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Wheel Design

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Noah Partida30/04/2020 08:20:17
5 forum posts
4 photos

Hello everyone!

As I continue with 3D modeling Martin Evans Eagle, I have come across another feature I do not understand. I don't understand when the wheel flanges will start to cut into the side and make the 10 degrees angle that is set by the N25GA. If anyone could explain to me geometrically where the angles of the flanges and threads start and end that would be great! Thank you very much!


Noah Partida30/04/2020 08:21:40
5 forum posts
4 photos

Original drawingWheel Cad DrawingZoomed in Wheel Cad Drawing

Mick B130/04/2020 09:26:52
1738 forum posts
91 photos

I'd think there's risk of overinterpreting the drawing to a level of precision not achievable outside an instrumentation lab.

The flange edge radius isn't specified because it's supposed to be derived from the 1/16" flange thickness at an arc where a 10 degree tangent each side touches the radius. In practice it'll be a bit over a 1/32" rad - I think I could probably work it out if I put the time into it. A more considerate draughtsman would have calculated the nominal value of that rad and put it on the drawing as a 'reference' dimension.

Then the specified 1/16" rad runs from the 10 degree tangent down to a tangent on the 3 degree taper. It would be best to blend that transition as smoothly as possible. Absence of a step is *probably* more important than the precise value of the radius, or the taper.

But hey - I know drawings better than I really know railway wheels. There will be people here who know far more.

Redsetter30/04/2020 10:38:37
134 forum posts

The 1/16 radius at the root of the flange is important, as that is what centralises the wheelset on the track.

The tread does not have to be tapered - a parallel tread will work equally well, possibly better,

The radius at the tip of the flange is not critical, on wheels of this size it is mainly cosmetic and can be formed freehand with a file.

blowlamp30/04/2020 12:13:45
1424 forum posts
87 photos

It looks like you've drawn it properly.

As long as you have used the 10 deg lines to trim the 1/16" diameter 'arc' and then constructed the 1/16" radius tangential to it, then it should work out OK. From there, a tangential line at 3 deg confirms the profile.

Actually, the 10 deg line that intersects with the inner face of the wheel appears fairly pointless to me, as if you use it to trim the 1/16" diameter 'arc', you get a tiny flat/chamfer of a transition instead of a nice smooth blend.


Mick B130/04/2020 12:28:38
1738 forum posts
91 photos
Posted by Redsetter on 30/04/2020 10:38:37:

The 1/16 radius at the root of the flange is important, as that is what centralises the wheelset on the track.

The tread does not have to be tapered - a parallel tread will work equally well, possibly better,

The radius at the tip of the flange is not critical, on wheels of this size it is mainly cosmetic and can be formed freehand with a file.

Not really my subject, but I'd appreciate understanding it better.

Somebody once told me that the taper on the wheel sets the position laterally on the rail, and that contact of flange with edge of rail is a fault condition. Presumably the rail edge radius is significantly smaller than 1/16"? Because presumably the further up the flange root radius the rail contacts, the more frictive that contact will be?

Paul Kemp30/04/2020 12:56:36
563 forum posts
18 photos

Not sure I properly understand the question but from my interpretation of it as written here goes. The flange position (and to an extent the thickness) will be determined by the gauge and the back to back distance of the wheel set. If you draw two opposite wheels on their axle the shoulders on the axle within determine the back to back distance. This is important to ensure the wheel set will pass through points etc which are fitted with check rails. If the back to back is too small the engine will derail as it will be forced to ride up the check rails, if it's too large then the check rails won't guide it and it will still derail! So the position of the flange is governed by the back to back distance and its thickness to still comply with the gauge (distance between the running rails). Hope that makes sense? I am not familiar with the N25GA standard but I doubt it requires thou precision for compliance. For greatest accuracy in production it is usual to finish the flanges with the wheel set mounted between centres when all the relevant governing dimensions can be checked.


Roger Best18/07/2020 10:51:11
104 forum posts
19 photos

smiley Hi guys

I am also dissatisfied with the dimensions given.

The wheel has two models of operation, straight and near-straight running on the coned tread, as illustrated in all books about locomotives, and scrubbing round "tight" radii, as in junctions and almost all model applications where the track is not dead straight, which Paul ably points out is actually more critical to safety.

In the drawing those two systems are not tied by a tolerance, so the 1/16"R is floating in space, with a multitude of tolerance build-up deciding where it is. Obviously the designer intended all dimensions to be absolute with a form tolerance on top, but it doesn't help the machinist at all.

As a total amateur machinist I would want to use a 1/16 radius tool, touch off the back plate and move across exactly the distance to put it in the right place, feed in to diameter, allowing for the tangent, then feed across on the compound slide at 3 degrees. Then all running surfaces are in the right place and accurate. Obviously roughing is required and I am sure the pros have more sophisticated methods.

