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Stuart triple crankshaft machining

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Matt Stevens 123/10/2020 02:15:17
75 forum posts
6 photos

So....getting colder outside - time to start the triple!

Crankshaft in the kit has the 6x counter weights cast, everything else has to be made from stock. I wanted to get some comments before starting the job...

The crankshaft and journals are made from 3/8" dia mild steel according to the drawing. I have some 1144 material but it is slightly undersize by about 3 thou and so i am wondering if i should use this or get some silver steel/drill rod which will be precision ground? My concern is that if it is undersize, it won't be a great fit in the bearings which will be reamed (once made) to 3/8". Thoughts?

Whats the best way to set the 3x sets of webs at 120deg from each other for drilling and pinning?

Any tips on registering the counterweights against the crank webs to keep them straight and located properly? Or would you advice throwing them away and machining brand new webs with counterweights from 1.75" stock (i have 1144 in stock)

JasonB23/10/2020 07:55:14
19178 forum posts
2107 photos
1 articles

Another option for shaft material would be PGMS (precision ground mild steel) which I use quite often.

If you are using Loctite and pins then I would make them up in sets of two webs and a crankpin using a spacer between to keep the webs parallel while laid on a flat surface. Drill and pin once set. You can then hold the shaft in an indexer or rotary table to loctite each set onto the shaft using a square against the flat side of a web index round to the next one once Loctite has set. Use a smaller reamer or bore the webs as you want a closer fit than the shaft/bearing one.

Weights should be a close fit over the webs and almost align themselves with just the screw to retain them. The separate weights are more a throw back to when the crankshaft was supplied as one piece so easier to machine without the webs. therefore you could go for integral weights with a built up crank

Matt Stevens 123/10/2020 14:15:17
75 forum posts
6 photos

Thanks Jason,

I am finding that precision ground steel in a grade like 12L14 is not so readily tends to be more Tool steel grades and so i suspect not so easy to machine. Because of this, i am leaning towards using Drill Rod as i know i can get a piece of 3/8" @3ft for quite cheap.

Is the trend leaning towards using loctite and pins compared to silver soldering? I guess its less clean up afterwards... I also guess less chance of any relief happening changing dimensions.

I am thinking to make the webs/weights from solid. Its probably just as much machining anyway.... But i guess i could machine up the cast weights just for fun anyway

Martin Connelly23/10/2020 14:49:02
1522 forum posts
171 photos

Some points to be wary of with ground finish rod, it may not be as circular as you want and the finish may be a bit rough and in need of polishing leading to slight reduction in diameter. Ask the vendor for details before buying so you have the option to complain if it is not as required.

Martin C

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