huangyhg

mixed metric designs
our electronics company has long designed our products in metric and the mating tooling & fixturing in imperial to continue making use of the in-house machine shop for tooling fabs.
i've recently began designing and submitting tools designed in metric, much to the shop's anger. the company continues to be split about which dimensioning scheme to use. the main argument being the cost of re-tooling the shop to metric "unnecessarily" - "just because the product is in metric doesn't mean the tooling needs to be" they argue.
their pushbacks have produced several compromise proposals:
1) don't convert - it's always worked; don't rock the boat.
2) dual dimension prints; since the shop (and our auxillary machining vendors) will be converting back to english anyway.
3) design dimensions in metric, but use english fastening components - english dowel pins & fasteners/tapping.
my responses -
1 & 2 - there's inherent conversion error introduced in switching from one to the other; additionally, since our english unit precision is governed by decimal places, this complicates conversion of tolerances
3 - i don't have a good response to this; just feels wrong to go half-way.
i'd be curious about advice on this battle and input on these objects from those who've gone this road before?
well, with regards to inherant conversion error, this can be addressed to some extent by 'adding' a significant figure as appropriate - there have been threads on similar here so maybe take a look.
if you are letting your title block being related to decimal places stop you, frankly that's verging on lazyness. title block tols are 'unless otherwise stated', i.e. you may have to directly tolerance most dimensions but so be it.
we have a mixture because we use a lot of off the shelf parts that come in a mixture of metric & inch. recently we've been trying to standardize a bit on metric threaded fasteners, but most drawings are still in inch. if we need to convert we do, if we need all a sig fig we do, if we need to directly tolerance instead of relying on the tol block we do.
there have been quite a few threads about similar, both on this forum and over in i think it was "mechanical engineering other topics" or maybe something like "industrial/mfg engineering other topics". do a google search of this site using the field at the top.
kenat,
there's no real reason to move to metric from inch.
measurement systems are arbitary. in the big picture view of the universe, there's no difference between inch or metric. metric has some advantages in that it easier to think in a metric system (base 10, sizing aligns with mm). but there are some economical advantages to inch in america. there are certainly economical advantages to sticking to one standard, regardless to which one is picked.
switching is much more complicated that you might be thinking, and the advantage of metric is fairly slight (there are disavantages to a purist metric adoption, as well).
if you are in an inch environment, just suck it up and do everything in inch, the same if someone who favors inch moves into an metric environment.
i would agree that the swtichover is unnecessary.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
if your in-house shop is setup for inches, stay with it. use metric for customer interface, if required.
chris
solidworks 08, catia v5
it's all about the money - if you can put together a convincing proposal for management that shows the bottom line ($) then that's what (should) happen. you could try arguing that conversion leads to mistakes, but do you have proof? can you find a batch that needed rework because of that particular issue and assign a cost to the issue?
be prepared to discuss the actual costs of switching, including training and potential new hires. honestly the print is about design intent - you want to put the little squiggles on there that will best represent what you want your design to be - that's why we (here) use black ink on standard sized white paper, with english notes, in imperial units because we know that our suppliers want to see them.
if your work is exclusively with length/position, then i can agree with fcsuper regarding limited benefits of switching to si. if your work involves the first, second and third derivatives of length with respect to time, or f = ma, or linking electrical and mechanical systems, then there are more advantages to si. if your work involves amount of substance, and/or linking to the periodic table, then there are enormous advantages to using si.
tdculbert,
how bad is this problem, really? can you easily purchase metric components and drills and taps and stuff?
your machines are indexed in inches and millimeters are they not?
is this a hardware problem, or is it a machinist problem?
jhg
my experience, it's usually a machinist and management problem.
chris
solidworks 08, catia v5
in my experience choose one or the other don't try and mix and match. good design is done in either imperial or metric.
at the end of the day features that matters say a location pin need to be a certain size it will probably be an odd size in either metric or imperial, features that don't matter should wherever possible be standard sizes and put in to "round figures".
it probably doesn't matter a jot if a support plate is 12mm or ?" or if bolt centres are at 25mm or 1" but it soon becomes a complete mess when you mix them up and costs unnecessary money if you start machining ?" plate down to 12mm, or causes huge problems if you just choose to ignore the odd 0.7mm.

and that it's ok to use metric hardware interfacing features called out with their proper metric callouts one an inch drawing.
matt lorono
cad engineer/ecn analyst
silicon valley, ca

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