its a dyna mechatronics DM2900 with a BT30 spindle

i took apart the drawbar in order to figure out why the tools seemed not to unclamp very easily sometimes. sometimes i had to put on gloves and pull down on the tool while hitting the unclamp button until it finally popped free, rather violently

so the draw bar is the standard ball bearing gripper type, with a pneumatic actuator

i did some measurements and calculations, and here is what ive come up with..please comment! i hope i did something wrong because the force seems rather small.

there are 49 belleville springs. 47 of them are back to back (series), the last two are in the same direction.

so according to wikipedia, the new spring constant for the stack is 1 / (47 + 1/2) = 0.021. multiple that by the spring constant of a single spring to get the stack constant.

the springs are 20mm x 10.2mm x 1.55mm carbon steel, as far as I can tell. mcmaster says that one of them has a deflection of 0.337mm at 342 lbs. So that gives a spring constant of 25777 lbs / in for an individual spring. Multiply that by 0.021 from above, and you get 541 lbs / in for this stack as configured.

When I start tightening the top fitting on the drawbar, I measured the distance the fitting goes from just touching the spring stack, to fully tightened, as 6mm. So thats the compression the stack experiences just sitting there with no tool clamped.

I then measured the distance the drawbar compresses when it tool is clamped in. This comes out to 5.5mm. (i had to swap the actuator in and out and meausre to the top of the fitting to accomplish this.)

So it looks like the system as a whole provides 11.5mm of compression to the spring stack when a tool is clamped in. Thats 0.453".

So that comes out to 0.453" x 541 lbs/in = 245lbs of drawbar force when clamped on a tool. That seems rather low, doesnt it?

The tech support at Dyna is very helpful but I'm not sure even they know what the drawbar force was exactly supposed to be on this 16 year old machine.

I could easily change the stack spring material (to CRV) or add more springs in parallel to make the stack stiffer, but I dont want to just redesign whatever the original intent was. But 250lbs seems low. The z axis can pull 400lbs according to the spec so I think that means it could pull the tool out of the drawbar if it were truly stuck in the work, like a big tap that was still threaded by accident during tapping when the Z axis goes to clearance.