How to persuade engineers to abandon silly tolerances?
How to persuade engineers to abandon silly tolerances?
(OP)
I'm the Senior Manufacturing Engineer for a mid-size electronics manufacturer, and our Mechanical Design Engineers have recently started to put GD&T tolerances in all sorts of strange places:
- 0.02" perpendicularity specs on PEM studs up to 1" long.
- 0.005"-0.015" flatness specs on machined 14" x 10.5" aluminum bezels.
- 0.010" parallelism specs between the front and rear faces of a bezel.
Now, these bezels are assembled to a sheet-metal PCB mounting plate and enclosure, and the entire thing mounts into a console. We have never had problems with poor flatness on our parts, and there is simply no reason to ask our machine shops to hold parallelism with a surface that is effectively just waving in the air. As for the PEM stud perpendicularity spec, I am told that this is to allow us to reject parts if they are damaged in shipping, and to ensure proper packing by our vendors!
Now, I used to be a Mechanical Design Engineer not so long ago, so I didn't come down in the last shower. But the trouble is that these engineers are largely insulated from the cost implications of these specs. Plus, they're quite jealous of their territory, and it's notoriously difficult to persuade them to change their drawings. So what I think I need to do is demonstrate in a pretty unambiguous way the cost impacts of adding these kind of superfluous tolerances. Are there any good sources for analyzing the cost impacts of tighter tolerances, or is it really just a matter of asking our vendors for case-by-case estimates?
- 0.02" perpendicularity specs on PEM studs up to 1" long.
- 0.005"-0.015" flatness specs on machined 14" x 10.5" aluminum bezels.
- 0.010" parallelism specs between the front and rear faces of a bezel.
Now, these bezels are assembled to a sheet-metal PCB mounting plate and enclosure, and the entire thing mounts into a console. We have never had problems with poor flatness on our parts, and there is simply no reason to ask our machine shops to hold parallelism with a surface that is effectively just waving in the air. As for the PEM stud perpendicularity spec, I am told that this is to allow us to reject parts if they are damaged in shipping, and to ensure proper packing by our vendors!
Now, I used to be a Mechanical Design Engineer not so long ago, so I didn't come down in the last shower. But the trouble is that these engineers are largely insulated from the cost implications of these specs. Plus, they're quite jealous of their territory, and it's notoriously difficult to persuade them to change their drawings. So what I think I need to do is demonstrate in a pretty unambiguous way the cost impacts of adding these kind of superfluous tolerances. Are there any good sources for analyzing the cost impacts of tighter tolerances, or is it really just a matter of asking our vendors for case-by-case estimates?





RE: How to persuade engineers to abandon silly tolerances?
Here's a clue.
If you have a 'Buyer' dealing with your vendors, maybe you can have a good, friendly, cordial relationship with them. Understand that the relationship is inherently adversarial, but can also be collegial and cooperative.
If you have a 'Supply Chain Manager' dealing with your vendors, then your relationship with your vendors will be evolving from adversarial to openly hostile as each of them is systematically screwed over. In this case, you will probably need even more tolerances, because your vendors will be trying to ship you garbage and your inspection will be trying to sort it out.
Mike Halloran
Pembroke Pines, FL, USA
RE: How to persuade engineers to abandon silly tolerances?
No, I haven't. Nor have our engineers attempted to justify them to me. But I was the original designer of the precursors to these enclosures in a previous stint with the company, and since they have changed remarkably little over several years I can state with confidence that there are "no expensive instances at the root of each one." For instance, what justifies a 0.010" parallelism spec on a display bezel? Why is it so important that the cosmetic, non-functional front face of a console-mounted display is everywhere within 0.010" of the rear? Heat dissipation :)? As I wrote above, the only justification for the perpendicularity spec on the PEM studs is that they might get bent in shipping. But this is using a sledgehammer to swat a fly--incurring an additional fabrication and inspection expense on each unit in order to deal with a hypothetical problem; that is, a vendor refusing to take responsibility for damage resulting from poor packaging.
We have a Buyer dealing with our vendors, yes, but I am the main technical point of contact, and our relationships are generally friendly, especially with those vendors I have known for several years. I could certainly ask them for estimates, but I'd like to save them the hassle if there is a good reference available.
RE: How to persuade engineers to abandon silly tolerances?
If they can't, they are probably only trying to show their ego's.
Usually when I see ".010" and ".005" typical all over the designs/drawings, it's a red flag of ignorance.
