Dealing with Mill Scale

Topics include, Machine Tools & Tooling, Precision Measuring, Materials and their Properties, Electrical discussions related to machine tools, setups, fixtures and jigs and other general discussion related to amateur machining.

Moderators: GlennW, Harold_V

Post Reply
KellyJones
Posts: 189
Joined: Mon Nov 29, 2010 10:10 pm
Location: Snohomish, WA

Dealing with Mill Scale

Post by KellyJones »

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)
User avatar
Harold_V
Posts: 20232
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Dealing with Mill Scale

Post by Harold_V »

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
Posts: 189
Joined: Mon Nov 29, 2010 10:10 pm
Location: Snohomish, WA

Re: Dealing with Mill Scale

Post by KellyJones »

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)
User avatar
Harold_V
Posts: 20232
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Dealing with Mill Scale

Post by Harold_V »

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.
AndrewMawson
Posts: 286
Joined: Sat Jan 04, 2003 5:46 pm
Location: Battle, East Sussex

Re: Dealing with Mill Scale

Post by AndrewMawson »

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
Posts: 2664
Joined: Sat Jan 04, 2003 7:12 pm
Location: Mid Tenn.

Re: Dealing with Mill Scale

Post by Al_Messer »

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"
Lew Hartswick
Posts: 775
Joined: Sat Jan 04, 2003 10:45 am
Location: Albuquerque NM

Re: Dealing with Mill Scale

Post by Lew Hartswick »

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
Posts: 264
Joined: Sat Oct 03, 2009 7:13 pm
Location: Eureka, California

Re: Dealing with Mill Scale

Post by atomarc »

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
Orrin
Posts: 307
Joined: Sat Jan 04, 2003 12:24 pm
Location: SE Washington State, near Moscow, Idaho

Re: Dealing with Mill Scale

Post by Orrin »

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.
User avatar
steamin10
Posts: 6712
Joined: Sun Jun 08, 2003 11:52 pm
Location: NW Indiana. Close to Lake Michigan S. tip

Re: Dealing with Mill Scale

Post by steamin10 »

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
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.
User avatar
Harold_V
Posts: 20232
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Dealing with Mill Scale

Post by Harold_V »

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.
Post Reply