View Full Version : Broaches

01-10-2018, 08:39 PM
Have been looking at the question of broaches and I am confused. I am primarily interested in cutting internal keyways ( for eg, a keyway in the bore of a pulley to lock it to a shaft).
I noted "1/8"B, 3/16"B, 1/4"C and 3/8"C" as a description for a kit. The fractions are easy enough - just the width of the broach (I hope!) but what does the B or C (I have even seen D) reference?

What is a size (or series of sizes) that are designated 00 (I think "zero-zero")

I looked on the Harbor Freight site and the manual for one set (http://images.harborfreight.com/manuals/41000-41999/41170.pdf) suggested that the broaches could also broach square or hex holes? The Dumont site http://dumont.com/) - with an enormous difference in quality/price - shows that there are specific broach shapes for these forms. Is the HF approach feasible?

If I need to broach a keyway (say 3/16 inches wide and I have a suitable broach) but lack the guide (circular piece with slot to accept the broach) of the right diameter can the latter be turned to diameter & a slot milled? Are there guidelines for the dimensions ( length of guide, diameter of guide (how much tolerance), diameter & tolerances for the slot? Suitable material? Presume steel but which one? For a one-of (or 3-of) home workshop job must the guide be hardened?

General thoughts on broaching appreciated as well.

Bob Fisher
01-10-2018, 08:56 PM
Mild steel should be fine , the only dimension in doubt is the depth of the slot. Measure the small end of the broach and start there.it usually reguires at least one shim to finish to full depth. I bought all of my broaches from KBS. (All three of them). I'd be a little wary of a broach from HF, although they might be OK. I use the mill to push a broach thru, seems to work OK. Bob.

01-10-2018, 10:00 PM
The letter designations are the width of the part behind the teeth. The teeth can be skinnier than the back, but not wider. The back is fitted to a standard size slot in the bushing. You can have multiple width of teeth, all with a back that fit the same bushing. The 00 size is probably pronounced as "double ought". Low carb mild steel is fine for guides that get occasional use.

01-10-2018, 10:03 PM
The letter code refers to the overall width of the broach (not the width of the cutting teeth, but the width of the smooth bit on the back). So any B broach (3/32", 1/8", 3/16", etc.) will work with any B bushing.

Mild steel should be fine for the bushing. I use a HF "12 ton" hydraulic press to push them through, seems to work OK, if inelegantly. An arbor press would be better, but it takes a fairly big one to have enough ram travel, and I haven't been able to justify it.

I have wondered if it was possible to make square and hexagonal holes with an ordinary keyway broach, as the HF instructions suggest, but I can't say I have tried it. Seems like you'd be restricted to full-width broaches only (1/8 A, 3/16 B, 3/8 C, etc.), and even then you'd need a custom guide bushing. Come to think of it, would you need a separate guide bushing for each side? Hmmm... It might be easier to just build an EDM machine. Or raise the money for a proper broach.


01-10-2018, 10:28 PM
...and the length of cut and diameter of the bushing; http://dumont.com/bushings-shims-presses/american-standard-bushings-by-broach-style/a/