I have always found wheel geometry baffling and I think I now know why. wink

Perko718/07/2020 12:17:27
353 forum posts
25 photos

Contrary to Roger Best's post, there is a geometric relationship between the various arcs and lines but it is not clearly shown on the drawing. It can be worked out if you really want to be that accurate, but in my experience it is not really necessary as long as the critical dimensions (effective wheel diameter, effective width and depth of flange, and root radius) are close to the specified values.

IMO the best way to go about it would be to first face off each side of the casting to give you the overall thickness. Then turn a flat tread of 3-3/8" diameter ending in a square flange 1/8" thick and 3/32" deep to give the 3-9/16" overall diameter. Then reduce the flange thickness to 1/16" while forming the 1/16" radius at the root of the flange. Next, using a form tool and files (or just files) form the 1/32" flange radius and blend it into the root radius. Lastly turn the 3deg taper on the tread.

If you really want the accuracy of the given dimensions then I can mark up a drawing showing the relationships, but you would be hard-pressed to meet them exactly for all wheels unless you made a form tool having that compound shape.

If you are uncertain that wheels not exactly meeting the profile shown will still work, have a look at some earlier O scale and OO scale model railway wheels with cookie cutter flanges and no fillets. They still went around curves corners at high speed without derailing (at least most of the time) laugh.

SillyOldDuffer18/07/2020 12:50:41
6346 forum posts
1395 photos
Posted by Perko7 on 18/07/2020 12:17:27:

Contrary to Roger Best's post, there is a geometric relationship between the various arcs and lines but it is not clearly shown on the drawing. It can be worked out if ...

Tubal Cain's Model Engineer's Handbook throws light on the subject. Wheels run on tracks!

The Handbook shows the wheel in relation to a check rail, and mentions an assumption about the corner radii of black mild-steel flat and the profiles of two commercial 5G rails, which determine the 1/16" radius. A table gives all the main dimensions for making wheels in various gauges, including Y 'position of edge of tool, radius RR, to form root'.

As the book's in print I won't copy the pages. My favourite Camden Steam don't seem to have it in stock, £7.70 from Amazon.

Not anything I know anything about, but Martin Evan's Drawing shown by Noah looks 'good enough' to me. Hard to believe the dimensions are critical.


Perko718/07/2020 13:43:17
353 forum posts
25 photos

Noah, will PM you with some more info. Geoff Perkins

Clive Foster18/07/2020 14:04:17
2389 forum posts
77 photos

Major league overthinking about the drawing forgetting its something that you just make, mosly by eye on unsophisticated machinery.

Start with a flat flange and wheel the right thickness and diameter. Use a concave tool to put the 1/16 radius on the wheel side of the flange. Probably end up with a thou or so undercut on the top but that is too small to worry about.

Use a convex 1/16 radius tool to cut the wheel taper stopping when the two radii blend. Again probably have a wisker of over or undercut. Polish it out and be happy.

Flip the wheel and put the radii on the back.

Do the rest.

With the right technique the theoretical drawing tricky stuff just falls out.

I wouldn't do it that way, heck I wouldn't build a loco anyway, but I have the gear to set up effectively the same thing by measurement and stops. But its still just a matter of relative tool cutting edge offsets and cut depths. Not circular geometry.


Edited By Clive Foster on 18/07/2020 14:04:32

duncan webster18/07/2020 14:33:59
2804 forum posts
41 photos

I used TC's SMEE dimensions to work out the distance from a roller held in the flange/tread join to the inside face, then using a tool with correct tip radius and approach angle machine along the tread angle to get the correct flange thickness, then round off the top of the flange with a file (that is the least important dimension). I used brazed carbide tool as if you don't have to sharpen it between wheels it makes getting them all the same size a lot easier

I've got it on CAD somewhere for 5"g. Send me a pm if you want a copy

Roger Best19/07/2020 12:46:22
104 forum posts
19 photos

Highly informative thread chaps, ta

I will be getting a copy of that book, but I won't ask for that CDA file just yet thank you Duncan as I will be working in 3.5" for some time.

Noah - are you satisfied?

Circlip19/07/2020 13:40:09
1198 forum posts

How did LBSC et al manage without 3D modelling and super glue ??????????????? and BEFORE anyone mentions drawing errors, SOME have the capability to solve errors without recourse to electrons.

Regards Ian

Roger Best25/07/2020 14:48:29
104 forum posts
19 photos

I think they made plenty of scrap, and enjoyed it. Maybe a few locos that listed and wore out their rod bearings.

The Model Engineers Handbook has arrived, very interesting it is too, with all sorts of stuff.

Page 13dot16 refers, specifying the radius position and tabulating the various dimensions. Unfortunately they differ to the diagram above in some dimensions but the principle of positioning the radius tool centre on the diameter point, and a specified distance from the back face is there.

The specified flange radius is only 0.030", so 1/32, and the angle 20degrees, so much less rounded than Noah's drawings. More to scale I expect.

Thanks for the tips.

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