Question about selecting Stock/Blank Size in part design 2

[Vah1D]

Hi,

I read this wonderful thread today about stock size callouts,

https://www.eng-tips.com/viewthread.cfm?qid=393666,

seems like the consensus was to leave the decision of what blank/stock size to use to machinists, but I still have a question about best nominal size to choose in order to minimize the cost, machining time, etc. I will ask it in form of an example:

Let's say I have the option of designing a 0.5" thick steel part, 3.00"(+/- 0.01") wide or 2.90" (+/- 0.01") wide, i.e. as long as the part is made within that tolerances, the part will function as expected regardless of the size.

My question is: if I go with 3.00", do machinists (in general) go with next stock size available (probably 3-1/2" or 3-1/4"?) and machine down to 3.00" to achieve that tolerance? If yes, does that mean that it's more desirable if I go with 2.90", so they can just use a 3" stock which is more readily available and also cheaper?

Thanks,

 

[TheTick]

When I was a machinist, the general assumption was stock edges needed to be finished unless otherwise noted.

As an engineer, I usually mark when stock edges or faces are permissible. Stock dimensions are usually tolerance per manufacturer spec and labelled as stock dimensions.

 

[chez311]

Vah1D,

To answer that question properly one would have to know exactly what manufacturing process your supplier intends to use as well as the exact specifications of your part.

I think for a standard cold rolled steel bar, if you can get the grade of steel you require in a bar of that width, the standard tolerances for width (ie: size measured by calipers) would probably be within +/-.01" however your form tolerances might not be (ie: banana shaped bow) - or it might be close, depending on the final length of your part as this is typically something specified per unit lenght (straightness per foot). This would be something to get from the material supplier.

If you have some wiggle room in the design which could be provided to manufacturing for cost savings/ease of manufacturing - I think the best thing would be to discuss this with your supplier, they would be the one to know best depending on your requirements and what processes they are going to use or have available and how they planned to buy the material for your job (bar or sheet). There isn't really a one size fits all answer.

 

[Vah1D]

Thanks @TheTick, (in my example, stock edges/faces are not permissible)
so does that me if you were the designer of the part in my question, you would specify 2.90" for width just so the machinist can use 3" stock?

 

[drawoh]

Vah1D,

Your machinist must achieve your tolerances somehow, and your surface finishes.

--
JHG

 

[Vah1D]

@drawoh,

I understand that, maybe I should have mentioned this, I heard this a lot that stock material comes in over-sized by default, so I was thinking maybe it's better just to specify 3.00" and they can achieve the finished size and tolerance even using a 3" (since it's oversized), but wasn't sure if that's always the case? When I check McMaster for example, it specified +/- and not necessarily oversized.

 

[chez311]

Vah1D,

I didn't initially see that stock edges are NOT acceptable on your part. If your part can be 2.900" wide then I would specify it that way, it guarantees that your supplier can use 3.000" stock if that is how they are going to buy their material.

Again form AND size variation will have an effect, if you specify a part 3.000" I don't think I would be comfortable that all edges would clean up on a 3.000" stock especially if your part is relatively long.

 

[chez311]

FYI most steel bar is undersized as far as I can tell.

 

[Vah1D]

Thanks Chez311, that makes sense.

Normally I'm not too worried about it, since I'm in custom design and each part is usually a one-offs, or limited qty's, so probably cost difference won't be as much if they go with 3" vs 3-1/2" stock; but now I'm in a situation that I have to make about 120 qty of a parts (all identical finished width but different configurations) and I thought maybe this is a good time to ask this question.

 

[superptrucker]

Generally, size it to whatever is most ideal for the design and let the machinist determine what stock to buy. They might buy extra size for reasons besides clean-up anyway. Maybe they already buy a lot of 3-1/2" stock for another part and it would be easier to just order more of that size even if you made your part 2.9". Maybe they need extra to hold it in a vice. Best thing to do if you're really concerned about this is to just talk with the machinist.

 

[drawoh]

Vah1D,

Here is Joe Osborn's excellent reference on Tips on Designing Cost Effective Machined Parts.

Good parts start with good prints
We pay attention to what you say and don’t say. We have to sell parts!

The emphasis is mine.



--
JHG

 

[mfgenggear]

Vah1d
a lot has to do with the condition of the stock vs the finished part. and if there are processes involved such as heat treat, forming, and how the material has to be tool for machining or forming.
in this if it is straight machining and no post operations, there has to be enough stock to true up. it has been my experience to just give the final blue print dimensions, what is exactly required,
and let the shop decide the stock size after it is analyzed for tooling purposes. and every shop is tooled, and have different types of machines. so stock size could change.
example the shop could request 3 ft or 6 ft lengths x 1/2 thick by 3.00 in wide stock and machine and part off. it really depends on the person processing the part.

 

[powerhound]

To add one more iron to the fire, keep in mind that when you identify a size as STK, general rule #1 does not apply to it. This may not make a big difference in your design but it's always good to be aware of unintended consequences.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[Vah1D]

Thanks everyone for their helpful comments (and Drawoh for the link to Osborn's dfm guideline), I have a much better understanding now.

 

[3DDave]

General rule #1 does not apply to variations in stock dimensions, but that is because the material specification usually does control that or similar characteristics. Thickness, width, taper, flatness, twist, bow, surface roughness, etc., are typically controlled. Unfortunately, many of these vary depending on the mill as-produced size. The thickness tolerance is typically larger for material rolled in greater widths, so material that is sawn out may have significant variation of the variation in thickness depending on the width of the original plate.

Similarly, material properties can vary significantly depending on the original mill size. This is one origin of problems where thick material is milled to a thinner condition, exposing unbalanced internal stresses to produce unexpected warping.

 

[emorrison]

Stock sizes are covered under various standards... if they are suitable, without machining, then tolerance them as such or simply label them as STK, etc.. If they are not, then you should refine the dimension and form such that the surface is machined to specification. Short answer... yes, if you want the surface machined, make sure you are outside the standard in terms of size and form, so that you can ensure a clean-up on those surfaces.

 

[ctopher]

I never call out stock size anywhere. Leave it up to the machine shop to order/use what they need.

ctopher, CSWP
SolidWorks '17
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