Chris
SolidWorks 11
ctopher's home
SolidWorks Legion
RE: How to persuade engineers to abandon silly tolerances?
Nope. These particular GD&T tolerances don't stack: Flatness specs on machined parts that mate with sheet-metal parts that do not have matching flatness specs, parallelism specs on surfaces that don't mate to anything besides air, and perpendicularity specs on mild-steel PEM studs (just increase the hole size!).
I'm on the review list for the drawings, and I could just be a jerk and reject them until they either stop doing it or give me an ironclad justification. Maybe it'll come to that, but for now I'm trying for a gentler and more persuasive approach.
RE: How to persuade engineers to abandon silly tolerances?
Clearly demonstrates that there is widespread lack of understanding of the purpose of tolerances within the broad engineering and design community.
RE: How to persuade engineers to abandon silly tolerances?
Can you institute some sort of "work sharing" scenario where some of the engineers that cause the problem have personal face-to-face encounters with the costly results? Can they be involved in the interface with vendors? Can someone show them that "additional specifications = additional costs"?
Another idea I have used to great benefit in the past is this: identify the offender that is the most respected by his peers. Take him/her aside and ask for his/her help. Tell them that you have seen a great benefit from the use of GD&T, and give concrete examples that you really believe in. Tell him that you also are afraid that its value is being harmed and that's where you need his help. Compliment him. Tell him you are approaching him about this because you have seen how the others respect him and will follow his lead. You might tell him you repsect his professionalism and judgment.
Explain how overuse of any tool wears it out. It becomes useless and you don't want that to happen to GD&T because it is too important. Explain how when the symbols appear all over a drawing its hard for the regular guy on the shop floor to know which ones are really important. The end result is either a part that does not meet the need, or one that costs much more than it should because of all the unnecessary inspection time. That means that not all GD&T symbols were created equal. Ask him if would agree that there are examples of GD&T being overused in some cases, and that overuse in effect devalues it. Ask him what his suggestions are to help make sure that the GD&T axe remains sharp. Ask him if he could be your go-to guy to help the guys in the shop using the drawings and demonstrate to them how to inspect the parts for all the GD&T symbols. That will bring him face-to-face with the issue. Ask him if he thinks he could help keep the GD&T tool sharp for the benefit of the company.
His subtle change in priorities can spread through the whole group. I've seen this work more than once.
RE: How to persuade engineers to abandon silly tolerances?
For instance, I've seen perpendicularity specs on holes through very thin parts. Most of us would say, "Who the heck cares about perpendicularity in a 2 mm metal plate; it is negligible!" But suppose that hole is a datum feature -- we want to relate it back to the primary datum, which is most likely the face of the metal plate. This also ensures that any gaging or fixtures will have everything tied together.
I don't know if this is justification for the items given in the OP, but it's something to keep in mind.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: How to persuade engineers to abandon silly tolerances?
Even I faced the same problem, we recently introduced GD&T in our organization. When we need to relate the datum features in the part, mainly while qualifying the datum features usually with form controls. There were many questions raised form sourcing about the introduction of new geometrical controls on the part. Even suppliers quoted more money for the same parts.
Assume a case of a rectangular block with only size dimensions. The same part is now with some datum features, flatness tolerance and some perpendicularity tolerance to relate the datum features etc., the supplier says that now he wants to measure all these new geometrical symbols which are put on the part which causes extra money for it. By introduction of GD&T don’t you think measurement cost will increase to some extent?.
Also while qualifying the datum features, to what tolerance it should be? One of the GD&T teacher suggested that the datum features to be qualified with the flatness or appropriate geometric characteristics to 10% of the desired tolerance of the features we are controlling? Is that true?
RE: How to persuade engineers to abandon silly tolerances?
You do realize you are asking people who apply tolerances for a living?
RE: How to persuade engineers to abandon silly tolerances?
Engineers need to understand that every tolerance on a print adds cost to a part. That cost is exponentially proportional to the tightness of the tolerance.
Also, there needs t be understanding of the tolerances that are possible with a given process. To simply mandate a tighter tolerance and demand that manufacturing "be more careful" is not enough. Also, manufacturing needs to provide these process tolerances.
Like most people, engineers are pressed for time and will copy specs without really thinking about them. Sometimes (like w/ PEM studs), these tolerances may come from the manufacturer of the purchased part.
In any case, the beginning of the conversation should be an objective examination of the need for the tolerance and how it got there.
RE: How to persuade engineers to abandon silly tolerances?
