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Drawing Standards for Castings?

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vtcnc

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#1
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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4gsr

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#2
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.
 

vtcnc

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#3
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.
 

4gsr

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#4
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
 

RandyM

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#5
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.
 

sanddan

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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.
 

vtcnc

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#7
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?
 

vtcnc

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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.
 

RandyM

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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.
 

vtcnc

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#12
There actually is an ASTM standard for casting engineering drawings. It is:

ASME Y14.8-2009 It is uploaded on scribd if you are a member.

Plus found a possibly useful reference:

https://www.sfsa.org/publications/hbk/s4.pdf (supplement only but worth a look)
Hi Tony,

Thanks for the additional information. Yes, I included the link to the sfsa supplement in my original post. I do have a Scribd membership, so I will download the ASME Y14 publication and take a look. That may be just what I need to encapsulate all of the help offered from the HM members.
 

Tony Wells

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#13
Ah, yes you did, Bryan....I missed the link. Sorry 'bout that.
 

vtcnc

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#14
Hi Tony, [snip] that may be just what I need to encapsulate all of the help offered from the HM members.
Well then...! Just finished perusing this standard (ASME Y14.8-2009) and I have to say Tony...this was the document I needed. If this isn't in the drafting standards repository, it probably should be. If you or Nels want me to purchase a legit copy as a donation to HM, let me know and I'd be happy to pony up the $ for this valuable addition from ASME if it isn't already in the library. Everyone can benefit from this document.
 
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Tony Wells

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#15
Any addition we get to our private library is welcomed. Although it is copyrighted, in the case of a purchase for an organization's library, I believe this is acceptable. It's no different that having it in a company library where a group of engineers share a copy. I know from past experience, we had to keep certain standards on file, and they had to be accessible to all parties with legit need. That said, it doesn't mean that everyone here can take a copy and post it on their own website and offer unlimited free downloads, as is the case with say, any of the Torrent variants. Personal, educational use only.
 

vtcnc

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#16
Any addition we get to our private library is welcomed. Although it is copyrighted, in the case of a purchase for an organization's library, I believe this is acceptable. It's no different that having it in a company library where a group of engineers share a copy. I know from past experience, we had to keep certain standards on file, and they had to be accessible to all parties with legit need. That said, it doesn't mean that everyone here can take a copy and post it on their own website and offer unlimited free downloads, as is the case with say, any of the Torrent variants. Personal, educational use only.
Went to the ASME bookstore and went through the process of understanding if there are any copyright/sharing issues. Apparently, they protect their electronic copies via FileOpen, which essentially encrypts based on user download location (i.e., if I download, you can't from your location if I forward it to you). We may be out of luck in this case. What I can say is that the Scribd subscription worked in this and many other cases. PM me if you don't have a Scribd subscription.
 
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Davd Flowers

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#17
I was completely unaware that there was such a standard and would be very interested in reading up on it.
Pattern making is quite fascinating, so many things to consider......
 
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