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Check my sums please

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Phil H104/04/2020 21:07:16
244 forum posts
25 photos

So the good news is that I have machined the axles for my Rob Roy build.

rob roy rolling chassis.jpg

All the wheels are a nice hand push fit onto their axle seats but I hit a snag with just one of the leading wheels. The wheel runs true on its OD but there is a front face runout (the front face of the wheel rim) of 0.025" or +/- 0.0125"!! All the other wheels are plum.

I initially thought that I would need to clamp the offending wheel to a face plate running true on its OD and bore it again, fit a sleeve (probably fitted with Loctite) because I would possibly need to remove a reasonable amount of metal and fit the wheel complete with its sleeve to the axle. However, I sketched the snag and if I am right, I might get away with another slightly over sized axle on a trued up wheel bore.

Please would somebody be kind enough to look at my sums to see if I am on the right lines?

leading wheel runout.jpg

leading wheel runout (2).jpg

Martin Connelly04/04/2020 21:20:39
1211 forum posts
146 photos

The triangle is radius and run out. I think it should be diameter and run out. The wheel diameter is about 6 times the wheel thickness. The error on the shaft hole shown on your diagram of 0.002" should be 1/12 of the run out. I think you should add 1/6 of the runout to the hole diameter for the new hole diameter.

Martin C

Clive Brown 104/04/2020 21:43:06
399 forum posts
11 photos

Just asking; does the back face of the wheel have the same runout as the front face. In other words, is the 0.5" wheel thickness consant around the wheel?

Phil H104/04/2020 22:33:39
244 forum posts
25 photos


Thanks. I'll look at it again in the morning. You could be right.


I am quite sure the rim is constant thickness all round but as with the sums, I will check it carefully in the morning. I will also go back and check the axle and wheel bore for debris and anything else that could be causing the run out. It is very strange that the other 5 wheels machined at the same time, with the same methodology and tooling are all fine with just one freak.

Gary Wooding05/04/2020 09:50:32
662 forum posts
166 photos

I reckon your sums are correct. The wheel is actually wobbling about the exact centre, so is the axle bore. Since the wheel diameter is 3" and it's 0.5" thick, then by similar triangles. the bore is off by 0.0125*.5/3 = 0.002. But this is for half the error, so to true-up the bore you need to make it bigger by 0.004.

DMB05/04/2020 10:36:37
992 forum posts

"All the wheels are a nice hand push fit on their axles".

Wondering why no one else has picked up on that remark? I really dont think that they are a tight enough fit to prevent later movement, un-doing the wheel quartering. I suppose it's possible that builder intends to loctite them.


Mick B105/04/2020 10:37:33
1540 forum posts
83 photos

It depends on how undesirable you think it is to have an odd-size axle. *Personally and on what I understand* I think I'd prefer your initial solution of overboring and bushing, so as to keep the axles all the same. It'd make future maintenance more straightforward. But hey, that's just me.

Baz05/04/2020 11:28:45
356 forum posts

If it were mine I would re bore the offending wheel taking about ten thou out on diameter, a five thou cut and make the axle to suit, ok you may have to make a new axle but it’s only a few inches of silver steel. I would not worry about the oversize wheel / axle, when the loco is painted you will never notice it and once the wheels are quartered I doubt they will ever need to come apart again. Certainly would not overbore and bush, that’s two loctite joints on one wheel to worry about, wheel to bush and bush to axle, in my humble opinion a recipe for disaster.

Phil H105/04/2020 12:14:45
244 forum posts
25 photos

Thanks very much for the comments,

Clive, I measured the rim thickness this morning and there is a very slight variation but it is less than 0.001".

Gary, Thanks. I checked my sums again this morning and I think I am about right. I reckon I will need to make the offending wheel about 0.010" diameter larger to eliminate the wobble.

DMB, yes I intend to use Loctite to fix the wheels. They are not glued and quartered yet but there is no slip between the wheel and axles when I clock them up. The wheels need to be teased on and off their axles very carefully so I think the runout errors are true.

Mick, I honestly think I'd like to avoid the bush if possible. I will keep the bush solution in my back pocket if I mess the re-boring operation up.

Baz, I am with you. I reckon about 0.010" bore increase will do it and I agree that it is just a bit of EN1 (I think it is) and a bit or turning. I also agree that nobody will spot a slightly larger bore of 0.010" even if I leave the joint as bare metal.

Just to confirm the issue;

This morning I put the wheel back on its axle (in the lathe collet) and measured the OD runout and it was less than 0.001". I put the wheel against the face plate with the live tailstock centre pushed into the wheel bore - just enough to keep the wheel against the faceplate as I clocked the outer diameter. This time the run out was 0.0035". To me, that suggests that the bore starts true at the back of the wheel (the direction it was bored) but goes for a walk towards the front of the wheel.

Just making some clever clamps for the faceplate and I will get a very stout boring tool ready.

Phil H

Phil H106/04/2020 11:41:55
244 forum posts
25 photos

I mounted the offending wheel onto a face plate with aluminium clamps and re-bored it. I removed about 0.012" from the diameter before I was confident of a nice, new clean bore. The run out on a new axle is now <0.001" - so thankfully it was successful. It is also very difficult to tell the difference between the new, larger axle/ wheel to one of the other wheel axles combinations.

wheel rebore - copy.jpg

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