Dimensioning and Tolerancing in Plastics: Where do get Training?

[JasonNicholson]

I am struggling with dimensioning and tolerancing especially as it relates to injection molded parts. I always strive to be one of the best at what I do. However, this area seems to keep causing me to stumble.

This is the process I go through and where I struggle:
1. Make a drawing of a 3d model for inspection and checking purposes but not for manufacturing.
2. Hold 2D design review meeting with other engineers to examine the drawing looking for mistakes and left out information. The drawings are corrected and the design review is repeated if needed.
3. The 3d models and 2d drawings are sent to the Toolmaker. We get 1st samples back with a list of measured dimensions from the 2d drawing. Some dimensions are in and some are out of tolerance. I "deviate" (allow out of tolerance dimesions) some dimensions according my discretion. I have trouble visualizing or figuring out of if dimension X is going to cause problems down the road. But if I have the tool maker fix dimension X, that costs us time which is money. So fixing a lot of dimensions is out of the question. Most of the time 1st parts are functional and assemble fine. So in some places I have tolerances that are to tight and in other areas I probably don't have the critical dimensions even on the print! ack! So frustrated!



Where is the place to start from scratch and really learn dimensioning, tolerancing, and GDT as it relates to plastics? I am willing study like crazy and learn this stuff. I want to reduce this problem as much as possible. Please help.



-Jason Nicholson

 

[KENAT]

Well, honestly it seems your issues aren't so much with plastics specifics as tolerancing generally.

Many Plastics molders give guidance on what tolerances can typically be achieved by their processes, often dependent on size. This helps give an idea of what they fundamentally achieve.

The other angle, and arguably the more correct one, is to dimension based on function - make tolerances as loose as you reasonably can while still ensuring function. Obviously sometimes you don't want to get carried away with how loose your tols are and in these cases using the process capability as a guide may be appropriate.

What drawing standards do you work to, ASME, ISO...? As well as ASME Y14.5 there is another spec ASME Y14.8 that applies to castings, forgings and moldings. It might give some ideas on what needs to be specified etc.

Some community colleges may have drafting programs that might help.

You can get GD&T training both self paced or consultants etc. but it seems your issues are at a slightly more basic level.

There are books on tolerancing etc which might help.

Ideally, working with an experienced checker or designer who bleeds over your prints for the first few years until you get sick of making corrections...

There are online resources, for instance the GD&T forum on this site has some good stuff. Tec-Ease and Drafting Zone have some online resources too.

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[MikeHalloran]

Your first design review, of 2D drawings or sketches, should be as early as possible, with your favorite molder, _not_ with a bunch of engineers, _not_ with your local GD&T fanatic, and _not_ with managers or other chaff present.

Issues to be discussed include how to make the tool, so the molder may wish to invite his favorite toolmaker, or not; his choice. Once you've collectively decided how the tool will be made, you and the molder can discuss what tolerances are achievable for each feature, and then you can evaluate how that affects the mating parts, and how the mating parts, or the design, need to be adjusted.

A book might help you use the right words when talking to the molder.
Or it may not; English may not be his mother tongue, and he may be self-taught anyway.



Mike Halloran
Pembroke Pines, FL, USA

 

[ewh]

Ideally, working with an experienced checker or designer who bleeds over your prints for the first few years until you get sick of making corrections...

Those were the days!

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

Are you molding the parts yourself and having tools made to allow this or are you just interested in getting the parts and a molding vendor makes them for you, it's not clear from your post?

Getting info from the people that will actually make the parts is very useful, even more so when you don't have any real familiarity with the process involved - in that case it's essential.

However, don't always take whatever they say as automatically gospel. In some cases they'll say they can do things that aren't optimal because they can charge for it or similar. Or they'll try and make you compromise on things for sake of manufacture that really are essential to function.

Design reviews are pretty useless for finding detail mistakes in my experience. They'll sometimes catch real big stuff but smaller issues - that may still effect production - get overlooked in the general hub-bub etc. Having one real experienced guy sit down with the print by themselves and looking over it should find more issues.

You need to catch all the functional requirements (which only you and your colleagues will know) while making sure it can be made cost effectively (which the molder will know far more about).

To be successful you need to balance both sides of the equation.

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[patprimmer]

Like KENAT and Mike say, but I would add, the comments of a toolmaker might help a lot as may the comments of tech support from a raw materials manufacturer, especially if it is a more difficult material to hold on tolerance or one that that the moulder, toolmaker or yourself are not fully familiar with.

Fibrous reinforcements can really complicate this as can post moulding shrinkage, differential shrinkage and swelling from moisture uptake from the atmosphere.

Regards
Pat
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