Difference between ASTM and AMS bar (machinability and tolerances)

[Kasey7]

Hi,
I work in a machine shop that manufactures engine components for various companies. We're often dealing with tolerances to <.0005", and there seems to be quite a difference when machining material supplied by our clients (usually AMS barstock), and material we buy ourselves (usually ASTM barstock).
Could someone explain/confirm the differences between ASTM barstock and AMS (both aluminum and SST), or even better point me to somewhere on the web that would offer an explanation?

I've been told, (from various sources that, again, I'm unable to confirm), the following:
• ASTM bar is extruded, AMS is extruded/drawn.
• A final product made from either ASTM or AMS bar offers identical properties (strength, corrosion resistance, etc.) to the customer.
• AMS bar, though more expensive (?), is better suited for precision machining.
• AMS is less likely to distort during the machining process (less internal stresses during the mfg of the bar?)
• AMS turns beautifully, ASTM seems gummy and often the surface appears to have fur on it when roughing.
• ASTM seem more prone to vibration when finishing.
• AMS barstock came into being primarily to satisfy the needs of the aerospace industry.

I've been told that alot of the problems I have had with turning some barstock we use is in my head, or the coolant oil/water ratio is wrong. Is there really no difference, where machinability is concerned, between ASTM versus AMS?

 

[redpicker]

About the only thing that is correct about "what you've been told" is that "AMS barstock came into being primarily to satisfy the needs of the aerospace industry". This is because AMS = Aerospace Material Specification, so by definition, this is correct. The rest of what is in your list are generalizations which may be true, false, or irrelevant, depending on exactly which ASTM or AMS specification you are referring to.

In general, yes, many AMS spec steel grades have tighter quality requirements than many ASTM spec steel grades, but that is about all you can say. There are no direct equivalencies between specificaitons from the two societies (there would be no need for two if there was). Basically, you have to know what it is you want to be able to determine if the specification you are using is going to be suitable. There is overlap between the two, however, and a lot of material can be certified to either, which makes much in your list somewhat contradictory.

rp

 

[btrueblood]

Most of what the OP lists could be more due to the particular cold work history and/or heat treatment history of the products (which is to some extent dependent on which mill processed it), and little or nothing to do with which specification was used in ordering the material.

Some generalizations that can be made:

Many AMS don't control physical properties of the materials, just the chemistry. If you are concerned about (for instance) the yield strength of the AMS bar you order, you specify testing of the lot/heat/batch to some level to verify strength.

ASTM standards typically don't control the chemistry as tightly as AMS standards, but there is usually an ASTM standard that covers your particular desired bar which includes strength testing automatically. YMMV, read the spec. to know for sure.

 

[EdStainless]

The improved machining of the AMS material is likely due to the cold draw process. This results in bars with more uniform dimensions and properties.
This is no news to you, but often a rough or irregular surface will leave chatter or variations in the machined finish even after multiple passes.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Kasey7]

So just to be clear, my question primarily concerned the realitive machinability of each spec. From btrueblod and EdStainless comments, AMS offers improved machinability due to it being drawn. Right? (Again, I wasn't sure if AMS was drawn to begin with; although perhaps AMS being cold drawn is optional). I would imagine drawn bar would have less internal stress as well.
Also, if AMS barstock is drawn, that should be the primary reason why AMS is more expensive if everything else about the two specs are equal.

In any event, finally learning some of the correct terminology has lead me to the following helpful links:
•

http://www.precisionkidd.com/pdf/DrawingProcess.pdf

•

http://www.productionmachining.com/articles/important-machining-factors-of-carbon-steels

So in a nutshell, it is the cold drawing process that improves machinability by work hardening the bar, reducing ductility, and reducing internal stresses.

 

[TVP]

Kasey,

One thing to keep in mind is that cold-drawn bars can be ordered using ASTM standards. In fact, the quantity of cold-drawn steel ordered to ASTM standards exceeds that of SAE AMS standards by a factor of 100 or more. I am not as familiar with quantities for aluminum, but it is probably similar. Here are a couple of links that show equivalent ASTM and SAE AMS standards, meaning the same alloy processed in the same way (cold-drawn vs. extruded), not necessarily every tolerance, etc.

http://www.amcastle.com/pdfs/Alum_CF_R_QG.pdf

http://www.kaiseraluminum.com/customers/products/extrusions/bar/

 

[Wrenchbender]

Kasey, Agree with other posters regarding cold-working as an explanation to your 2nd, 3rd, 4th and maybe 5th bullets. A material condition offering less tensile elongation generally makes for better chip formation during machining, so I would say 'yes' to your 29 Feb 07:08 post.

 

[Kasey7]

Well, this has been a useful post for me. Though sadly, it seems the barstock Ø for cold working maxes out at Ø8". Probably a third of the stuff we work with is between Ø9-15".
I guess the next thing to consider is forging versus barstock, as forging is probably considered another form of cold working.

Some more terms I came across during this informal investigation are isotropic and anisotropic. Cold working a bar makes its crystaline structure much more anisotropic, as does forging. Besides inproved strength, metals that are more anisotropic than their isotropic counterparts (hot rolled bars, castings, etc) have better machining characteristics.

 

[btrueblood]

"forging is probably considered another form of cold working. "

Forging by definition is hot working; i.e. some degree of annealing/stress relief is occurring simultaneously with deformation, and the material doesn't work harden as much. But yes, do check forged material as it may machine better for you.

"metals that are more anisotropic than their isotropic counterparts (hot rolled bars, castings, etc) have better machining characteristics. "

Well...yes, generally because you cut across the grains (i.e. the machining/cutting direction generally proceeds perpendicular to the long axis). But also, the anisotropic materials are worked materials and thus have less ductility, so form smaller chips or at least shorter curls.

I do like Ed's explanation of a smoother starting surface helping to reduce chatter/vibration.

 

[Kasey7]

"Forging by definition is hot working" That's certainly true enough. I was comparing forging to casting; red hot versus liquid metal. Forging is certainly not "cold". :~/
I guess I was refering to the type of deformation taking place. I think cold working and forging both involve some type plastic deformation, whereas casting does not.

 

[Wrenchbender]

Not that it matters much, but large diameter bar stock, in the size to which you refer, whether steel or Al, is already forged.

At the melt shop molten metal is poured from the furnace into molds; after freezing the ingots are typically sent to the forging shop or rolling mill (because a 10 ton ingot is not very useful for making anything). Ingots are reheated and wrought (e.g., reduced in section by hot-rolling or open die forging) to the rough shape ordered. The hot working eliminates coarse grain structure and chemical segregation (unmixing) from the ingot. Additional heat treatment can follow, with turning or rough grinding to the size ordered by the customer.

Somewhere in there, an optional cold rolling after hot, can impart a better surface finish and mech properties, but I didn't think cold rolling was usually done in your size ranges.