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Re-making a centre hole in a small crackshaft

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gerry madden27/06/2021 14:02:36
201 forum posts
97 photos

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.

dscn8211.jpg

There is some damage to the thread and to the taper behind it that I would like to carefully address. Here's the problem....

dscn8210.jpg

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 ?

Gerry

John P27/06/2021 14:31:00
314 forum posts
212 photos

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.

John

sprung centre  dog .jpg

Dave Halford27/06/2021 14:40:54
1669 forum posts
19 photos

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 227/06/2021 22:13:47
1666 forum posts
20 photos

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 P28/06/2021 09:45:41
314 forum posts
212 photos

Posted by Nigel Graham 2 27/06/2021 22:13:47
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.

Hi Nigel,

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 have not used this for turning operations but have for grinding,here in the photo this worm shaft is hardened ,the open end has a small chamfer which is a centre ,the OD 's are ground between centres
the fixed steady is placed in position and the pads adjusted.Two driving dogs are fitted one for the driving the other has 2 coil springs to provide the tension to hold against the head stock centre,
the tail stock is removed to allow the internal wheel to finish grind the bore.The springs have about 15 to 20 lb tension which would be enough even for turning operations, obviously some care would be needed on long slender work to prevent the springs pulling and bending the work.

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.
Setting up would involve no more than holding the job on the headstock centre and gripping the
shaft end in the drill chuck in the tail stock and the bringing the fixed steady pads to support the shaft.
Providing the centre was turned with a small boring tool the slight misalignment if any would be no worse
than turning a tapered shaft with an offset tailstock.

John

spring centre 2.jpg

worm .jpg

Dave S28/06/2021 12:16:44
201 forum posts
41 photos


Assuming a suitable sized lathe I would approach like this:

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:

182aa982-5e0e-4572-ae1c-6f90441f5114.jpeg


Bore in situ then you know that end is running true. Setup fixed steady on other true portion. If using outer race secure a drive to on of the jaws, and make sure the con rod isn’t flapping about.

Now if required can slap on some weld to build up if needed and then machine and remake centre.

Dave

not done it yet28/06/2021 13:39:08
6271 forum posts
20 photos

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 Gilligan28/06/2021 14:15:40
avatar
18710 forum posts
915 photos
Posted by not done it yet on 28/06/2021 13:39:08:

.

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, […]

.

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.

MichaelG.

John MC28/06/2021 15:58:21
avatar
352 forum posts
40 photos

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.

John

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