At home to Mr cock up today - Your assistance please

[britannia]

I am changing two (one axle) of the driving wheels on my 5 inch Brit - don't ask why, too long and uninteresting a story.  Anyway, I have had the 2 wheel castings in stock for some years and never got around to doing the job, you know how it is.  So I turned the casting to drawing accurately if I can say that myself.  Just needed to position and drill the 3/8 crank pin hole.  Lost all sense of sanity at this point being 'on a roll' as they say, and drilled one of the wheels to drawing. 2.5 seconds later after switching the mill off, realised that I should have made the hole position to the wheels coming off the loco.  Today I took the old wheels off, and yes of course, the position of the crank pin hole is a nice (say) 1/16 (3/64?) further down the wheel centreline.

Ideas on going forward please (only one wheel was drilled).  So I could tap the hole, fill it with MS/SS? and using the old wheel as a jig, drill it out again, but given how small the error is would the tapped metal simply drop out - maybe not if glue in the first place?  If doing this, would I be better using a D bit cutter which might not be tempted to wander like a drill?  Other ideas?  As the crank pin is wider at the surface of the wheel, a larger hole would work, and would take out the error so to speak - but how on earth would you drill that hole??

I could swear, but that won't help (tried it), and buying a new wheel casting and starting again would start the swearing off, so not a good idea, particularly for the bank account given the cost of casting.  Don't yet want to go down the route of retreading the old wheel with a steel tyre.

Would greatly appreciate your positive support here.

Richard

[firebird]

Can you turn an off set crank pin ??

Cheers

Rich

[davewoo]

Have done a similar repair to a part built 5" Brit bought by a friend, the crankpin holes were all over the place, most could just be bored oversize in the right place. But two were drilled really out of centre to the pear shaped wheel boss. Made up a " top hat" out of MS with a thin flange at the back of the wheel and held it in place with two small countersunk screws and Loctite. And then drilled it as if it were a new casting. One of the pins had the return crank on it so drilled and loctited a pin diagonally through it ( might have been a grub screw), I think we had to spot face the back of the boss to give clearance for the flange, which was about 1/16" thick. At the time the general opinion was that it would not last ten minutes, this was in the late 80's and it's still going strong. The annoying thing about this model engineering thing, one lapse of concentration can cause such a load of grief (and swearing)
Good luck

Dave

Thinking about it  I'm sure we drilled the old crankpin hole out and made the top hat plug bit oversize so the new hole was drilled entirely in the steel plug rather than breaking into the cast iron boss.

Thinking about it yet again you could drill the hole oversize with a slot drill indexed from the wheel centre on the mill, that shouldn't follow the existing hole, and make the crankpin part that goes into the wheel oversize to suit, might save the faff of my first idea! Pretty sure we used a slot drill to do the wheels mentioned above that were not too bad, the two we made plugs for were very far out. It was a long time ago!!!

[Nigel Bennett]

Rich's suggestion of an offset crankpin would be the easiest solution!

If you plug the hole in the wheel, I'd try to fill it with cast iron. Preferably thread it and use a decent Loctite (638 or 603). I wouldn't use stainless for your plug, but mild steel would be an alternative. You'd need to ensure a dissimilar material plug was big enough to accommodate the new crankpin hole within itself and not burst out of the side into the cast iron. Let the glue cure well! Then as you suggest, use the old wheel as a drill jig. If you use a cast iron plug, you should be able to make it smaller and get away with "bursting out of the side". Perhaps use a good-fitting slot drill in the old wheel's crankpin hole (you could sleeve it with a thin-walled bush so it fits well) to form your new hole as it will be less prone to wandering.

Steel tyres... not a bad idea really, as they give a much better grip on the rail than iron. Not too difficult; all my recent locos have them and I fitted a friend's Maisie with a set when a chunk of CI flange snapped off. (Don't drop locos on to a concrete floor...) You could perhaps contact Ed at Model Engineer's Laser to see what he could supply for steel rings. Turn rebates on wheel and tyre so that there's a positive axial location for when you fit the cold wheel centre into the nice hot tyre. I suggest you'd need about 0.006" of interference fit for your wheels.

