Countersink Through Hole Size
This month we will discuss another common issues that we encounter on sheet metal part prints: Improper countersink through hole sizes. Many of the parts we produce include countersinking. Depending on part configuration, material type and thickness, countersink size, etc… we can employ two methods of adding countersinks to parts. We can machine them in after the parts are laser cut or punched, or we add them to the part right on the punch press (and sometimes a combination of both).
The issue with countersink through hole sizes occurs when the size called out is smaller than is mathematically and physically possible given the combination of the major diameter of the countersink, the angle of the countersink, and the thickness of the material. When the depth of the actual countersink exceeds the thickness of the material the through hole is dictated by the factors mentioned.
So, we can end up with the situation on a print where the through hole is called out in relation to the size of the screw being used and then a countersink size call out for the same hole that will result in the actual through hole being significantly larger and well out of tolerance.
We have several recommendations to help avoid this:
1) Visually evaluate the hole you added the countersink to in your 3D CAD system. If there is no remaining through hole, meaning the countersink extends all the way through the thickness of the material, then the countersink is likely making your through hole larger.
2) After adding a countersink feature in your CAD program, take a measurement of the resulting through hole to see if it has grown (if it has, then the countersink parameters are dictating the through hole size).
3) When dimensioning your prints snap a dimension on the through hole to see if the actual diameter is in conflict with any other dimensions given for the through hole size (sometimes a dimension can be auto-created for the through hole, but the size shown may be based on the information in the model from when the hole was created and may not reflect any change to the hole after the countersink was added).
Happy designing everyone!