Cutting an Hexagonal hole

I need to cut about a dozen Hexagonal holes, about 7mm across the flats. If it was just one, for a one off I would sacrifice an Allen key, but these need to be presentable.

Material is aluminium about 20mm dia by 12mm thick a hole in the centre and then broached.

I may need to make more than a dozen eventually so thinking of making a tool. Suggestions anyone?

Thanks,

graham.

Assuming that it's a through-hole ..

Pull Broaching would be the obvious choice.

Something along these lines would be nice.

MichaelG.

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P.S. ... I recall you suggesting that someone else should outsource his job:

This looks very much like a case where you might take your own advice.

... As you will see; it's very easy with the right kit.

Graham

There is something called a Wobble Broach that may be suitable, pictures here, and I think it has been covered elsewhere on this forum.

Cheers

Martin

PS Or

Machine a piece of silver steel to hex size with relief behind cutting face, harden, temper & sharpen. Then drill aluminium work piece to just under size and press the broach through to cut hole. With suitable relief on the cutting tool/broach it shouldn't jam in the hole, hopefully.

Normal comments re Granny and eggs etc. 

Edited By Martin W on 07/07/2014 00:12:55

Have a look at Michael Cox's web page he shows a really simple method of broaching there

www.mikesworkshop.weebly.com

cheers

Bill

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Bill,

That is a very useful method for short holes; but, if I understood Graham's post correctly, he wants a 7mm A/F hole 12mm long ... I think anything other than pull-broaching would be very difficult [near impossible].

MichaelG.

I could send them out and have the hole wire eroded or buy a push broach which are a bit easier to put through than a pull one if you don't have a puller. Not the cheapest though you would need to offset the cost by the time you will spend making a tool. Hex broach at 7mm from someone like Steelman's is £240.

You can make similar tools from Silver steel, 7mm dia lead pin on one end tapering out to 7mm AF and then turn grooves to form the teeth and back clearance.

Hi Graham,
As the material is aluminium I would try to make a broach from a long Allen key. Using a carbide tool I would turn a number of grooves spaced a little over 12mm apart They only need to take the points off the hexagon. I would then turn the first section to 7mm dia and the following sections to slightly larger diameters until the last section was the full hexagon. It should be possible to force this through a 7mm round hole as each step would only be removing a small amount of metal and only one groove would be cutting at a time. This is only an idea - I have never tried it.

Les.

Michael, it is true, I did suggest the chap outsource his production run. He was planning on hundreds of parts a month. I am planning on maybe twelve, in total.

Balance is hard to achieve, not in this instance though

Wobble broaching, rotary broaching, looks like the answer. Not hard to make the kit either. May even come in for other jobs too.

graham.

Can you be sure a wobble broach won't make a twisted hole on such a long run? I suppose there's only one way to find out.

Neil

I think Michael G is correct rotary broaching is normally limited to about 1.5 time the minor diameter of the broach. If the broach is too long then it becomes very weak at its narrow point.

Does the hexagon hole have to go all the way through the aluminium? Could you recess the back by 2 mm so that the thickness to be broached was only 10 mm?

Mike

Edited By Michael Cox 1 on 07/07/2014 11:13:14

I did one by planing it in the lathe, using a square tool wide enough to create the flats, and gradually moving the cross-slide out until I'd got a full flat. Indexing the job round 60 degrees isn't hard if it's in a 3-jaw chuck. Yes, it was tedious but it ended up OK. Bopping a suitably-sharpened hexagon down a slightly undersized planed hexagon hole might be a better way of ensuring consistency. Don't take too much out, or if you do or it'll look bad when it bursts out. Countersinking the hole beforehand would help.

I have never tried this, but could you use an ordinary keyway broach passed through 6 times, once of each face of the hexagon? You would need to make at least 1 special guide so that the faces are at the correct angle to the previous one. I guess the suitability of this option depends on the availability of broaches of the correct size.

Baldric

Does the specification require a perfectly formed hex hole?
I assume the part or parts slide on a hex shaft of some sort.

If you were able to enlarge the hole slightly it would greatly reduce the force required to push or pull the broach through the hole. You often see this on mass produced parts. The centre part of a hex nut sliding on a shaft has very little influence on the load carried by the part or the accuracy due to the geometry, it is at a tangent to the imaginary circle located at the mid points of the flats on the hex shaft.

Regards
John

Taking John's idea further, if you made holes in just the right places to mimic one of those 'wavy' sockets you coudl get a snug fit with nary a straight line in sight!

Neil

Suggest sending it out to a water jet company or laser cutting if you need better finish on the edges. Will

The tool called a wobble broach above is available commercially from Italian companies Brighetti Meccanica and Poliangolar. I own a Poliagngolar with MT4 shank. The long version of their 7mm hex tool has a 15mm long working area, so should be enough for you application.

This won't create a spiral hole, because both the tool and the work piece rotate at the same speed. The wobbling action of the tool just makes it easier to push the broach into the hole because you apply pressure on only one corner at a time.

There's also a nice video on a home made version, with explanation how it works on youtube:

https://www.youtube.com/watch?v=wpx76-_lPkM

If you have to do several holes it might be worth your while to make something along those lines.

Cheers,

Harm

Just tried it and its not a problem to do.

Took a spare 6mm allen key as I could not be bothered to search for a 7mm one, hacksawed off a 1" length, faced and then turned the end slightly hollow.

Drilled a 6mm hole in some ali bar, applied a bit of tapping fluid and pressed the key into the hole using the bench vice.

Not too shabby

If doing it again for a proper job I would cut a slightly steeper cone into the end of the punch, make two punches, one with the corners turned off so the first punch will remove 50% material and the second take it out to size and lastly make a simple guide to keep the punch true while pressing it in.

J

I have had a little bit of a redesign and I can machine the thickness down to 5mm by turning the end cap into a cup shape, will save a bit of weight too. But turns the machining operation into a two stage process, suppose you can't have it all ways up.

I like Jason's idea, cheap, simple, fast. Just need to make it accurate and I am all sorted. I have a couple of presses so that isn't going to be a problem, maybe the fly press. I have looked at some of the ideas and as always, very creative. I still like the rotary broach idea and that is something I could tuck away for future reference - and construction.

The hole doesn't need to be a perfectly formed hex because it is just for an Allen key to remove the cap. It does need to be central though, quite important.

Thanks very much for all the suggestions guys.
graham.

Good result

I stand happily corrected.

MichaelG.