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Member postings for John Bentley

Here is a list of all the postings John Bentley has made in our forums. Click on a thread name to jump to the thread.

Thread: Machining Crankshaft
06/02/2012 21:59:13
John (Bridge) didn't actually say that his crankshaft was unusually hard and in fact mentioned that he had already removed the surplus metal at the weld. It wouldn't surprise me if he has finished the job successfully by now.
 
I have not found the Stuart crankshafts particularly flimsy while turning them normally. They cut very well without distortion in either my Peatol or my Chinese mini lathe. Similar freelance versions (up to three throws) made from solid bar have turned out equally well for me. I didn't have any experience when I started out and haven't had any trouble at all with conventional crankshaft designs like these. Success is probably the result of no more than using sharp tools at proper height with gentle cuts at sensible speed.
 
Jason's recommendation of LSBC's tool looks like a good one and I will be trying it out next time.
 
John
 
05/02/2012 22:15:36
Hi John - What I forgot to mention was that I used two tools - a left hand and right hand version. However a tool like the one in the picture, but ground back a bit farther looks like it would clear the webs for the entire job. As far as the shaft dimensions are concerned, you can absorb a little error if you haven't machined the bearing sides yet - I was only advising against wanton disregard for the numbers.
John
05/02/2012 21:19:52
Hi John - I'd vote for making simple fixtures and turning between centres. I did one Twin Launch and one Compound that way, using a round-ended version of a parting tool. Keeping the tool razor sharp, setting the height precisely and taking light cuts are probably the key to a trouble-free job. I might suggest that you don't take a cavalier attitude toward the dimensions of the crankshaft (the journal and web widths and locations). Obviously the sides of the webs are used as the thrust portions of the main bearings in this engine, so the shaft fits into the base like the last piece of a jigsaw puzzle. A flexible overhead lamp is the most important tool - you need good light coming from the correct angle for each surface as you are cutting. It really isn't a difficult task... looking back it was much easier than milling the top surface flat on the gunmetal soleplate (I didn't like that job too much!).
Best of luck - I hope we will get to see a picture of your finished crank. -John
 
 
 
 
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