Designing/Drafting - Which Material?? 3

[Matt_Eng]

Hi Eng-Tips Users,

We have had some difference of opinions in the office and i would like to put a question of 2 parts to you guys.

If/when you are/were designing a piece of equipment, how do you select the materials to use? Do you have any hard and fast rules? There are some obvious examples, such as something which requires extremely high tensile strength or has is very corrosive resistant etc.. but what we design here is general automation type equipment and alot of the pieces do not require specialised properties.

Second part of the question...for the same parts that do not require specialised properties, how do you call out the materials on your drawings? I know as engineers we like to be specific, but this can be limiting to the manufacturer. Do you call out a class of materials, or write a specific material and add "or equivalent" or do you approach this in a different way?

Thanks

 

[hydtools]

You are the designer, you get to choose. It would be wise to ask the manufacturer about their common practices. Specifying results in a consistent product. Saying "make it out of something soft" will get you variable results.

Ted

 

[JStephen]

More in the tank/vessel fabricating end of things here-
But in general, the material is specified by calling out ASTM (or equivalent SA) numbers. We're using the same materials all the time, so we know what's available in what material. If it's something new, we'll be working with a vendor or our purchasing guys to make sure what we're showing is something that actually exists and is workable.
Minor purchased parts, we might call out the McMaster-Carr stock number. If it's something that the local hardware store likely carries, we'd put the "or equal" after that.

 

[Jboggs]

I try to give the fabricator/manufacturer as much leeway as possible, within functional requirements such as temperature, strength, machining or welding concerns, corrosion resistance, etc. Most of what I do isn't governed by some overall code or corporate policy, and quantities are very low, so its not uncommon for me to just specify something like "low carbon steel", or hot rolled plate ("HRS"), or cold rolled bar ("CRS"). That way the shops might be able to use raw material they already have, save me some money, and still produce a functional part.

 

[handleman]

There are actually quite a few factors to consider here...

Availability. You may be able to spec a theoretically lower cost material from a performance standpoint. However, for automation equipment, quantities are generally in the single digits. Therefore sourcing an inexpensive but rarely stocked material can actually cost you more (both in money and in delivery time) than specifying a higher cost, yet commonly available material.

Machining vs material: If you specify inexpensive material, you may end up losing any savings by spending more on machining if the stuff is more difficult to process, or has residual stresses, etc. Raw material cost is typically a small portion of the cost for low-quantity custom machined parts.

Required processing: As you design things, consider stock material sizes, and which surfaces of your parts require machining vs. could be left un-machined. Also, sometimes it's less expensive to purchase stock that is already precision ground etc. Especially if your machine shop is internal to the company, it may be beneficial to understand what sort of suppliers and materials that you can purchase already-machined plates/blocks/etc. that significantly reduce fabrication time. It's generally much less expensive to purchase a 100mm x 100mm x 15mm precision blank than for your shop to cut one from a plate, square up everything, and then put your holes/features/etc in it.

Corrosion resistance: If this is required, it may be less expensive to spec more expensive material if it means you can avoid post-processing like painting/plating etc.

Welding vs machining: When you have larger or more odd-shaped parts, sometimes it's better to weld, sometimes machine from billet. Fabrication method makes a difference in which material works.

Most of these considerations actually have very little to do with the final performance of the part.

 

[BEMPE16524]

doesn't matter what type of materials, i will prefer something that is easy to machine or form and cost effective and meet the requirement.

R.Efendy

 

[BrianPetersen]

It is up to you as the designer to take all of the applicable considerations into account, including required performance of the end product, and cost to manufacture of the complete assembly (not just each piece), and the cost to manufacture depends on the cost and availability of the specified material, and what it's going to take to manufacture the part ... including considerations for what can be manufactured in-house versus what has to be sent out to a specialist shop ... and don't forget that there are times that a specialist shop could make something for less than what it would cost in-house, e.g. if it's something that the specialist shop deals with every day and your in-house shop does not.

