expanding lap: how to make one?

[havoc]

I took my rotary table apart to find out why the handwheel was so stiff. Well, I found out: the axle on which the worm gear is mount doesn't fit very well in the eccentric piece. There is a part of the hole in the eccentric bush that is a tiny bit too narrow.

I tought of going through it with a honing tool just going a few times over the offending area until it fits. But I cannot find one small enough (bore is 14mm). So I wanted to make an expanding lap. I know I have somewhere drawings of it in a book but I cannot find them. Has anyone here a sketch/drawing/plan of an expanding lap of about 14mm diameter?

[SteveW]

Havoc,

Have you tried a piece of carefully sized home made wooden dowel with a saw cut (or two) down it to take a wrap of wet and dry?

It's sometimes necessary to to put a wood screw down the centre or a wedge to expand the thing. A Soak in Brasso will provide a good finish. A variation would be a diagonal cut through with a central longitudinal wood screw to pull the ends together and expand it.

[Noddy]

Steve's got there first, and with a better explanation than I was coming up with!

Another possibility is to visit the local car spares place, and see if they have a brake cylinder hone. it has 3 very fine stones on spring loaded arms.

I think sparey had some stuff on laps. I'll look tonight

[modeng2000]

Polly Model Engineering www.pollymodelengineering.co.uk/sections/bruce-engineering/tools.asp sell hones.
l
"Cylinder Hones and spare stonesfor finishing bores
4 sizes to suit a range of bores from ½” to 7” diameter. 3 in 1 set also available."

John

[havoc]

I visited the car spares shop, but the hones only start from 3/4". Too large. I'll take a look at the polly ones.

[havoc]

Since I don't have anything else I tried making a simple (non-expanding) lap. I put a brass plug of about 4cm on a steel bar and turned the brass to the same diameter as the axle of the circular table. Rounded off the ends a bit and tried it in the hole. It started to bind there were the axle binds.

So I loaded it with some fine lapping compound and started. I got about 0.5cm inside the hole when the whole thing locked solid and cut a fine nice pieces of flash out of my hand and fingers...

Over to the experts for more ideas. Please...

[SteveW]

Havoc,

Try a bit of tree-wood. It comes in the bigger sizes and is a lot cheaper than brass. A sharp tool raked for alloy will do the business getting to size.

Also, once you're finished you can use the bits to boot up the old loco.

[petercolman]

First get the set up so that the lap will run parallel and true to the hole, the lapping wants to be under power if possible. The lap itself can be made from copper or ali and split so it can be expanded using a screw in the end (tap a hole with a taper tap and use a grub screw)
make the lap just so it will pass through the hole and then load it with a small spot of lapping paste and away you go. when done wash all paste off.
Peter

[havoc]

I have lapped the bearing, but it didn't help. There is something else wrong.

The lap used was in brass and I set it up in the lathe using the fixed center. Afterwards a night in white spirit to get everything out of it. Now the axle passes very smooth into the bearing. But...the bearing surface on the axle is in 2 parts with a smaller bit between them. Before lapping I could only insert the axle from each end up to a certain spot, then it binded.

After lapping, I can insert the axle from each side, until it comes to the second bearing part of the axle. Then it binds hard. So it looks as if either the axle or the hole is out of line.

Now I see a few solutions to that problem, but nothing simple with the tools I have.
If it is the hole, then I could make a new piece or I could turn out the hole and insert bronze bushes. But I'm not up to turning a straight hole for 150mm if it is only 14mm diameter. Neither can I turn the piece around and line it up so that when starting at both ends the holes line up.

Making a new axle is out possible but of the question as I cannot get the worm gear off.

Simple solution will be to order spare parts...