Dealing with Mill Scale

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KellyJones
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Location: Snohomish, WA

Dealing with Mill Scale

Post by KellyJones » Mon Jun 20, 2011 1:28 pm

All:

I have been trying to make stuff from pieces of what I think are hot rolled steel. The steel is generally soft, but both upper and lower surfaces are covered with what appears to be mill scale. The scale is rather hard - so hard that is eats up my tools very quickly. Last weekend it ate a HSS tool bit I was using to face it with a fly cutter. In less time than it takes to tell, the HSS toll had completely worn away (I mean in seconds!) and the machine stopped cutting. With some effort this scale comes off with a belt grinder and the remaining steel is relatively easy to machine.

Is this an expected behavior? Is there an alternative explanation? It is getting extremely frustrating around my shop.

thanks
Kelly Jones, PE
A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing.
George Bernard Shaw
(1856-1950)

Harold_V
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Re: Dealing with Mill Scale

Post by Harold_V » Mon Jun 20, 2011 1:48 pm

That's typical of mill scale. You can minimize the problem by the use of carbide, and taking cuts that are well below the surface. A skim cut, in the hopes of removing only the scale, is not likely to be met with much success, even with carbide. In such a case, a surface grinder can be quite useful.

The rule of getting well below the scale applies across the board. HSS will last longer, although you often experience wear on the tool at the interface of the scale. Tool geometry can help--just insure that the edge that contacts the finished surface doesn't have to plow through the scale .

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

KellyJones
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Re: Dealing with Mill Scale

Post by KellyJones » Mon Jun 20, 2011 1:52 pm

Harold:
How deep would you say a fly cut would need to be to meet your recommendation? 30 thou?

thanks
Kelly Jones, PE
A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing.
George Bernard Shaw
(1856-1950)

Harold_V
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Re: Dealing with Mill Scale

Post by Harold_V » Mon Jun 20, 2011 2:12 pm

KellyJones wrote:Harold:
How deep would you say a fly cut would need to be to meet your recommendation? 30 thou?

thanks
Corner radius of your tool would dictate. If you keep the radius small, and provide some lead, that should work pretty well. It won't be a perfect solution, but it will yield greater tool life.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

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AndrewMawson
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Re: Dealing with Mill Scale

Post by AndrewMawson » Mon Jun 20, 2011 3:43 pm

I have a feeling that you can dissolve mill scale in citric acid, which is a relatively safe acid to have in the workshop. It is also very good at dissolving rust.

I've just sucessfully dissolved the rust from four 1.5" diameter acme threaded lead screws that had 3" long bronze nuts firmly stuck on them following a very major fire.These were the bed lifting screws of a 60 ton hydraulic press that I bought fire damaged. De-oiling and 48 hours soak in citric acid did the trick.

Useful stuff to have arround!
Andrew Mawson
Battle, East Sussex, UK

Al_Messer
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Re: Dealing with Mill Scale

Post by Al_Messer » Mon Jun 20, 2011 4:39 pm

I use Hot Rolled Steel on most of my projects. I just clamp it up in the Shaper vise, set the depth of cut to about .035", feed rate of about 20 thou., Medium speed and turn it on and forget it. Tool contacts from the edge where the scale is already gone and flakes the rest of it out of its way as it traverses the work piece. I LOVE my Shaper for stuff like this!

Al
Al Messer

"One nation, under God"

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Lew Hartswick
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Re: Dealing with Mill Scale

Post by Lew Hartswick » Mon Jun 20, 2011 5:36 pm

For the last quite a few years I've been using Hydrochloric ( Muratic ) acid to etch the scale
away before I ever take a cut on a lathe or mill. But now that we don't have the controlled
access to the storage place, like I did before the new shop, I think I'll try the acetic or citric
acid route. I think it'll take alot longer but be a lot safer if some idiot student messes around.
Haven't I read something about adding a saturated salt ( sodium chloride) to the acids to
help with that.???
...lew...

atomarc
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Re: Dealing with Mill Scale

Post by atomarc » Mon Jun 20, 2011 5:41 pm

Running a torch over the material will pop the mill scale right off. It fly's off in little meteors so be sure to wear safety glasses. :)

Stuart

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Orrin
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Re: Dealing with Mill Scale

Post by Orrin » Mon Jun 20, 2011 9:52 pm

Ordinary vinegar with table salt dissolved in it will remove the scale. An excess of salt (with some undissolved in the bottom of the container) is what the recipe calls for.

I have an abrasive blasting cabinet, so I usually don't have to resort to the slower chemical method; but, I have tried it and it works.

Orrin
So many projects, so little time.

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steamin10
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Re: Dealing with Mill Scale

Post by steamin10 » Mon Jun 20, 2011 10:27 pm

Mill scale or rolling scale, was handled with salt on a hot strip. The practice was abandoned because of the corrosive effects to the equipment as well. Commonly Sulfuric is used for cold metal, and HCL heated to 160* or better is used to treat hot roll. The Trick is HCL must be 'activated' or have Ions available to exchange with the oxides. That means Ferric chloride, which is the product of the pickling anyway, so the reaction speeds up, as the liquer gets used. Amazing what you can learn when you pay attention at work..... 8)

Disn't mean to bore ya..
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
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Harold_V
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Re: Dealing with Mill Scale

Post by Harold_V » Mon Jun 20, 2011 11:36 pm

Lew Hartswick wrote:For the last quite a few years I've been using Hydrochloric ( Muratic ) acid to etch the scale
away before I ever take a cut on a lathe or mill. But now that we don't have the controlled
access to the storage place, like I did before the new shop, I think I'll try the acetic or citric
acid route. I think it'll take alot longer but be a lot safer if some idiot student messes around.
Haven't I read something about adding a saturated salt ( sodium chloride) to the acids to
help with that.???
...lew...
While I have yet to try the concoction, salt and vinegar is reputed to dissolve rust----and I expect mill scale as well. The interface of the material with atmosphere tends to be a corrosion point, so the material should be fully submerged to avoid damage.

HCl (muriatic) has one very negative attribute--that of rusting (via fumes) anything ferrous in the near vicinity. It is best kept out of the machine shop. I used to buy it by the 55 gallon drum and am all too familiar with how aggressive it is.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

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