Sheet Metal Inside vs Outside Dimensions: Which to Use
Sheet metal inside vs outside dimensions: dimension to one side, usually the outside, with inside bend radii, and go inside only where the fit needs it.
Machined-part habits trip people up on sheet metal inside vs outside dimensions. A bent part has a thickness between the two faces, and that thickness varies from sheet to sheet.
Should You Dimension a Bent Part to the Inside or the Outside?
Pick one side and stay on it. For ordinary press-brake parts that's usually the outside, with the bend radius given on the inside. Go inside only where a fit depends on it.
An r/SolidWorks user posted a drawing and asked what the standard practice is. People who bend parts had practical replies. Put the thickness in the title block, not on the views. Use a bend table. One said they couldn't count how many customer prints are useless because the shop can't pull bend dimensions from them.
Sheet metal inside vs outside dimensions in the references
The Drawing Requirements Manual, in its sheet metal drawings section, says brake-formed parts with straight bends are dimensioned "to outside mold lines and to inside bend radii." It reserves inside mold lines for cases where thickness tolerance can't be allowed to affect the inside dimensions. It notes those parts usually need die forming.
Fabricators say much the same. RP Proto's sheet metal design guide says outside dimensions should be used unless the inside is critical. It also says to dimension in a single direction where possible, because every bend adds variation.
When the inside wins
If a part slides over something, the inside width is what has to fit. Dimension that one inside, note it, and leave the outside free or mark it as a reference. A reply in the same thread made the point: make the overall a reference dimension, because the inside and outside counteract each other. Never give the same flange a hard inside and outside dimension.
A title-block note like "dimensions to outside unless noted" saves a phone call. Tight inside widths also limit what the press brake can reach, as narrow U-channel bends explains.
Those choices feed straight into tolerance stack-up. See how tolerances stack on a cut-and-bent part and why bend deduction matters.
Uploading a bent part
The sheet metal capability page lists "±0.13 mm cut, ±0.5° bend angle" and takes STEP, STP or DXF. Mark which side your critical dimensions sit on. If you're not sure how we'll read inside and outside dimensions on your drawing, ask us when you quote.
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