stiffness measurements cnc mk3

[EddyCurrent]

I can't see the angle brackets having any movement at those forces but I do think the FK bearing might have play in it, I've greased it but not spent much time yet with fine tuning using shims. As you say it may not be a concern, also did Jonathan use 200N in his measurements ? and did he publish the deflections ? I didn't see them.

[routercnc]

Hi Eddy,

Yes, Jonathan used 200N but my scales were only up to 5kg so I could only apply 50N. The response will be linear anyway at these loads, and as the load is divided by the deflection it works out the same (4 times load gives 4 times deflection). The stiffness values will be the same in both cases. We also both used 50mm Z extension so the machine condition is the same and results are all therefore broadly comparible.

Jonathan has not published figures to my knowledge, just this info in the 'sufficiently strong machine' thread:
'. . . .By applying a force of 200N and measuring the deflection I found the stiffness in X and Y with the Z-axis at 50mm extension. I did the same test on my milling machine (Clarke CMD1225C column mill) and hence found that this machine is a similar stiffness to my milling machine, although a bit weaker in Y . . . '

Would love to see some figures if you have them Jonathan ! . . .and anyone else if you've measured at 50mm Z extension.

Going back to the freeplay in the FK bearings, if you only tested in one direction and took a few readings (as I did) then the freeplay would all be used up in the first test and should not be a factor in the repeat tests. It would only matter if you went back and forth. I wanted to seperate free play from actual machine stiffness so measured in one direction only.

I've currently got my AC bearings out of the housings ready to shim up as they have quite a bit of freeplay even when loaded.

[EddyCurrent]

Going back to the freeplay in the FK bearings, if you only tested in one direction and took a few readings (as I did) then the freeplay would all be used up in the first test and should not be a factor in the repeat tests.

Also referring to Jonathan's point about where the measurement was made, I applied the force at the end of the spindle, with the collet nut removed, and had the DTI on the other side right at the extreme of the spindle shaft. I removed the collet nut because being slack, there was play in it.
If this thread is going to be a repository for peoples measurement then maybe a set of rules should be made to ensure everyone tests in the same way.

[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.