Haha, yes. I was one of those people until a relatively short time ago, and I may well be one of them again one day. I understand the point of tolerances and wholly support their prudent use, but friends don't let friends apply pointless tolerances that only serve to drive up cost without adding value to the product.
Oh, an anecdote that halfway answers my own question. When we got quotes for a prototype run on one of these parts, the price came in shockingly high. When I asked our vendor what was driving the cost up, he cited these GD&T tolerances. I requested authorization from the project lead to waive those tolerances for the prototypes, and the price dropped by 45%. Of course, the parts were perfectly fine; as I mentioned, we've been making variants of this enclosure since 2004 without these tolerances. OK, point proved, I thought, only to see the exact same GD&T tolerances turn up on the next drawing revision.
Hence my inquiry about the availability of estimating tools, so I can make the point in a more general fashion. Our vendor has also suggested we switch from 6061 to MIC 6 for these parts specifically to cope with these GD&T specs (since it has very good flatness and parallelism), but I am reluctant to incur the increased material costs just so we can continue to use pointless tolerances.
RE: How to persuade engineers to abandon silly tolerances?
PEM, for their part, claims a 0.5° standard perpendicularity tolerance on their studs and standoffs, and a 2° worst-case tolerance.
RE: How to persuade engineers to abandon silly tolerances?
So the bottom line is that GD&T is a language allowing you to control certain characteristics based on the function of the part. If you think the GD&T is making the cost go up, your design engineers need to evaluate the GD&T numbers, not chuck everything out the window.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: How to persuade engineers to abandon silly tolerances?
Right, of course--as you may gather, I understand that objects retain qualities of flatness, perpendicularity and parallelism, to a greater or lesser degree, even when we don't explicitly specify them using GD&T callouts. And, as I've already stated, I think GD&T is a wonderful tool when used appropriately--I have used it many times myself as a designer.
But these are parts that were manufactured for years without these GD&T specs, sourced from around seven different vendors over that time, with the price remaining roughly consistent, all else equal. I assembled some of the early units myself when I was the designer, and they did not present any issues of this nature ... nor did any of the subsequent units, to the best of my knowledge. After all, these are electronics enclosures made mostly of sheet metal, made to fit into cutouts in sheet-metal consoles or into 19" electronic racks--they do not generally have stringent fit requirements or tight assembly tolerances. Then all of a sudden, the mechanical designers started adding GD&T tolerances to the machined parts (already our most expensive metalwork by far), and behold! The price went up! I just drew the natural conclusion.
RE: How to persuade engineers to abandon silly tolerances?
My hesitation: If a drawing gets hauled into a courtroom, we have to be sure that all the possible characteristics are adequately addressed.
Here's an example -- I was helping design a flat pipe flange once. The outline (perimeter) of the flange was a triangular shape, but the shape wasn't critical because nothing touched that perimeter; it just saw air. When I tried to put a GD&T profile tolerance all around that perimeter, there was great resistance from the others: "That's not a critical feature." "Now I have something else to inspect." Etc.
But I pointed out that if we didn't use the profile tolerance, then the triangular shape still had to be toleranced somehow, and the plus/minus method would lack many of the advantages of GD&T. So I showed them how a profile tolerance could still be used, but the tolerance number we agreed upon was something like 3 mm! Not expensive at all.
In summary, you might have a point about things like flatness and parallelism, because those characteristics were already implied via the size (unless it's a non-rigid part). But I'm just cautioning against thinking that GD&T is only to be used on critical features.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: How to persuade engineers to abandon silly tolerances?
At another company, we took a standard drawing and sent it out for a quote. We then took that same drawing and retoleranced it to reflect the machining processes used. The revised quote was 30% less for basically the same part. These were molds for glass ware, so we had also looked at the final product cesign and its tolerancing. When we went to the mold designers to ask them to loosen the tolerances, they bulked and would only loosen about 20% of those that we asked them about changing.
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RE: How to persuade engineers to abandon silly tolerances?
Powerhound, GDTP S-0731
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RE: How to persuade engineers to abandon silly tolerances?
No, I think this is just the common engineering practice of adding specs because it doesn't cost the engineer anything to do so, plus it adds a little of that "real engineering" flavour to a job that is, in reality, pretty unchallenging--designing one sheet metal box after another. I did that job for 31 months and found it grindingly tedious after a short while, jumped at the first proper mechanical design job that came my way, then came back to the company four years later as a Manufacturing Engineer.
RE: How to persuade engineers to abandon silly tolerances?
