Practicalities of lapping

[Lykle]

Hi all, once again I turn to you for help.

I am making the part as shown in a CAD rendering below.
Everything is done and nice and tight, the bronze bearings are fitted and I put a reamer through them while they were mounted. Everything looks like it is ok.

The only trouble is, the shaft is a very tight fit with the bearings. I can get it on a little but not all the way and also 1 bearing has a slightly smaller inner diameter apparently as the shaft will only go in for about 5 mm and then it is jammed.

Now I want to lap the shaft and the bearings to a good fit. I always thought I knew about lapping but now the practicalities of it are giving me some trouble.

Do I lap with the proper shaft? Or do I make a new shaft only for lapping? I tried to help it along with some sandpaper (fine) wrapped around a stick on the lathe (yes, ways are protected) but that doesn't seem to do much. Maybe it was too loose.

Anyway, any suggestions, practical advice and gotcha's out there?

Oh yes, this is what I am making.

Lykle

[vtsteam]

I've made laps before from lead -- cast into an end for a mandrel (tapped the lap and the mandrel stub screws on) -- or even cast directly on a mandrel. Turn the lap on the lathe to final diameter after it is mounted on the mandrel

There can be a slit in the lap and a screw or screws bearing against one side to widen the gap and expand the lap as you gradually lap the bore.

A lap for a rod is the same principal only in reverse -- a hole on a lead cylinder that can be closed down.

Laps should be softer than the softest material in the part. The softer material will tend to get impressed with the abrasive.

If you use a hard lap (or the original shaft) while lapping with compound, it s likely the bearing will get charged with abrasive.

A lap must be kept moving back and forth to prevent uneveness as well as rotating evenly along the bore and extending a short way through it.

Laps take a long time to remove a small amount of material, so tolerances should be tight but close before lapping.

[vtsteam]

Here are some pix of the one I used on the hot air engine No. 83.

This one shows the slit and strain relief hole, and mandrel screwed into the lead lap



And this one shows a tapped hole and set screw (grubscrew) to expand the lap

[vtsteam]

Should add that I've heard of laps being made of wood in a pinch, though I've never tried it.

Softwood, I imagine.

[David Jupp]

Should add that I've heard of laps being made of wood in a pinch, though I've never tried it.

Softwood, I imagine.

I've always seen hardwood recommended for this - it will still be soft relative to most metals.

[Lykle]

First of all, thanks for the comments, very useful.

Now, wood lap, that is a great idea! I will start with that and see how it goes.
Only issue I see is that I want to lap the two sides on the same lap, to keep them concentric and together.
So I will nee a long lap to cover the 70 mm of bronze.

And for lapping compound I think I will have to use valve grinding paste as I can't find lapping compound here and I can't wait for it to come from CTC or such. Do you think that valve grinding paste is a good idea?

Lykle

[vtsteam]

Well, as in anything, there is the ideal, and the real.

I've used valve grinding compound. But can't say it's ideal for what you want. Don't know what kind of finish you expect, how fast you want to take off material, or exactly what material you are lapping. I'm sure there are specialty abrasives for any purpose.

Remember, lapping is a slow process, and keep the whole lap moving axially back and forth along the bearing. the lap ends should enter the bore to keep the lap wearing evenly, but not so far that you jump out inside of the bearing. housing.

Don't expect to take a lot of material off -- this is more of a finishing operation. Not sure how you're going to expand the wood as it wears or adjust it to the bore. I prefer the lead lap. But I'm sure wood is do-able, as people do use it.

[Lykle]

Hello good people,

OK here is a little update.
I made a wooden lapping rod. This is a rod cut lengthwise with an angled cut.
So by moving the top part to the left I increase the height of the rod.
All I need to do now is find the right lapping compound and I can go to work. I need to remove next to nothing in the bronze bearings so it should go fairly quickly.

Thanks for all the help.

Lykle

[vtsteam]

Well since you're going home brew materials, old time lapping compound for brass was apparently ground glass -- read a warning in a 1904 Model Engineer that it should be used with that metal instead of emery..

Gingery describes making glass powder by taking a piece of pipe with an end on it, standing it upright, and dropping a heavy length of rod in it as a plunger/pulverizer.

Or, you could buy some modern proper compound for bronze.......

[Alan Haisley]

Since you have very little to remove, try baking soda or Bon-Ami or borax powder or even toothpowder or paste. Not shure what English for Bon-Ami is, only American. It is a milder abrasive than most kitchen scouring powders and the crystals apparently break down after a while.

Alan

[Lykle]

Thanks to all your comments, the bearings are now lapped.

I suddenly remembered that I had a pot of Aluminium Oxide powder, for my airbrush sandblaster. So I oiled the wood, sprinkled some powder on and presto, one lapped part.

Oh wait, I will go to the workshop and take a picture. Least I can do for all the help I have been getting.

The part is now fitted and it has a nice smooth action. After I replace the oil with a thinner one, it will be perfect.
Yes I know, the surface is not great yet, that will be done later.

This is the part in place. I also added an overview to show you the whole thing.