4 WAYS TO REMOVE MILL SCALE

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 an operator that does not enjoy, you did not wake up this morning and say I got to prep all this plate, I am one happy, happy boy. We know we don't have that. So what we have to do is, we have to make you happy. We can do it with either a grinding wheel, if you still want to go back to a grinding wheel. We can do it with a segmented flap disc. But what we're going to change, and you can see from the color even the difference, the zirconia's blue and that's not natural, by the way, that's done that way. This is silicon carbide, this is designed, for instance, to work on stone, to work on masonry, to work on glass. And the idea here is instead of being an impact grain, it's a pick. It literally comes in and picks this material up. So you're going to make a pass and you don't have to lean into it. - [Bob] Oh, I'm already there. - [John] And by you not having to lean, it's going to finish it. So you're not going to get all these wave marks. So let's put this one on. Still 40 grit. We haven't changed the grit size, we haven't changed anything else from here. - [Bob] Wow. As soon as I started, I reduced my pressure a lot, and I tried to lay this thing down as flat as I could, and I took off and walked across there pretty quick, really. But I literally, when I, soon as I felt it, I reduced, I wasn't pushing on it at all. Matter of fact, that was like floating on there. I thought I was going to, I thought I was going to miss, like I was trying to do over here, trying not to scratch it. So looking at this, I haven't made those individual cuts. - [John] Exactly. - [Bob] So I'm just, like you said, picking it off of there. - [John] You put in less effort, you're letting the speed of the grinder do the work, you don't have to get frustrated, 'cause wherever you point it, it's going to come off. And so, and the idea is, that you're not building up as much friction 'cause you're not pushing into the material. I mean in the same respects and stuff you'll see, 'cause we looked at this two of the three these and we said, you know, one of the points you brought up, it's really, really nice to get that mill scale off but sometimes you grind further out than you need to weld, and then you have to, later on, you're going to paint it, and you really don't want those marks to come up and show. - No, really don't. - And so we've reduced that somewhat. You can see the real deep marks from the grinding wheel, the lighter ones from the zirconia but still marks, and this kind of sculpting. - That, that right there, painted in the right light, it just, it looks like somebody went over and hammered it. - Exactly. - Beat up. But this is still flat, I don't have these bumps that look like I hammered the daylights out of it here from the, every time that it came over here and cut, so this is better. - It is. Now all that's great in a reality for somebody and stuff that's doing small batches, that are people that are doing some, just some fab shop work or whatever. You don't have to make any big changes or anything around here. But now you've got some commercial work, you've got to do a lot of these plates, you got to do a lot of this work and such, it's going to be painted or powder coated, cosmetics are going to be important, and you've saying to yourself, is there any way I can do this and get off the mill scale but really control the amount of scratch to the point where I can reduce, later on, post work? So what we did was, we went back to the drawing board. And so what we did was, we went back and we said, what's the biggest problem we still have here? And that's a directional scratch. - Um hmm. - Even though it's lighter, it still has a direction that's going to show light unless you later on go and sand it or do some other work with it. - Um hmm. - So what we did is, we manufactured a stainless steel wire brush and then we took that filament, that actual wire, and we sent it to our manufacturing facility and they actually grew, what is called grew, diamond on it. - Stop it. - So this is a-- - I can see that now, that you turn it in the right light. - So the whole idea, you see, you can see that almost half of the length of the wire actually has kind of a gritty appearance. - Sure. - That's pure industrial diamond. And we do that because we literally can take this and grow, take diamond segment and it's called growing diamond on each one of these filaments. So now what I've done is, I've taken an abrasive product, this time industrial diamond, and I put it into a format that you can come on to mill scale with, and very effectively. And because these wires are all going to move, you're not going to have a directional pattern. - Not going to have directional. - Now you're going to lean into this a little bit. It's not like a regular wire brush where you're just using the tips, you got to kind drag that diamond through. - You got to get into it some? - Right. And with about the same effort as you did here. - Okay. - You're going to see it's not much. Now what you've got, is you've got an area you'll notice the mill scale's off, the machine isn't building up any kind of heat. - A big area. - It's doing a big swap at a time. - It's like eating it off of there. - And it just leaves you with a very light, non-directional real finish. - That's strange, I mean, you know, when I, first of all when I started to grind it, I didn't think it was on. - Well . - I mean, it just kind of went to nothing, because of the super low RPM, and then I could actually feel, you know, when I got up here and just got the right angles, so that this would flatten out somewhat, you could kind of feel it just kind of biting into this and then, when I pulled it back. That's amazing. - And how about, you used wire brushes in the past, how many wires you got sticking in you? - I don't feel any, you know, this big old fat thing here catches all the wires and I get 'em stuck in my jeans and, that's amazing. - The reason being is, the wires are not running at so high a speed, that in reality of stuff they're allowed to do their work. - Um hmm. - And so you're just completely controlling that. - This fascinates me. I see some huge benefits to this. - The other thing is, because you haven't scratched the surface up so badly, for this grinding wheel or even this initial disc, because of all the effort you have to put in, to get it to paint I have to do at least two more steps. - Um hmm. - With this, I can immediately go to the 150 grit non-woven disc and I'm ready to throw it to paint. - I know this helped tremendously. I mean, this answered a lot of my questions that I've had over the years. I've tried various products. This, to me, is fascinating. I mean, there's a lot of our viewers that ask us technical questions about abrasives on occasion and I'm lack the technology and the science behind abrasives, and always been fascinated by that. So you have answered a lot of my questions. - Excellent, thank you. - Well thank you, I appreciate your time. This is very interesting and I know a lot of viewers are going to benefit greatly by this so, seriously, I mean this is something that I've always been annoyed by mill scale, and this easy solution of just wiping this stuff right off here is pretty fascinating. Thanks for watching weld.com. Make sure you check us out on Facebook and Instagram as well. He's unhappy. I got him in the booth. I got him right where I want him. - Oh not again. - You should have ran. - Okay. - You should have been able to run faster, that's-- - Look at me. - I know. - I haven't run since 1968, man, when the rubber bullets were on my . Don't start this, okay? Do not start this. And I had incentive then, that was a 19" color portable. All right, just tell 'em this is John from Pferd and, you know, and I'll just take my chances with the Feds. - [Camera Person] Hah, I was still recording, what do you know? - And you-- - Why are you turning red, I wasn't sayin' nothin'. - Hey..

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