Grinding thin material
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    Default Grinding thin material

    Hi guys could do with some help, long story short our old hands have retired/left and left me somewhat in the dark with surface grinding. Iím not a total newbie but Iím stumped here, grinding 90mm dia blades 1mm thick, tolerance is 0.005mm, D2 hardened to 60rc, Iím having major issues with burning and warping, prefer to use the cnc grinder as itís the only one with coolant, but that doesnít seem to help much, tried with and without coolant to no real success. I have managed to get one part finished using the manual with no coolant, hit tolerance in places but blade bent out of shape. Using a 46 wheel and settled on 0.01mm doc. I am shimming the high points mag half on, then flipping over, full mag till complete. Any help would be great thanks

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    What sort of 46 wheel? White, synthetic, hardness? Usually if I get burning and overheating it means the dress is too fine. You should be running the diamond across the wheel at a pretty good clip, once only. You may have to delay at the end of each pass so things can cool down. I'd go half your DOC and flip the part multiple times. Others may have more to say.

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    My wheel knowledge goes as far as the grit size unfortunately itís blue 3SG46KVX if that helps any? Iím dressing 0.1 in 2 passes at 1mpm

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    3" 1/2 x .040 to .0002 D2 hardened to 60rc,
    cnc grinder as it’s the only one with coolant, CNC Surface Grinder?
    46 grit size, k hardness, ceramic. A very good general-purpose wheel.

    that is a fairly tough job. *How much stock to take? what take amount per pass - what step over.

    incremental grinding - stepping over with cross at a set down feed dial likely your action.
    Stepping cross too much puts a lot of wheel in the action to overheat the surface and drag down RPM.
    The wheel selection might go to an H wheel or if surface finish not a challenge a more course grit. A very open wheel has more space between grits and is often the better wheel for thins. That is called concentration with 100% having as many grits and they can fit in the space, 75% is open spaces between the grits. We used to use a wheel that looked like Pop Corn because of the spaces between grits...It was a cool running wheel.
    Stress relived the parts would have less chance to stress warp.

    You might try a very small step over .015 - .020 perhaps - feed on the grind side only - travel well off the part at both sides of long travel to give needed cool down time- be sure the diamond is turned to a sharp facet- have a space between parts to give spindle full rpm return.
    I have use a cut off wheel to cut groves in a wheel OD to make then act softer but only a grinder hand should do such as thet. You may have to buy a more open wheel. softer (h might be good) A white wheel might fracture and relase grits better, my have to go to q 36 grit.

    If you were in ths USA I would tell you to call Radisc grinding wheels and ask for their advice...
    Hate to say this but a Blanchard shop might make easy of the job. they use a course wheel and it cuts cooler.

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    They run in a series 0.4, 0.275 and 0.040 thick on a spigot, Iíve managed the thicker ones, but the same technique doesnít apply. 0.004 per side stock, wired edm post hardening to minimise heat treatment distortion. Itís a cnc surface grinder yes

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    One thing that has helped me in the past is to grind in toward the center, then go to the other side of part and grind to center. That will keep heat to center (more mass to absorb heat) so you are not running the heat all the way into one side. Especially good on round parts.

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    Forgot to mention 1.5Ē keyed bore through center, so itís an interrupted cut so to speak, also only doing 1 at a time until I can prove it works. Currently using a 0.040 step over

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    I would not go two ways with cross...would put perhaps 4 across if the chuck is true enough ..thia to justify the small incruments at cross. likely change the wheel.

    Make big bold changes if you run a bad part. add plenty of time because scrap cost more than time.. certanily a CNC has plenty of time because you can do other things as the CNC takes baby steps.

    I was still writing as you moved ahead so you might go back to post #4.

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    I would find someone with a double disc grinder to grind them

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    I've ground a fair amount of hardened D-2 though usually not as thin as your example. The wheel you're using should be a good enough general purpose wheel but would very much agree that a more "open pore" wheel should grind cooler. Also agree that your dressing technique/speed will affect the performance. Also agree that the total amount of material to grind off makes a difference in how many times the wheel needs to be dressed.

