| Gallery, Projects and General > How do I?? |
| Practicalities of lapping |
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| 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:
--- Quote from: vtsteam on March 24, 2013, 11:34:03 PM ---Should add that I've heard of laps being made of wood in a pinch, though I've never tried it. Softwood, I imagine. --- End quote --- I've always seen hardwood recommended for this - it will still be soft relative to most metals. |
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