Wheel Castings - Centering in the chuck

[Smifffy]

I'm just about to start machining my driving and coupled wheels for my Royal Scot. At just under 7.5 inches diameter, they're quite chunky.

I'm going to machine the backs and bore each one before turning a mandrel and finishing the fronts/flanges/treads.

I'm after some advice on what to centre them against for the first cut. Do I use the external diameter of the casting, the inner diameter of the tread or the centre diameter ?

Obviously all are a bit variable as they are rough castings. But an error here will lead to the whole wheel being off centre and at approx 70quid a go, I don't want to get it wrong.

As always any help will be very much appreciated :-)

Mark
(No squirrels caught yet!)

[alanstepney]

Not an easy question to answer, but at least you have thought of it well in advance.

Most people, including yours truly, only consider possible problems, after making a cut or two, when, Whoops, often covers it!

Having made that mistake in the past, what I now do is look at the casting carefully, working out a rough centre point (on the front, as that is what one sees) then seeing what parts are eccentric to a true circle based upon that centre.
You can usually find a compromise that makes all of the errors (if any) as small as possible, and therefore less likely to be seen.

Having found a centre that achieves that, the trick is then to transfer that dimension on to the back, so that you can bore and face the back.

[jackrae]

No doubt some will think this method daft but what I'd suggest is that you draw or print a wheel onto celluloid film and then lay this over the casting face. Position the film for a "best fit" onto the casting and then mark the "best" centre position through the fim onto the casting.

Drill a small hole through the casting using the marked centre and then use this hole to set the wheel central in the chuck for machining the back face

This method will (should) ensure that the face of the wheel has enough metal in the right place for subsequent face machining.

Jack

[baggo]

The way I do it is to grip the casting by the flange in the 4 jaw (using the outside jaws) with the outside of the wheel facing the tailstock. (If the outside of the flange is very rough then hold the casting by the tread first and give the flange a quick skim) The casting is then adjusted in the chuck so that the inside of the tread and the outer face run truly and a skim is taken over the tread to true it up. The casting is then turned round and gripped by the tread in the 3 jaw and the back faced off and bored for the axle. This ensures that the inside of the tread runs truly with the axle. Nothing looks worse than an eccentric tread!

If your chucks are too small to hold the castings, you'll have to bolt them onto the faceplate which is not quite so easy.

It pays to check how much thickness you've got to play with when machining the front and the back. I've got wheels where whoever machined them didn't take enough off the back and consequently machined too much off the front to get the tread the right thickness, leaving the spokes sticking out further than the tread!

[simonwass]

Baggo's spot on here, always bore a wheel from the outside in, if a casting is done on a board the 2 halves when cast may not be 100% central. When you bore using the back face as your reference, the front can be a way out.
Bore from the front and the faintest skim to true the inside edge of the rim is all that is needed.
If your wheel needs to be machined on a faceplate, a spindle bore taper fitting is useful, machine it to suit a custom made plug for each size axle hole you use. This way each wheel gets centered on a spigot on your faceplate, turn the wheel around and its still centered (so long as the wheel -just- fits the spigot).

[simonwass]

Just thought of something as an aside.
My Paget's wheels dont have central axle holes, the hole is eccentric to the rim and the axle has eccentric wheel mounts. Eccentric + eccentric = concentric!
This is all due to the cranks being so close to the wheels as to not allow any room for an outside crank web!

[SteveW]

Guys,

I've been considering this one for a number of years now and this is what I've come up with. Feel free to comment.

1. You've got to start from somewhere so I reason that the front of the wheel, rough as it may be/will be the closest to the original shape that was used to make the hole in the sand.

2. The back face is likely to be the least useful in the respect so needs to be the first to go. The best approximation is it should be parallel to the best known face, i.e. the front.

3. Any truing cuts to the back face need to be parallel to the front but not necessarily concentric to it unless there is a boss but that can be left over size and finalised last.

4. Once you've got a machined face that's true the wheel can be reversed and mounted either in a four jaw or on a true face plate and a best true established on the hub outer or rim inner surfaces. Just think about which would look badly eccentric when finished.

5. A guide hole and now be drilled/reamed that is now the best centre on the front wheel profile.

6. I know the theory is that the rear face and axle are usually machined from the same side but so far it's been only truing cuts.

7. Either way the front and back surfaces are now more or less parallel and you've got the lot centred. You can either leave it on the face plate/back surface and complete the front or reverse and follow and convention.

[baggo]

Sounds reasonable to me Steve. Once you've got the back of the wheel flat you could turn the tread and flange, and machine the axle bore all at the one setting if using a faceplate. Would save having to turn a mandrel.

[Smifffy]

Hmmmmm, so many variations :-)

I've already skimmed the back of one wheel and have been giving some serious thought on where to go next. As Steve says, the wheel does not need to be concentric at this point. So far, I've been very loathe to take the wheel out of the chuck as I will lose it's position.

I think I will follow Steve's advice here now and remove the wheel, reverse it in the 4 jaw ensuring the back is flush as best I can and centred on the inner rim.

I'll then face the front and drill/bore the spindle hole.

However, as I don't have a face plate I will now remove from the chuck and fix it to a mandrel machined to the bore size and do the flanges and treads from here.

[chris vine]

Hi Smiffy,

don't forget that unless the mandrel is really large and stiff, you may have trouble machining the tread etc because of vibration and chatter.

do you have a catch plate which could be used as a make shift face plate?

Chris.

[Shawki Shlemon]

I normally clean the wheels and use the tread as a base ,machine the back, the flange o/d and bore and then do the front ,tread etc on a mandrel . In this case the wheels are large and chatter is possible , may be using face plate is a good idea.

