Beginner's questions on hand lapping

[homebrewed]

Have never imagined that plastic will work ! I have a piece of ABS block collecting dust. Will it be good for the purpose ? ABS is known to be not very resistant to chemicals so I am wondering if stuffs in lapping compounds will attack it.

ABS probably would work better than aluminum, but not as good as HDPE or other similar abrasion-resistant plastics. If I had a choice I'd go with HDPE -- it's widely available. Something to think about: HDPE anti-friction tape, something you might find in a woodworking supply store. Stick it down on whatever you've got that's the right size. It can be fairly thick so would hold up awhile and is easy to renew when needed.

For your task at hand you don't need anything so refined but the knowledge on how to do it better can come in handy at times.

 

[compact8]

For your task at hand you don't need anything so refined but the knowledge on how to do it better can come in handy at times.

That's very true. I have quite a lot of experience with sand paper and I know sand paper will serve the purpose well in this work ( except for the minor issue of cutting down more near the edge of the workpiece ) but I would like to try out something new this time. Experiencing new things is fun and the cost of failure will be minimal.

edit : Just ordered a HDPE block, 2 bucks, cheaper than ABS.

 

[homebrewed]

The whole process of lapping just gets more and more involved and the scale gets more and more focused as the process progresses. I felt like one of the problems I was constantly dealing with was the fact the friction caused by the abrasive diamond or paper would cause more pressure on the leading edge so I seemed to be having the opposite problem Tom and Robin talked about where the middle of the work was getting dished out. I had great flat area out to the last 1/8” from the edge that would be slightly relieved and the only way to get rid of it was by extreme lapping as there was less friction. The Shamrock 12” cast iron lapping plate I have came charged with really fine diamond dust so is wonderful for final finish but literally impossible to really remove much material. If i was going to be doing that for any larger area I’d use my shaper(closest to ground finish), then surface grinder and finish with lapping. Lapping by hand is a HUGE time sink and can be frustrating to say the least.

Your experience w/regard to the edges getting rounded off is similar to mine. One way to reduce that (but I don't think possible to completely eliminate) is to use plastic lapping film The base usually is Mylar with a thin coating of the polishing media in it. It's stuck down with surface tension on a very hard surface like glass, and the combination has very low "give". That's what helps flatten everything out up to the edge. Not cheap, available with various grinding media all the way up to diamond, grits up to something like 100,000 (!). The layer with the polishing material is very thin so the stuff is very easy to damage. You have to be a fanatic when it comes to cleanliness when using the stuff so no errant bit of larger grit gets in there and mangles your lapping film.

It might be possible to "charge" a sheet of Mylar with diamond grit to make your own lapping film. I never tried it but it might work. Or not

The "charging" procedure involves dusting a thin layer of grit over the sheet and using a steel roller to embed it into the substrate. I made one out of a bearing and it seemed to work pretty well. Not original to me, I found the idea somewhere on the internet.