Dimensioning question. Am I losing my mind?

Jim Canale

I'm beginning to think I am losing my mind.  A couple weeks ago, my employer re-ordered some sheet metal panels I designed a couple years ago.  The panels arrived, and the oval holes are oversize, just as they were the last time we took in a shipment.   I expected ovals .210" long by .125" wide, with an .06' radius.  Instead, I've got holes .246" long by .172" wide.   I didn't get a chance to ask the vendor, because my boss says I should "let it drop".   

I don't work with oval holes too often.   Is there something wrong with the way I am dimensioning them?     (screenshot attached)

Joe Dunfee

 The method you dimension the slot is fine.  It may be that the company that made them simply used their old program they used to create it, and reproduced the same error they made the first time.  And, they will certainly do the same error next time. If the problem was never pointed out to them, it should be.  If it was, then they need to suffer financially or loose your business.

-Joe

Jim Canale

Thanks Joe.  It is a relief to me to find out I'm not losing my mind.  You know, I am the only designer at my employer's business.  No one else knows squat about mechanical design, drawings or dimensioning.   So there's not anyone around here to ask.

John Gaunt

If it is cut by NC machine, some data entry works by taking each dimension from the previous point.

For that situation, I would probably be asked to add a dim across the flats.

Not strictly correct to 'double up' dimensions like that, but it could be a help in such a case.

Eric Fredrickson

Jim,

Some times I dimension based on who is getting the drawing.  Toolmakers sometimes would like dimensions all referenced from a single surface because that is how they do the machining. But dimensioning that way would lead to a stack up of tolerances that allows much more variation in the part. So I dimension parts based on where I want the variation to occur.

Since you mention that you are looking for a .210 long by .125 wide hole, then I suggest dimensioning it that way.  When the part arrives, it is going to be difficult to check center to center dimensions of radii, but everyone can measure the length and width of the oval.  If you want to make it easier for someone programming (though it is not difficult to subtract 2X the radius from the overall length) you can put the center to center distance as a reference dimension.

Jim Canale

Eric and John,

I try to dimension my drawings thinking about the tooling the shop will use.  The coordinates to the center of the hole (not shown on my screen shot), were given, relative to a corner datum.     As this is a piece of thin (.032") aluminum,  I figured they would use a 1/8" endmill or router bit, plunge at the X/Y coordinate, and move +/-.042 to make the slot.   I'm not sure how they would inspect the holes, but perhaps I should've shown the length and width of the hole on the print, as well as the radius.

John Gaunt

I should say your dwg is quite correct, Jim. Just suggesting how the shop might have come to make the mistake despite it.

I have seen a similar shape, larger in perspex - the cutout piece was actually presented as the finished job.

RevCloud.jpeg

Tim Neumann

We just dimension the overall size of the slot, not center/center. Keep in mind though we fab large items where the tolerance is usually plus/minus 1/8" & no cnc machining involved.