Just out of interest Julia, did you turn the printed wheel centres true before fitting the tyres?

 

Paul.

Hello Paul.

 

No, I didnt do any machining, just cleaned them up with a stiff brush and some IPA.

If the 3D printing is not reliable, you could try making the centres slightly undersize and build a jig to get the wheel centre (based on the axle hole) and tyre concentric. Use adhesive to fill the gap.

You suprise me Julia, but I'll bet that's where the problem lies. As you say leaving the axle hole to be drilled after tyre fitting could be the way to wobble free running.

 

Paul.

As I have said before, I would be very wary of stressing FUD by force fitting the wheels onto the axles.

In reply to the title, for some it would appear that "enough is too much"!

 

I'm not sure how tight a fit your 3D prints are on the axle, but I suddenly see why Bill Bedford makes his deliberately loose!

Thanks for the comments folks .

 

Rather ironically the broken wheels occurred while the wheel center wasn't fitted to the rim, I have never had a broken wheel while fitted to a rim. The rim acts as a good 'clamp' containing the stresses within the wheel, a bit like fitting a steel rim to a old fashioned wooden cart wheel I guess.

 

M

Hi Julia,

 

I can't offer any advice from my own experience (other than tea and sympathy regarding wobbly wheels) but have you read the article by Chris Pendleton in the latest MRJ? It's all about curing the deficiencies of less than perfect 4mm scale wheels, conducted with Chris's usual level of attention to detail. Obviously some of it is 4mm specific but I suspect that a lot of what he's talking about will be relevant if you're trying to build your own wheels.

 

Regards, Andy

Julia,

The old fashioned cart wheel rims were heated (red hot) before fitting, without forcing, the shrinkage of the steel on cooling "clamping" the wooden components to give a firm wheel. 

A few thoughts and questions on the problem:- Bear in mind however that my experience stems from shrinking starter gear-rings onto car-engine flywheels many years ago!

Would the FUD plastic allow you to heat your rims slightly, say in boiling water, to ease the strain while forcing the centres into the rims??

How do you ensure that the components are "square" to each other when force fitting?

What are the tolerances of the FUD centre's outer diameter? Consistency here would be essential, (I think!) perhaps it may be necessary to machine the FUD as implied by Paul (Halfwit)?  

I assume that you are machining your own rims, and are happy with the tolerances of the inner diameter.

Option 4 seems the way to go, and if a slightly loose fit to axle results, (A la Bill Bedford) then Loctite is your friend.

 

In admiration of your determination to push the boundaries...

 

Don 

 

Edit... re-reading the other items above

"Rather ironically the broken wheels occurred while the wheel center wasn't fitted to the rim, I have never had a broken wheel while fitted to a rim." implies that an axle being inserted caused the fracture. Doing so would put the outer diameter of the FUD into tension (equal and opposite force to the force-fit compression axle-to-wheel).

This then raises the question of what are the tolerances of the 3D centre's hole?

 

Don

I'd be interested to know how aerodynamic the Peckett is and what top speed it achieved during flight. Could be a new world record

 

David V

Me again.

 

Im glad you grasped the last bit of the previous message Don. I was worried that the cart wheel analogy would be wrong.

 

The wheel rim is a 'snug' fit onto the wheel (a 2mm SA standard wheel rim btw) and doesnt have to be 'forced' in (I dont like the word 'forced') and the same goes for the shaft too although the wheel / shaft fit is a difficult one to get right. If you fit the wheel center to the rim before the shaft the forces involved in fitting the shaft are transmitted through the wheel center to the rim (which is more than capable of dealing with the forces) and should then make a stronger wheel. Its when I fit a shaft onto a wheel center without a rim that causes the problems because the stress/force cannot be transmitted through to the rim.

 

Well that was my thinking behind it anyway, please, someone tell me I am wrong...!!

 

EDIT: Remember though that its not the fit that I am having problems with, its the concentricity of the wheel once its on the rim and shaft.

 

 

I'd be interested to know how aerodynamic the Peckett is and what top speed it achieved during flight. Could be a new world record

David V

 

 Hehe, there is a bit of weight to the chassis for something that small so it left a little dent. A perminent record of me loosing my temper!

 

M

Apologies in advance cos this is truly awful, but I've now got 'Only You' by the Flying Pickets in my head. You see what I've done there? Pickets ... Pecketts ...

 

I'll get me coat ...

Julia,

Your edit above re. concentricity takes us (ie you!) back to the original option 4!

 

Don

Its when I fit a shaft onto a wheel center without a rim that causes the problems because the stress/force cannot be transmitted through to the rim.

 

Remember though that its not the fit that I am having problems with, its the concentricity of the wheel once its on the rim and shaft.

