Turning an Elmers #25 crank disk

Hi guys,

I'm trying to plan the machining operations for an Elmers 25 wobbler.

The crank disk is a bit of mild steel, a bit less than an inch across. To make this, I have a length of steel bar the right thickness - I plan to mark it up, cut off a square length the right size - drill the crank-shaft hold in the middle, then mount it on an arbour and turn it down.

The bit I'm not sure of is if this will work :^)

What I will end up with is a square shape that I've got to knock the corners off to turn it round - but the mounting will be an bolt on an arbour held in a collet. I'm pretty sure this will spin in the arbour rather than cut as the tool hits the corners of the work piece...

Never having used (or made) an arbour before I've no idea how securely it will grip the work piece...

Is there a better way of making this piece that doesn't involve buying more material (round BMS rod of the right thickness)? (or am I making a problem for myself that $10 will solve

cheers. Jerry

Use a bit of your square bar say 1.5" long, hold in your 4 jaw chuck and just turn it down to 11/16" dia, face the end and bore.

Take the 4-jaw off the lathe with the bar still in it and fix to the mill table to machine the shape and drill crankpin hole

Back to the lathe and part off

Edited By JasonB on 20/08/2013 07:31:14

Thanks Jason - yep, if I had square bar that's what I'd do... but I've got rectangular bar, the thckness of the crank disk and width enough to get 11/16th out of it...

Should I just put that back in the spares bin and go buy some rod?

I would do as Jason suggests but start with 3/4 in dia round bar and use the three jaw chuck. Much quicker than starting with your rectangular bar and much less waste.

Russell.

Whats the exact dimensions of your bit of bar.

Russel he is trying not to buy round bar but use whats to hand, we just need a better idea of what Jerry has. Sounds like a bit of 5/32 (or 4mm) flat stock or sheet

Yep, that's it, 5/32 flat (rectangular) bar...

but it sounds like I'm creating more work for myself - a bit of 3/4" BMS rod won't break the bank...

thanks guys

If you have a 4-jaw, you can holds a rectangular bit in it as easily as a square bar. But any bit of scrap bar will do for this sort of job. I've used old mooring pin, but my 'best' ones are made from some lovely free-cutting but medium carbon hex.

Neil

Given the small dia of the part you should not have a problem with your method of drilling (preferably reaming) the 5/32" hole and then mounting on an arbor.

To make the arbor chuck a bit of say 3/8" (10mm) bar and turn down 1/2" of that to a good fit in your 5/32" hole, thread most of its length, M4 will do nicely and then you can slip your roughed out disc onto the arbor followed by a washer and nut and away you go.

Don't take massive cuts.

Edited By JasonB on 20/08/2013 20:32:22

I built an Elmer's 25 as a first project. Trying to make it as simple as possible I opted for a plain crank disc without the notches.

For the disc I used a 3/4" round bar. Chucked in the 3 jaw it was faced, centre drilled and then drilled the correct diameter for the crank shaft (deeper than the disc thickness). After turning to diameter the bar was removed and a slice cut by hand with a hacksaw. The crank shaft was fitted to this slice with a drop of loctite and then left to set for 24 hours.

I then proceeded to grasp the crankshaft in a collet chuck so that the disc could be faced to the correct thickness, thus ensuring that the crank-pin disc face and the shaft are perpendicular.

My original description if it is any help : Elmer's 25 crankshaft

Thanks very much Clive, and thanks for taking the time to document your build - I've already read it through several times and it's provided a great deal of confidence! Much appreciated

cheers. Jerry