Cast iron laps - correct depth/spacing of grooves?
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    Default Cast iron laps - correct depth/spacing of grooves?

    I'm making a set of 3 cast iron laps. These are round, 250mm (10") diameter, about 14kg = 30lbs each. I have flattened the faces on a lathe, and will lap them against each other (Whitworth's method: AB, AC, BC,...) to get them really flat. I have some Timesaver green lapping compound that should work well for this.

    My question: I first need to cut "swarf grooves" either on the bandsaw or with a slitting saw. What is a good choice of width, depth and spacing for those grooves? I was thinking width 1-2mm (0.040-0.080"), depth 1mm (0.040"), spacing 12-15mm (0.5-0.6"). Can anyone give me some good advice about this? Or if you have a similar lap which works well, measure it and let me know.

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    I have a 12" round lap made by Crane company. The slits are .062" wide with .500" wide flats between. Works well.

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    I have several cast iron lapping plates also, their groove spacing is the same as ironsmith's. They range from 8" × 12" to 12" × 18" in size.

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    My thought is tailor the slots and pitch to the size of parts you'll be lapping.

    Smaller parts get a finer pitch and slot width (say, 1mm x 10mm), bigger parts perhaps 2mm x 15mm.

    Depth is up to you, deeper means more life before having to recut the slots, but more of a pain to clean when changing abrasive grit. And I do think it's a good idea to thoroughly clean the laps each time, so you don't introduce rougher grains into a finishing process.

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    ironsmith, eKretz, Milland, thanks -- this is very helpful advice. First thing I will be doing is removing about 5 microns (0.0002") from the top of my surface plate. After that, general purpose use. Reason I am making 3 is so that I can easily recondition them after use.

    I'm probably going to cut the slots with a ganged slitting saw. I saw an interesting variant here on PM some time ago. Someone had cut circular grooves in an overlapping "olympic rings" pattern. They single-pointed it with a "fly-cutter-like" tool ground to cut thin slot. That would be faster for me, but I am not sure it's equivalent, so am going with the traditional square grid design for the grooves.

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    If you are using silicon carbide on the laps or even diamond which is not completely embedded, then the laps will probably get flatter all by thmselves as you lap the much larger surface plate.

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    Quote Originally Posted by Mark Rand View Post
    If you are using silicon carbide on the laps or even diamond which is not completely embedded, then the laps will probably get flatter all by thmselves as you lap the much larger surface plate.
    The amount I have to remove ranges from 0 to 11 microns as I move over the plate, and averages about 5 microns. That is so little material that (especially at the start) the lap may not have a chance to flatten itself. So I want to start with it very flat. Also, I have never used the Whitworth 3-plate method and this is a good excuse to try it out.

    My plan is to start with very fine SiC. But first I need to make the laps.

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    Quote Originally Posted by ballen View Post
    The amount I have to remove ranges from 0 to 11 microns as I move over the plate, and averages about 5 microns. That is so little material that (especially at the start) the lap may not have a chance to flatten itself. So I want to start with it very flat. Also, I have never used the Whitworth 3-plate method and this is a good excuse to try it out.

    My plan is to start with very fine SiC. But first I need to make the laps.
    Too bad you aren’t closer—-I could cast the plates for you They could be nicely designed with 3 feet, ribs, etc.

    Denis

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    Here is a company who has been making lapping plates for years. Email them and see if they can ship you a set. If Not I sold a set of King-Way Straight-edges that I UPS'd them to NYC and they were added to a container going to Norway. Also I taught a class in Stuttgart at a machine rebuilder who had cast iron plates they made. I'll write them and copy you.
    Challenge Precision|Workholding|Angle Plates|Lapping Plates|Precision Manufacturing

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    It sounds like he has already got them just about made himself... No need to purchase. Lapping plates aren't very complicated to make either. I lap mine against each other in sets of 3 for maintenance just the way he's planning to; works great.

    BTW, a quick and easy way to cut the grooves is with a cutoff wheel mounted in the surface grinder.

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    from my limited experience making these:
    a1.jpg
    (20mm on center and 1.5mm width, 2-3mm depth, using bandsaw)

    initially I was trying to take off a lot of material (hardened steel plate, HLV-H dovetail top), and the narrow/shallow grooves would clog up quickly and make it difficult to generate the pressure needed to actually cut the part - that was with the coarse diamond grit (40 micron)
    once I switched over to 10 micron or less diamond it stopped being an issue, but still seemed to hydroplane

    if I had to redo it, I'd cut the grooves even deeper and increase width to 2mm also, maybe reduce spacing, 12-15mm you planned sounds about right

    from what I understood searching and reading about these is that the small grooves and large flats are essentially for finishing small parts, when you're lapping something larger than the lap itself, increasing groove width and reducing land size makes it easier to generate more pressure to cut material quicker, probably not an issue if you're taking off just few microns, but still, 1mm depth sounds too small

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    I see in this new lapping machine ad, has some good info on lapping too: What is the Lapping and Define the Process?

