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| Joules:
Follow on from Marks (RotarySMP) Pimping his 7x12 lathe thread, I fancied trying a 3D printed lap for an 18mm shaft. A few issues bothered me initially, heat and rapid wear of the lap with abrasive and so many contact points within the layers of the print, plus not having a hard edge like a metal lap for scraping the slurry. To try and solve one issue and get a better average contact I am trying paper inserts. These will carry the lapping compound, and slurry. They are replaceable meaning the lapping tool shouldn't experience wear directly. The lap can be washed and loaded with fresh paper and different grades of compound without cross contamination, even strips of Emery paper, though the paper allowance is 0.1mm radius and Emery paper would need 0.25mm My initial print is undersize, I printed the bore at 18.2mm but as seen in the picture a snug fit round a 17.95mm shaft. I knew this would be the case and intended to bore out the tool to size including the paper. Before I bore out the tool I will try it as is, you can see the lines of contact on the paper, it might be that with abrasive and oil the paper swells and makes better contact as it wears. This may only work for actually polishing than lapping, it is all experimental for now, and thanks to Mark for the suggestion of 3D printed laps. |
| RotarySMP:
Cool. The "rapid" part of rapid prototyping is very powerful. I look forward to your experience with the different options of paper and grit, as that test will need a lap later this week. It wasn't my idea, as it was suggested by Bob in the LinuxCNC forum . https://forum.linuxcnc.org/26-turning/38280-pimping-the-mini-lathe#157528 Mark |
| Joules:
Didn't find the steel bit I was after, but the mystery brass I have that turns horribly should be a quicker test. The rough start was about as good as I could get turning this piece, it also tapers to the chuck down 0.05mm. My first try and to get a feel for the lapping tool was just paper, Autosol polish and oil. I wanted to see the contact pattern of the lap before deciding where to go next, the result was surprisingly even bar a couple of hollow sections on the lap. I guessed this pattern might wear down as some heat flattened the lap high spots, I'm not sure that's what i got, but it did improve. The lap was washed between paper changes to make sure no slurry was left behind, just warm water and washing up liquid. Next up was a hybrid of 50/50 Emery paper and plain paper, the Emery was 360 grit and really did lap the bar. I took out the taper and had it less than 0.015mm across the bar in a short time. I didn't fancy hunting down some finer grit paper, so jumped to final polish with Brasso. The finish is dull but to be fair the surface needed much more work getting the grit down. However, you can see by the dull polish that it has smoothed the surface quite a bit and the contact paper is more even. Once finished the lap was again washed and checked for wear, none present, the lapping tool did open up with the paper and Emery and is now a loose fit on the Delrin shaft it was tested on. You have good feel with these laps and can identify high spots by the increase in drag of the lap. The spring in the lap has been useful, I think if you needed more pressure an O ring would be adequate round the lap. I was able to squeeze one side to apply pressure and adjust a high spot near the shoulder. I am impressed, for a tool you can print in just a couple of hours and tailor to pretty much any size, not wasting aluminium as you only use the material you need to print the lap. As this one is PLA you won't feel so bad if it ends up in the bin at some point. The CAD model I drew for this incorporated 0.5mm radius corners at the slots, so the Emery paper or beer can lap wouldn't have a bulge at the corner. My worry over the slurry was not a concern as the slot allows a place for the slurry to accumulate. Yes Mark, give it a go be interested to see the result you get on harder metal. |
| sorveltaja:
That's an interesting idea and a test. If one uses PLA for printing it out, probably some kind of 'annealing' (I have only read about it) could make it a bit more heat resistant. |
| Joules:
Don't anneal it, the lap will go out of shape. You are right on the limit of sag when annealing PLA, where we have annealed precision (ROFL) parts, we have had to peg any location holes so when the print changes shape, location holes for fasteners don't move. It just isn't worth it for a dimension critical part. |
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