|gerry madden||27/06/2021 14:02:36|
|201 forum posts|
Good afternoon chaps, it's pick your brains time ! Many years ago some horny-handed yoof, almost certainly me, butchered up the end of my Cyclemaster's crankshaft by repeatedly removing the magneto flywheel with the shock of a hammer. Its now well and truly rounded and eccentric as you see in the pic below.
There is some damage to the thread and to the taper behind it that I would like to carefully address. Here's the problem....
Apart from the centre hole in the far end of the shaft, the only 'as new' surface is the one shown in the picture. I don't want to break down the flyweight/big-end assembly so how could/should I hold the 'ok' section in order to drill a new centre hole concentrically to it ? (For info the shaft is around 13mm diameter and 6 inches long.)
...ha ha this always happens, as soon as I start to write I think of things. I have a Stevensons block which is through-drilled so I can hold the 'ok' portion in a collet. All I have to do now is to hold the block in the mill and centre it up with a DTI. Any other ideas ?
|John P||27/06/2021 14:31:00|
|314 forum posts|
On the assumption that you have a lathe with a large enough swing you could try this ,using a spring dog as in the photo below to hold the good end in a headstock centre and a fixed steady to support the unworn good end directly or with a bush would be and easy job the drill and then machine a new centre with a small boring tool set over in the topslide.
|Dave Halford||27/06/2021 14:40:54|
|1669 forum posts|
If that big disc comes off I would consider mounting the crank end in the 4 jaw or faceplate and spend 1/2 a day getting the OK section to run true in the fixed steady.
I'm pretty sure John Stevenson would have welded up the threads and the off centre hole for you and 'machine while wet'.
I would take very light cuts as the crank pin might move.
|Nigel Graham 2||27/06/2021 22:13:47|
|1666 forum posts|
I would suggest using the lathe is the eaiser option because to use the milling-nachine as you suggest, may mean having to set tthe shaft etc in the "corner" off the table.
I have done that for one or two awkward tasks, and it is not easy!
You mark the cra nk end as having weard bands but are there bands between them on which you can place the DTI? I am thinking of being better to set the steady at the chuck end then slide along to the support point.
That of course assumes a constant diameter but I see the outer end both steps down and appears to have a groove extending from the key-way, and which might create problems for the steady unless it is much narrower than the steady's contact areas. I think if you can mount the steady right by the step you might be OK with very careful machining, but as John suggests, putting a bush on the shaft would be better.
Obviously you can't face that end back more than enough to just clean it up, but I would try using a small knife tool to ease the damaged area of the centre-drilling down enough to give re-drilling it a better chance of concentricity. It doesn't look as if the shaft is especially hard there so HSS tools ought cut it.
Incidentally you learn something new every day here! I have never previously heard of a "spring dog" yet lo and bohld, there is one, on the drawing of the lathe set-up. As the "words and music" bear out, it holds the work back onto the live centre, since the outer end is not constrained axially.
|John P||28/06/2021 09:45:41|
|314 forum posts|
Posted by Nigel Graham 2 27/06/2021 22:13:47
The illustration there is from a re-print of a book that is nearly 100 years old ,back then folk really new how to do things.
I suppose for that crankshaft job the fixed steady pads could be profiled with an end mill or reamer fitted in the chuck to provide a better contact around the area of the slot on the end of the shaft.
|Dave S||28/06/2021 12:16:44|
|201 forum posts|
Given the bearing by the crank there is a reference surface there. Either remove the bearing, or use its outer race with some greedy soft jaws such as these:
Now if required can slap on some weld to build up if needed and then machine and remake centre.
|not done it yet||28/06/2021 13:39:08|
|6271 forum posts|
I would check that, although deformed on the surface, whether the centre may still be central at its inner end. If that is the case, it may not be too difficult to arrange to clean up the current deformation, perhaps holding the crank at the tailstock and cutting with a cutter in the lathe chuck.
Alternatively, I would seriously consider dismounting the shaft from the web and sorting. There would, presumably, be timing issues if this course was followed.
|Michael Gilligan||28/06/2021 14:15:40|
18710 forum posts
If that is the case [and yes, I agree it is quite plausible] … it may even be possible to do the preliminary clean-up by hand ; which could simplify any subsequent machine setting.
|John MC||28/06/2021 15:58:21|
352 forum posts
How about cutting the externally threaded bit off and replacing with an internal thread? It would, I'm guessing, depend on how deep the key way (woodruff?) is. What size is the existing thread?
For the taper repair I would set the top-slide by machining a taper on a bit of scrap until the tapers mate properly. Then grip the far end of the crank in a chuck and support the end to be machined in a steady. True up the taper, remove the damaged thread and drill and tap for a new fixing.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.