stiffness measurements cnc mk3

[routercnc]

I've been making some stiffness measurements of my machine - build thread here:
http://www.mycncuk.com/forums/gantry...ercnc_mk3.html

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

X direction:

Y direction:

Z direction:

(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:

X 1250N/mm
Y 1000N/mm
Z 2450N/mm

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.

Thanks