a perfect 90deg joint - 15mm Alu

[Jonathan]

Why would that be then.? . . . . I've tapped 100's M6 threads deeper than 25mm.!

I meant significantly more. If the tap does not have a reduced shank then clearly the depth it can cut is limited to the length of the tap, which for some of the ones I have is around 20-25mm. Either way it doesn't change the point I'm making.

[dsc]

Four M6 bolts would probably handle several hundered times that before shearing! As mark said, you don't need the bolt holes to be deep - bear in mind you'll struggle to tap M6 more than about 25mm deep anyway. The strength of the joint depends on the friction between the surfaces, and thus the clamping force and surface finish. Increasing the length of the bolts beyond 20mm of thread wont make much difference to the force you can apply.

If you don't mind me asking, what is the part for? It can help a lot to know since we might be over examining this, or worse vice versa.

Jonathan, I wasn't worried about shearing the bolts, I simply thought that 80mm long bolts will offer more support for the horizontal plate.

As for the use of the part, it will be part of a coffee grinder with these used to crush the beans:

http://www.portafilter.net/uploaded_..._LL-752918.jpg

This means there will be mostly clockwise turning forces acting on the mount and the carriage, although I don't know what the torque is yet.

Regards,
dsc.

[dsc]

I'm contemplating a similar problem at the moment, so I thought I'd add it in here rather than start a new topic.

Here's a corner joint, with three 15mm plates (Eco Cast, so should be 15mm consistent across):

http://i1120.photobucket.com/albums/...rner_joint.png

Idea is to get the edges aligned perfectly and easily. Plates are held using M5 bolts, with 5.5mm through-holes on the plate faces to allow some wiggle room on the edge threaded holes. This must come up quite often in various CNC machine builds, so I'm curious what you normally use.

So far I've got the following solutions on my list:

- pocket cuts to match the plates against each other

- corner jig to align plates, so no further machining

- locating pins

Dead square box is needed, I'm open to any suggestions.

Regards,
dsc.