X3 mill with rails, ATC, new motor

[Jonathan]

Ok, broken tap situation resolved...

I intend to buy the fasteners I need for this tomorrow, so time for some rough calculations to check what grade I should use. From the THK documentation, I gather that the tightening torques for the M8 and M6 fasteners in the rails should be 30.4Nm and 13.7Nm respectively. They don't recommend a grade of fastener (as far as I can see), so we are left to work out the tensile strength required ourselves.

First step, use the torque value to calculate the tensile force on the fastener. This is the same formula as working out the axial force you get for a given torque on a ballscrew, so I don't even need to look it up (although can be more accurate by including more in this):

.

This gives the force, for a given torque (tau), coefficent of friction (mu, ~0.2 for steel) and pitch. Machine screws are graded based on the tensile stress, so we need to divide this value of force by the cross sectional area of the screw to arrive at the stress, in N/mm^2. The stress area is roughly the minor diameter of the fastener, so roughly the diameter of drill you would use. The area is then clearly just the area of a circle.

So, e.g, the M6 fasteners are 1mm pitch, 13.7Nm and steel, so the force is 13.7*2*pi*0.2/0.001=17kN. Divide that by the stress area, 17000/(pi*2.5^2)=865Mpa. Bolts are graded such that the first number is 1/100th of their tensile strength rating, so e.g. a 10.9 grade fastener is rated for 10*100=1000Mpa. A rule of thumb suggests to only tighten a bolt to about 80% of its tensile strength rating, so in my case I will be OK with 10.9 grade fasteners for the M6 screws (especially as the actual stress for the given torque will be a bit less due to things I missed out for brevity).

Now I just need to remember to bring my torque wrench home...

Also noticed that RS have the caps to help prevent debris getting into the linear rails, so I'll get some of those.