I am trying to find the formulas for making bends in sheet metal. I understand there are formulas for bend allowances. My issue is how do I determine where to bend if I want the inside dimension to be an exact number and if I want the outside to be an exact number. For both of the below questions the material is 6061 aluminum .080 thick with a .1 inside bend radius and 90 degree bends.

For example for the picture I included. If I wanted the piece to be 7" on the inside and the sides to be .5" I would do 7 + .5 + .5 + (2 x bend allowances) to get the total length of the plate I think? but how do I know where to put the bend lines to get 7" exactly on the inside?

Also on a totally separate plate how would I go about getting the total length of the unbent plate plus how would I know where to place the bends for an outside measurement of 7"

This seems like it might be so trivial but I just can't seem to grasp what is going on.

To clarify it would be 7" between verticals for the first one and 7" on the outside of the verticals for the second one.

This is not an exact part I need. It is an exercise in how to determine the lengths needed for what I am asking for. This information seems to be harder to find than gold.

Edit: I have read How do I size metal plates to get the correct dimensions after folding? and still don't understand. How does bend allowance and bend deduction help me. I am confused as ever and also extremely new at this.

I have done a little bending on my brake and seem to have something so please clarify if you can.

On a piece where I need the inside verticals to be exactly 7" I drew a centerline and measured 3.5 + .05( half of radius) from center and drew a bend line. Did same on opposite side. It seemed like I got 7" but it could be just luck.

Also on a piece where I need exactly 7" on the outside of the verticals I drew a centerline and did 3.5 + .05 - .080(thickness of the sheet metal) from center and seem to have gotten 7" on the exterior verticals.

Are these two formulas even remotely correct? C shape

  • $\begingroup$ As you mention, the bend must have a radius (which you define as equal to 0.1). This just raises something to clarify (by editing your question): when you mean you want the inside to be 7", do you mean that the distance between the verticals must be 7" or that the horizontal length between bends must be 7" (meaning the distance between the verticals would be 7.2")? Likewise for the second part of the question: is 7" on the outside between the exterior faces of the verticals or between the bends? $\endgroup$
    – Wasabi
    Sep 21, 2015 at 17:52
  • $\begingroup$ Is this a duplicate?engineering.stackexchange.com/questions/2589/… $\endgroup$
    – hazzey
    Sep 21, 2015 at 19:23
  • $\begingroup$ What are your tolerances on the 7" dimension? $\endgroup$
    – GisMofx
    Sep 22, 2015 at 1:38
  • $\begingroup$ It is not super critical. It is for a box that will be powder coated. in the 1/100's i would guess. $\endgroup$
    – BrianB
    Sep 22, 2015 at 12:23

1 Answer 1


In Inventor, you can select the location ("datum") you want to use for referencing where you intend the bend to be located. First, make a sheet metal part:

Inventor 1

Then, make the base face whatever you want the back of the part to be. Click the "Flange" button, then the edge you want to fold, and the dialog box will show the options:

Inventor 2

The options, from left to right, are:

  1. The edge you selected should be the back/outside of the flange
  2. The edge should be the start of the bend
  3. The edge you selected should be the front/inside of the flange
  4. The edge you selected should be the outermost projecting part of the bend. Note that if the bend is 90 degrees, this is the same as option 1.

In general the bend radius is determined by the sheet metal thickness; this link has a lot of explanation about the bending process. At the end it says that "the inside bend radius should equal at least the sheet thickness."

I've used Inventor and Solidworks to get parts manufactured and I've never had any issue with any of the default bend radii. Inventor has a free version available to students. If you're not using a CAD package to create the drawings I think you're probably wasting a lot of effort; any local university or even community college for that matter probably has a CAD lab you could use to get the drawings done.


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