Dimensioning Folded Sheet Metal

[RichardStarrbuck]

We are bending sheet metal to create a razorback for an equipment rack. It will be one length of sheet with multiple triangular bends like so:

b
a_________/ \___/ \__/ \___/ \__c

The bends are to be right angles (90 degree) at the peaks and aprox 135 degree at the crest.

Speaking strictly about the bends, should dimensions go from the sheet metal edge to the center mark of the bend radius?? As in: horizontally from left to right (a to c), then vertically (a to b) from the bottom face of the sheet to the center mark of the bend radius?
Can you dimension from an edge to the outside of the bend surface?

Basically, what is the proper way to dimension folded sheet metal?
Is there an ASME spec?

 

[drawoh]

RichardStarrbuck,

I usually dimension to the nominal sharp corners. Generally, with sheet metal, the radii are not well controlled, and you are interested in the straight sections. The first think you should do is chat with your fabricator and inspector, and see what they want. They are the ones who must use the drawings.

It will be fun locating holes accurately on this thing. You may want to drill or punch them after bending.

--
JHG

 

[KENAT]

Try to dimension on the 'inside' of any bends so you aren't adding material tolerance into the 'equation'.

As Drawoh says, dimensioning to the projected sharp corner can be better since the radii are not primary features but arise from the air bending process.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[KENAT]

Sorry, goes almost without saying that dimensioning should be driven first by functional requirements and then secondarily by ease of inspection and manufacturability.

However, you do need to be able to hit required functional dimensions/tolerances by a cost effective means which is where what you are asking about may come in.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[berkshire]

RichardStarbuck,
When dimensioning sheet metal products, do it in a manner that makes it easiest to inspect the final product. Don't worry about the fabricator. Remember if you do outside dimensions the fabricator has to deduct the material thicknesses and bend allowances, whereas with inside dimensioning he/she merely has to add the bend allowances , however if you give an acute bend a dimension to a theoretical intersection of the two outside faces, this is easier to inspect by laying two rules on the outside faces, this is better than trying to measure the center point of a radius on the inside of the part. When making 90 degree parts, dimension to the outside or inside wall of the part, this allows for variations in the bend radius due to variations in the equipment used to produce the part.

B.E.

You are judged not by what you know, but by what you can do.