Poka-Yoke 3

[davej]

I am setting up a work cell to form a part in a 3 stage stamping die, then projection weld two nuts in location. I am interested in help with mistake-proofing the welding process. Does anyone have examples of methods used to make the welding failsafe?

 

[kendo]

One suggestion could be to fit proximity sensors into your fixturing and wire them into the cycle start of the machine to prevent operation if the nuts are missing. Verification of the system is important at the begining of each shift

 

[revdode]

Verifying the presence of the nuts in the weld station and in a station immediately afterwards helps, unfortunately it doesn’t always guarantee the nuts will stay in place.
If the weld is bad it may not fail until the component moves to assembly or final use. The normal approach is to carry out some form of press test, an alternative would be to monitor the parameters of the welding set during the operation and stop the process if these go out with set ranges. I haven’t seen this approach used for weld nuts but it does work for spot welding sheet metal components together.

 

[steveh]

Projection welding is one of those things that respond well to process monitoring. There are two things you need to know out of this process: 1) Weld current is delivered.
2) Nut is displaced downward (due to melting of the interface area between nut and part).

You can use a current monitor for the first and an LVDT for the second. Then make them report back to the PLC - if they aren't happy with the weld, PLC declares a fault.
Steve

 

[CATdude]

Beyond the inductive sensor aready suggested, several other sensor technologies allow a greater sensing range plus the ability to prove proper orientation. Convergent mode lasers, high-res remote-head style ultra-sonics, even economical camera-based pixel counting sensors have been succesfully used.