stiffness measurements cnc mk3

[Jonathan]

Excellent thread...

I can't see the angle brackets having any movement

Put the indicator on it and you'll soon find out. The reason I suggested in your build log making your own 'better' bearing blocks is that you can make them stronger than the originals by incorporating the brackets and bearings in one solid block, or whatever seems appropriate. However, I expect the distance measured parallel to X between your spindle centreline and the Y-axis ballscrew is the main contributing factor.

As you say it may not be a concern, also did Jonathan use 200N in his measurements ?

Yes, I used 200N as the deflections would have been too small to conveniently measure using a smaller force. To get more reliable readings you should test over a range of forces then plot a graph of the deflection vs force. The gradient of that graph is your stiffness. If the graph turns out to be non-linear, that could help explain what's contributing to the compliance.

and did he publish the deflections ? I didn't see them.

Unfortunately I lost them - however I remember the stiffness measured was greater than my milling machine (by the time you include the table), so about twice the stiffness routercnc has measured. However, we need to be careful to all take the readings at the same point, as I did it by putting a 1/2" (or 10mm, can't remember) bar in the collet and positioning the indicator on the bar about 5mm from the collet. That means the readings include the stiffness of the spindle. If you just push on the spindle body or Z-axis then clearly the spindle stiffness wont be included and you'll get better readings.

Also, when I measured the 'sufficiently strong machine' stiffness it had less then half the bolts in the gantry, which might make a difference...
I've lent my friend (who has the machine) a couple of dial indicators, so hopefully we will soon have some more reliable readings.