Surface grinding technique: roughing corner/finishing corner
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    Default Surface grinding technique: roughing corner/finishing corner

    A few years ago, when I got my first surface grinder, Buck suggested a technique that I now use a lot.

    Case in point: I cut a 50 x 400mm strip of 1.2842 (90MnCrV8 or AISI O2 tool steel) from a 10 x 150 x 400mm plate. The plate was ground parallel both sides (Blanchard) but after I cut out the strip, the strip was bowed about 0.3mm (0.012") from internal stress. It needed to be straight, and I'm no good at "heat straightening", so I needed to hog 0.35 mm (0.014") off the strip.

    Following Buck's method, I used a 46 open wheel, 25mm wide, plunging straight down along the rear long edge, dropping 2 microns in each traverse, lots of coolant. When I reach full depth of 0.35mm, I then crossfed across the part, about 0.2mm per pass, pushing the part directly into the wheel. This breaks down the front corner of the wheel, but puts enough pressure on it that it doesn't tend to clog up. Then, when I got to the front of the part, I downfeed a few microns, and then crossfeed back full width in the other direction. That's it: one plunge and two crossfeeds, finished.

    The point is that the front corner of the wheel gets all of the abuse from the roughing, and the rear corner of the wheel is kept in good shape for the finishing pass.

    Is this a standard technique? You beat up one corner of the wheel for roughing, and baby the other corner for finishing?

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    The lead-in edge when incremental grinding ( making long travels and a crossfeed each time off the part breaks down the lead in corner of the wheel and also takes off the bottom of the wheel at a slow angle..so leaving the follow or last part of the wheel still flat for the surface finish needs.

    I have straightened a bow part with C clamping it down or set in a straightening press to m measy=ud amount.
    Perhaps I push/bend it with two stacks of .060 and it doesn't straighten so I go a little more .100.. and the .150 holding it there for a few seconds...finally, it will/may straighten a little and I become more careful.
    And I make it straight or straight enough to grind.

    A journeyman blacksmith taught me that method..going a little more (bending) might break a hard part, that is why you use the measured spacers to straighten.

    Grinding to straighten I consider first if the part has been grinder burned..if so stresses of contraction likely are pulling tight the concave side. Taking off .005 may reduce the bow 50%, so if possible I like to take .005 off that side.

    If I think internal stress is at fault I like to shim under the bow and grind the top almost flat..but expect to still have some stress in the part so flip-flop the part a number of times so to see where it might go.

    What Bruce describes is good to keep in mind that the front lead is loaded so likely will heat the part much more than the fresh lead at the wheel back. The wider wheel does not bottom break all the way across so still has a finisher flat bottom portion.

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    Just may need to use a little softer wheel with that method I think. It will be a lot gentler on the wheel so it won't break down as easily. I like to run ⅓ wheel width stepover for finish grinding at high table speed and shallow depth. The slow cross may be better for roughing in some cases. And the straight downfeed with no cross then raise, step over and come down again of course has its uses.

    So I guess for me anyways, the method depends on the application, and there really is no standard technique, per se.

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    QT ekretz: And the straight downfeed with no cross then raise, step over and come down again, of course, has its uses.

    This is a very good practice with the right wheel and part material..It is surprising that a lot of grinder guys don't do it more often. Down feed off the part and long travel down to about +.005 in as many sections that gets the whole part to +005 (or so). Re-dress and skim to size.

    Very often this is the fastest way to take a part to sizs.

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    Quote Originally Posted by eKretz View Post
    And the straight downfeed with no cross then raise, step over and come down again of course has its uses.
    The move across wheel width and downfeed might have been faster, I didn't think of that here. Let's see:

    At two microns per downfeed and 350 microns off I need 175 downfeeds. At 12 traverses per minute (5 sec per) I get 24 downfeeds per minute, so 7 minutes per downfeed cycle, two needed on part, 14 minutes.

    I was crossfeeding 0.2 mm, so to go 25mm, I need 125 crossfeeds. OK, that's slightly faster. As would have been doing the 0.2mm crossfeeds from the very start instead of going straight down at the start.

