Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

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RJS100
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Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

Post by RJS100 »

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
Attachments
Pseudeo Z Axis Brake.jpg
martyscncgarage
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Re: Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

Post by martyscncgarage »

In general, if the machine has dovetail or box ways on Z, the friction does not "generally" let them slide down.
If they have linear ways then most certainly you need an axis motor with a brake. OR if the head is considerably heavy and not counterweighted, you would need an axis motor with a brake. There are brakes that can be ADDED between the axis motor and the ballscrew but are typically more expensive than buying an axis motor with a brake.

I am not so sure I would short the windings together like you have it shown. Though I have never done that. Normally you use AXIS3BRAKERELEASE output to control the brake mechanism. It energizes to release the brake at the time the axis motor is holding the axis.

Marty
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RJS100
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Re: Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

Post by RJS100 »

Thanks for your thoughts Marty. I too am not sure what issues I may have by shorting together the motor windings. I can't imagine this creating much current as the torque required would be minimal. I was just curios if anybody had ever tried this approach. Fortunately, I have a backup plan as the 1700 oz. stepper I plan on using is also available with a brake. I just happen to own the model without the brake.

Thanks again... Richard
martyscncgarage
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Re: Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

Post by martyscncgarage »

Look at this:
http://www.automation4less.com/store/pr ... 34-24-0500

Probably cheaper to buy the correct motor with a brake IF its needed.

I would not mess with the stepper windings. Do it right, control it correctly.
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RJS100
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Re: Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

Post by RJS100 »

I am surely leaning that way. Thanks for your thoughts.... Richard
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Re: Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

Post by Muzzer »

Much simpler to fit a counterweight or an air spring (piston) to offset the weight of the head. Then you only need to worry about friction and inertia. This of course is quite a common approach.

Presumably, shorting the windings will only generate torque if there is movement, which by definition is too late, in which case this scheme would simply slow the movement down. Accidentally opening or closing the relay when the driver is operating could trash the drive, so there are plenty of opportunities for this to go wrong.
RJS100
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Re: Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

Post by RJS100 »

Thanks for your thoughts! I was thinking about adding an air spring to off load the Z-axis and glad that you mentioned it. I was not thinking about it with regards to solving the issue with the Z-axis slipping when not under power. Thanks for bringing it to my attention.

On the bench, when I short the motor windings, the shaft becomes virtually locked into position and strongly resists motion of any kind (like a very strong brake has been applied). I believe as you torque the shaft, the slight movement of the coils through the magnetic field from the permanent magnets induces a current in the shorted wires. Since the relay contacts would kill power to the stepper before shorting the motor windings, I don't think the driver could be trashed. I contemplated using a adding a resistor to limit the current.

In either case, I think the use of an air spring is the best solution yet.

Thanks again... Richard
martyscncgarage
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Re: Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

Post by martyscncgarage »

Reminder, for support please follow this post: viewtopic.php?f=20&t=383
We can't "SEE" what you see...
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ShawnM
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Re: Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

Post by ShawnM »

I think an air counterbalance is the way to go and it has a few advantages over just a brake. The Biesse Rover that I’m currently retrofitting had Yaskawa motors with no brake on the Z axis but it does have dual air cylinders to counterweight the nearly 50 pound HSD spindle and the large, aluminum Z axis plate. There has to be nearly 85 pounds of “stuff” moving up and down with the Z axis. There’s no doubt it would fall without the pneumatic counterbalance. I can dial in the proper pressure on the regulator to easily make it float anywhere and it doesn’t fall when powered down. This leads to the second advantage, quicker Z axis moves. Since all that weight is now balanced the Z axis motor works as easy going up as it does going down. On a typical machine the Z axis is the slowest axis because of gravity and having to lift all the weight of the Z axis. When it’s counterbalanced it has much faster moves up and down and can decrease machining times not to mention less wear and tear on the motor and drive. So using pneumatic cylinders or gas struts has its advantages. Just my 2 cents.
Last edited by ShawnM on Mon Sep 20, 2021 8:42 am, edited 1 time in total.
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Re: Pseudo Z-Axis Brake & Stepper Size - Love your thoughts!

Post by rk9268vc »

I used servos on Z axis (which have no detent) and removed air strut on my PM-30 build. It has no counter balance and the head weighs approx 100lbs
It is a dovetail machine with tight gibs and the head doesnt move when powered off

I bought an inline motor brake. They can be had for around 270 freedom bucks
Here is the one I used and recommend (not sponsored), just give it 24VDC to release the brake
http://www.automation4less.com/store/pr ... 34-24-0500
Just slap it in-between the motor and the ballscrew
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