Might also verify that your step pulse active states are set properly.

Increasing the pulse width slightly might also be worth a try.
Russ
 

I have these nema 23 motors

http://www.ebay.de/itm/From-German-WH-3pcs-Nema23-dual-shaft-stepper-motor-425oz-in-CNC-Free-Shipping-/201017269054?pt=Motoren_Getriebe&hash=item2ecd901b3e

I will try to find those settings in bios. Maybe it helps.

I can mount nema 23 but a thought that bigger is better so I didnt try the smaller nema 23  425oz.

Some things are indeed better when bigger - but this doesn't necessarily include stepper motors. Unfortunately your 23s aren't ideal either - they're quite high inductance and would require approx. 100Vdc to get their *max* torque/speed potential.

All this said - don't make the mistake of chasing speed for the sake of it. As long as you can achieve the cutting speeds you need, rapids can be somewhat less important. The thing often not considered is what acceleration can you get - now that IS important.

Another thing to try is this.

Go into the PC Bios, and disable C1E or EIST if you have them. This allows Mach3 to run much better on some multi core PC's.

However, most 1600oz steppers are very slow. Ebay is flooded with them, and I see this all the time. People buy them, thinking bigger is better. In actual use, a 450oz Nema 34 will often run a router at 5x faster than a 1600oz motor.

I disabled C1E and I think that did the trick.

Now I'm running 4000 per min with  no problems.

This is more than double  of what I was getting earlier.

It can go probably up to 5000 per min but for my steppers this is more than enough.

Thanks a lot to everyone.
This is one of the best communities and people here certainly know what they talking about.

Thanks a lot again

Might also verify that your step pulse active states are set properly.

Increasing the pulse width slightly might also be worth a try.
Russ
 

My drivers are set to

DIR low active  (here is active low cause I have to change direction)

STEP low active   (when I set high active the drivers dont work)

Where can I increase the pulse width ?

All this said - don't make the mistake of chasing speed for the sake of it. As long as you can achieve the cutting speeds you need, rapids can be somewhat less important. The thing often not considered is what acceleration can you get - now that IS important.

This is wisely said.

look on the motor tuning dialog for increasing the step pulse width. Unfortunately your driver data sheet doesn't specify what minimum pulse width is required (Chinese... what can you say?). Also your Chinese chappy is fond of using slow (as in cheap) optos but if it's not missing steps you're probably ok. The dir pulse value is the extension to the default min time the dir signal must be changed before the next step pulse. This should only need changing if you find you're losing the first step after a direction change.

I'm still a tad mystified by your early comment that you got stalling when you tried to move more than one motor at a time. Is this still happening?

When I have disabled the C1E in bios now everything work very smooth and fast.

I have set my rapids to 3000 now (before the max was 1600).

I can go to 6000 without loosing steps but I have to reduce my acceleration.
So I set it to 3000 with very fast acceleration.

For the job that  took 1h and 8 min to complete, now it is done in 48 min.
The cutting speed is the same.

And you were right about the motors running not coupled to anything,
I have mounted them on the CNC and they work perfectly.

Thanks again for all your help and advice.