hardest plastic for joining axles?

[mmagliaro]

As I ponder the design of the engine that will be built around my scratchbuilt steam loco wheels (other thread),
I am thinking about the plastic tube I will use to join the two half-axles in the center.

I can use a piece of solid rod to start out.  I will just bore a 1.5mm hole in it. 
But I want to cut a partial flat on the tip of each half-axle, so I can push it into the tubing, then drill and tap
a hole in the tubing and put a set screw in there to guarantee that the axle will never slip.

So I want a plastic that's hard as can be, that I can tap threads into.

Common materials I could buy some rod in would be lucite, ABS, nylon, Delrin.   What would be the hardest, sturdiest stuff?

Thank you for your thoughtful replies!

[nkalanaga]

I have no idea, but are you going to use the plastic as a bearing surface?  If do, you might want to consider friction and wearing ability along with hardness.  Another factor is that very hard materials are often brittle, making drilling and tapping risky. 

If you can't find the perfect material, how about a retaining pin instead of a set screw?  Drill through both the tube and the axle, insert a piece of brass rod, and file the ends flush.  It can be driven out if needed, but wouldn't need tapping.  Drill the half axles for a fairly tight fit, and the tubing for a slightly looser one, and there shouldn't be any danger of the tubing cracking as the plastic ages.

After all of that, I'd say Delrin, as it seems to be almost indestructible, can be drilled and tapped, doesn't seem to be bothered by most model lubricants, and is "slick", if you use it as a bearing surface.

[peteski]

Without doing any research I would say the hardest would be the old-fashioned Bakelite.  The stuff the old phones were made of.  But Bakelite is also quite brittle (but it is readily machinable).  However because it is so hard and brittle I don't think it will be suitable for your application.

Delrin is probably a good choice. But I'm not sure that a set screw and a flat on the half-axle is realistic for N scale. What size set screw would you use. 000-160?  The best way to positively align the half-axles would be like it is done on the Kato Mikado, but that is not doable at a hobbyist level.  The options I see as viable is either linear knurling on the half-axles (that would be tough to do with the tools at hand) or drilling holes in half-axles/tube and pinning them with a piece of thin rod.

[PGE_Modeller]

Max,

The 2mm Scale Association uses what they term " acetal" for their axle muffs - the tubes holding the two half-axles in line.  Delrin is a DuPont trade name for acetal homopolymer, which Colorado Plastics recommends for applications requiring a plastic rod (or tube).  Colorado Plastics' website, http://www.coloradoplastics.com/blog-0/bid/106023/What-is-the-Difference-Between-Delrin-and-Acetal, provides a description of the difference between Delrin and generic acetal.

Cheers,

[mmagliaro]

Thanks for your thoughtful information, everyone.

Peteski, I could use a smaller-than-00-90 screw, that's for sure.  "Set screw" is probably a euphamism.
I can make the cavity as open as I need to in order to clear a screw head, so it doesn't really have to "set" down
into the axle muff and be flush.  It can just be a regular screw.  I can use screws all the way down the M0.5, which is
might darn small.

Perhaps, yes, the best solution would juse be some drilled holes and pins.  I sure wish there was such a thing as
a "non conductive metal"... ha ha.

[Chris333]

I have a rod of Delton at home, bought it on eBay a while back for the lathe and never used it. Think it's about 1/2" Dia if you want it.

[peteski]

Thanks for your thoughtful information, everyone.

Peteski, I could use a smaller-than-00-90 screw, that's for sure.  "Set screw" is probably a euphamism.
I can make the cavity as open as I need to in order to clear a screw head, so it doesn't really have to "set" down
into the axle muff and be flush.  It can just be a regular screw.  I can use screws all the way down the M0.5, which is
might darn small.

I wasn't really worried about the set-screw length - just it's diameter.  Drilling a #61 hole in the muff for a 00-90 screw would probably weaken the plastic muff dramatically.  That is why I mentioned a smaller diameter screw.

[robert3985]

Delrin.  Black.  I use it all the time for mechanical applications and have built a lot of tactical, weapons-mountable instruments using it.  I've cut precision threads into it, tapped it, bored it, and otherwise machined it for set-screws, keepers, lens focusing rings and pins.

And, it's readily available at your local plastic supply house in all sorts of shapes and sizes.

There is a gluing system specifically made for acetal plastics which will glue Delrin to dissimilar materials such as metals.  look here for more information: http://www.cypox.com/systems.html ...which may obviate the need for set screws or pins in your application if you don't need to do any adjusting...or, you could adjust using the set screws, then when you've got everything just right, bond it.

Cheerio!
Bob Gilmore

[Doug G.]

Could you have the two axle halves drilled out and leave the plastic axle solid? The opposite of what you are proposing. I know some commercial locomotives have used that configuration.

Doug

[mmagliaro]

Could you have the two axle halves drilled out and leave the plastic axle solid? The opposite of what you are proposing. I know some commercial locomotives have used that configuration.

Doug

The steel axles will only be 1.5mm in diameter, so I don't see how I could bore them out to insert a
plastic center.

[mmagliaro]

On the heels of the idea of drilling a tiny hole through the plastic tube and through the axle, and then
putting a piece of wire through there to lock the axle so it can't turn in the hole...

I am planning to use ball bearings on these axles, not the usual bronze friction bearings.
Could I therefore get away with using brass for the axle instead of hardened steel rod?
It seems to me that the brass would not wear in ball bearings.

That would make it pretty easy to drill that locking hole through the brass rod with a wire drill.
1.5mm is about .059".  So... drill a #78, say (about .016").  That's 1/4 of the diameter of the rod and should
not compromise it, or the plastic tube, and using steel music wire, there is no way it could shear off.

Yes, I could do this with hardened steel axles, but it would take a lot of effort.
We're talking cobalt wire drills and a lot of patience.

[nscaleSPF2]

Max,

Do you need a mechanical connection for the axle parts?  You expect to take it apart for maintenance?

Because if you can use an adhesive to join the plastic and metal parts, I have an outstanding recommendation for you.

Using a wire to join the parts may not work either, unless the wire is a press fit into the metal/plastic joint, and can withstand the torque transmitted by the axle.

Keep going, Max.  This project is starting to get interesting.

[mmagliaro]

I don't care about taking them apart.  I'm all ears.  What is the outstanding solution?   

[nkalanaga]

If the ball bearings are the kind where the shaft fits into an inner collar, which then rotates with the shaft, almost anything would work for the shaft.  As long as it's solidly mounted to the inside of the bearing the only wear will be internal, where the balls roll on the moving parts.

That was the bearing type used on the check sorting machines at my last job, and I've used wooden dowels for shafts in them, just for fun.  I'd get worn-out bearings and use them for wheels, with the dowels for axles.

Now, if  the balls will roll directly on the axle, then I'd say that brass wouldn't work.  Hardened steel balls will eventually roll a groove into the axles, increasing the amount of play, and ruining the bearing.

[glakedylan]

is there possibly room enough for a flywheel
using flywheel as the connector
each of the shafts fitting 1/2 way in
pressure fit

just thinking out loud of other possibilities
fwiw....

sincerely--
Gary