Author Topic: 3D Printed External Lapping Tool  (Read 1354 times)

Offline Joules

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3D Printed External Lapping Tool
« on: February 17, 2020, 10:38:28 AM »
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.
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Offline RotarySMP

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Re: 3D Printed External Lapping Tool
« Reply #1 on: February 17, 2020, 10:52:02 AM »
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
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Mark
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Offline Joules

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Re: 3D Printed External Lapping Tool
« Reply #2 on: February 17, 2020, 03:02:34 PM »
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.
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Offline sorveltaja

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Re: 3D Printed External Lapping Tool
« Reply #3 on: February 17, 2020, 03:28:34 PM »
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.


Offline Joules

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Re: 3D Printed External Lapping Tool
« Reply #4 on: February 17, 2020, 03:42:40 PM »
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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Offline Joules

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Re: 3D Printed External Lapping Tool
« Reply #5 on: February 18, 2020, 04:09:09 PM »
I dug out some finer Emery papers, 600 and 800 grit.   Need to get some finer stuff, the new 600 paper soon ate 10 micron off the brass in just a few mins.  Had to go gentle with the 800 paper, then finish with normal paper and Autosol at about 1000rpm to polish.   Very pleased with the result, though working up-to a small shoulder isn't a great idea.  The Emery papers just needed a wash and should be good to go many more times.

The little SU1 mill just earned itself another star, as a lapping machine, very easy to mic the work in this setup, it uses much less energy than the big lathe.  The combination of Emery and paper really works well, the paper holding slurry and oil.  I will try a piece of steel when a bit just over 18mm diameter is to hand, picking up some 1200 grit paper is also on the shopping list.
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Offline RotarySMP

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Re: 3D Printed External Lapping Tool
« Reply #6 on: February 18, 2020, 05:02:24 PM »
What is the tolerance on the diameter along the bar?
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Mark
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Offline Joules

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Re: 3D Printed External Lapping Tool
« Reply #7 on: February 18, 2020, 05:57:21 PM »
+10 micron over 60mm with a 20mm wide lap.  The Emery paper was more aggressive than I expected on the brass and ended up with a hollow I couldn’t work due to the small shoulder.  I didn’t go below 18mm, steel should be interesting.

No detectable out of round from the lap anywhere along the length lapped.  From the hollow here to the front is +5 micron, I should have undercut the material at the shoulder for the lap to over run.
« Last Edit: February 18, 2020, 07:35:35 PM by Joules »
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Offline RotarySMP

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Re: 3D Printed External Lapping Tool
« Reply #8 on: February 19, 2020, 02:51:49 AM »
Thanks. I look forward to trying this on the test bar. Still have to finish the taper.
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Mark
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Offline RotarySMP

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Re: 3D Printed External Lapping Tool
« Reply #9 on: February 23, 2020, 05:21:56 AM »
I tried out your method of using sandpaper with the 3D printed lap. Worked pretty well.  Bit of a PITA to load the sand paper into the lap, but you get the hang of it by the time you get to 2000 grit.

The bar is an unhardened stainless steel. I managed to get it to within 3µm over 200mm with a couple of hours work.

Thanks for testing that Joules.
Mark
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Mark
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Offline Joules

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Re: 3D Printed External Lapping Tool
« Reply #10 on: February 23, 2020, 10:13:00 AM »
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.
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Offline RotarySMP

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Re: 3D Printed External Lapping Tool
« Reply #11 on: February 23, 2020, 02:58:27 PM »
Good ideas. Thanks.
Mark
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Mark
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Offline efrench

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Re: 3D Printed External Lapping Tool
« Reply #12 on: February 23, 2020, 06:08:52 PM »
Would a longer lap, perhaps 10X the diameter, work better?

Offline Joules

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Re: 3D Printed External Lapping Tool
« Reply #13 on: February 23, 2020, 06:38:41 PM »
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.
« Last Edit: February 24, 2020, 10:22:49 AM by Joules »
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Offline RotarySMP

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Re: 3D Printed External Lapping Tool
« Reply #14 on: February 24, 2020, 10:00:06 AM »
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
Best regards, Meilleures salutations, Mit freundlichen Grüßen, Cu salutari
Mark
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