OK. Some thoughts.
I've read a lot of surface grinder threads, and there are usually a few areas of the conversation that are lacking.
People talk mostly about how much material in thousandths can be taken off in a single pass pattern. I say pattern to differentiate from a single pass left-to-right. It takes a pattern of these to cover the workpiece.
A common misconception is that a 2" contact wheel would save time by covering the workpiece in one pass, or in very few passes.
The fallacy is in not understanding that mostly only the leading edge of the belt or wheel is doing the majority of the work. The table normally steps in or out with every pass, from left to right and then right to left.
So, when one sets the amount of Y axis feed or "stepover," that in combination with the depth of cut selected at the spindle controls the amount of material removed in one side to side pass. This amount is then repeated on the next pass, and so forth, until the workpiece has been covered completely by the wheel/stone.
So, imagine that a flat piece is sitting on the chuck, the surface has been "found," and that the spindle has been lowered by .005" depth of cut.
Then, it remains to set the amount the table will jog in or out with each pass- my grinder can be set from zero to about .200" stepover on each side, or none or all on either side. In this case, we'll set stepover to .050", a reasonable amount for significant material removal.
We are taking a rectangular piece of the cross sectional area of the workpiece then, with each pass. In this case, a rectangle .005" x .050" in area. It takes a given amount of horsepower to do this, and puts a given amount of heat into the workpiece. If we double the depth of cut, we double the amount of work and heat, or if we double the stepover rather than the depth, the effect is the same, of doubling the work performed per pass.
So, stepover is just as germane as depth of cut to the discussion of what a grinder will do.
After setting up flood coolant on this machine with Trimsol soluble oil, and using the rough AlOx stone that came with it, it would take a .005" deep cut at about .050" stepover before the spindle would begin to bog. That's a lot more than many grinders would do, but still if taking .150" off each side of a big ladder billet, even with auto feed, it takes FOREVER.
Additionally, even with coolant, hogging at this rate with a stone will begin to cup the stock up on the ends, the thinner the worse. Forget about AEB-L at .100"!
That's why I decided to change over.
I have been settling into the new grinding head conservatively, at this point I've been able to take .010" depth of cut with .100" stepover per pass with no sign of slowing the spindle. At this point though, the stock will begin to warp if it's thin or AEB-L. The belt introduces quite a bit less heat to the work, but when taking this much off it does heat up after a while. My mag chuck is not fine pole, which I believe would help with this- although it is a nice old powered electro-mag with built in de-mag feature. I may look into setting up a mist system for it, although that would take some added shielding to keep coolant from spraying all over the shop.
Another thing that is a bit misunderstood in grinder discussions is stone grit and finish. It's actually possible to get a quite decent and smooth finish with even a 36 grit wheel, quite unlike a 36 grit finish on a hand-held workpiece. It help to understand that the process is actually "abrasive machining."
What I mean is that the surface of the stone, on a micro level, is made up of a bunch of tiny little sharp cutters, each one not unlike the sharp tooth of a lathe bit or end mill. What controls the smoothness of the surface is mostly how uniformly and at what depth and speed the work is fed under the cutters.
Much like a lathe or mill, the work is often "hogged out" to remove most of the material, then a final pass or two taken with a lighter cut and higher spindle speed to get a finer surface finish. It is true that on a surface grinder, if a particularly fine or accurate finish is desired, the abrasive material or grit size can be changed as well. Of course as is true of almost all machining, the cutter material or grit must be chosen to match the qualities of the material, at the outset.
Much of this applies to surface grinding with belts as well. I've been using a half worn-out 36 grit red ceramic belt- a sharp new belt would likely cut more aggressively and cooler. And, of course if I switch to say a Gator A45 next the surface finish I can obtain will be finer than that possible with even a finely-fed 36 grit belt.
That's all the rambling I have for now, on the subject. I'm sure there will be more. I just wanted to shed some light on a few things for any future hack machinists like me, learning online...
