stiffness measurements cnc mk3

[routercnc]

Hi Eddy,

Yes, been following your build - coming along nicely. So with those figures X and Z are >5000N/mm, and Y = 1250N/mm.

The lower Y stiffness could be the mounting angle brackets for the AC bearing but not sure why it is similar as you move around the Z carraige as the moment would be changing. I also notice your ballscrew is right at the back of the gantry which is another reason for it being a bit lower than the other axes as it is the furthest away from the tool with the highest moment compared to the other ballscrews. But if the limited cutting I have done on my machine (with a lower stiffness) is anything to go by it's not a concern.

[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.

[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.

[routercnc]

Look forward to the results from that.

You can see in my first post that the initial stiffness measurements were on the collet nut - DTI on the round part just above the flats and load applied next to it on the flats. So I did include the spindle in the measurement, but not the tool as that would depend on the tool used for each cut. You could argue the reference setup in many ways, but that's what I did. If you agree Jonathan, would you mind reading off the collet as described when you repeat? If Eddy has copied my method then we'll have 3 comparible results to get a sense of the range of stiffness vs performance vs design to add to the general knowledge on the forum.

[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.

[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.

[routercnc]

Hi Neale,

However, it's difficult to know where "acceptable" is on that kind of scale, which is why seeing some measured numbers helps. Is your machine currently usable? Can you relate the stiffness numbers you have obtained to actual cutting speed, DOC, surface finish, etc, results?

I've been cutting out a few 20mm thick upgrade parts (will post in the build log soon) and have taken some photos of the edges of one of them. These were cut using the following:
6mm carbide 2 flute
1.0mm DOC
600mm/min feedrate
occasional spray of duckoil

cutting in the X direction
(there was a fair bit of what sounds like cutter resonance in this direction)


cutting in the Y direction
(much less chatter/resonance sounds in this direction)



My X & Y axes are currently 1000N/mm and 1250N/mm respectively at 50mm Z extension (measured on the collet nut). But these parts were cut at a longer reach, closer to 100mm as they were almost on the bed so the actual stiffness for these cuts would be lower.

I don't have an airline at home so can't clear the cutter path and suspect some of marks are chip re-cutting damage. But some look like cutter resonance as they are so regular. This suggests if you regularly want to cut a lot of aluminium, and want a nice finish, then you need a stiffer machine than my mk3 machine, perhaps more like 2000-3000N/mm in X&Y @ 50mm Z.


For the latest part, the Z axis motor mount plate, I followed the cutter with the workshop vacuum cleaner to remove as many chips as possible and got a better finish in terms of surface nicks etc. It's better but the resonance type stripes are still present although in this photo you can't see them that well. Here is the start of the job pocketing for the stepper motor:



Hope this helps.

[EddyCurrent]

I just put some figures into this, FSWizard - Free Advanced CNC Speed and Feed Calculator, and it suggests 600mm/min is too slow, though I've not tried it myself.