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| sbwhart:
Well this post is going to be a tail of Triumph over near Disaster. Made a start on the crank shaft, drawing calls for a solid one piece crank shafts, all crank shafts I've made to date have been fabrication, but this being a one throw crank shaft I thought i'd make it in one as the drawing. Ok first part using a 3" long bit of 32mm dia free cutting mild steel turn the main journal 3/8" dia. Turn it round in the chuck and drill a 2.5 mm hole 2" deep for the oil way. Centre drill then a jobber 2.75 mm then finish off with a 2.5 mm long series drill, drilling part way with an over size drill gives a bit of clearance for the swarf reducing the chances of it clogging and breaking, also keep clearing the drill back:- job done. OK turning the offset for the crank. Plan A :- Mark off set with a small centre pop then with a pointed bar in the centre pop clock the bar up turn dia. Marking out Mark centre line and set centre line vertical Touch on top of bar Bit of maths Mark position of off set Centre pop and her we have it in four jaw ready to be set up. At this point tried getting it roughly set on the bench, and realized that with one jaw set over the next two jaws wouldn't close down, the diameter was too small. Ok plan B. Make a bush with a 3/8 bore clamp the crank shaft with a grub screw positioned in a none important place (grub screw would mark the shaft) then I remembered that if you put a packing piece between the job and the chuck jaw you could get the off set, so decided thats what I would do. Her we have the bush on the shaft with the packing piece. And job part turned OK spotted the deliberate mistake. :doh: I'm using a three jaw, the geometry is all changed I used a 1/4" packing piece to get a 1/4" throw I should have used a bit of trig and worked out a thicker bit of packing or used a self centering four jaw. A quick check and phew I still had enough meat left to get the job done. Plan C:- well back to plan A really but using the sleeve to give more of a diameter to grip on. Still had the off set marked with a centre pop on the bar end. So her we are clocking it up And part cut:- interesting shape with the near mis not quite cleaned away. 1/4 dia turned and drilling the oil way. Turned crank And in the housing Well that came out all right in the end I'm sure glad I did a quick check as to how the throw was coming out when I did. Cheers Stew |
| Stilldrillin:
Stew, Once again, you can say you`ve had a good day. :thumbup: All`s well, that ends well........ :wave: David D |
| andyf:
Stew, you've already made a nice job of it, but if you ever do need to offset something in a 3-jaw, the formula is Offset = 2/3 shim thickness. So, to get a 10mm offset you need a 15mm shim. I didn't work it out for myself :smart:. It's explained towards the bottom of this page: http://homemetalshopclub.org/news/aug03/aug03.html Andy |
| sbwhart:
Thanks Andy :thumbup: Thats filed away in my little black book. Great link by the way. Have fun Stew |
| NickG:
Stew, Well done with that, you've done a great job there. I thought about doing the same for the offset in the 3 jaw. I would have used 1/4" packing for 1/4" throw too - will have to look at Andy's link to find out why it's not! I hate doing those, just hate the interupted cut - what size cuts did you take, did it take long? Maybe I had the speed too low. I did it with a split bush on my rocking engine as remember the hassle I had using the 4 jaw on my stirling. But I drilled the offset hole in the bush in the milling machine and it must have wondered off because my crank disc wasn't uniform thickness. Was the main dia. of the crankdisc part too thin to grip on in the 4 jaw? Nick |
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