Does anyone have any experience with machining Dowmetal?

crossthread · Dec 19, 2019

An acquaintance of mine that I used to work with has asked me to duplicate a broken balance beam on a piece of analytical equipment. He says that it was originally made with dowmetal which he can supply. He is currently in Sydney so I don't want to commit until I know a bit more about what I am getting into. I did a little research and found that it is a magnesium alloy but can vary widely in the content and I couldn't find anything on how it machines. The job would incorporate milling and lathe work but nothing overly complicated. Has anyone out there ever worked with this material and comment on the machining capabilities? Thanks very much.

Milland · Dec 19, 2019

If the bulk is Mg, then (while I can't guarantee it) it's likely to machine like a more brittle Al. In other words, fairly easy when using sharp tools at moderate RPM and feeds.

Keep the work are clean, frequently remove chips, and check your programs (if CNC) to make sure you don't have a crash that could send sparks into the chips. Store chips and small waste in sealed steel cans, large pieces are not a hazard.

Joe Gwinn · Dec 19, 2019

If it was intended for construction of balance beams, it will be quite stable dimensionally (no strange phase changes over time or temperature) and most likely will be easy to machine with sharp HSS tools.

From Quora: "Dow metal is an alloy of magnesium. It constitutes more than 85% of magnesium and remaining are aluminum and manganese. ... Dow metal : A brand name applied to any of various magnesium alloys containing more than 85 percent magnesium, characterized by extreme lightness."

crossthread · Dec 19, 2019

Thanks guys. Joe, I got the same definition but nothing about how it machines. My normal procedure, on what I deem mystery metal, is sharp HSS but I thought someone may have some experience with it. Thanks again.

Joe Gwinn · Dec 19, 2019

crossthread said:
Thanks guys. Joe, I got the same definition but nothing about how it machines. My normal procedure, on what I deem mystery metal, is sharp HSS but I thought someone may have some experience with it. Thanks again.

There was recently another thread on machining pure magnesium. The danger is that magnesium chips burn nicely, hot enough to destroy the machine tool. And perhaps burn the shop down. The problem is that magnesium can burn underwater, stealing the oxygen from H20 and releasing hydrogen. Using cutting oil (not oil emulsion in water) excludes oxygen from the magnesium, and keeps it cool while cutting.

Milland · Dec 19, 2019

Joe Gwinn said:
There was recently another thread on machining pure magnesium. The danger is that magnesium chips burn nicely, hot enough to destroy the machine tool. And perhaps burn the shop down. The problem is that magnesium can burn underwater, stealing the oxygen from H20 and releasing hydrogen. Using cutting oil (not oil emulsion in water) excludes oxygen from the magnesium, and keeps it cool while cutting.

I did a lot of aerospace Mag work (still have a pair of Faraday Cups working just fine on the WIND satellite since the Nov 1994 launch), and never used coolant or oil for machining. Dry cutting, performed as described in my previous post. For small jobs like what's being described, keep it simple and keep the machine clean.

Don't spark and life is good...

Joe Gwinn · Dec 20, 2019

Milland said:
I did a lot of aerospace Mag work (still have a pair of Faraday Cups working just fine on the WIND satellite since the Nov 1994 launch), and never used coolant or oil for machining. Dry cutting, performed as described in my previous post. For small jobs like what's being described, keep it simple and keep the machine clean.

Don't spark and life is good...

It sounds like the OP is making only a few, so just being careful could work. But don't allow the chips to accumulate.

For machining zamak zinc-aluminum alloy, I found that to get a good finish it was essential to continuously blow the chips away with compressed air. This can be combined with a shop vac. Which may prove sacrificial.

crossthread · Dec 20, 2019

I am just making one. Thanks Joe.

steve45 · Dec 21, 2019

Is that anything like Rearden Metal?

Sorry...

Mike1974 · Dec 21, 2019

Why do people seem to always say "sharp HSS"..? Why not sharp carbide? I mean I guess if you are using a bridgeport you don't get the full effect of carbide (not enough rpm, un-controlled feed, etc)

Uncoated 3 flute carbide endmills will be as sharp as HSS, and likely to hold their edge better too.

Milland · Dec 21, 2019

Mike1974 said:
Why do people seem to always say "sharp HSS"..? Why not sharp carbide? I mean I guess if you are using a bridgeport you don't get the full effect of carbide (not enough rpm, un-controlled feed, etc)

Uncoated 3 flute carbide endmills will be as sharp as HSS, and likely to hold their edge better too.

It's a bit of a holdover from earlier, coarser grain carbide tools. Back in the day HSS were sharper, because the larger grains in carbide didn't support a keen edge.

Now, with smaller carbide particles a sharper edge can be held, lessening the gap to HSS. But at the extremes, a HSS tool, with it's still-smaller microstructure can take a sharper edge. How long it keeps it over carbide in similar cutting conditions is another story...

Mike1974 · Dec 21, 2019

Milland said:
It's a bit of a holdover from earlier, coarser grain carbide tools. Back in the day HSS were sharper, because the larger grains in carbide didn't support a keen edge.

Now, with smaller carbide particles a sharper edge can be held, lessening the gap to HSS. But at the extremes, a HSS tool, with it's still-smaller microstructure can take a sharper edge. How long it keeps it over carbide in similar cutting conditions is another story...

