Tolerances and part costs

[cjccmc]

We always throw around the well worn phrase that tighter tolerances increase costs. I can understand that in many respects but what about the cost of inspection fixtures, like shown in the link, that have to be an order of magnitude more accurate than the part itself? On a low quantity purchase is there much cost difference if ask for +/-.002 vs .008 on the dwg if I need to make a fixture that is +/-.0001?

Link

 

[MikeHalloran]

Inspection fixtures are cost effective if you're making 10,000 parts a day.
For low volumes, not so much.

Luckily, sort of, there are CMMs, which are equally costly at low and high volumes.


Mike Halloran
Pembroke Pines, FL, USA

 

[dgallup]

Functional inspection fixtures are only capable of checking relatively large tolerances. If you need really small tolerances then you need some sort of measurement system. CMM is only one of many ways to measure parts.

What tolerance doe the part need to function correctly? That is what goes on the drawing.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[truckandbus]

I've asked the question to a Purchasing Director several times so he could educate us engineers as to how much we were driving part prices higher and higher but never got a response, so I suspect that unless you get into extreme situations, the impact is not that great. Makes me think some of the cost increases are returned in golf outings and vacations....

 

[ctopher]

It depends of the fixture's function. If it sees heat, you have to look at expansion, etc.
Use tol on the part(s) that work in your design, design the fixture tol so it fits.

Chris, CSWA
SolidWorks 13
ctopher's home
SolidWorks Legion

 

[cjccmc]

thanks for the replies

DGallup: I agree that tol should not be any tighter than what is reqd. However, I often find that I can take multiple approaches depending on how tight I can get a feature on the part, and if tighter is not really that much more costly why not ask for it. For an everyday example consider the feel of a well made tool vs. a cheap one. Both can do the job but who doesn't prefer the better made one.

T&B: Same at my company, next to impossible to get feedback as to why some parts which appear to have equal complexity are very different in price. I have seen some get purchased at $X and next round of quotes come back with half or double $X. Maybe depending on how hungry the shops are?

 

[MikeHalloran]

One cost driver that we sometimes forget is the number of piece parts that one can get out of a standard quantity of stock.

E.g. if a supplier can get 90 parts out of a sheet of something, and you order 100, he has to buy two sheets. He may be able to use the remainder for some other project, but you will pay for that material anyway. So it may pay to minimize the size of your parts, or design them to nest neatly, or select your order quantities carefully, coordinating with your supplier and his supplier.

Similarly for bar stock.




Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

Purchasing variability beyond tolerances easily mask the contribution to cost. Unless one is asking for tolerances much smaller or larger than customary for the field, it doesn't seem to matter. Yes - I've had manufacturing kick back a drawing because the tolerance was 'too large.'

What I think is expensive is narrowing the 3 sigma distribution of variation, but that can pay off in lower assembly and warranty costs, which are even harder to track on small quantities.

 

[ajack1]

Tolerances are often less of a factor than they used to be. With so much work now done on CNC machines it really doesn’t matter unless it involves extra processes, if you are putting in a series of holes it wouldn’t cost more to hold a 0.15mm limit than it would a 1mm limit.

As Mike Halloran says how much oversize the original stock has to be or how many parts you get from a sheet/ bar will have far more impact and something like one small radius as an inside corner on a deep pocket will have a major impact.