    Lapping Polishing Machines Compounds and Consumable Products

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    Quote Originally Posted by dgfoster View Post
    Too bad you aren’t closer—-I could cast the plates for you
    Thanks for the offer! I already have the three 250mm rounds, 40mm thick, faced off and ready to be grooved.

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    Quote Originally Posted by eKretz View Post
    BTW, a quick and easy way to cut the grooves is with a cutoff wheel mounted in the surface grinder.
    That's a good idea -- I think I have a cutoff wheel that's about the right thickness, would probably be faster and cleaner than doing it on the mill.

    Would you suggest a single deep pass at creep speed or lots of shallow passes? My grinder can auto-downfeed about 6 or 8 microns per pass so one mm would be about 150 passes, say 2 minutes. I need about 20 grooves in each direction for each plate, so a total of 40 grooves per plate. Thus I'm looking at a minimum of about 90 minutes per plate x 3 plates, probably six hours in all. Full-depth slow passes might be better, but not sure if it's a good idea.

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    Quote Originally Posted by jz79 View Post
    from my limited experience making these:
    Those look very nice. If I had a vertical bandsaw I'd use it for making grooves, but I only have a horizontal bandsaw and that would be tricky.

    I think you've convinced me that 2mm depth makes more sense than 1mm.

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    SG with a parting wheel or dressed with wheel...
    One trick is to touch and take. Suppose you wish grove depth to be .040 and find it takes .055 down-feed to make that depth with adding wheel breakdown, so you touch and take that amount. For step across choose an easy same amount so next time it is easy to match.
    You may go back to start number and take a true-up but likely that wont be needed.

    I used to make them leaving a little wheel radius to intersect the top edge so a wipe with an oil rag would bring some lapping compound back up to the surface.

    Yes, one can grove a much wider plate than the chuck width with placing a wide plate way off the chuck at front and back to grind the area that is on the chuck.

    Good to clean the plate going from one lap grit to another. Popsicle stick to bring up and to wipe grit back to the correct container.

    Diamond grit takes a good amount of cleaning because it imbeds into the plate.

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    Quote Originally Posted by ballen View Post
    That's a good idea -- I think I have a cutoff wheel that's about the right thickness, would probably be faster and cleaner than doing it on the mill.


    Would you suggest a single deep pass at creep speed or lots of shallow passes? My grinder can auto-downfeed about 6 or 8 microns per pass so one mm would be about 150 passes, say 2 minutes. I need about 20 grooves in each direction for each plate, so a total of 40 grooves per plate. Thus I'm looking at a minimum of about 90 minutes per plate x 3 plates, probably six hours in all. Full-depth slow passes might be better, but not sure if it's a good idea.
    I'd do it just like Buck suggested. The wheel will definitely break down, so to keep a fairly uniform depth you'll want to do light downfeed and rapid table traversal.

    I would keep the grooves sharp-edged though. A rounded groove edge may pull oil and lapping compound up out of the groove when the part goes over it too, which may cause float due to hydrodynamic lift.

    The mention earlier of problematic "hydroplaning" is something you want to avoid if you want the flattest parts you can achieve. Another surefire way to have problems with that is to move the parts over the lap at too high a speed. If you want the very flattest parts you can get, move them at a slow and even speed over the lap.

    Also, do not use too much lapping compound. A little goes a long way. Excessive amounts will also contribute to float and flatness issues.

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    Another lap method is using automotive wet paper on a true flat plate using a solution of washing soda and water or a thin oil like spindle oil. Mosly this is to aid surface finish ... Care/caution to not make a convex surface on the part..

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    Quote Originally Posted by ballen View Post
    Thanks for the offer! I already have the three 250mm rounds, 40mm thick, faced off and ready to be grooved.
    Mostly kidding ash
    i am over-committed right now, anyways. But it is interesting to think how purpose-made.castings could be convenient. For one thing, like much classic designs in metrology equipment, the company that has made plates for decades, provides plates take “two men and a boy to move” them. Check out the weights. A well-designed casting of similar dimensions could weigh 1/2 to 1/3 as much and be much more practical in use. And it would still be more than stiff enough. Lifting 140 or 220 pounds around is not what most people would enjoy. Having the plate on 3 (removable) short legs could making lifting it up and situating it stably on a bench top without a tendency for rocking. Including a few pockets near the bottom could simplify wedging a flat bottom plate of a bench or table. It is pleasant to imagine the possibilities.

    Denis

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    2 Quick questions a lil OT. I have 12”h x 12”w x 1” and 12x12x3” cast iron slices both square.

    I want to make a 12x12 CI transfer plate using my surface plate (and lots of practice).

    My questions were. How thick should my cast iron plate be.
    And what grade surface plate should I use, I was thinking 18x18 AA? Or should I just go grade A
    Last edited by Homebrewblob; 12-11-2020 at 02:34 AM.


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