Steppers stall at low speed

[spiral72]

I aquired a home-brew 2 axis CNC about two weeks ago. It's taken me
some time to research steppers and drivers etc and get this thing
running. Finally after about 8 hours of sending bit sequences to the
interface I finally got it figured out.


It come with Tubro CNC, a DOS based communication program with the
interface. The interface is based on the UCN5804B drivers.


I cut my first "sample" over the weekend. All lines, manually entered
G-code. Whew! (I don't have a CNC program yet)


Straight lines cut great. Max linear speed is about 10ipm. I'm cutting
foam with a hot wire. The trouble come when I tried to cut a few arcs
and circles. As the wire round each quatrant of a circle, the steppers
slow down respectively (as they should)..... x slows as it reaches the
90 & 270 degree quadrants.... y slows as it reaches the 0 & 360 degree
quadrants. I hope this makes sense.


The trouble is, as the steppers slow they stall and begin to vibrate.
The program doesn't have a feedback from the steppers so there's no
way for corrections to be made, and we get irregular arcs/circles.


As far as I know the steppers are NEMA 42 running 5VDC. Linear
movement is through a fine threaded rod. The drivers are set up for
wave drive mode.


Any ideas? I'm going to try two phase mode the first chance I get. As
far as I know half step is at a loss of torque right?


Thanks for you're time and any info. I'll check back frequently if
there's any questions I can answer.

[audioandy1762]

5 volt DC seems a little low normaly you would run between 10 and 20 times the rated voltage of the stepper motor with current limit depending on the drive you are using.
Do a search on the forum for stepper motor power supply.

Andy

[spiral72]

Thanks for the information. The steppers are 5v, I assumed the supply was also. Maybe 5v at 10A or something. I guess I'll check the supply voltage with a meter and report back.

Forgive the ignorance. I really did quite a bit of reading on the subject, but there comes a point where someone has to hold your hand or you do it the hard way.


I considered building a bracket to hold a router for small wood working projects. Instead of a stepper, do you suppose using a solenoid and an adjustable stop for the z-axis would work? The wood I plan to cut is balsa, 1/4" or less. No oak or plastic. I haven't found another cnc works a z-axis this way..... maybe for good reason.

[spiral72]

The supply voltage read 28.0VDC

The voltage stayed constant when I jogged the machine, so I'm assuming the supply is keeping up just fine.

Do I need to run more voltage? or should I borrow a meter and find the current delivered under a stalled condition? My meter won't do it.

Would running the steppers in 2-phase mode do anything? The board is hardwired wave mode now. I'll have to some board mods to make the change. It'd been nice if it was a jumper or DIP switch.

[spiral72]

Full step drive (two phases on)
This is the usual method for full step driving the motor. Both phases are always on. The motor will have full rated torque.


Wave drive
In this drive method only a single phase is activated at a time. It has the same number of steps as the full step drive, but the motor will have significantly less than rated torque. It is rarely used.