Grinding wheel question

[earlgo]

The question arose in a conversation with a fellow hobbyist as to what is the reason that a hard wheel is recommended for grinding soft materials and a soft wheel for hard materials. I looked this up in two references dating back to 1913 and the recommendation was the same but the reasoning was not given, other than it worked. Rogers Machinist's Guide circa 1913 and Advanced Machine Work circa 1925 both said the same thing but with no real reason. Any suggestions?
--earlgo

[BadDog]

Grinding wheels cut by exposing a large number of sharp edges to the part being ground.

Hard material rapidly breaks down those edges, making the wheel dull, and rapidly heating the material being ground with ever slower progress. A soft wheel refers to the binding agents, and being soft, when the individual grains become dull (and hotter), increasing the stress on the bond, they slough off exposing new fresh grains with sharp edges. If you don't use a soft wheel with hard material, you'll find yourself having to repeatedly dress to keep it cutting well.

Soft material doesn't dull the edges of the grains nearly as rapidly, and thus don't need to break down as rapidly to expose new edges. Thus the wheel, and a dress, lasts much longer while still continuing to cut well.

There are also other things to consider like material loading, particularly with metals like aluminum, but that isn't part of the hard/soft discussion. This also doesn't apply to things like diamond or CBN wheels.

[liveaboard]

Hard material dulls the cutting edges of the abrasive wheel sooner.
A soft wheel sheds material sooner to match, revealing fresh sharp abrasive for cutting.

Using a soft wheel for soft material is wasteful. material is lost from the wheel while it's still good.

[Russ Hanscom]

Good explanation, Thanks guys

[mcostello]

If You want to ever see a wheel disappear quickly, just grind a piece of steel with a green silicon carbide wheel. It has to be seen to be believed.

[liveaboard]

I want to have a separate room only for grinding. The grit is just horrible.
I try to do as much as possible outside [handheld grinders].

What solutions do you guys use?

[earlgo]

My Delta Toolmaker is in a separate room, and the swarf blows against a wall. When a lot of grinding is to be done I hook up the shop vac to collect the swarf as it flies off the wheel.

Delta 24-105

--earlgo

[BadDog]

My surface grinder is on the opposite side of the shop from all other machine tools, same side as my wood machines. Mine was built with an incorporated vacuum system. When I turn it on, the vacuum runs, so as long as I keep the cast iron catcher adjusted properly, it makes very little mess. The vacuum is located in the base of the machine, and the filters look like hanging file folders. When they get full of grit, there is a bar you use to bang on them to drop the debris into the bottom where it can be raked up with a small dust bin for disposal.

[SteveM]

I'm planning on putting the surface grinder and the bench grinders on a bench in the corner near a window.

I will put an exhaust fan in the window.

I don't think I can build walls, but I will set up a retracting curtain around it, like the kind around a hospital bed.

I went to a machinery auction where they had two very late model South Bend heavy 10's. Took a look at the beds and other than one I saw years ago where you could measure the bed wear with a ruler, these were the worst I had ever seen.

One of the thing this shop had was a lot of bench grinders (I won 1 or 2). The problem was that rather than the grinders being in a separate room or in the corner, they were between each of the machines.

Steve

[BadDog]

I have several of my bench grinders and all but one of my belt grinders (huge G8) on a heavy cart I bought for near nothing at auction years ago. When I have hard grinding to do, rusty, cast iron, etc, I just wheel it outside. It has it's own built in subpanel with one longish 10ga power cord with 4 quad receptacle boxes for powering bench, belt, and angle grinders (and more) outside. Works well in Phoenix, maybe not so well in more frigid areas.

[Bill Shields]

drop down canvas or plastic curtains (tarps) are also a big help...

anchor the top of the tarp to the joists above and tie a piece of pipe to the bottom so that it can hang under its own weight...or use a cheapie pull / down roll up window shade or three...

Anything for the grinding dust to hit and fall to the floor is better than having it airborne around other machinery

[Conrad_R_Hoffman]

Don't grind dry. That way the mess will be localized. Dressing the wheel goes everywhere but drops quickly and I just sweep if off the cement floor.