    Your way is kinder on the wheel corner, but might also tend to clog up the wheel (very dependent on material and wheel of course). Will try it next time.

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    Haven't run a surface grinder in many years, but if I had a warped piece, and had enough stock to remove, I would determine the direction of the warp by rocking or using an indicator, then I would turn the part so that the warped ends were up and softly peen with a hammer that had M2 welded to it in the center and work my way out till I got it as close to flat as I could get it that way, then I would indicate to see how much warp it had and shim the ends, grind at .0005" or less per pass with LOTS of coolant after each pass I would take a sharpie and scribble the whole thing then make the next pass and repeat till all the scribbles were gone, then I would remove, thoroughly clean the chuck, flip and repeat without shims. That is the way it was taught to me.

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    Quote Originally Posted by SteelrFn View Post
    then I would turn the part so that the warped ends were up and softly peen with a hammer that had M2 welded to it in the center and work my way out till I got it as close to flat as I could get it that way
    That didn't occur to me. I've used peening on sheet metal. Would it work on 10mm thick unhardened O2 / 1.2842 / 90MnCrV8?

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    Here find a heat straightening event by Keith Rucker, a very sharp guy.
    I might have let it cool a little before wetting, but Keith is likely sharper than I am so his way is fine.

    We used to straighten hard spline round broaches that were about 5 feet long, with using a very hard rounded carbide chisel point and a rapid air hammer to expand the inside curve to a bend by hammering a dent in the flute radius. they would come pack from a production run with warps all over the place and we had to get them back to .002 by placing them between centers and mapping all the warps/bends.
    Warps are all different and each has a best or Ok method.

    Grinding out a warp one has to consider that the wheel cutting forces(pressure) will push out some of the warp like a spring, so that action meay restrict /reduce the grinding ability...and that releasing stresses may make the action more or less than what one might expect.
    A part not having enough grind stock may need some whacking or heat to get it in the range of grindable.

    If you can push the part with one hand (elbow swing not your weight) and a hammer handle on the part then likely the wheel will push it that much.

    A good way to see wheel pressure is to push a part onto a bench grinder wheel and count the sparks, to make tha same sparks on the SG you use about that same pressure.

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    And use the most openly dressed, free-cutting wheel possible until you have the part nice and flat. After that you can get your finish. I don't like to peen or use heat to straighten unless absolutely necessary. I find that can make it tricky to keep the parts flat sometimes. Those stresses you add in may also relieve themselves over time, or be so uneven in the material that it moves unpredictably when you grind some away. It can also be very time consuming. So if I can, I avoid that. Sometimes there's not much choice though. The method above mentioned by SteelrFn is good, using a very large quantity of gentle taps over a large surface area - that keeps the stress added by the peening evenly spread out in the material. And yes, it should be very easy to peen 10mm thick material.

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    Quote Originally Posted by SteelrFn View Post
    I would turn the part so that the warped ends were up and softly peen with a hammer that had M2 welded to it in the center and work my way out till I got it as close to flat as I could
    Quote Originally Posted by eKretz View Post
    The method above mentioned by SteelrFn is good, using a very large quantity of gentle taps over a large surface area - that keeps the stress added by the peening evenly spread out in the material. And yes, it should be very easy to peen 10mm thick material.
    If the part is generally bowed, wouldn't it be better to bend it straight in a three-point setup using some spacers on the ends? I would have thought that distributes the stress more evenly, whereas peening might be necessary if the curvature is localized?

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    Quote Originally Posted by jwmelvin View Post
    If the part is generally bowed, wouldn't it be better to bend it straight in a three-point setup using some spacers on the ends? I would have thought that distributes the stress more evenly, whereas peening might be necessary if the curvature is localized?
    Pressing will not generally work well to take out a warp, peening is much more fine tuneable. Like for instance if your bar has 3 corners touching flat and one up in the air. It is also much easier to do the final tweak with peening since it's much more gradual. Press is more for roughing in for me. Either way, I avoid both if at all possible. I'd rather shim when it's practical.

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    Quote Originally Posted by eKretz View Post
    Pressing will not generally work well to take out a warp, peening is much more fine tuneable.
    Makes sense, thanks. With peening, the idea is to put the concave side up and stretch that surface with the peening blows?

