Bushing support
The support nut for the bushing is created by drilling a 6mm hole through the middle of the pipe. The hole is widened to snugly accomodate an M6 nut. Then an M6x20 bolt is put through the hole and fastened with the nut.

The purpose of this design is to hold the bushing in an accurate position. It can be removed (which you need to do if you want to take the extruder assembly off), and then put back in exactly the same place without recalibrating anything (hopefully). The top point of the nut should be marked so that you can ensure you put it on in the same orientation when re-assembling.

The groove obviously only needs to be cut in a rectangular shape with a straight metal file because the sides of the hex nut are parallel (this means it slides up or down to disassemble). The depth isn't too important. It just needs to be deep enough to key the nut nice and accurately without any chance of slipping, but it can be quite deep too because the top and bottom of the tubing will support it even if you cut all the way through the copper. It should also only be on the inner side of the tubing, otherwise it can't be tightened. The filing should be done slowly and checked frequently so that there is no slop in the nut positioning.

Any excess bolt length can be cut/ground to sit roughly flush with the end of the bolt. The exact position of this is not critical.

The next step is to attach a larger M10 nut to the M6 nut, which will be the bushing housing. To begin with, you only want a temporary joint (rather than brazing them together) because you might need to adjust their relative positions to correct for manufacturing errors. I am just using a hot glue gun because the "glue" can be peeled off cleanly afterwards so it won't interfere with the brazing. Later on, once the final position is determined and glued in place the nuts can be removed and then clamped together. The glue can be peeled off and then the nuts cleaned and brazed.
- posted by Simon McAuliffe @ 07:30