You won't likely change the engineers' minds; you have to find and re-educate whichever Heavy Hitter got talked into it.
Mike Halloran
Pembroke Pines, FL, USA
RE: How to persuade engineers to abandon silly tolerances?
Mike Halloran
Pembroke Pines, FL, USA
RE: How to persuade engineers to abandon silly tolerances?
You won't likely change the engineers' minds; you have to find and re-educate whichever Heavy Hitter got talked into it.
I'd never thought of it that way, but "tin shop" just about covers it. The parts in question are machined bezels, but everything else is sheet, and nothing is high-precision. It's actually a little painful: I designed this stuff when I was still pretty junior and had no experience in sheet metal, so it makes me wince to see that these frankly rather poor designs just keep getting repeated over and over with slight variances.
These two guys are the only mechanical design engineers in the company (the other mechanical engineers are in Project Management or Manufacturing). The "heavy hitters" supervising them are mild-mannered electrical and software engineers who don't tend to question mechanical design decisions, since all the really critical and interesting work is in the electronics. If their equipment sits in a box that meets all the client's requirements, they're pretty happy. On the other hand, now that I've demonstrated the cost implications of these new specs, they'll be more likely to ask questions the next time the drawings come round.
RE: How to persuade engineers to abandon silly tolerances?
Good question. Is parallelism of .250 too much? .500? Flatness of .375? For certain, most suppliers hate having rejectable characteristics added to drawings, so they jack the price. Or they don't know what it means, so they jack the price. Pick a value at which you'd throw the part out and give the engineers a hint. Also, start looking for suppliers that aren't either untrustworthy or ignorant.
RE: How to persuade engineers to abandon silly tolerances?
Made fine for X period...
Then, one day some parts don't fit, but they're to the letter of the print.
If they've done the calculations to justify the tolerances (seems unlikely on the bezel paralellism) then are they really wrong to specify it? Or is your view point that they aren't taking into account the slightly flexible nature of the parts - or that the shop floor guys are used to parts not quite fitting so they'll "Hammer to shape, File to fit, Paint to match"? Or is it just that you think it's unlikely to be a problem?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: How to persuade engineers to abandon silly tolerances?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: How to persuade engineers to abandon silly tolerances?
One would be to have a parts quoted with tight tolerances and the same part quotes with open tolerances. Procnc.com has a tech newsletter from September 2010 that goes through the cost benefit of optimizing tolerance call outs. Quantifying the savings goes a long way toward inspiring people to change their habits. Setting the part cost as a design goal should also be a motivator.
The other approach is to have workshops with the vendor and the design team to go over the limitations of the vendor's tooling. This usually comes across as the design group coming to the rescue of the poor vendor rather than a critisism of the designer :P My company has avoided countless wholesale redesigns for 'cost reductions' simply by letting the vendor do what they are capable of instead of pushing them to meet fictional requirements
RE: How to persuade engineers to abandon silly tolerances?
RE: How to persuade engineers to abandon silly tolerances?
An example would be the face bezel you are talking about. Does it need to be relatively flat? Yeah sure, cosmetically, it does. Does it need to have flatness and parallel tolerances? Yeah maybe, to an extent. Do they need to be within a few thousandths? No, likely not.
I would say, bring a few of these designers into the production pits and show them how their designs are used, put together and functioning, how they are made, what tools are used, the capabilities of the machine. Its likely that they have had very little experience with these things. Maybe it would simply take SHOWING them that the face bezel is just there for looks and doesn't mate with anything that requires it to be so flat. Just a thought.
RE: How to persuade engineers to abandon silly tolerances?
“…It looked as if some of the tolerances were assigned much closer than should be necessary, and I started to try to find out how they had been fixed. To accomplish this, on every occasion when I met an engineer I asked him how he decided the tolerances in his branch of the subject; I fear I bored a great many people at this time. I got a variety of answers which sometimes explained things a bit, but often not at all, and though I discussed it with quite a number of men, many of them occupying prominent places in different branches of the profession, I came away with the impression that scarcely any of them were really interested in the subject of tolerances. To exaggerate the picture which I got as the result of my inquiry, I concluded that in designing a new machine the chief engineer drew it freehand with dimensions to the nearest inch, and sent it to the draughtsman to work out the detail to the nearest thousandth, who then gave it to his junior assistant to mark in the tolerances. Instructions were certainly always given that tolerances should be as easy as possible, but only lip service was done to them, and the junior assistant, anxious not to get himself into trouble, would, as a general rule, think of the smallest number he knew and then halve it.”