Losing X steps, stepper to small??

[matth]

In need of some help from all you guys.

I have got to the point with my 4' x 2' gantry router of cutting in anger and of course the tune up requirements and problems are starting to show!

So, I'm at the stage where I want to router my cutting top flat (this is the biggest, longest job I've run).
Every time I try to complete this job my router losses half an entire x move then starts again after the next y move, this drives the gantry into the x limits and the job fails.

I can't replicate this problem with smaller jobs or long jogging moves.
Now, the y axis is running a 400Oz/inch with a 20mm lead ballscrew, but the x is running an identical stepper on a 2mm lead screw.
Obviously the x stepper is having to run a hell of a lot faster to drive the x axis plus it's having to move the weight of the gantry.
When I encounter this problem the x stepper is very VERY hot, is it possible that this motor is stalling due to the heat (motor is running at 70V and maximum bipolar current)?

My drivers can run upto 8Amps and I have my eye on a 1600Oz/inch stepper to replace the x motor, would this help.

Input anybody???

[ger21]

What is the current rating of the motor, and what is it set at? What's the voltage rating? How fast are you trying to cut?

When you say very, very hot. Can you hold it? Heat is usually from either too much voltage, or too high of current.

A bigger motor may not help, because larger motors usually have a lower top speed.

A higher lead ballscrew may be a better answer.

[matth]

I can't remember the motor spec at the mo but I know its running within its ratings.
The Y axis is running exactly the same motor, voltage, current settings with no problems.

The axis is setup to run at 2300mm/min. The motor is to hot to touch for more than a couple of seconds.

I don't really want to entertain the idea of replacing the ballscrew as this was a second hand purchase and the entire router has been built around this.

Maybe gearing the stepper and the ballscrew 1:2 or last option swap to a servo (I asume these are better at higher RPM's).

WOW, just revisited my MACH motor tuning settings. The X ballscrew is only 0.5mm lead which means that the x stepper to do rapids of 2300mm.min is running at 4600 RPM! I didn't even know a stepper could run that fast.

[ger21]

If you are actualy seeeing 4600 rpm, you probably have virtually zero torque at that high of an rpm.

So your X screw needs to turn 40 times faster than Y to go the same speed? You definately need to change something. Gearing 1:2 might help a little. 1:5 would be better.

If you can touch the motor for a few seconds, it's probably not too hot. Most are rated at ~100°C

[matth]

Thanks for the replies Ger.

Now I am confused, I have had the machine running with 2300mm/min rapids and pushing against the gantry I have not been able to stall the axis.
The cutting that stalled today was with a 1/4" router bit in a 1 3/4HP router cutting 2mm per pass.
I persume that the very small lead of the screw is reintroducing the torque lost at the stepper.
So if I add a 1600Qz/inch stepper geared at 5:1 that will mean that my stpper only has to run at ~900RPM but I will only see ~320Oz/inch torque at the screw. Would this amount be ok??

If the motor is not running to hot, where should I look for the cause of the x axis stalling during a long job?

[ger21]

Originally Posted by matth I presume that the very small lead of the screw is reintroducing the torque lost at the stepper. So if I add a 1600Qz/inch stepper geared at 5:1 that will mean that my stpper only has to run at ~900RPM but I will only see ~320Oz/inch torque at the screw. Would this amount be ok??

It's more complicated than that. There is a formula to figure out how much force a screw will deliver when turned with a given torque. I don't have it handy, though.

The higher the lead, the lower the force. A .5mm lead screw will provide 40 times more force than a 20mm lead screw. The problem, is that steppers lose torque as rpm's increase. Full torque is only available when the stepper is NOT turning. As soon as it starts to spin, it's torque starts to drop. At some rpm, you won't have enough torque to move the machine. Usually that would be under 1500 rpm, but it varies with different motors, and with the voltage applied. Higher voltages will give you more top speed. With larger motors, torque usually drops off faster with rpm than with smaller motors. So, a larger motor may actually have less torque at higher rpm's than smaller ones

You need to see a torque curve chart for your motor to see where the torque drops off. You should gear the motor to stay in the higher torque range of the motor.

One other thing, is that you could be getting binding somewhere as the machine warms up. But I think your trying to spin the stepper too fast.

[dertsap]

if you add a fan to the motor the motor should run much cooler , i have 400oz motors and they get extremely hot due to the voltage i'm pumping to them ,enough that i forgot to turn off the system and when i went into my 10x12 work shop i could feel the heat in the air , which could save me from having to heat the shop in the winter , a couple days ago i got some pc type 12v fans from the local surplus store at 3 bucks a piece , what a difference that made , much better now ,the motors would have crapped out sooner or later from the heat

[matth]

Originally Posted by ger21 It's more complicated than that. There is a formula to figure out how much force a screw will deliver when turned with a given torque. I don't have it handy, though. The higher the lead, the lower the force. A .5mm lead screw will provide 40 times more force than a 20mm lead screw. The problem, is that steppers lose torque as rpm's increase. Full torque is only available when the stepper is NOT turning. As soon as it starts to spin, it's torque starts to drop. At some rpm, you won't have enough torque to move the machine. Usually that would be under 1500 rpm, but it varies with different motors, and with the voltage applied. Higher voltages will give you more top speed. With larger motors, torque usually drops off faster with rpm than with smaller motors. So, a larger motor may actually have less torque at higher rpm's than smaller ones You need to see a torque curve chart for your motor to see where the torque drops off. You should gear the motor to stay in the higher torque range of the motor. One other thing, is that you could be getting binding somewhere as the machine warms up. But I think your trying to spin the stepper too fast.

I think I agree with your last statement here.
So my plan is to gear my current 400Qz stepper 5:1 and then see what I can do and then if necessary upgrade to the 1600Qz.
I'll rerun the last program at half the feed rate to see if the axis stalls again.

[matth]

LOL, I need a coffee or some other simulation for my brain!
I'm sorry for making you guys waste typing time but...

5mm lead not 0.5mm meaning that the stepper is running at 460RPM at max feed.

So, the question still stands. What could be making the x axis stall during a long job?

[dertsap]

acceleration setting ?

[matth]

Acceleration is set in Mach to 250. I have however completed some very long small jobs (milling very complex pcb) which obviously involves a lot of x acceleration and decceleration with no problems!

[pminmo]

Maybe your drive (or other electronics) is overheating and as a consequence falting.