Maybe very good brand name HSS (niagara? - OSG... etc) but I have used alot of junk HS too, that wouldn't cut you a slice of bread. On the other hand, never seen badly sharpened carbide new- from manufacture, re-grinds well... :bawling:

)

Milland · Dec 21, 2019

Yeah, a lot of current HSS comes from foreign production that's sorta crap. I won't buy HSS on Ebay unless it's a reputable NOS US brand, as you have to worry about counterfeit packaging with the newest stuff.

crossthread · Dec 21, 2019

Well Mike I have never heard anyone say "get yourself a nice dull HSS tool" but I have found that a very sharp HSS tool is sharper then most carbide bits. In my opinion HSS still does a better job on some material. I am talking lathe here not mill. I also like the fact that after the almost complete demise of Armstrong type holders, it's a lot easier to adjust the rake angle with HSS then carbide. It only takes a minute to regrind an HSS bit to eliminate heel drag or chatter. With a carbide bit in a QCTP you are pretty much stuck with what you've got. I'm not trying to be old school or anything. Carbide is amazing and if you have the power you can drive it like you stole it.

Mike1974 · Dec 21, 2019

crossthread said:
Well Mike I have never heard anyone say "get yourself a nice dull HSS tool" but I have found that a very sharp HSS tool is sharper then most carbide bits. In my opinion HSS still does a better job on some material. I am talking lathe here not mill. I also like the fact that after the almost complete demise of Armstrong type holders, it's a lot easier to adjust the rake angle with HSS then carbide. It only takes a minute to regrind an HSS bit to eliminate heel drag or chatter. With a carbide bit in a QCTP you are pretty much stuck with what you've got. I'm not trying to be old school or anything. Carbide is amazing and if you have the power you can drive it like you stole it.

Have you used any aluminum specific inserts? Those things are fucking sharp!!! I mean like look at it wrong and you got a cut on your finger. HSS has it's place (drills, taps, etc) but for outright cutting stuff.... carbide all the way!

Can't find a linky now... but you should look them up. I bought some aluminum specific inserts for a boring head and I cut myself installing the insert on more than one occasion..

In bold - if you dull/hone a flat on the edge on a HSS drill for some brass/copper/bronze it does work better.

johnoder · Dec 21, 2019

Do read and take seriously the parts about fire hazard - and the simple ways to circumvent that. Ignore and be prepared to get the scare of your life - a rapidly blossoming brilliant white fire impossible to put out - and making it tenfold worse - in the most spectacular way - if you douse it with water

There are (or were fifty years ago) purpose made dry compounds (think floor dry similar) useful in smothering the conflagration

Joe Gwinn · Dec 21, 2019

johnoder said:
There are (or were fifty years ago) purpose made dry compounds (think floor dry similar) useful in smothering the conflagration

Use sand or kitty litter.

WizardOfBoz · Dec 21, 2019

Joe Gwinn said:
If it was intended for construction of balance beams, it will be quite stable dimensionally (no strange phase changes over time or temperature) and most likely will be easy to machine with sharp HSS tools.

From Quora: "Dow metal is an alloy of magnesium. It constitutes more than 85% of magnesium and remaining are aluminum and manganese. ... Dow metal : A brand name applied to any of various magnesium alloys containing more than 85 percent magnesium, characterized by extreme lightness."

Joe, I think that the attractiveness of Mg/Dowmetal for the balance beam application is its low weight. I believe it has about the same coefficient of thermal expansivity as Mg, that is about 70% higher than steel. That coef is nearly as high as Al.

Joe Gwinn · Dec 21, 2019

bosleyjr said:
Joe, I think that the attractiveness of Mg/Dowmetal for the balance beam application is its low weight. I believe it has about the same coefficient of thermal expansivity as Mg, that is about 70% higher than steel. That coef is nearly as high as Al.

Low weight is certainly desirable, but so is dimensional stability. Some metal alloys have complicated crystal phases, which change abruptly over temperature, when phase conversion occurs. I saw a big discussion about this on the design of a high-sensitivity pendulum-based tilt sensors, one that could measure long-term seismic motion of the ground, but no longer recall the details.

Added: It was the Taylor/Hobson units called Talyvel Clinometers.

thermite · Dec 21, 2019

johnoder said:
Do read and take seriously the parts about fire hazard - and the simple ways to circumvent that. Ignore and be prepared to get the scare of your life - a rapidly blossoming brilliant white fire impossible to put out - and making it tenfold worse - in the most spectacular way - if you douse it with water

There are (or were fifty years ago) purpose made dry compounds (think floor dry similar) useful in smothering the conflagration

Might be thinking of the GI-ubiquitous "Purple-K"?

Standard-issue around combat airfields & crash trucks "back in the day". Or our Oxygen & Acetylene plants. Still widely used. Works rather well.

Usta bee, big bare-iron hayrake wheels, tank of the powder, Nitrogen cylinder to heave it a fair distance was our "nuclear option".

ISTR is it sort of ablative?

Purple-K - Wikipedia

Anyway, it can leave an air-excluding crust that's a lot more useful than porous sand, kitty-litter, diatomaceous Earth, or vermiculite.

Even if it pretty well trashes whatever survives of what it is used on, it has saved a LOT of lives and avoided worse disasters, yet.

That said, Magnesium has gotten plenty of NEW attention since it is more recently found all around us, not just in warbirds or wormbirds (VW beetle engines..).

Firefighter Training: Extinguishing Magnesium Fires

Worth preparing to reduce the risk of more "Bad News".

I'd far rather grumble about extinguishers bought and never needed than the other outcome.

The day twenty-two OTHER extinguisher had failed to even slow-down a damaged
Acetylene production plant hopper fire. Until one brave man went right into the fireball with an Axe to get the hopper wreckage clear to pour the Purple-K to it.

Does anyone have any experience with machining Dowmetal?

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