Practical CNC Failures

[Torchhead]

Stepper or servo? Does the belt actually "slip" or the motor stall ? If it's servo their control has no form of fault shutdown so an axis will fault and stop turning while the others keep trucking.

I have seen the idler arm get sloppy and actually cause a bind when the motor runs in one direction. The idler is just a sleeve bearing and will wear out over time.

Another weak spot is the trans-gantry shaft. It's tiny for the amount of torque it has the withstand. The pinions are held on with set screws and over time will work loose and that can lead to the gantry racking and jamming things up. There have been several reports (on servo machines) of that shaft twisting and eventually breaking.

Motor speed is controlled by the step & dir signals from the PC. You can stall a stepper if you ask it to move faster than it can do at a given torque requirement. IF your motors are indeed stalling (and not the toothed belt jumping out of the grooves and slipping) then it's either from:

Something is binding and raising the torque requirements (remember that steppers lose torque with RPM)

The motor has an intermittent connection on one winding

Since it's just one axis things like noise into the PC are pretty remote.

If it's servo (with encoders) then a dirty encoder on the X motor can cause strange motions and often faults the drive

TOM caudle
www.CandCNC.com

[Hatty62]

Thanks Tom, You are right it is the step motor stalling. Sounds just like the gear belt slipping. The bearings, shaft, and gear seem to be in good shape so I guess I better look at the idler arm if it is stalling putting that heavier spring didn't help. I need your web site I need to talk to you about some parts. So the motor stalling doesn’t mean it is bad right? If I fix the overload problem and the motor still stalls can I get one from you, do you sale them? I have never used anything but a plasma cutter on this table so it hasn’t had to work very hard. Thanks again and when I find a number I will give you a call.

Kyle

[Torchhead]

Typically steppers don't go bad. They are just magnets and two coils. About the only thing to wear out is the shaft bearings. Stalling comes from:

a: trying to spin the motor faster than it can handle at a given load. Steppers lose torque with RPM. When the torque required to move the load exceeds the available torque at a given RPM the motor stalls. If the velocity values in the motor tuning have been changed it could be causing the stalls (trying to spin too fast).

b: If the driver goes bad or things are binding then the stall point may be really low. On most drives there is a setting to control the max pulse current. On Geckos for example it's a resistor. If that value has changed the current may be too low to handle the load. Steppers get torque from current and speed (RPM) from voltage. Since the current is controlled by the drive it sets the max torque. Max torque only happens when the motor shaft is locked. f the current setting is too low the whole torque curve moves down. The best way to check motors and drivers is to swap them around (substitution).

You can swap the X and Y motors and just jog using the Y to jog X and Vice versa. If the problem stays with the X drive then it's for sure not the X motor. Then you have to work backwards. The Drive is the next suspect. If you are sure the velocity and acceleration are correct (x should be very close to Y settings) then the next step is to see if the Y drive can move the X axis at full jog speed without stalling. If it stalls too then you have a mechanical problem on X that is causing it to bind and stall. If Y runs X just fine THEN it's probably the X driver I have no idea what brand of stepper drives PCNC ended up using. They burned their bridge with Gecko.

I know a lot about the PCNC tables because I have been involved with dozens of retrofits of our controls and have actually been on site a few times and done the install myself. We have furnished probably 25 to 30 THC upgrades to PCNC tables and switched them over to work with MACH3 Their stepper systems were okay and biggest issues was with mechanical issues or with their THC that did not work.

Their Servos were really poorly implemented and the controller was based on the Rutex drives but without their (Rutex) motherboard and were not wired according to Rutex specs. The Controller itself was a giant wad of hand wiring. We have a stack of the old PCNC servo boxes out in the shop. Only good thing in the unit is the transformer.

Our contact information is on my website. We do not sell individual motors. Website is on all my signature lines.


TOM caudle
www.CandCNC.com

[Scott V]

I replaced my gantry shaft with better quality material, and also put in some real bearings/holders. I have not been set up to use for a few years, but it really is a decent machine with Toms controls and the stepper system. I really could not do much about the drive gears on the Y axis. With locktite and a notch in the shaft to hold the gear from spinning, It's not a issue. The shaft now runs smooth, unlike the PCNC drive shaft. I think it was bent mild steel too. My friend repairs Prob shafts in work boats, and I followed what he thought was best for a shaft.