Direct Drive Servo or pulleys

[sendkeys]

[Swede]

I'm a fan of direct drive when practical. It's simply easier, less components, zero backlash. You can always use timing belts/pulleys later if you need more torque, or continue direct drive but use a bigger motor.

As a CNC new guy, I was quite surprised at just how powerful a bigger NEMA 23 was for my bench mill, let alone the 34 that does the Z. I ran my mill continuously yesterday for hours and the motors simply did not get warm.

So I vote for direct for simplicity, assuming the torque is there.

[doug6949]

One factor to consider is the length of your screws. You want to keep them below critical velocity if possible. RBS, Thomson, etc have charts to determine how fast you can spin a particular screw. If 5tpi goes above this number then you can use 2.5tpi.

[balsaman]

What drives? Gecko 320's like encoders under 500 ccr.

[sendkeys]

Thanks everyone

[balsaman]

Those will work.

Eric

[bunalmis]

Originally posted by doug6949 One factor to consider is the length of your screws. You want to keep them below critical velocity if possible. ....

What is the crtical velocity?

[doug6949]

Critical velocity is the speed (rpm's) that corresponds to the screw's natural resonance. It is sometimes OK to go up or down past this speed but if you stay near it very long the screw will act like a guitar string. The nature of motion control dictates that you stay below critical velocity in your design.