centre. Keeping things "short" will be of benefit. With this in mind I would also suggest mounting the tool with a morse taper rather than a parallel shank in a drill chuck.

This version of the tool also uses 3 bearings (I think), one of which is a thrust bearing. Omitting that bearing would reduce the overall length by a useful amount.

I like Neil's idea with regard to the design of an external broach. I have a number of components to make that will have a 9mm AF hex, worth a try I think.

Thanks for the feedback. When I do get around to making it, I will keep the kit as is, and use bigger bearings and make it for MT2 direct fit to the Myford S7. While making it, I will make different holders for using different sized HSS while at it. I am hoping that I can also incorporate the over cap setup for OD broaching to 8mm or so . I may have to do the OD broaching on the mill yet, not sure. After making things and doing some testing. I am sure that there will be some small things that can make a big difference to how the process works. Today when I was running the nc lathe, I tried some different inserts to see how much difference was in the spindle loadings. The spindle power difference was as much as 5% from the insert geometry being tested. So it got me to thinking about the rotary broaching. The insert that made the smallest chips, used the lowest amount of power.