    DOC you're taking shouldn't be a problem if the wheel is dressed correctly for the material. If you can't grind off .01mm (.0004) per pass then your wheel isn't correctly dressed (approx. 2-3 seconds for 13mm width wheel) or you have a very dull diamond. Have you inspected your diamond under magnification? If it looks rounded it needs to be replaced by a new diamond (or rotated in the holder, try that first) so there's a sharp edge to the diamond. A dull diamond will not fracture the wheel structure properly, it will only round off the grain edges. The burning you speak of is when the wheel surface is glazed and is just skidding across the material surface (leaving burn "skid marks"), not "biting" into the material and abrading it off as it should. I don't feel you need the CNC grinder for this but using coolant, even in a spray bottle, will help control heat/distortion/warping. Try creating a small "tub" from modeling clay around the piece and keep it topped up with coolant from the spray bottle.

    Another thing that helped quite a bit with grinding hardened D-2 was the use of a spindle speed control (VFD) that slowed the wheel to 60-75% of full RPM. By slowing the spindle speed it increased the grinding force and caused the wheel grains to fracture, thereby exposing fresh sharp edges, where full spindle speed would cause the edges to dull. In effect it made the wheel "softer" and kept it from glazing/burning as readily. You may not have a spindle speed control available (ask for one) but following the suggestions we've all made should help considerably. If not then something isn't right and needs to be addressed. You should at least be able to get a new diamond from the budget if nothing else. Dressing should only be about .03-05 mm at a time, more than that is not good for the diamond, especially when doing a fast dress.

    One more thing, D-2 is a more abrasion resistant material and generates more heat during grinding than most other tool steels. You have to keep a constant check on how hot the workpiece is and stop to cool it down or allow it to cool down. I've had jobs grinding blade punches that required me to take no more than 2-3 passes at .01 mm DOC and then allow the material to cool down or it would start to warp/deform/burn. After cooling off, (too hot to touch is too hot to grind) I could begin again. Your final pass to size should be with the material at room temp or cold to touch to ensure size/flatness within tolerance. I'm not the last word in grinding but a tool and die maker has probably had the most experience grinding hardened D-2 so hunt one up, buy him/her a cold one, and ask for advice. Hope this helps, good luck.
    Last edited by AD Design; 06-29-2020 at 09:07 PM.

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    Thanks guys, a load of different things to try here, surely between them something has to work. My diamond isnít the sharpest but I rotate it daily so just assumed it wouldnít be to bad, definitely start there and work with everyoneís suggestions

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    FWIW, everybody likes SG wheels but me. I think they're great for heavy stock removal on a heavy grinder, but are a poor choice for a smaller machine and light low-Ra cuts. I have almost the same wheel in a 60 grit and it tends to heat and burn unless I go very conservative on DOC and feed, plus the finish is rarely as good as you'd expect from a 60. Norton recommends their own diamond to dress it, which I don't have. Try a simple inexpensive open structure white wheel and see if things improve.

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    We always preferred the SG wheels and for that application dress it normally, then downfeed a couple of thousandths and run the diamond across rapidly. My best results on difficult to grind stuff was to use a grinding wax.

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    Another away to look at heat caused burning a surface of a thin is that heat makes metal expand outwardly at the surface. With a thick part the part’s strength can resist surface change other that a minimal growing upwardly but a thin part with its surface trying to grow outwardly has enough strength to cause the part to bend convexly and try to lift it off the magnet at center area.
    The swelling up can cause the follow surface to contact the trying to grind perhaps another .001 - .010 with the entire wheel, much more than the intended .020 or what you wish to cross feed into the part.
    So, everything one can do to aid less heat is to one’s advantage. I have read that ceramic grits are not as sharp edged as aluminum oxide. I will research this further with setting grits under a microscope. If true then ceramic might cause more heat.
    The concentration (grits per space or open space between grits) is a major factor for heat.
    Hardness or how easy dull grits will be let lose is another big factor along with
    grit size, Amount of stock removal. Amount of cross increments.

    It is common that the rapid cooling causes much of the part distortion/warp.

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    D2,thin, close tolerance, cnc grinder. This equates to a 100 grit CBN wheel. D-2 is tough to grind with AlOx wheels, which will dull quickly, which creates heat, which...
    CBN wheels are much less expensive to grind with (they last many times longer than conventional wheels) and make the toughest tool steels grind (almost) easily.

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    Quote Originally Posted by Red James View Post
    CBN wheels are much less expensive to grind with (they last many times longer than conventional wheels) and make the toughest tool steels grind (almost) easily.
    -Seldom used CBN wheels on a SG, company budget driving that choice. Did use CSN almost exclusively on the jig grinder for dowel pin holes and "buttons" or on die sections headed to the WEDM, worked quite well on hardened D-2.


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