stiffness measurements cnc mk3

[Neale]

There's a familiar theme - "Are you ever going to actually make anything?" "Well, as soon as I've finished this machine to make that tool to finish off those other bits I started a year ago last Christmas, I'll be ready to go..." OTOH, I have been able to respond to the "I need a turntable to display my embroidery in an exhibition. It has to do about 3RPM, it's Friday evening, and I need it ready and painted for Monday morning" kind of request so I just about keep my head above water re brownie points.
I know about wood bending - my current router is an MDF build (cold-rolled cow dung, as a friend unkindly called it), and you can judge the humidity in the garage by the sag in the bed. And the Z platform. And the gantry. I'm about to stiffen the bed with a couple of bits of steel tube I recently found, although given that the bed currently has the stiffness of crocheted cooked spaghetti, that won't be too difficult.
There are a number of build threads on this forum where people have gone to considerable lengths to build a really strong machine, and I have to agree that a machine that is a bit too strong is a much better result than a machine that is a bit too bendy. 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?

[routercnc]

I've machined some holes in the brackets for the bed (see my mk3 build log) but don't want to do much more until the limit switches, e-stops, extra X axis limit switch, etc is made and set up. It's too risky to run in this condition.

From that limited experience it feels like the machine is going to be reasonably OK for aluminium in terms of build stiffness, but as per my other post on 'machining aluminium with a 1.5kW spindle' I'm having problems relating to having to use too high an rpm and melting the aluminium to the tool. I was running at 0.5mm DOC, 300-600mm/min feed, and 10,000 - 12,000 rpm (!). The surface finish on the inside of the holes looked fairly good but I can't comment further without cutting doing something a bit more demanding.

I won't be machining anything else for a bit whilst I finish bits off so hopefully someone else can make similar measurements and comment further if you need a better answer sooner.

[routercnc]

double post

[routercnc]

. . . .Can you relate the stiffness numbers you have obtained to actual cutting speed, DOC, surface finish, etc, results?

I'm still playing around with DOC, F&S, but here are some photos of holes I cut in the bed to allow nuts to be added for clamping things down. Finish is not too bad - you can see the finish on the sides of the hole, and at the bottom of the hole.

[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.