[millman]

Bore a hole in the correct place large enough to remove evidence of your misplaced hole and just turn the crank pin from larger sized material. Loctite pin in wheel and jobs done. Hole needs boring though, drills and even slot drills will wander depending on rigidity of your mill, if you are doing it on something larger than a Bridgeport you can ignore that comment.

[britannia]

Apr 1, 2024 15:30:20 GMT millman said:

Bore a hole in the correct place large enough to remove evidence of your misplaced hole and just turn the crank pin from larger sized material. Loctite pin in wheel and jobs done. Hole needs boring though, drills and even slot drills will wander depending on rigidity of your mill, if you are doing it on something larger than a Bridgeport you can ignore that comment.

 Probably the best route to go down I think.  Cheers

[timb]

This might be of use:-

www.youtube.com/watch?v=mbynP_gEoUA

[oldnorton]

A good idea to repair the wheels by inserting plugs. I would use cast iron then any machining across a joint is into similar metals and the tool is not forced into the softer one. Loctite will hold the plug and the crankpin will still hold fine in a new hole that is half old-wheel and half new-plug. It only becomes tricky when there is not enough material in the wheel to accept the plug. But I do agree that the oversize, stepped crankpin might be the logical fix for a small displacement error.

[keith1500]

This might be of some use…

Jan 4, 2021 9:17:08 GMT keith1500 said:

The throw on the main cranks was out considerably when compared to the coupled wheels. I felt this had to be corrected rather than bodge the coupling rod holes which are already slightly elongated !

I set up a mandrel on the mill. I recall Roger using this method so took a leaf from his book.

Walschaerts valve gear by GL5Keith1500, on Flickr

I figured I could move the pin back by 12 thou and simply make a new crank to suit the hole. The hole was blued so as to show when the boring bar was cutting all around.

Walschaerts valve gear by GL5Keith1500, on Flickr

Then the cranks where checked on the surface table using the height gauge and a dial gauge to show when the surface gauge was being deflected upwards and by what amount. To do required a test pin fitting. The outside diameter matching the coupled wheels so as to keep maths to a minimum.

Walschaerts valve gear by GL5Keith1500, on Flickr

I wasn’t satisfied with one wheel and so had a second go.
Fingers crossed all this theory works outs.

I wonder how many measure their cranks before assembly or just hope for the best?

From…


modeleng.proboards.com/user/921/recent?page=11

[britannia]

Apr 3, 2024 9:34:59 GMT oldnorton said:

A good idea to repair the wheels by inserting plugs. I would use cast iron then any machining across a joint is into similar metals and the tool is not forced into the softer one. Loctite will hold the plug and the crankpin will still hold fine in a new hole that is half old-wheel and half new-plug. It only becomes tricky when there is not enough material in the wheel to accept the plug. But I do agree that the oversize, stepped crankpin might be the logical fix for a small displacement error.

 So in the end, I plugged the incorrect hole using MS (surprisingly I didn't appear to have any cast iron in stock).  I taped the hole and threaded the plug, inserted the plug with Loctite and remounted the wheel on the face plate and cleaned up both sides - perfect so far.  Then I used the old wheel to act as a drill jig on my mill machine, with a spigot through the centre of the old and new wheel to ensure precise position.  The new hole actually drilled out OK leaving a small crescent moon on one side of the old hole.  Made up and inserted new crank pins, then yesterday made up a new axle, and quartered the wheels using another pair of driving wheels off the Brit as a template.  Tested the new wheel set today with the connecting rods temporarily re installed on the crank pins.  All was well with the wheels turning freely.  Phew! 

Must try keep concentration in future!

[britannia]

Apr 2, 2024 10:57:39 GMT timb said:

This might be of use:-

www.youtube.com/watch?v=mbynP_gEoUA

Thanks.  Really interesting treatment of the crank pin holes