Default motor screws too short

The instructions for call for M4x6mm button head screws to attach the motors. This screw length has been the same since v6.0.0. The 6mm thread is quite short to make a strong connection with a thread in plastic, and the cable tidy covers for v7 further reduce the effective screw length left to hold the motors to the lugs. The cable tidy lugs are nominally 1.5mm, which prints as 1.6mm at 0.2mm layer height.

There have been reports on the Forum of motor screws not holding tightly. The ones that I can find quickly are: https://openflexure.discourse.group/t/finished-my-first-build-high-res-v7/1933/3 and https://openflexure.discourse.group/t/first-time-build-of-v7b2-low-cost-version-using-labcrafter-kit/1785/2

I find that the screws that I have been using habitually are M4x8 button head. They are longer than recommended, but I have still thought that they are quite short.

The motor lugs themselves are nearly 12mm tall in the straight part on x and y, with through holes, and 10mm deep blind on z.

We should specify a longer screw. Maybe M4x10mm? There should be enough depth on z with the motor body and cable tidy cover taking up some of the thread. The only other downside of a screw being 'too long' here is that it takes more time to screw in.

• Test M4x10 screws for length on z-axis, with the motor but without the cable tidy.
• Decide on M4x8 or M4x10 screws
• Make z-holes slightly deeper to fit M4x10, if necessary. (in module motor_clearance() in gears.scad)