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Long Keyway

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John Purdy30/09/2020 17:56:13
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I have to cut a 1/8" keyway in a 1/2" dia hole that is ~2.5" long.   A standard series "B" broach will cut a keyway to a max of only 1 11/16" long.   How do I do it?   I have a couple of ideas, I have the broach and the bushing, but any suggestions or ideas would be most welcome. Thanks.
John

Edited By John Purdy on 30/09/2020 17:56:57

Edited By John Purdy on 30/09/2020 17:57:56

Martin Connelly30/09/2020 18:50:34
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The broach should go all the way through the required distance but needs a long bush to support it over the whole length and a suitable piece of material to push it when it is down the inside the keyway. The other issue will be that you are trying to cut over a long engagement length. The best way to avoid this is to make multiple slots in the new bush(es) so that only one or two teeth engage on each pass. Probably more than one bush required or a lot of shims to suit.

Another option is to use the lathe like a shaper, should be plenty of previous talk on this.

Filing will be hard to keep accurate over this depth of bore.

Martin C

JasonB30/09/2020 18:56:59
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I've done long ones by making a longer bush and shim then using a bit of flat stock to drive the broach once it goes below the surface of the bush or you can buy these. Depending on material you have to watch for the gullets fitting up with the longer cut

John Purdy01/10/2020 18:26:00
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Thank you Martin and Jason for your replys. The reason for the query was that I was given to believe that that the max length of cut of a broach is governed by the gullets between the teeth plugging up with the cuttings and the broach jamming. I have had a broach jam in the past and it was a nerve racking process to remove it without breaking the expensive broach or damaging the work piece.

Your suggestions are pretty much what I had come up with, a longer bush to support the broach and do it in multiple passes. An 1/8" keyway with a "B" series broach is normally cut in two passes, first one just using the bush alone and the second with a 1/32" shim in the bush. I was thinking of doing it it say 4 passes by making the slot it the bush 1/64" deeper and using a number of 1/64" shims between passes. Question is how to hold a 1/2" rod to cut the 3/16" wide slot that deep and 1/64" deeper than a standard bush. As can be seen in the picture there is very little meat left as it is in the normal bush. The other possibility I was considering was to cut an initial slot in the lathe by planing it part way then finish with the broach using two 1/64" shims to reduce the chip load.


John

keybush.jpg

Edited By John Purdy on 01/10/2020 18:28:11

JasonB01/10/2020 18:31:41
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Hold bush in vice with sacrificial packing against each end so cutter does not hit vice jaws.

John Baron01/10/2020 20:36:40
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Hi John, Guys,

You could always try my method of cutting small keyways into long holes !

04-09-2020-006.jpg

This picture is cutting a 3 mm keyway into an aluminium timing pulley. The hole is 28 mm deep. 50 mm shouldn't be any harder.

04-09-2020-005.jpg

For some reason the png drawing has not been placed in the album. But the pictures show how it is done.

20-09-2020-01.jpg

The grub screw is used to push the cutter out in small increments, and the spring is used to hold the cutter in place and act as a returning force to keep the cutter pressed against the grubscrew.

The hole and bore in the bottom is there to allow the swarf to escape by falling through. I found that in the initial broach I made the swarf got pushed up behind the cutter causing it to become very tight to remove.

I'll rename the drawing and see if the forum will accept it.

Ah, seems that it did work this time.

keyway broach.jpg

 

Edited By John Baron on 01/10/2020 20:42:11

John Purdy02/10/2020 00:24:05
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212 forum posts
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John

That idea looks like a possibility. The sleeve I need the keyway in is steel I was planning to use my hyd press to push the broach through so I can see a problem keeping it aligned with the hole. In your picture it looks like in is being held in the drill press chuck therefore maintaining alignment, but I don't think I would want to use the drill press to push it through steel. It could be used in the lathe tail stock as well which might be more feasible. It would certainly be better than a boring bar in the tool post. Certainly food for thought! Thanks.

John

John Baron02/10/2020 09:07:21
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328 forum posts
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Hi John,

The longest keyway that I've cut using this method was a 4 mm one 50 mm long in steel though the bore was 1/2" rather than 12 mm. You do have to do it in little bites, the ram is nowhere as rigid as a hard HSS broach and you need lots of cutting fluid. I used Trefolex, I've also used RTD, of the two RTD is better, its more runny. I've also used this method with a ram sat on the cross slide of the lathe.

Good luck with your deep hole.

John:

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