i recently purchased one of these machines as an introduction to cnc..
aside from the loose rails on the gantry,  dismal earthing in the controller, the machine functions. my big problem is the axis motors XZY - i cannot set the velocity/acceleration to a level that works. if the settings are high the axis move fast when jogging then stalls and screams. Lowering the settings gives a loss of steps perceived by a light finger on the motor handwheel the screaming stops tho'
this is causing some concern as i am assuming that a loss of steps intermittently will affect the accuracy. as the failure is intermittent/random its difficult to pinpoint.
the supplier has been reasonable in that they have dialled in and checked out the Mach3 settings and the internals of the controller. I think now they have run out of ideas.
i have taken off the Y motor so eliminated any mechanical problem and it shows the loss of steps whilst jogging. i am beginning to think the problem may be in the PC. this is a

Dell optiplex 740 running win7 with the properties for Mach3 set to run in a winXP environment.

ive checked all the steppers for shorts/earths - the only problem i have seen with the router is I can measure 50vac between the router and the controller earth when the spindle motor is running. originally the earthing was so bad on the controller that the start-up / down of the spindle caused the axis smotors to go berserk, so scraping the paint off the connections cured that although the  voltage still exists. the motor terminals and associated cabling dont show any measurements (ohms) to earth. wether that is causing a problem i am unsure - disconnecting the motor from the controller didnt show any effect on the stalling problem.

Are there any interrupts coming from the PC that could cause the signals to the controller to miss a step or two? im clutching at straws here thinking of interrupts unfortunately i am not in a position to change the PC.
im just thinking 'intermittent, random'. its something common to all axis so - controller PSU, interface card, parallel cable, PC ?

any thoughts from anyone would be most welcome.

being new here Im not sure where this posting will end up so my apologies, maybe the administrator could put me right?

I can measure 50vac between the router and the controller earth when the spindle motor is running

What you are describing is a serious and hazardous fault which could possibly result in loss of life.

Before you do anything else cure that problem.

Fo any equipment that is connected to incoming mains electricity, every metal (or electrically conductive) part that an operator can touch must be correctly bonded to protective Earth. There are variations (such as double insulated) so check the electrical regulations that apply in your country.

Tweakie.

thanks for that tweakie

I will bond the table and controller together that will earth any stray volts - but that doesnt appear to cause the problem with the steppers
as with it disconnected completely the steppers still fail

Richardw,

I have a Dell Optiplex 740 and can't use it for Mach 3 with the Parallel port.
( We have tested numerous different Dell's and they all sucked!)
The quality of the output signal generated from the pc is poor due to onboard grahics or whatever.There is NOTHING much you can do to correct the signal so consider anexternal motion controller or different pc. Another problem will be the pulse frequency stability and actual value.Go to Diganostics and the pulse frequency should be very close to Kernel SPeed and not fluctuating. On some Dell's the pulse frequency was say 24000 instead of 25000 AND the frequency would drop to say 14000 when running a program.


Tweakie already commmented about the grounding and it is important.

My opinion of the Chinese Controller......, problematic junk, and personaly just don't even bother providing support for any of the 30xx series.

Sorry wish my commnents could be positve but you have what you have.

Just my nickle,

RICH

Thanks for that info Rich, I must say I half suspected a problem with the PC but being a newbie it took me a while to get to grips with the machine/ controller / Mach3 / g-code package.
I think I'll invest in another no-dell PC (I did have another lined up but forget that) so eBay here I come. Your feedback has helped a lot so thank you very much.
On the grounding issue, I assumed that the socket in the controller providing the spindle motor, having an earth connection on it would have grounded the motor via the plug and cable. It transpires the cable did not have a core for an earth!
So an earth strap fitted and all is OK.
So thanks again.

as an update, the PC was finally replaced with an Asus one and looks good!
the PC was $25 from a local ebay seller.  I did check the motor earthing again
and found that the 800W spindle motor spec'd an earth on pin 4 but there isnt one
connected internally of the motor (manufacturing fault?) and there was an earth wire
 in the supply cable.Ive also found the diagnostic
package on Mach3 that checks the clock frquency and this is good at 25kHz it reports.
so all should be good i hope, although i am attempting to get information back about the
controller (PRT-E750) from the supplier as there appears no information on the web from
lengthy searches. the information ive asked for is stepper motor spec',
front spindle motor panel setups and switch settings on the driver cards, all for future reference.

nothing forthcoming as yet!