Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!
Posted: Thu Sep 09, 2021 12:15 am
I am working on my very fist CNC conversion. I will be converting a Precision Matthews PM-833TV mill to CNC (I am waiting for delivery of the mill). I have completed most of the electrical work at this time and bench tested it successfully. The components I will be using are:
-Acorn board
-Three closed loops stepper drivers (Stepper Online CL86Y) - Can accept AC input power.
- One 1700 Ounce closed loop stepper (Z-Axis)
- Two 1200 Ounce closed loop steppers (X,Y-Axis)
- 70V/35V Toroidal transformer
- Proximity sensors to establish home position.
- Two additional din rail mounted axillary transformers
- Heavy Metal double ball nut conversion kit.
The articles I have read regarding proper sizing of stepper motors suggest that bigger is not always better, and that a properly sized stepper will perform better than an over sized stepper.
Heavy Metal (the folks that sell the ball nut/ball screw conversion kits for this mill) use a 2800-ounce stepper motor for the Z-axis. When I questioned why such a large stepper, they said that is what their customers all request, but commented that it was probably oversized, and a smaller unit would probably be fine. BUT… they had not tested a smaller stepper for the Z-Axis.
I searched a few forums and found that the larger stepper was being used only for it’s high detent torque to stop the Z-axis from moving downward and crashing when the power to the mill was shut off. I have a hard time accepting this because Arizona CNC kits makes a conversion kit for the PM-940 (a much heavier machine). They use a 1700 ounce closed loop stepper for this conversion. Both conversion kits use 5mm pitch ball screws. I am challenged here because Precision Matthews will not provide information on the weight of the heads of either of these machines, or even if one is heavier than the other.
Seeing that all agree that a 1700 oz. closed loop stepper would have plenty of torque, but that the Z-axis may slip when the mill is turned off, I came up with a possible solution. What if I use a relay/contactor that upon shutting off the machine will perform the following: One set of contacts would close and shunt one of the motor windings. The other set of contacts would open kill power to the stepper driver. Upon shunting the motor windings, the stepper would lock in place just as if a brake was applied, while the other contact would kill power to the stepper driver so no power could be applied to the motor windings while they were shunted. Below is a crude drawing. Please let me know your thoughts. I am completely new to this so if this not a good idea…. Don’t be bashful. If it is a good idea, please let me know where I may find a din rail mounted contactor/relay for this job.
Thanks….Richard
-Acorn board
-Three closed loops stepper drivers (Stepper Online CL86Y) - Can accept AC input power.
- One 1700 Ounce closed loop stepper (Z-Axis)
- Two 1200 Ounce closed loop steppers (X,Y-Axis)
- 70V/35V Toroidal transformer
- Proximity sensors to establish home position.
- Two additional din rail mounted axillary transformers
- Heavy Metal double ball nut conversion kit.
The articles I have read regarding proper sizing of stepper motors suggest that bigger is not always better, and that a properly sized stepper will perform better than an over sized stepper.
Heavy Metal (the folks that sell the ball nut/ball screw conversion kits for this mill) use a 2800-ounce stepper motor for the Z-axis. When I questioned why such a large stepper, they said that is what their customers all request, but commented that it was probably oversized, and a smaller unit would probably be fine. BUT… they had not tested a smaller stepper for the Z-Axis.
I searched a few forums and found that the larger stepper was being used only for it’s high detent torque to stop the Z-axis from moving downward and crashing when the power to the mill was shut off. I have a hard time accepting this because Arizona CNC kits makes a conversion kit for the PM-940 (a much heavier machine). They use a 1700 ounce closed loop stepper for this conversion. Both conversion kits use 5mm pitch ball screws. I am challenged here because Precision Matthews will not provide information on the weight of the heads of either of these machines, or even if one is heavier than the other.
Seeing that all agree that a 1700 oz. closed loop stepper would have plenty of torque, but that the Z-axis may slip when the mill is turned off, I came up with a possible solution. What if I use a relay/contactor that upon shutting off the machine will perform the following: One set of contacts would close and shunt one of the motor windings. The other set of contacts would open kill power to the stepper driver. Upon shunting the motor windings, the stepper would lock in place just as if a brake was applied, while the other contact would kill power to the stepper driver so no power could be applied to the motor windings while they were shunted. Below is a crude drawing. Please let me know your thoughts. I am completely new to this so if this not a good idea…. Don’t be bashful. If it is a good idea, please let me know where I may find a din rail mounted contactor/relay for this job.
Thanks….Richard