I would agree with you about the rim containing the stresses, but have seen nylon/ABS wheels where the centre have cracked after being stresses too long. As for the concentricity, I think you have hit one of the intrinsic problems of using press fits on plastic centred wheels. That is that unless the axle is perfectly true and concentric as it is pushed into the wheel, it will score the inside of the bore and never run true. A press of some sort is really needed for this sort of work unless you have supreme* skills. Having the axles already mounted in the frames before mounting the wheels makes the whole job many times harder.

 

*OK there is a way of doing it without a press, but every time I have suggested it to modellers they have back away in horror.

Hi

Rather than give up the project all together, would it not be better to sacrifice the prototypical looks of the spoked wheels, In favour of disk wheels.?

I agree with you about the stresses in the wheel, its not ideal but where there are any press fits you will get them. Its all about getting the forces down enough so the material will cope with them.

 

I do think though that if I had a choice between making something a press fit and making it a clearance fit I would choose the former. The reason is that to me it is much easier to locate something that is a press fit, if something is a loose fit you then have to rely upon some kind of jig to locate the two parts whilst they are stuck together to maintian the wheel back-to-back, run out (wobble), and concentricity, that plus the fact in this situation you would have to allow for accurate wheel quatering too as it will become very difficult, if not impossible to do this after they are stuck together. After all its how the current range of 2mmSA wheels work, i.e. relying on friction to position them accuarately.

 

I was hoping that 3D printing would be accurate enough to produce a wheel center that didnt require any post manufacturing machining and I think its nearly there but not quite. After all I have managed to get 2 wheels together succesfully.

 

I would love to be proved wrong but right now I just cannot see how adding a clearance hole would improve things.

 

EDIT: Don / Bill I would be more than willing to print a set with clearance holes if you would like to show me how they could be easily assembled...

Option 4 sounds to me like the only way forward, the tolerances on FUD obviously aren't good enough for wheels in 2mm scale.

 

I also agree with Bill, above, that some kind of press is required for fitting the wheel to the axle - some of the wheels on my 37 aren't quite as good as they should be, a press would have prevented that.

 

Remind me to never upset you...

Hi Julia sorry if i  missed this some where in the post but have you tried machining the wheel centre before fitting the tyre? 

One way to go in 2mm would be to have them made in brass by i.materialise. http://i.materialise.com/materials/brass It is more expensive and they don't like doing builds with multiple pieces, but it would give you something that has know qualities.

We should not lose sight of the fact that all 2mm Association driving wheels are conductive to avoid having to use pickups, leading to better running. Whilst wheels with plastic centres will be suitable for converting commercial steam locos all metal wheels are a better bet when scratchbuilding. I assume FUD is a non conducting plastic but I believe some metal powders can be 3D printed which may be suitably conductive.

Many years back we did have some plastic centred wheels in the Association and I believe Geoff Jones had a hand in these.  These wheels had a wire down one of the spokes to provide conductivety between the tyre and axle.  A few members however experienced these overheating and causing spokes to melt under short circuit conditions. Wheels of this type could be now more vulnerable with the widespread use of DCC where with the extra current available under fault conditions failure could be more common. Its really a case of horses for courses.  Some people poo poo the idea of things melting but it is a fact. It does not matter if it welding or melting or acting as a fuse it is basically the same process, metal melting. I have seem two instances on F H of the drive shaft between loco and tender acting like a fuse and melting when a loco short occured but was not a full short cicuit with a resistance low enough to trip the DCC power supply. Both were  higher resistance shorts with power maintained for a definate time period..

One way to go in 2mm would be to have them made in brass by i.materialise. http://i.materialise.com/materials/brass It is more expensive and they don't like doing builds with multiple pieces, but it would give you something that has know qualities.

 

According to the iMaterialize website they print a wax model and then use lost-wax casting to produce the brass item, so the finished item would need to be machined before fitting into the rims. This is the way that the 2mm driving wheels are produced at present - the waxes are made by an octogenerian gentleman in a village on the North Yorkshire Moors, cast by a foundry at Kirbymoorside, sent to a chap in Lancashire who turns them and fits CNC-machined rims and then they go to another 2mm member who checks them to ensure they meet the necessary standards (appearance as well as dimensional accuracy) before they then appear in the 2mm shops. Quote an involved process, and reliant on a lot of skilled craftsmen (which is their main problem, and why a small team (including Julia) is looking at developing an alternative that doesn't require the same level of skilled labour in their production).

 

Andy

Almost all the distortion and shrinkage that you get with 'normal' lost wax casting comes from the use of rubber moulds. If you go back to you company in N Yorkshire you will find that most of its production waxes are made in hard, i.e. aluminium, moulds. They do this, in part, because of the problems inherent in the use of rubber moulds. I believe the same company has one of the machines that i.materialize uses, so maybe you should approach them with a view of at least getting samples of your wheels printed and cast.