accurate boring

Good evening all, Some help please. I want to bore two crankwebs for a single throw crank. The webs are approx 2"long by 1" wide x 3/8 thick bright bar.

Two holes in each so the crank and pin can be inserted and silversoldered and the piece of the main crankshaft then cut away. I have tried to drill these singly,as a pair,in the drill press and bored against the 4 jaw chuck but all results have come out with the holes slightly off right angles to the face of the webs at various angles. error doesn't seem to be consistent. Anybody got an easy trick or solution to get all the bores aligned and square to the web face please.

Many thanks

B.K.

Accurately mark out both parts so the centre distances are the same, clamp to the faceplate and use a dti to pick up one centre, drill to about 1/16" less than finished and then bore to the required fit. Repeat for the other 3 holes. If you leave the bar as one piece 4" long it will make it easier to hold and the saw in half afterwards.

If you have a mill then hold teh bar in the vice on parallels, drill and bore as above with a boring head.

 J

 PS if your faceplate is not true then skim it before starting work.

PPS That sounds a largish engine it would be worth pinning the joints after soldering so it won't budge. Or cut from solid, this and the next few picks show a crank machined from 2"x1" bar.

Edited By JasonB on 28/03/2012 17:46:42

Edited By JasonB on 28/03/2012 17:49:13

I bored the crankwebs for my hit 'n' miss engine on a lathe in a similar way to that described by JasonB; except for the following variations. I used the 4 jaw chuck rather than the faceplate, as it's difficult to clamp small parts to my faceplate. The crucial difference is that I made the two crankwebs overlength and bolted them together before boring. That way you only have two holes to line up, and even if you're a bit out, the holes are guaranteed to be in line.

My crankwebs were a similar size; I loctited the crankshaft together on a simple jig and then drilled, reamed and loctited 3/16" silver steel pins in each joint. I didn't bother making the crankshaft in one piece and cutting away the centre afterwards.

Best Regards,

Andrew