Machine design material selection

[cntrlnr]

I am in the process of designing a machine. I wish to drill and tap the frame to receive screws. I plan on using SAE grade 5 fasteners and need to select a suitable steel for the base material. In reviewing the ASTM specs I spotted A284 as a possible good material (steel, weldable, for machine design) spec. Having a significantly lower yield strength than the grade 5 fasteners, it adheres to the rule of thumb for nuts vs bolts (soft and ductile vs strong and tough). Without having to go through strength of male and female thread calcs is it reasonable to use this material?

cntrlnr

 

[ERT]

I think that you will find that ASTM A36 is far more commonly used for welded steel frames. ASTM A36 is available in plate, bar, angle, and C channel. For round and rectangular tube (HSS) a typical specification is ASTM A500 Grade B.

ERT

http://www.akeng.com

 

[diamondjim]

Looking up the mechanical properties of
ASTM A284 listed some high tensile and
yield properties for this material?
A36 only has 36000 psi yield strength.

 

[metengr]

I believe there is more to consider in machine frame design than simply using your rule of thumb mentioned above. The selection of the steel needs to based on some minimum yield and tensile strength requirements, minimum ductility requirement for forming, toughness requirement for dynamic loads in service, and dampening capacity for vibration.

So, the selection of one or more suitable steels needs to be based on design use of the machine versus your statement regarding fasteners.

 

[dvd]

Deflection of members is another important factor to consider in designing machine frames. Steel generally has the same deflection for a given load regardless of the strength properties because of Young's modulus. So I vote for the A36 if machine frame deformation is the controlling factor.

 

[ERT]

1. Yes dvd, good point - many machine frames are limited by deflection (and therefore vibration) characteristics. If so, the stresses are relatively low, so a high strength steel is not required or beneficial.

2. Dampening is minimal within a welded steel frame. If this is the key objective, consider using a cast iron frame if you have the product volume to justify it. It is advisable to increase fastener engagement depths when tapping cast iron.

3. Regarding ASTM A284, or the A283 standard which has replaced it - can anyone confirm diamondjim's comment regarding it being higher strength than A36?

ERT

 

[Philrock]

I agree with metengr about using more than just rule of thumb. Your material selection may indeed depend on thread strength. If you are limited as to thread depth, you will have to use a material that will give you adequate strength threads for the depth you can go to.

 

[cntrlnr]

Thank you to all who responded. I clearly did not provide sufficient supporting information with my request however the scope of the responses has provided me with the sense of direction I was lacking.

cntrlnr

 

[unclesyd]

Something else to cloud the mind.
You might want to look at this company for all the machine base, parts of, or the material for. I've found this to be one of the least expensive approaches to fabricating machines.

http://www.americangrinding.com/

 

[rorschach]

My data for Astm A283 grade C shows 55-65ksi UTS min and 30KSI Yeild min with 22% elongation

Grade D gives a Min UTS of 60-72ksi and a min Yeild of 33 ksi and 20% elongation

A36 has a min UTS of 58-80 and Min Yeild of 36 and elongation of 20%

Diamondjim, in my world, all of this is junk iron. you can't hardly GET steel weaker than this.

If you need strength, weldability, and availability, look at ASTM A572 GR 50 (50 KSI Yeild) or GR 70 (70 Ksi yeild)

Gr50 can be welded using the same weld procedures as A36, Gr 70 is a bit more demanding but only slightly.

 

[bklauba]

One Q that probably should be addressed is: Do you need to stress relieve the frame prior to machining, so as to maintain the functional dimensional accuracy and stability required?

If you don't need much stability or accuracy, perhaps because your design allows adjustment or it is just not needed, great. If you need to stress relieve, then your choice of material should be integrated with that decision.

BK

 

[WarrenPowers]

What aluminum alloy is the least porous?
I am designing an o-ring-plate-capped, thick-wall extrusion that will hold 10-20 PSI Nitrogen over 10 years. I know aluminum is a lot like "Swiss-cheese", can you recommend a place to find physical characteristics of various types of aluminum alloys? Possibly recommend a surface treatment to seal aluminum? The right thing to do is use stainless steel for this product, but I have machined a lot of stainless and I just would rather work with aluminum if possible.
Warren Powers

 

[bklauba]

Is this a new Q?? (sure looks like one)

BK

 

[WarrenPowers]

I should have placed my question in a different location.
Sorry.
Warren Powers