by Bob Moffatt
- [Bob] I have Mr. John Thompson, National Technical Sales Director for Pferd, and I have some technical questions for you. I want to get into this piece of plate here and I want to clean it, but I don't want to get into the base metal. What's the most common method?
I'd start out with a quarter inch zirconium wheel. And I know from doing it all these years, that when I try to remove mill scale the abrasive just loads up and it's just sittin' here and it's not doing anything. So the first thing I would do would be angle it up. Now I'm cutting and I don't want to cut into this, I want to maintain the surface integrity, I just want the mill scale off. - [John] All right. First of all, let's take a quick look at what the mill scale is. When they were making this material and it's hot, all the impurities come to the surface and so there's 100% adhesion, okay? That scale is hanging on to all the little nooks and crannies and such that are here. So coming in with a regular grinding wheel like you do, when it first starts to take this from this kind of gray color to a shiny black, because you're really not getting it all off, now frustration takes over. - [Bob] Yeah, it does. - [John] And the first thing you do is start picking this thing up and you're just going to get it off. But remember, you're running at 160 miles an hour, that's what this wheel is running at, it's your regular high speed grinder. And your control at that point, to keep from digging too deep, is gone. So what we do is we've actually sat down and said, what is it going to take to get rid of mill scale? And so let's start out with the wheel you would have had on the pegboard. Grabbing that wheel and saying, look, I'm going to make a push and we're going to take a look. Now it may, cause it's brand new. - [Bob] Sure. - [John] You may initially get some of the scale off, but I want you to take a look at the amount of scratches you're putting into the material and remember, later on you're going to have to paint this, and you want it to look halfway decent. - [Bob] All right. - [John] So let's do that. Wrong grain, wrong speed, wrong control. Let's see what happens. - [John] Okay, here's a perfect example. You see, first of all you've got a really deep scratch. - [Bob] I do. - [John] And this is a standard 24 grit. And it's a brand new wheel. - [Bob] It's a brand new wheel. The other thing is, you'll see how you're skipping because you're spending at least 40% of your time in the air. - [Bob] Well, yeah, I'm trying to keep a light pressure but-- - [John] No, but-- - [Bob] Yeah, yeah, you're right, and then I'd have to go back and clean that off, and then I'm just overlapping deep scratches with deep scratches and deep scratches. - [John] Exactly. Abrasives only remove. So in reality all you're doing is taking more and more of your actual material thickness and stuff down, possibly even creating a weaker point, if it's going to be a stress point later on. The other thing is you're not getting it all off. - [John] You've got to get this all cleaned up. Mill scale melts at a higher temperature than the clean base metal. That's another reason. So let's go to a flap disc instead, okay. - [Bob] Okay. - [John] You're not going to bounce, you still buy these out at your local supplier, it's a flap disc, it doesn't bounce around as much as a hard wheel. Costs a lot more, okay. We're just going to put this one on, same thing, mounts on the same as anything else, speed stays the same, you're still using it on your grinder. And we're going to come a little bit above on here. Now this is actually rated at 40 grit. So we're not talking about anything real cosmetic. This was a 24 grit wheel. - [Bob] Okay. - [Bob] So this is 40 grit and that's about as coarse as most of the time you can find them, right? - [John] Yes. - [Bob] Which you, you'd think would be pretty aggressive. But I've grabbed one and I'm thinking, wow, this thing isn't cutting at all, it's just floating. - [John] For now let's take a look at what's happening. - [Bob] As an operator, I feel like I'm kind of floating out here. And when I pulled the grinder down toward me a little bit, I'm changing my angle, obviously, it's biting. I'm making overlapping gouge marks in here, I'm making waves. - [John] Right. But look at also, you've gone from the gray color, now you're going to a shinier black color. And you can see how it's so broken up. Here it's a very crisp edge, and here it's all broken up. The reason being, and you can see the disc already, has started to glaze up. - [Bob] Yeah. - [John] So it isn't going to take you long, all right, when you start to pull into this, because now the scale is down low, you got to work to get that out. - [Bob] Right. That's where my frustration level goes through the roof. - [John] Exactly. Because again, and I'm looking at it over here in the light, and it's glazed. - [Bob] And so when that stuff, is it heat that's making it glaze up? Is it, I mean, I realize that once it glazes up, to me it's floating, I don't feel anything and so that's the, that's where you get hacked off and you start pushing down and trying to make things work when they're not going to work. - [John] So, what is mill scale actually made of? It's made out of the impurities from the steel. It's made out of the excess coke that's in the material, it's made out of oils, it's made out of drawing materials, it's made out of all the things that got mixed in. Your heating them back up to about 1800 degrees, 'cause that's the actual friction you're creating here. And what you're tending to do, especially on the oils and the debris, is all you're doing is starting to float 'em. And now you're floating 'em and now you're floating 'em and now you're floating 'em. - [Bob] I'm just rearranging them. - [John] All you're doing is moving 'em back and forth. Think about a coat of paint on a car, three four coats of paint on a car, and you come in, you're going to grind all that off, and you literally liquified and it lumps over here and you push it back over here and it lumps over here, and you say, what is going on? You've exceeded the temperature of the mill scale and all you've done is starting to float it all over the place. And the madder you get, and the harder you press, the more you just float that all over the place. We can't give you anything that you have to lean into, and we can't give you anything that's going to build up a lot of heat and friction, which means you can't stay on it for any length of time. - [Bob] Okay. So what's a good solution here? What do you recommend here? - [John] The whole idea is that we have taken a look at the actual issue that you've had, and we've said it cannot be done with an impact grain. Impact grains are aluminum oxide, zirconia, ceramic. It just can't be done. These grains are designed to bang on the material. So what we're going to do, is instead, we're going to completely change the grain that you're using. Because here's our constant. We know we have a lot of mill scale, we know we have a high speed grinder, we know we have