Thread: stiffness measurements cnc mk3
I've been making some stiffness measurements of my machine - build thread here:
Initial basic measurements were made as follows:
Y axis jogged to centre of gantry (worst case)
X axis jogged away from home, position arbitrary
Z axis lowered 50mm from home position, in a typical machining position
DTI clamped to bed with probe touching round section of collet nut (just above the flats)
Force of 50N applied (5kg) by hand using digital kitchen scales - guide figure for machining aluminium
DTI reading taken
(although this photo looks like I jogged the machine onto the scales I did not do that - I pushed the scales up into the collet to take the DTI reading)
For the X direction I applied 50N and measured a deflection of 0.04mm. Stiffness is then calculated from 50/0.04 which is 1250N/mm. The other axes were calculated the same way and were:
I was hoping for at least 1000N/mm to give 50N load and 0.05mm deflection during aluminium machining (simplistically).
Although I have just about achieved that the bed stiffness has not been measured (although I think that should be pretty good), and I was hoping for a bit more than 1000N/mm.
So exploring the Y axis stiffness in more detail I took measurements at various distances on the Y/Z axis components, starting at the tool location and working back to the gantry centre. In this way I could plot the loss of stiffness from the machine gantry all the way out to the tool location. These results are below, with a photo showing a ruler to help you see where the measurements were collected on the machine.
Y axis plate, distance from gantry face 10mm, stiffness 5000N/mm
Z axis rail, distance from gantry face 32mm, stiffness 3333N/mm
Z plate, distance from gantry face 55mm, stiffness 2500N/mm
Tool Centre, distance from gantry face 130mm, stiffness 1000N/mm
These were taken along a line shown by the ruler.
You can see that the stiffness quickly drops off as you move towards the tool centre, and the importance of minimising the distance and maximising the design at the concept stage.
Out of interest I also measured the stiffness of the gantry sides, where they meet the gantry and found a stiffness of 10,000N/mm. So the basic gantry is fairly good, but the losses build up as you progress across the Y and Z axes and to the tool.
I've got lots to finish on the machine but it has given me ideas about where to go next to get a bit more out of it.
At a quick glance, it looks as if a fair bit of the loss of stiffness as you move out from the gantry towards the tool is actually coming from play or flexibility or otherwise in the Y and Z rails, plus a component from gantry twisting. Your first figure is presumably bending of the gantry, primarily, and then the other effects become larger as you move out from the gantry. Maybe the Z bearing spacing could be increased? Reduce thickness of Z moving plate to reduce leverage? Deeper gantry? I'm playing with ideas for my own design and just finished my first pass (on paper) at the whole Y/Z assembly so very interested to hear your thoughts. I think that by reducing clearances and cutting clearance grooves in moving and fixed Z plates you can just about fit a ballscrew nut in between while still using 15mm rails but it's all a trade-off, of course, as you are weakening the plates a little.
Thanks for giving the data - it's good to see real numbers to start to get a feel for what is happening in the structure.
Glad it is of interest. I'm hoping that the method and setup is simple enough for others to try and compare to. I know Jonathan has measured something similar so perhaps others have too.
Yes the first figure (10,000N/mm) is the basic gantry stiffness and everything else drops away from this due to various factors you have mentioned. What you cannot see in these photos is that the Y axis ballnut screws to a wooden panel on the Y axis (!), so this probably accounts for some of the 10,000N/mm gantry side dropping to 5000N/mm on the Y plate. An upgrade to aluminium should help here. It's a carry-over part from an old design when I had less machining capability.
I've always felt I could have done a better job, especially on the Z axis, along the lines you have mentioned above plus other ideas. This data shows there is still room for improvement although I have been accused recently of only using the machine to make a better machine, and not actually making anything with it. Easy to get drawn in . . . .
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?
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, thanks for posting your stiffness measurement method, out of interest are you using grease in the bearings ?Spelling mistakes are not intentional, I only seem to see them some time after I've posted
No problem Eddy.
The profile rail bearings were second-hand (fa-system ebay) and arrived with grease in. I've never added any since (about 3 years ago!) as they seem to keep running smoothly. I don't get in the garage much, perhaps 1 hour per week, and most of that time I'm doing improvements or other non-cutting work.
I've greased the ballscrews every now and again.
I just tried mine now it's mostly together mechanically. Build thread here http://www.mycncuk.com/forums/gantry...y-eddy-23.html
Using a Mitutoyo digital DTI with 0.01 mm resolution, deflections with 5kg force applied as above.
X 0 mm
Y 0.04 mm
Z 0 mm
I think the Y deflection is due to the ball screw system because wherever I apply 5kg load to the Z carriage I get the same deflection.
Last edited by EddyCurrent; 22-03-2014 at 06:51 PM.Spelling mistakes are not intentional, I only seem to see them some time after I've posted
By routercnc in forum Linear & Rotary MotionReplies: 7Last Post: 03-05-2015, 10:55 AM