Crown Wheel Run-Out

[Bob Culver]

Hi KC
.004 is not very much. If the mounting pedestals are marked and measured with a mike or vernier callipers for thickness it should not be difficult to reduce one .003 progressively reducing for those each side. Could press with thumb on  a very fine flat file and/or sharpen an old file to a smooth dead square end for use as a scraper. Check with blue. Seating must not rock. Hopefully will all be OK on first assembly. 
If the cw runs out radially a bit more tricky.
CW mounting bolts need to be consistently tightened.

And Roger, what is the lathe with the double raised v bed please? Hopefully not a diabolical Indian peasant made file finished Metro as mine.

[Roger]

Bob,
My lathe is a Boxford 4 1/2” model A fitted with screw cutting gearbox, a very useful late for A7 work.

Alan,
I agree with your comments about checking the mating faces of each half of the carrier.
I mounted each one in my home made mount on the lathe and each face proved to be a few thou out, I took as little off as possible to achieve full contact all the way round on each carrier, also the crown wheel mounting face.
Surely machining all three contact faces is a must to achieve total accuracy.
Below is the other half after facing.

Roger

   

[Chris KC]

I confess I've been struggling to get my head around this but I think you are both right. Truing up the CW face alone might do it if you are lucky but it won't address an overall 'bend'.

[Roger]

Chris,

The “bend” as you called it, was what I was trying to eliminate, I blued the two faces together and had a good result.
I then attempted to fit the bolts! The holes did not line up.
Using smaller diameter bolts the crown wheel ran true. 
So what do I do with the holes? Attempting to force the bolts in will take me back to square one I guess! 
Can I ream them out but what bolts do I then use.
Does this also mean that the six planet gears are also misaligned.
The exterior of this carrier is not as visibly damaged as my original one, which had serious scars on it.

Roger

[Chris KC]

Yes that's what was worrying me. You can true up the joint face but if the thing is 'bent' then everything else will still be slightly out. There must be a practical limit to what can be recovered. Did the dowel pins for the planet gears engage in the opposite half OK Roger? Then it's tempting to believe that their alignment will be adequate. The bolts are a tight fit in the holes (or should be) even before any misalignment creeps in. I'd be wary of applying too much brute force getting them in though or it'll be back to square one.

[scrooge]

I must admit I have been reluctant to turn my carrier faces to try and eliminate my run out.  I figured that if I try to correct then the planet wheel holes will become less true to the pin axis.

Yesterday at Beaulieu I found an early diff carrier with a very good, and pitting free, crown wheel.  One broken half shaft as well but no pinion. £40 though.

I thought id wash it and test the run out but this time I put a small marking out table across the lathe bed and then placed the diff carrier assembly vertically sat on the thrust bearing on the plate and with a suitable preload weight on top.   When the carrier is in the axle assembled, I think the 4 thou preload is more simulated than I was trying by loading the two hub type bearings outers,  The mag mount DTI sat on the lathe saddle to bear on the rear of the crown wheel face. A 1 thou run out found.

Ive swapped the new crown wheel to my other original carrier and have the same 1 thou.   I do think it may be the pitting on my original despite my best attempts to avoid it.

I'm glad now I wasn't smart enough to figure out how to skim the carrier. 

I also had a master class in setting up the axle from a very knowledgeable type.   I don't think my run out is worth worrying about .

Jack French's 1953 notes in the companion suggest 4 thou and you should start looking in the bulletin advert- tricky nowadays though ?

charlie

[Roger]

Chris,
I have now machined all three faces on the carrier and checked with blue the contact areas were good and only needed a quick rub with fine stone.
On assembly of the crown wheel the bolts were too tight a fit so I opened the bolt holes up with the next size up metric drill only removing a few thou. The bolts went in much better and still held the crown wheel with no movement. I then clocked the crown wheel and the result was 0.003”, a big improvement from the original 0.016”.
The planet wheel shafts entered the opposite hole ok.
I think I can conclude that machining all three faces is a success, I will fit it to the car when the cold weather arrives so I can keep driving for now.

Roger

[Chris KC]

Thanks for the update Roger, and good luck.
I have come back from Beaulieu with a couple of spares to try and will decide what to do next once I've clocked them.