Please help with this build Albert

[bigal999]

Just a quick few questions stuck on my design
1. When joining into the end of 15mm aluminium should I use 8mm or 6mm Allen screws and how deep into the aluminium should I tap the hole.
2. When joining 15 mm aluminium 125mm wide how many screw do I need
Albert

[routercnc]

In the end you would need to decide what works in your design but based on the information provide I would do the following:

8mm screws (there is still plenty of material left in the 15mm plate). 8mm are tightened to a higher torque so your clamping force on the joint will be much higher. Plus the heads are larger so make better contact with the piece they join to. You need to check the other material can accommodate the size of the head if using cap heads and they are countersunk.

Anything deeper than about the 1.5 x D for threads does not improve the joint. 1.5 x 8 is 12 so drill min 20 and tap to min 14 to allow you to use M8 with 12 mm engagement.. If you have a bottom tap you can drill a bit shallower but make sure you can tap enough whole threads.

For 125mm width I would choose 3 screws if the design allows. We could go into joint calculations and stress triangles but I’m not sensing you want to go there so at least 2 and ideally 3. One would be in the centre and the outer 2 would be set in around 15mm or so from the edge.

[bigal999]

Hi thank you for the advice it's just what I needed I helps a lot on the design I was planing to use cap heads and sink them level with the surface.
This brings up another couple of questions.
1. Over a 600 mm length can I expect any flex on the gantry this will be an aluminium plate 600x150x15.
2. Should the bolts used be stainless steel or high tensile ones.
Thank you
Albert

[routercnc]

Hi thank you for the advice it's just what I needed I helps a lot on the design I was planing to use cap heads and sink them level with the surface.
This brings up another couple of questions.
1. Over a 600 mm length can I expect any flex on the gantry this will be an aluminium plate 600x150x15.
2. Should the bolts used be stainless steel or high tensile ones.
Thank you
Albert

A simple plate for the gantry like that will flex and vibrate more than you think. I would go for something with a proper depth like box section or aluminium profile.

Both bolt types will work for what you need. But high tensile would be used in an application where the loads being applied are enough to break a regular bolt. That shouldn’t be the case for a CNC machine where you are after stiffness not ultimate strength and they are probably more expensive so stainless will be fine.