Making a 6.0mm x 19.0mm Keyway Broach Guide?

[Ed DuffnerParticipant @edduffner79357]

Hello guys,

I would like to make a keyway broach guide/bushing Ø19.00mm (for a 6.00mm broach). I'm a bit stuck on working out the depth I need to cut the groove into the bushing and the reference I can use to start the cut.

My understanding is that 3.0mm of the key should engage into both the shaft and the mating part, a coupling in my case. Is the 3.0mm taken from the tangent plane of the shaft or from slightly lower where the 6.0mm key width would intersect the shaft diameter.

The highest tooth on the broach is 16.66mm and the supplied shim is 1.46mm thick, so in total 18.12mm. The broach is 9.64mm wide along its spine.

If I measure the back to back distance from one side of the motor shaft to the opposite outside face of the key I get 22.00mm.

Can I just touch off the cutter on the 19.00mm diameter of the bush and mill to a depth of 18.12mm or should it be a little deeper? What is or is there a correct way to do this?

Is this the kind of info that would be in a Zeus Guide or Engineer's handbook?

EDIT: Forgot to mention, the narrow end of the broach is 14.65mm and the height of the first tooth is 15.00mm

Thanks,
Ed.

Edited By Ed Duffner on 06/05/2020 15:22:00

[Ed DuffnerParticipant @edduffner79357]

How deep to mill the slot?

[Martin ConnellyParticipant @martinconnelly55370]

Do a number of different depths so that the first cut is not too deep. Measure the difference in height between the first and last tooth and this is the difference between each slot depth.

.

Martin C

Machinery's Handbook has, for 3/4" and 1/8". I will add a picture of the table. 

 

Edited By Martin Connelly on 06/05/2020 15:24:09

[Brian WoodParticipant @brianwood45127]

Hello Ed,

There are standard tables for broach guide bush depths, I'm sure you can find such information on line

Regards Brian

[HOWARDTParticipant @howardt]

A 6mm key way, tangental depth in the shaft is 3.5mm +0.1, and in the bore is 2.8mm +0.1.

In simple arithmetic, if 19+2.8 is depth across the bore, minus 16.66+1.46, then the thickness left in a 19mm plug is 3.68mm.

In reality the depth of the keyway in the hub doesn't matter, so long as it is deep enough for the key to fit in. Keys are normally side fitting, with clearance at the top and you could always file the top off the key if it is too tight.

[Martin ConnellyParticipant @martinconnelly55370]

[Ed DuffnerParticipant @edduffner79357]

Wow, that was quick. Thank you very much indeed guys.

Regards,
Ed.

[JasonBModerator @jasonb]

I've made quite a few bushes for various odd diameters and even tapered bores, I simply measure the broach from back to first tooth and cut the slot that deep, any less and you are cutting too much off with the first tooth any more and you will not get the final depth with the correct size shim(s)

The simple maths above does not take into account the shims or even the thicknness of the spine of the broachas if you cut the slot to final dimensions the broach won't cut on the first pass

[DC31kParticipant @dc31k]

Posted by Brian Wood on 06/05/2020 15:24:07:

There are standard tables for broach guide bush depths, I'm sure you can find such information on line

To slightly temper that remark, the standard tables assume a standard size of broach (body thickness, first tooth thickness, increment per tooth, number of teeth). With the multinational nature of tooling sources, this assumption may not be justified.

It is better to measure what you actually have, as the OP has done and make some common sense decisions based on that (the slot has to be deep enough to allow the broach body to pass through the uncut hole; the first cut of the first tooth should be of a similar depth to the cut between the first and second teeth; the last cut of the last tooth needs to be made with an easily-available total shim thickness). In a hobby setting, you are not seeking maximum metal removal rates and if you have to make an extra shim and push the broach through one extra time, it does not matter.

[Clive FosterParticipant @clivefoster55965]

The one and only time I did it I cut the slot in the guide bushing to maximum depth so the final tooth on the broach just cut deep enough to produce a flat in the bore.

Then I used shims to set the cutting depths. Verifying that I got the depth expected each time. As I recall it I didn't get my sums quite right and had to mill down a "funny" size shim for the last cut. Very much the slow way but was in control of every step.

Always feel that first time through a job is not the time to hurry and be expert efficient. Step by step and measure each time may be slow but its way faster than "Drat, now I have to do it all again.".

Clive

Edited By Clive Foster on 06/05/2020 17:19:11

[Ed DuffnerParticipant @edduffner79357]

That's all making sense, thanks again chaps.

I did find that using the term "key seat" in google gave much more info than just advertising and sales pages. One reference also includes standards for width tolerance. Link

Ed.

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