Why use a 8 mm lead screw?

Why is everyone using a 8mm lead screw i.e. 8mm movement per rotation? The advantage 8mm lead is faster Z movement. But the huge drawback of a 8mm lead screw is that if the steppers are not powered, the z axis drops down. The mill bits can then break or damage the work piece. I am seeing a lot of parking frameworks just to get around this issue.

I am using a 2mm lead screw (2mm movement per rotation) for my lowrider2, and that works very well. The Z axis stays in place, even with unpowered steppers. And the Z movements are still fast enough.

So why use a 8mm lead?

 

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That’s interesting. Is there a significant price difference?

You’ve got about 3200 steps/mm then? We used to use an allthread 5/16" screw for the mpcnc and it had about 4500 steps/mm and there were a lot of problems with the 8 bit micro crashing if it went too fast. Marlin has gotten a lot better in that respect. Also, at 12V, the steppers don’t have as much torque when going faster, ao you’re closer to losing steps. Those are my two guesses.

With a 24V or just limiting it more in Marlin, that seems like a very food option.

I use marlin on a ramps1.4 with A4988 stepper drivers in 16x microstep mode on 12v. It uses then 1600 steps per mm. The Z feed rate is limited to 15mm/sec maximal. 20mm/sec also worked. At 30mm/sec the stepper started loosing steps. Possible even that could have worked if I had turned the stepper current higher.

Because the movement is 2mm instead of 8mm per rotation, the torque required on the stepper for a 2 mm lead is thus only 1/4 of what it would have needed on 8mm lead. In other words the available force on the z axis is 4x higher.

At 15mm/sec I tried to stall the stepper by blocking the z movement, but that was almost impossible.

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The trouble is that without more voltage, the driver actually can’t reach the current limit before it steps again. There is a limit there, and it’s in rev/s on the motor, and if you exceed it, your torque will drop significantly.

You make a good point about the increased leverage though. IDK what at what speed with 12V the torque would be reduced by 4x.