Banjaxed hexagons

I think this very simple question might be a product of my mathematical inadequacies, but I’ve been trying to cut hexagons on a mill using a rotary table to turn the bar (bronze) through 60° six times. It’s been a frightful banjax of which I am deeply ashamed.

Two nasties occur. Firstly the flats are not equally wide and secondly they are not true axially. I don’t think there is a problem with the rotary table, the chuck I’m using to hold the bar, nor the mill itself. Moreover, I think I may not be calculating the tangents correctly to ascertain the depth of cut (with an endmill) necessary to create equal flats.

Thus I think a collet block set is the answer.

But what I don’t understand with collet blocks, is, again, how to cut to the correct depth. Obviously the dimensions of the blocks themselves don’t vary. So, for example, if one wished to cut a hexagon on a bar 15 mm in diameter you would start by putting the stock in the appropriate sized collet and clamping it in the block. But when you have the block clamped in the vice on the mill, how do you know how much to machine off for the first flat, before you turn the block for the next? Do you take, say, a millimetre, turn over, take another millimetre, turn again and so on; and keep going until you have a perfectly even (hopefully) ‘hex’?

Have you tried the other way, namely machining less than you think necessary? Take the first 6 passes, which leave you with 6 flats but with curved gaps, then take a little more etc. If the bar is in the centre of the rotating table the flats should all be the same.

Hi there Take light cut then turn the hexagon over 180 degrees. Measure the thickness and deduct from af dimension. Divide result by two and take a cut this deep. All flats are machined to same dimension. Use the same method on the rotary table. If the chuck is true to the rotary table the flats will be the same. No need to buy collet blocks.

Cut depth from circumference of bar, to a hexagonal flat where corners of hexagon juts touch the original bar surface

r (1-Cosine 30degrees) = 0.134 r

where r = radius of bar stock. Substitute D/2 for r

to get 0.067 D

Hope that helps.

There's a small free computer app called Shopcalc.exe which you should find somewhere on the net

According to that, you should go in 1.005mm, or basically, one mm

Also, the max size of any flat should never exceed 7.5mm (A-B)

Edited By Ady1 on 12/07/2015 12:28:01

You will get shopcalc here

That's really brilliant everyone. Thank you very much. I should now be able to do this either by 'slightly trial and error' or if I get really clever using the maths so kindly explained.

I do hope my old maths mistress is looking down on us and duly impressed Nutland, once a thorn in her side, has bothered to concern himself with this!

Again thanks all.

Martyn

Sort of related so I will add to this thread if the OP doesn't mind. (too late if he does )

If you wanted a hex with an accurate centre hole eg to make a collet block, is it better to cut the hex first or the hole?

Don't think it matters, for an accurate job it would be best to cut the hexagon oversize or bore the hole undersize & adjust if the two features are not concentric to another.

Tony

Dear David Jupp

Sorry to be a real pain.

Could you possibly work this through with me line by line?

15 MM bronze bar...

Martyn

From earlier post depth of cut = 0.067 D , where D=15mm

so depth of cut = 0.067 x 15 = 1.005 mm

Probably check the actual bar diameter first and use the measured value - not sure how close it will be to nominal.

Looks like your hexagon will be very close to 13mm AF (1.00 mm off each side).

I usually find actually measuring the depth of cut (as opposed to simply relying on the machine dials/DRO) to be a relatively inaccurate process. In a case like this, I prefer to measure the piece that remains .... i.e. from the flat you just cut to the circumference on the opposite side which is a simple micrometer measurement.

If D is the diameter of the bar and A is the desired AF dimension of the hex then that measurement should equal (D+A)/2 at the correct depth.

Thanks for the sums. I keep that formula to hand.

Martyn

Edited By jason udall on 13/07/2015 10:11:49

If you are cutting to the same depth for each flat one of those must be at fault. Try holding a piece of bar in the rotary table chuck, put a dial guage on it, and rotate the table. There should be very little movement.

Russell.

I'v got a 1/2" UNC L/H thread nut to make for a mates old bench grinder. I'll make the hexagon first, then bore the hole and cut the thread, should be fun.

Ian S C