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    Quote Originally Posted by jwmelvin View Post
    Makes sense, thanks. With peening, the idea is to put the concave side up and stretch that surface with the peening blows?
    Yes, that's correct.

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    Every warp has its best solution. peening, heat, just grind out, bend the other way...

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    Is this a standard technique?
    I read it on here when i bought my first surface grinder & use it ever since.
    Forget the old guy's name, but he was almost as sharp as Michigan Buck!
    When i used to mention it in repsonse to grinding problems, or worse show pictures, it just seemed to antangonize the natives.

    It saves constant dressing on the wheel, and it puts less direct heat into the part (less additional heat warping) especially for volume removed. unless alternately the stock is reduced by tickling off a couple tenths at a time.

    Of course every method has its place, and some places in might not belong/be ideal. But generally i rough the way you describe, and then might or might not take a finish pass of a few tenths with wider steps.

    It does take a lot more dress off the wheel, but the benefit is how much can be ground without dressing, losing wheel position, etc. Also, if both edges of the wheel can be used (on grinders where loading the back-side of the wheel is OK, not, say Harig) even more can be got out of the wheel and original dress before running it over the diamond again.

    Don't use too soft of a wheel, it will build a ramp instead of a step, and then the effect is lost as the ramp widens. I like J hardness, open, coarse 36gr is really nice for hogging normalized and medium hardened steels. But i've been stealing NOS 46g for a while and have a few dozen of them. Maybe I hardness for harder steels sometimes.

    smt
    Last edited by stephen thomas; 10-22-2021 at 09:55 AM.

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    Hi Stephen,

    Quote Originally Posted by stephen thomas View Post
    It does take a lot more dress off the wheel, but the benefit is how much can be ground without dressing, losing wheel position, etc.
    Thanks for your post. Yes, one of the reasons I like this is because I don't need to dress as often. The roughing corner 'self-dresses' by breaking down, which preserves the rest of the wheel, and the rear corner only does the final finish pass so lasts a long time.

    Quote Originally Posted by stephen thomas View Post
    Also, if both edges of the wheel can be used (on grinders where loading the back-side of the wheel is OK, not, say Harig) even more can be got out of the wheel and original dress before running it over the diamond again.
    I didn't know that there are machines on which you are not supposed to grind on both front and back corners. Is it OK on my J&S 540? I've been doing it since I got the machine, didn't realise that it might not be OK.

    Cheers,
    Bruce

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    On Harigs, as i understand it, (only ran one briefly in a shop i worked nights - not my favorite surface grinder) the spindles are spring loaded back agains the bearings. So "heavy" grinding with the back side of a wheel is not recommended because it can cause the spindle to float forward. Probably won't grind accurately to an internal corner or face on that side, at least without operator awareness and long spark-out.

    "normal" surface grinding feeding both ways with light downfeed between passes should not be a problem. Maybe someone with deeper Harig experience will chime in.

    Never heard of it as a problem with heavier grinders like my B & S & DoALL, or your J & S.

    smt

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    Hi Stephen,

    Quote Originally Posted by stephen thomas View Post
    On Harigs, as i understand it, (only ran one briefly in a shop i worked nights - not my favorite surface grinder) the spindles are spring loaded back agains the bearings. So "heavy" grinding with the back side of a wheel is not recommended because it can cause the spindle to float forward.
    I think all grinding spindles have some sort of biased preload, to allow thermal expansion. It's no different on my J&S 540, if I shove the spindle hard from the rear, it moves slightly forward. As soon as the force is released, the preload springs move it back again. But since I am using the front corner of the wheel for the heaving roughing, and the rear corner of the wheel for finishing, the "moveable spindle" should not cause problems.

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    The old plane bearing Brown & Sharpe spindles were solid one way and would give the other way, so that need be considered when taking heavy or light axial feeds.
    OD feeds wee solid all the time.

    Some old clunker grinders might take 6 0r 12 millionths more going one way (cross)rather than the other way(cross).
    So really good to know your grinder.

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    on peening and heating: local temperature rise during grinding can be substantial. doest this reverse the straigtening?


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