stiffness measurements cnc mk3

[EddyCurrent]

I improved the FK bearing today and carried out the measurement again but this time using a spring balance like this one Rolson Pocket Spring Balance | Hand Tools

With the Z carriage halfway across the gantry, Z lowered 50mm from home position, force applied to spindle shaft around flats for spanner, collet nut removed and DTI 2mm from end of spindle shaft.
Force applied = 200N (20Kg approx)

X deflection = 0.11 (1818 N/mm)
Y deflection = 0.15 (1333 N/mm)
Z deflection = 0.04 (5000 N/mm)

For Z I hooked the spring balance into the collet holder and applied slightly more force than 200N because I was pulling at a slight angle, I was going to calculate a force vector diagram taking the angle into account but just estimated it.
I managed to get the DTI under the spindle shaft so got a measurement including the spindle mounts.

Edit: Sorry, sorry sorry, I later realised thet because the DTI tip was resting on collet nut threads this was changing the reading as well, so I fitted the collet nut tightly and measured off that.

So new readings

X deflection = 0.09 (2222 N/mm)
Y deflection = 0.13 (1538 N/mm)
Z deflection = 0.04 (5000 N/mm)

[routercnc]

I improved the FK bearing today and carried out the measurement again but this time using a spring balance like this one Rolson Pocket Spring Balance | Hand Tools

With the Z carriage halfway across the gantry, Z lowered 50mm from home position, force applied to spindle shaft around flats for spanner, collet nut removed and DTI 2mm from end of spindle shaft.
Force applied = 200N (20Kg approx)

X deflection = 0.11 (1818 N/mm)
Y deflection = 0.15 (1333 N/mm)
Z deflection = 0.04 (5000 N/mm)

For Z I hooked the spring balance into the collet holder and applied slightly more force than 200N because I was pulling at a slight angle, I was going to calculate a force vector diagram taking the angle into account but just estimated it.
I managed to get the DTI under the spindle shaft so got a measurement including the spindle mounts.

Agree we need a standard method so everyone can compare. Jonathan - since you intend to re-measure are you OK with the method posted above by Eddy? If so I'll also re-measure to the same condition although my machine has been partly stripped for fine tuning etc. so will be a little while.