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| 3D Printed External Lapping Tool |
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| Joules:
Hi Mark, you got a hollow next to a high spot as I did here, I think that shows the lap can tip but weigh up the convenience of a printed lap against a metal one seems a fair compromise. On loading paper I concertina paper and emery for half the lap rather than each segment, also print a boss to go in a larger lap to support the paper, then you can slide the lap across onto the work piece. Makes loading much easier, once the lap has been used a while the paper takes a set. |
| RotarySMP:
Good ideas. Thanks. Mark |
| efrench:
Would a longer lap, perhaps 10X the diameter, work better? |
| Joules:
I think at 10:1 length to bore ratio you are going to hit problems with inconsistent pressure over the lap as the print will distort or rock as your print gets further away from the bed. About 2:1 might be a better ratio, but you probably also want to beef up the outer diameter. I managed 5 micron accuracy with my first experimental lap. I have since printed a 30mm bore lap to test on some steel stock and see if I can improve on average accuracy. Mark got good results in his video, though I don’t know what the ratio of the lap was. I had problems hitting the small shoulder during lapping, causing the other end to dig or close up resulting in a hollow. A metal lap might have stopped at the shoulder, but you have to weigh up the convenience of a 3D printed lap and as you gain experience how much tighter tolerance can be held. Adding to my thinking on a longer lap, you probably want at least 3 times the lap length of clear shaft to make sure the lap properly clears areas being lapped. If the lap consistently covers a section of shaft, you may preserve a defect in the printed lap onto the shaft. My 20mm bore lap, at 200mm long just wouldn’t be practical on a 60mm long shaft. Hence my 3:1 ratio for clear shaft to lap length, minimum. The lap must be kept in motion at all times you hold it. You can let go and let it spin, but as soon as you grab it again get it moving, slow movement isn’t beneficial with a printed lap as the ends of the lap can dig at the micron scale. Sorry, that should have been 18mm bore, at 180mm long. |
| RotarySMP:
I didn't put much thought into it, and had a 20mm wide lap for a 24.2mm shaft. Not even 1:1. Good points you guys are making about a wide lap averaging out errors better. Mark |
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