[Lurkio]

Smiffy,

When I machined my wheel set I soon realised that to get things right a lot of thought and planning had to be put into the job. As you say, so many variations. The few articles I had read in books seemed to me to cut a few corners, so you are right to plan ahead for each stage of machining. I do know the job took longer than I imagined with many operations on each wheel, but they worked out to my satisfaction.
First job was to check the depth of the spokes on all wheels to get an idea how much could be removed from the rim faces, as Baggo suggests.

My own method for the initial machining:
Begin machining by setting up in the 4-jaw, with the back of the wheel to the chuck, and adjust the rim of the wheel to run true(ish), axially and radially, compromising as Alan says where necessary to minimise runout of the centre boss.
A skim across the outer diameter (flange) and front face of the rim now allows the wheel to be turned around and mounted up to the chuck or faceplate etc. Using a DTI on the skimmed flange gets it running true. Now the rear can be faced off and the axle hole bored. Once this is done all critical operations can be carried out with the wheel mounted by the rear face of the rim.

Regarding your lack of faceplate, you could obtain a large lump of steel, mount it in your chuck, face it off and use it as a mounting fixture. It provides a rigid 'backplate' to work from, and you can drill and tap holes in it where required to secure your wheels......easier than using a faceplate. Bang a pin in the centre and turn it to size to use as your spigot. Facing it off ensures all is running true.
I found this method invaluable for all the machining operations on the wheels, once I'd completed the initial machining as outlined above.

Regards,
Lurkio.

[Nigel Bennett]

If you do manage to make a mess of a wheel tread beyond what can be salvaged, it's not a difficult job to shrink a steel tyre on over it...and it's more accurate to prototype! From what folk have said, it grips better, too.

[Smifffy]

Regarding the comments on chatter - why does having a big lump of metal improve the situation. Excuse my ignorance!

I did have a major problem with Chatter when doing my bogie wheels - I'm now put off the idea of mounting the wheel in a mandrel :-(

I do have a catch plate - how can I use that ?

Ta again,

Smifffy

[baggo]

Mark,

I used a spare chuck backplate as a faceplate for turning some smaller wheels. Rather than machining the backplate, I screwed an aluminium disc onto it and then faced that and fitted a threaded stub to locate the wheels. I then turned down the outside diameter of the aluminium to a bit less than the finished wheel diameter so that I could machine both sides of the flange.



You could do a similar thing with your catchplate but you will probably need a larger disc to screw on to it.

I got away with just holding the wheels by a nut on the stub but you would probably have to put a couple of clamps over the spokes as well so the wheel does not slip.

John

[chris vine]

Hi Smiffy,

the trouble with a mandrel (with only a small flange behind the wheel) is that it is not very stiff (compared to the lathe mandrel). Also the wheel with spokes is not very stiff either.

If you hold the wheel onto a faceplate, it is the faceplate which is giving stiffness to the outside of the wheel and cutting out all the flexibitility of the small mandrel and the spokes.

Once I have one side faced flat and the hole reamed/bored, then I put a locating mandrel in the lathe taper and just hold the wheel up against the face plate with a rotating centre in the tailstock (and a piece of metal with a centre in it to push on the centre of the wheel). I usually put a piece of paper or cardboard between wheel and faceplate to increase the friction. Although you could use a bolt or peg between the spokes.

The advantage of doing this is that it is very easy to take one wheel off and put another one on. working this way, you can just do one very simple operation on each wheel at a time. EG face to thickness, turn tread, flange, put cone angle on etc. You only have to concentrate on getting one dimension right at a time and can use the thimbles on the feed screws without having to alter them.

In addition, if you get one dimension slightly wrong (which of course I never do cough cough Ahem!!) then they will all be the same and no one will be any the wiser.

Hope that helps
Chris.

[SteveW]

Guys,

The main thrust of where I was going above was to get the wheel roughed out and a good centre on it that fits with front from where folk notice.

The point of using a faceplate is to have a good and true surface to clamp the wheel to and once clamped you get the added rigidity and support of the plate. You also get a flywheel effect that can reduce chatter. A sheet of paper between the plate and wheel can also help. This method does assume you can get clamping bolts through the spokes. It's less useful (useless!) if you have to clamp over the rim from out side. It's also worth giving the plate a skim to remove any burrs and get the faceplate true first.

Once the wheel is truely centred (and secure) the axle hole can be finished to size (yea I know, from the front) and the rest of the turning completed. EXCEPT that it for the final cut(s) on the tread/flange.

All the wheels need to be EXACTLY the same size/tread angles. This is where a MT2 mandrel with a spigot that fits the axle hole and a faceplate for the reasons above come in. It'll give you easy and repeatable setups for each wheel in turn.

You need a repeatable system to mount each wheel in turn so that all the treads are cut from the same clamped saddle position.

The alternate is to complete each axle with the paired wheel and complete the final cuts between centres. Trouble is there's no way to get the necessary support/drive so Mister Chatter comes to visit.

I should also add that I haven't done this yet. Just done an aweful lot of thinking and reading up about it.

Something I did do a while back was to buy a 3/4" x 6" disk of cast iron for a back plate between a spare 4 jaw and my rotary table. Getting a CI disk in the 4-jaw you can make a wheel turning jig with tapped holes in the right places to fix the wheel through the spokes.

[Smifffy]

Wow guys, lots of good stuff here and educational too (the chatter explanations) :-)

So I have an immediate decision to make:

1. Get a backplate;
2. Get a big lump of metal and make a backplate using a catch plate (I have a spare).

The latter is probably the easiest for me in the short term, so I'll give that a go.

Thanks again for the help.

Mark