11 replies to this topic

[tjallen]

When an axle goes through an axle tube or a bushing, is one is slightly larger or smaller than a standard measure?
 
For example, if I am 3D designing an axle that is 1/8 inch diameter going through an axle tube that has an inside diameter of 1/8 inch, do you make one of them slightly smaller or larger, so they fit? How much larger, or smaller? Is there a standard amount of gap?
 
There must be standards for these things, but I'm not educated in this area.
 

[old & gray]

This link should be a reasonable place to start:

ANSI Limits, Fits and Tolerances Calculator
 
Measure the axle you are going to use as a starting point.

[Bill from NH]

Jimmy, to fit your 1/8" 3D axle in a 3D tube with a 1/8" inside diameter, I would say to program your axle a couple thousands smaller & the tube's I.D. a couple thousands larger. That would seem to allow the axle to be inserted in the tubing. I'm assuming 3D printing adheres to tight tolerances.

 

In real life slot cars  , I like to see axles & armature shafts to be "close on" to eliminate much hand polishing. I like bushing holes to be tight or undersized, because I can open them up to size with a straight reamer.

[MSwiss]

I sell only Koford M418 axles for 1/8" use. At $1.49, they are a superb bargain, and all the various 1/8" crowns and wheels always seem to fit well.

I don't keep anything real precise at the raceway to measure shafts, like a micrometer, but with calipers, they measure .1235"-.124".

Are you looking to run that cool 3D chassis, you had printed?

I doubt that material will be a very good bearing surface.

On real Jail Door cars, the brass tubing wasn't all that great. That's why it got ditched for real bushings/oilites, or ball bearings.

[Bill from NH]

I didn't think using bushings and bearings is legal according to the IRRA Jail Door rules. Are you running under a set of local rules? The BPR racers found a way to eliminate the slop in rear axle tubing on these cars.

[olescratch]

tjallen, I've noticed that when you use a 1/8" axle (4/32") the bushing will be 5/32" ID. Whether or not this is the standard I don't know, but that's what my dial calipers show. It would be impossible to fit two parts together that are the same size unless you are connecting them end-to-end.

[tjallen]

The ANSI Hole and Shaft Calculator from old & gray's link above gives the following answer to my questions. I entered .125" and selected Free Running, but Close and Medium gave similar results:
 



 
So, according to this calculator, the hole is .125" or a little bigger, while the shaft is always less than .125". Am I reading that right? If this is right, this is exactly the type of answer I was looking for. But I'm sure there are always other ways of doing things, too!

[MSwiss]

tjallen, I've noticed that when you use a 1/8" axle (4/32") the bushing will be 5/32" ID. Whether or not this is the standard I don't know, but that's what my dial calipers show.

You can't accurately measure the inside of a small diameter hole with calipers.

The (tight) curve and the thickness of the jaw don't jibe.

[Cheater]

It would be impossible to fit two parts together that are the same size unless you are connecting them end-to-end.

John,

Strictly speaking, and I admit it has no relevance to Jimmy's question, this is not exactly accurate.

Interference fits (where a larger diameter object is installed in a smaller diameter hole) are used all the time, for example, when valve seats are installed in cylinder heads or steel tires on cast-iron steam locomotive wheels. Typically, the larger part is heated and the smaller part chilled so the sizes at time of installation are changed to allow them to fit together.

[old & gray]

Interference fits can sometimes be assembled without heat/cold if the interference is small. Each of the materials "gives" a little and if sized correctly the spring back forces hold the pieces together.

 

Of course if the pieces get hot and expand at different rates your planned clearance can completely disappear.

[MSwiss]

tjallen, I've noticed that when you use a 1/8" axle (4/32") the bushing will be 5/32" ID. Whether or not this is the standard I don't know, but that's what my dial calipers show. It would be impossible to fit two parts together that are the same size unless you are connecting them end-to-end.

  

You can't accurately measure the inside of a small diameter hole with calipers.The (tight) curve and the thickness of the jaw don't jibe.

 

Thinking about your post a little more, I'm confused.

How could the calipers measure 5/32"?

Measuring a (small) inside hole/bore, with calipers, should give you a smaller dimension, not a larger one.

[Martin]

Mike, when you have those cheap multi function digital calipers set on fractions they just jump to the next fraction. Worthless.

Jimmy, my advise, get a good quality 1/8" ream. Maybe even a .1245" while your at it. That should get you close.