Tolerance achievable for Sheet Metal Part with some weld - 0.4mm TP

[vinmech32]

I have a bracket C shape with dimensions 300mm height 200mm length and 70mm width. It has some welding too after forming. Its made of CRS ASTM 1008. We are facing a dimensional NC with two holes which has a position tolerance of 0.4mm wrt a plane. We are machining the hole after all the process (forming and bending). Is this tolerance achievable or is it over design. Because we are facing only 70-80% conforming to specifications. The variation is high in some parts.

I couldnt figure out whats the approach to solve this problem. Either i need to focus on design or process to get it right. My first suspect was design.

Need some technical suggestion on this.

 

[Jim Wynne]

I have a bracket C shape with dimensions 300mm height 200mm length and 70mm width. It has some welding too after forming. Its made of CRS ASTM 1008. We are facing a dimensional NC with two holes which has a position tolerance of 0.4mm wrt a plane. We are machining the hole after all the process (forming and bending). Is this tolerance achievable or is it over design. Because we are facing only 70-80% conforming to specifications. The variation is high in some parts.

I couldnt figure out whats the approach to solve this problem. Either i need to focus on design or process to get it right. My first suspect was design.

Need some technical suggestion on this.

It's hard to say what the issue might be without knowing more (such as what "machining the hole" means) but fixturing is certainly a good place to look, both for production and inspection. You need to make sure that your production fixture is based on the datum scheme of the drawing and that you and your customer are measuring the same way.

 

[mdurivage]

I agree with Jim. The clamping force necessary to hold the part to drill the holes could be affecting the 'final' location of the holes.

 

[vinmech32]

Thanks for your reply. The hole was punched during the initial stages but we didnt conform to the true position tolerance of 0.4mm. then we went to machine the hole in order to improve accuracy of the hole location. But what will be the normal tolerances used for a sheet metal part. Is there any reference material of such kind. Its clamped manually. Fixture is the next one which i am more concerned after design tolerances since there is no proper fixturing for this part. The supplier is insisting on stringent or non-achievable tolerances as the root cause. I am really trying to gauge the tolerance capability of a sheet metal part.

 

[Golfman25]

You should be able to meet that tolerance without too much difficulty if your process is properly designed. Your best bet is to put the holes in after forming.

 

[Jim Wynne]

Thanks for your reply. The hole was punched during the initial stages but we didnt conform to the true position tolerance of 0.4mm. then we went to machine the hole in order to improve accuracy of the hole location. But what will be the normal tolerances used for a sheet metal part. Is there any reference material of such kind. Its clamped manually. Fixture is the next one which i am more concerned after design tolerances since there is no proper fixturing for this part. The supplier is insisting on stringent or non-achievable tolerances as the root cause. I am really trying to gauge the tolerance capability of a sheet metal part.

You can't machine an existing hole and change its location without messing something up. If there is no proper fixturing, I think you've found your problem. There is probably no way of consistently achieving conformance with (and verifying) a 0.4mm position tolerance without proper fixturing. On the other hand, with the proper fixturing, and again without knowing more about the application, you should be able to meet the specification.