Wednesday, 14 December 2011

Bearing All

Last week I mentioned that I would take my pretty patterns to the local steel foundry. Unfortunately my foolhardy foundry foray floundered. The foundry wasn't at all interested in a small run of one offs. I still have a couple of less local options before I try even farther afield, this will all have to wait until the new year though as New Zealand stops for a month at Christmas for the summer holidays. If this fails then I'll just have to fabricate the bits from scratch rather than cast them. Sigh.

Also, I went to pick up the tubing for the backbone from my local supplier. A piece of 1 3/8" 0.049" chromoly that I ordered two months ago. It seems that my supplier isn't. I'll have to look further afield for this as well. Trying to use local companies can be too trying.

So, in the meantime I'm going to start work on the wheels as a complete break from the recent disappointments. The front hub has a most complicated arrangement of bearings, featuring a double row of balls per side and then a live axle running concentrically through the hub on a pair of bearings. The hub has 6 bearing surfaces that all have to be concentric, I'll need to plan how to machine this carefully.

However before I can start on the axle, I need to make the bearing races that screw into the bearing housings, the knees, previously made.

Starting with a lump of 4140 steel that happens to be ground at 1.75" already

Centre it carefully and cut a 26tpi thread along enough of a length so that I can make all four races without removing the work from the chuck so that the threads and the bearing surfaces are all concentric.
Checking the fit.
Bore a big hole using my fabulous new boring bar.
Rough out the bearing surface as a plain bore with a 45 degree bevel at the inner end. 

The idea is to minimise the amount of material to be removed by the form tool as much as possible. This photo is from the second bearing off the bar as I forgot to take a picture of the first one I made.

Then use a form tool to profile the bevel to a radius of the correct size.
 The picture makes this look easy but in practise I had a nightmare on this job. Using a form tool on an external surface is hard enough what with all that chattering going on distracting you, but internally it all gets harder still, literally. I initially used a ground piece of HSS as my form but found that if it rubbed even a little, then the 4140 work hardened and I got extra grumpy. I had to use carbide to prevent this and slow the lathe right down. Carbide plus slow is not a normal combination, but it works really well in this application. Note that I also had to relieve the tool holder to prevent fouling on the work piece.

With the form cut I can then polish the bearing surface.
I'll need to do this again after I've had them hardened.

Then part off each bearing with enough meat left to clean up the inner, parted off, edge.

The inner races have an extra flange extending inwards, 
this forms part of a labyrinth seal with the hub flanges, more later.

I still need to machine the castellations on the outer races that allow the adjustment to be locked. I'll need to figure out a set up for that on the ever versatile Myford.

In other news, I've set up a small work bench with a vice in another part of the shed for the kids to play with. I'll get them a selection of tools for Christmas and let them cut off digits at their leisure. They don't read this blog, so I won't be spoiling any surprises.

Note that the vice is on the left, both my kids are sinister despite my wife and I being dexter. 
Weird eh?

1 comment:

  1. A great article, many thanks for taking the trouble to put it together. hydrodynamic bearings