Walk down to your shop foreman with a sketch. "We need something that does this. What do you think would be the best way to build it?". Ask the people who are actually going to build whatever it is that you are designing. Feel free to do this frequently. "Do we have the equipment to make this?" "Do you see any ways we could simplify this?"

I realise that this might not be possible in every shop ... only in the good ones.

 

[CWB1]

Your purchasing dept or process engineers should be providing a breakdown of material and process costs for each commodity type annually to help guide you. For castings that may simply be a list showing cost/lb for common materials. Others like machined parts may really be two lists, cost/lb for material and cost/hr for different machines/processes. Challenge anything that doesnt make sense but keep an open mind and let the data drive your decision making. At one previous employer grey iron was dirt cheap bc we poured our own and aluminum castings expensive to purchase, at another aluminum castings were 75% of our business and the use of iron expensive comparatively. Don't guess, engineer.

 

[ctopher]

Do you call out a class of materials, or write a specific material and add "or equivalent" or do you approach this in a different way?

If you don't care about the material, and does not make a difference in the design, OK.
But, I would never call out "or equivalent". If you leave it up to the vendor to use whatever they feel is "equivalent", you will not have consistent parts (especially if from China).
Call out a specific material, and it's spec.

ctopher, CSWP
SolidWorks '17
ctophers home
SolidWorks Legion

 

[BrianPetersen]

"Or equivalent" is open to interpretation. If you want to add some flexibility then add "... subject to written approval" or something of that sort. "Equivalent" in someone else's eyes might not be equivalent in yours.

Many moons ago, we (automation builder) had an issue because of liberties associated with "or equivalent". A particular timing belt drive was going to be operating in an environment subject to being exposed to contamination. We (engineering) specified a material that in our experience would withstand the environment. Purchasing looked at the size of the belt and the number of teeth and without telling us, substituted the cheapest one they could find that was the right size, tooth profile, and number of teeth, but paid no attention to the material. We kept getting warranty claims. Finally they sent young me out with the field service tech to see if I could find the problem. I did, the moment they took the cover off. Plain ordinary fabric reinforced rubber belt (wrong) - soaked in oil (which we knew was going to happen). Went back to the shop and read purchasing the riot act. Of course, their order to substitute the cheapest alternative had come from management, so I got to chew my boss out, too. (Yes, I kept my job. I've never found doing the right thing, to be the wrong thing to do. If it is, you don't want to be working there anyhow.)

 

[Matt_Eng]

Guys,

Thank you for your responses. Much appreciated. Has certainly gave me food for thought and some solid advice.

Have a good weekend!

Matt

 

[HS_PA_EIT]

Ashby diagrams are handy for early stage material selection. They chart roughly where various materials fall in terms of different material properties.

 

[pylfrm]

Second part of the question...for the same parts that do not require specialised properties, how do you call out the materials on your drawings? I know as engineers we like to be specific, but this can be limiting to the manufacturer. Do you call out a class of materials, or write a specific material and add "or equivalent" or do you approach this in a different way?

Provide a list of acceptable material specifications. For some parts the list might have only one item, and for others it might be rather long. Balance the cost and risk of adding options against the potential savings in manufacturing.

If you find yourself repeating the same few lists on lots of drawings, consider moving them to a separate document that can be referenced by the drawings.


pylfrm

 

[verymadmac]

The current stock holding of stores is always a good place to look, when you want to narrow down you range of options for material and hardware selections.

 

[Tmoose]

For many of MY projects the first choice of materials is what I have pack-rattd from beside the road on trash day.

 

[Tim Moss]

Hi

It all depends on the function of the part. For instance, if strength is required then you need to choose a grade of material which offers this property.

The next factor to consider is suitability for manufacture. A Metal turning company would need the part to be from something available in bar turning grade. See typical bar turning grade materials machined.

For basic parts I would go with the most cost-effective option in regard to material choice. Factors like availability and cost are critical.

Regards

Tim