Datums on Injection Molded Part 1

[PRuggiero]

Guys,

I am working on the drawing for a plastic injection molded part and am running into an issue. How can I put datums on surfaces that have a draft angle to them?

 

[CheckerHater]

Just do it

On more serious note: What datums, what surfaces, what draft - could you be any more specific?

Datums:
1. RFS, MMC, etc. - how you grab the part?
2. You can use datum targets, if you cannot grab the entire feature.

Surfaces:
1. You can use the entire surface of a part (or "this side of parting line" approach) to create a datum, especially if you work to latest standards.
2. It was discussed many times on this forum how to use tapered holes for datums.

Draft:
1. You can specify draft to be within hole tolerance, in this case round hole with draft is no different from regular round hole .

So you definitely need to provide more information about your datums/surfaces/drafts situation.

 

[pmarc]

Yep, the problem is quite complex.
You may want to take a look to ASME Y14.8-2009 which is specifically dedicated to dimensioning and tolerancing of injection molded parts.

 

[PRuggiero]

checker,

Starting simple, imagine i had a rectangular block with no holes in it. If it were a machined part one would typically have the bottom surface as A, the long side as B and the short side as C.

Now if we go from being machined to being injection molded, say all the sides had a 1° of draft angle on them. Now my datum A is still good, but for B and C what does one do?

Can you still do centerline datums using datum targets? Meaning, if I had that simple block I could put the datum B inline with the dimension, meaning it should be centerline. How does one do that with datum targets?

I'll have to check that spec out.

 

[CheckerHater]

PRugiero,
You probably would put your machined "brick" into vise with flat jaws.

Now imagine fixture with "legs" or "buttons" that only touch your part in certain spots.
This is the idea behind "datum targets". They show in what points your part is to be rested / clamped.

Look thru ASME Y14.5-2009, parts dealing with datums, datum targets, possibly using irregular features as datums, and also ASME Y14.8-2009 that pmarc suggested as well.

If you don't have access to actual standards try to at least see good textbook, or research on the Web.
Datum targets is basic concept, and if you were not familiar with it before, you may need some primer.
Sorry, if I misunderstood you, though.

:-0

 

[PRuggiero]

appreciate the help, i got asme 14.8 2009 and read it, helped out a lot. Some of the stuff I don't quite get.

1. For target datums it looks like at a minimum I need 3 for primary datum, 2 for the secondary and 1 for the tertiary. It seems like if you wanted to you could do more than the minimum correct?

2. If I have a nozzle which has a tapered thru hole, what is the best way to set up the axis of the hole as a datum? Currently I have the big hole as datum A and the small hole as datum B. Then I use the axis as A-B is that good? Does A-B just create a single axis as my datum?

Thanks,
Pete

 

[CheckerHater]

1. Yes you can use 4, but you'll have to deal with "wobbling" part, which is another long story.
2. Yes, you can establish the axis by grabbing ends of cone, if the part will allow it. (please, don't hurt me, you know who you are )

Once again, hard to tell without some sort of sketch of what is going on.
Try to follow 2 rules:

1. You are the boss (at least you are the boss of a part - you are the one to decide how to manufacture / inspect it)
2. Think of GD&T as a "secret language" you use to describe how to fixture and inspect the part: "I am going to hold it here and there and measure that in relationship to this."

Sorry for the delay and I hope it helped.