Broaching , a question

I'm getting ready to broach my first keyway with an actual broach . My question is about the shims , and how thick they need to be . Measuring the difference between the first and last tooth on the broach I get .062" and I'm wondering if that is the correct thickness for the shims . Snag

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My 3/16" keyway broach came with an 0.050" shim. For the others I shear strips of scrap sheetmetal and add one between passes until the key fits. My guide bushings are all shop-made to eyeballed depth.

Thinner shims leave less unsupported broach, but if the shim is too thin to hold the bend the broach may pull it through.

Is it a shop made broach or bought. All my Dumont broaches come with appropriate shims to cut the keyway to the correct depth.

It's a couple of broaches only from eBay . I machined a guide slug for it from some 416 SS I bought to "have some on hand in case I need it" and pushed it with my 12/20* ton HF press . I made shims from some .050" galvanized sheet I have on hand (being a Pack Rat is often a Good Thing) and it worked out quite well . I'll never broach another keyway on the lathe ...

I made shims from some

Sometimes you gotta do what you gotta do. Never say never. That oddball will certainly curse your work shop now.

I ran a 12 ton air jack on a 12 ton press, and I still tore it apart. I also bent a 20 ton with a 20 ton jack. That being said, the 6 ton arbor press has done all the straight broaching I've needed to do so far.

I made a couple of shims from some .050" galvanized sheet . Probably going to make some more of varying thickness so I can fine tune the depth . I made the slot in the guide bushing the same depth - measured from the round - as the width of the broach at the bottom end . The project got delayed a bit because I stripped the old DRO off the mill . The new ones are a lot better but there was a good bit of fabrication to adapt to my mill .

I made a couple of shims from some .050" galvanized sheet . Probably going to make some more of varying thickness so I can fine tune the depth . I made the slot in the guide bushing the same depth - measured from the round - as the width of the broach at the bottom end . The project got delayed a bit because I stripped the old DRO off the mill . The new ones are a lot better but there was a good bit of fabrication to adapt to my mill . Snag

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IIRC the proper depth of a key slot is half the width, measured from the cut edges, so a square key will fit half way into each part. I tend to need oddballs such as different width slots, the motor/pump shaft (metric?) existing and the pulley matching one of my broaches, and a custom T shaped key. There are even offset keys to advance timing on small engines modded for racing.

Maybe some of the information in the Dumont catalogue would be of use. It gives the broach lengths and the thickness and number of shims required.

I figure the shim should be the same thickness as the difference between the first and last tooth - mine measure .062 . This will keep the "pilot" on the end in contact with the bottom of the groove being cut . I noticed a variation in the depth on that keyway I cut yesterday ... I think because the broach was not bottomed out in the groove in the pilot bushing .

I've seen that with mine before like yesterday where the top of the broach was leaning towards me a bit, cutting edges forward, so I corrected it for the second pass for a 4mm key. I'll check my broaches tomorrow and see how the shim thickness correlates to the height increase along the broach and report back.

This was a case of I started the cut on the lathe , cranking the carriage forward and back . I think the taper would have self-corrected if I'd used a .062" shim instead of the .050" .

This is what I measured.

Broach list Dumont No 10 set Size    Start     End     Diff    Teeth     /Tooth    Shim

The /Tooth is Diff /(Teeth - 1) . Looks like your original assumption about the shim thickness is correct.

Thanks for getting back with that info . It seemed intuitive , but ya never know . Now we know .