Turning job – persistant errors

[Steven GreenhoughParticipant @stevengreenhough56335]

Crank disc keeps coming out wrong (a bit ‘wobbly’)

[Steven GreenhoughParticipant @stevengreenhough56335]

Hello guys, some time no speakey, I’m making up for it now though, Happy New Year!

As you may be aware I’m making a very long-winded job of building a “Cracker” 32mm gauge locomotive.

After a period of inactivity, I’ve resumed work only to find that I’d lost the crankshaft and flywheel that I’d made a while back; No great problem as it was suffering from a slight wobble, so would probably have been better remade anyway…

For those unfamiliar with the model, the crankshaft assembly is simply a shaft a little longer than the width of the frames. It has a flywheel on one end, runs through a bush in the frame, has a pinion at the mid point, runs through another bush incorporated in the cylinder stanchion, then terminates in the crank. This is the plan:

Things I’m doing differently: I’m using 2mm silver steel for the crankpin AND the shaft as this fits the pinion I’m using, and I’m making the crank as a disc rather than the offset ‘egg’ thingy (although I may file/mill the disc so it has a ‘web’ look once I’ve got my problem sorted)

Ah, yes I’ve not really explained my problem, have I? I simply cannot get the whole thing running true. No matter what the crank ends up with a sort of “Walkers Crisp” thing happening when it is turned, when viewed side on.

My procedure is:

1) Stick an oversize bit of brass in the 4 jaw and get it running true (near as dammit), then face the end.

2) Use a small centre drill to mark the centre of the work very lightly, then use a two-prong compass thingy to scribe a circle with a radius of 5.5 mm. A gentle punch mark is then put on the circle adjacent to a chuck-jaw.

3) Increase the centre drill mark large enough to start a 1.7mm drill, then drill overdepth. Thread the hole M2, using 1st, 2nd, plug.

4) Reduce external diameter of job to 14.5mm, by about 8 or so mm to give me room to work.

5) Take a deep breath, curse and sulk as I reset the work in the 4 jaw to bring the crank-pin mark to centre. I did this by slakcening the 3 & 9 o-clock jaws ever so slightly then tightening 12 o-clock and loosening 6-clock until the mark was roughly on line with a tailstock centre, then using a centre finder and tdi.

6) Centre drill, drill 1.7, thread.

7) Just like number 5, only in reverse, to bring crank hole back on centre. Centre finder not used, final setting done with clock on job.

8) Take a light facing cut to tidy up and remove centre drill tapers, turn little shoulder bit at 0.5mm, then part off (carefully through intermittent crank-pin hole) at 3mm. Clean/polish scored face on fine emery with oil.

9) Thread the end of 2 pieces of 2mm ground silver steel, M2. Twice I did this in the lathe and once in the drill press. The die is too small for the tailstock holder I have, so I mounted the steel in the drill chuck and the die in an ER collet on the spindle. In the drill press I put the die in a presicion vice. File ends so that when seated does not protrude through crank disc.

10) Assemble crank, and try frames/bushes. Become somewhat disheartened when disc displays slight out of trueness when turned.

I’ve done this 3 times today, 4 in total if we count the ‘lost’ effort, and I’m not happy with any of them. I’ve threaded new shafts/pins each time and I’ve tried swapping them between sets to see if 1 combo will run better – they’re all about the same.

I suspect that it’s near enough not to stop the engine running, but I know and it looks a bit rubbish. It also makes me wonder how I’ll get on when doing other 4-jaw work such as eccentrics (on future models).

I’m using an 8mm indexed cutter like this:

**LINK**

with this insert:

**LINK**

Taps are mid-range eBay jobs, die as cheap but seems to cut nice clean threads.

Any advice or encouragement would be appreciated…

Steve

Edited By Steven Greenhough on 01/01/2016 20:55:12

[Ian PParticipant @ianp]

Its not completely clear from your description but are you saying that the crankshaft and the crankpin are held in the disk by each being threaded and then screwed into the disk? If so at what point do you stop screwing them in?

I if the OD of the thread is the same as the shaft then you dont have enough material for a shoulder so there is nothing to keep the disk square to the shaft.

I think you would stand a much better chance if you made both shafts a press fit in plain holes. An alternative would be to leave the disk oversize, screw onto the crankshaft until the threads bind solid and then clean up both faces of the disk in the lathe. Obviously easier if you have collets but even so machining the back of the disk will need very light cuts.

The above only gets the disk true to the shaft, to get the pin square I think you will need a press fit after drilling the pin hole with the crankshaft offset in the 4 jaw.

Ian P

Without knowing what equipment you have available

[Michael GilliganParticipant @michaelgilligan61133]

Two suggestions, Steven:

1. Forget the screw thread … just assemble with 638 Loctite
2. Make the disc a little over-width and then true it up with a parting tool, after assembly to the shaft. [You may find it easier to make the shaft over-length, with a pointed end, and use a female centre in the tailstock]

Other options are, I am sure, available.

MichalG

[Roger Provins 2Participant @rogerprovins2]

In the past I've had similar problems which were solved (as Michael suggests) by …

"Forget the screw thread … just assemble with 638 Loctite"

[Steven GreenhoughParticipant @stevengreenhough56335]

Ok thanks for the suggestions I’ll try again tomorrow and ditch the threads.

[Steven GreenhoughParticipant @stevengreenhough56335]

I've not managed to get in the shed today

[Author]

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