Drawing Standards for Castings?

vtcnc

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I'm a bit embarrassed to ask, but figured this would be the place to do it safely - everyone is always helpful here. In all my years of drafting, design, engineering and production management, I've never had the opportunity to create a casting drawing. (See my other project thread for how this opportunity came to be)

Drafting Standards: this was the best I could find. What I'd like to know is the convention for calling out a feature dimension that has draft. For example, if there was a boss on a face, do I call out the diameter at the base or the diameter at the top? The pattern maker would know to add draft, but what is the industry convention - or is their one?

So here is a screenshot of one of the many gear hobber castings...These aren't meant to represent patterns, but rather "as cast". My intent is that somebody who cared to recreate these could at least work backward into their wood patterns from here.


The part in this example will have the female dovetail milled into the two rectangular ways on the flat base. Hopefully you can see the dilemma: in the right side view do I call out the 1" width at the bottom of the dovetail way, or do I call it out at the top? In the top view, do I call out the 1-3/8" at the top of the ways, or do I call it out from the bottom? The bottom has fillets...how does one consider the dimensions of these types of features with draft and fillets?

In short, what is the preferred method of calling out features with draft?
 
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There's really no set standards for draft angles, material allowance, etc. The 21/32" dimension is out, round it up to 11/16". Same way with the 4-3/32" round that up to 4-1/8". The pattern maker will know what draft angles to put into the pattern as well as the shrink factor, 1/8" per foot if you are casting iron. Take your 1" dimension for the ribs and place at the top of the rib. That would be an easy pattern to make out of aluminum and send to the foundry for mounting on their board.
 
Thanks for the quick reply. Ok, I guess better to err on more material than less, If I understand you correctly?

I was wondering about the precision of 1/32 callouts. I'll start utilizing 1/16ths.
 
Try not to use fractions smaller than 1/16". Most pattern makers will allow just a little more material for machined surfaces. They generally want to see the machine drawing also. Ken
 
The convention is to add a note to the dimension of +DRFT or -DRFT. For example, your 1.000 dimension would read 1.000 -DRFT.
 
I usually dimensioned the more important feature. IE if the bottom of the slot had to be 1" dimension that or if the top was the important feature then dimension that.

Usually I would design with a standard draft angle and only use a different value if the design required it. You could then add a note stating " all draft angles are 2 deg unless noted" and then dimension the areas that were different.

It gets trickier if the draft surface is large and a fit surface to a mating part. Then you might have to add more detail on the print outlining the feature.

One thing to note in your drawing, if you have a dimension shown in 1/8ths of an inch you are implying that your tolerance is in 1/8ths also. In other words, 2 1/8 +- 1/8. I would convert all dimension to decimal and pick a global tolerance for all features if possible. Example, 2.125 +- .010 or 2.12 +- .12. Global tolerances are usually shown in the title block but can also be shown in the Notes.
 
The convention is to add a note to the dimension of +DRFT or -DRFT. For example, your 1.000 dimension would read 1.000 -DRFT.

Hi Randy, do I understand this correctly? If I detail the bottom of a feature as 1.0 -DRFT, then the feature would taper to less than 1.0? Conversely, 1.0 +DRFT from the same feature would mean it drafts to more than 1.0?
 
I usually dimensioned the more important feature. IE if the bottom of the slot had to be 1" dimension that or if the top was the important feature then dimension that.

Usually I would design with a standard draft angle and only use a different value if the design required it. You could then add a note stating " all draft angles are 2 deg unless noted" and then dimension the areas that were different.

It gets trickier if the draft surface is large and a fit surface to a mating part. Then you might have to add more detail on the print outlining the feature.

One thing to note in your drawing, if you have a dimension shown in 1/8ths of an inch you are implying that your tolerance is in 1/8ths also. In other words, 2 1/8 +- 1/8. I would convert all dimension to decimal and pick a global tolerance for all features if possible. Example, 2.125 +- .010 or 2.12 +- .12. Global tolerances are usually shown in the title block but can also be shown in the Notes.

Thanks sanddan, yes, I do plan to provide a standard tolerance and note the exceptions. I will do the machined features of the castings in decimal format and the castings will remain fractional since that seems to be the prevailing convention.
 
Hi Randy, do I understand this correctly? If I detail the bottom of a feature as 1.0 -DRFT, then the feature would taper to less than 1.0? Conversely, 1.0 +DRFT from the same feature would mean it drafts to more than 1.0?

You understand perfectly.
 
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