[Old_Phart]

My Z axis drops when the motors are disabled. I'm using Clearpath motors on an AVID Z stage with a 22 pound 3 kw Water cooled spindle so I imagine that's the reason its an issue for me and not a bigger one for more people.

When motor is enabled or running there is no problem at all, but if i screw up and fault them, which I often do as I learn to use this, or simply when I shut it down for the day, the Z drops and I have to re-zero and/or the weight of the assembly drives the bit into the material.

I have seen springs on some DIY kits so I must not be alone. I have also seen a vid where someone put a brake on the zaxis motor but I don't think he was completely satisfied with that solutions release/engage timing.

I have an air cylinder I was going to fit to provide an air spring counterbalance but before going to that complexity I need to ask if anyone has solved this more simply.

[Jim Dawson]

There are four ways I have seen this done, you mentioned two, coil spring and air cylinder, the others are gas spring and counter weight. Of those choices, I prefer the air cylinder because you get a constant spring rate with very little resistance to movement. Gas springs act like a shock absorber and have resistance to movement, however there some available that have very little shock absorbing action built in.

When using an air cylinder, I tee off the cylinder to a large air reservoir rather than rely on just the regulator to react quick enough to maintain constant pressure. On my machine I use a 5 gallon portable air tank as the air spring reservoir. Some say just the regulator is enough, but we all have our opinions.

The electric brake works OK, and you can set up the timing for no movement in normal operation, but that timing fails in an E-stop condition where it's really needed the most.

[Old_Phart]

Sounds like I am right on target then. I have 8 inches of Z travel so conventional and gas springs would be inconsistent over that length and gas springs are indeed resistive to sudden movement.

There exists a constant tension spring assembly that would be compact and simple to mount (kinda like a tape measure in construction) but its cycle life is really short.

If I went air cylinder I intend to put a 2+ gallon air accumulator and as large a line as I can from the cylinder to the accumulator to keep the air spring pressure constant over a larger range. But it just seems like a lot of work for this kind of things and I hoped someone had a different solution like a linear brake or some widget that ignores physics.

If you have sen the Air cylinder used, could you point me to some pics? I'm having difficulty settling on a compact mounting solution for a 10 inch long air cylinder that doesn't involve pulleys and the ensuing mechanical trouble.
I figured on using small roller chain so I can capture the chain onto the sprockets more easily than cable.

EDIT: I found a bunch online that looks just like I would do this.

[Jim Dawson]

I have been working on an Anti-Grav device, but not quite ready for production yet.

Here is the system I built for my mill. It uses about 1/2 of a Yamaha Banshee brake cable. Has been in service for about 5 years now. The air cylinder is a 1 9/16'' and operates in tension, normally set around 45 PSI. In this case, it's really an anti-backlash device rather than a counterbalance due to the design of my Z drive train.

The cable is secured in a slug similar to a motorcycle brake cable attachment, held by a set screw. Just simply pulls up on the quill. The cylinder end adapter for the cable is simply screwed onto the the end of the cylinder rod and the cable is held by a set screw.


Using a cable system allows you to mount the air cylinder in any convenient location.

[joeavaerage]

Hi,
do Clearpath do a model with a brake?

I recently bought a Delta 750W B2 series servo with a brake and its very good. It operates within 10ms of the enable signal
(or Estop or a number of other programmable circumstances) and it has a holding torque somewhat in excess of the servos
rated torque. The electromagnetic brake will release with as little as 100mA but I have followed the manufacturers recommendation
of a 24VDC supply form which the brake sinks 250mA.

Your proposal of air springs with accumulators and so on sound like they could be as or more expensive than getting a braked servo.

Craig

[Old_Phart]

A Cable.. Thats genius!

CP doesn't make a braked servo , there is an aftermarket generic one for nema34 motors but it's pricey.

[Jim Dawson]

A Cable.. Thats genius!

Thank you

[mactec54]

My Z axis drops when the motors are disabled. I'm using Clearpath motors on an AVID Z stage with a 22 pound 3 kw Water cooled spindle so I imagine that's the reason its an issue for me and not a bigger one for more people.

When motor is enabled or running there is no problem at all, but if i screw up and fault them, which I often do as I learn to use this, or simply when I shut it down for the day, the Z drops and I have to re-zero and/or the weight of the assembly drives the bit into the material.

I have seen springs on some DIY kits so I must not be alone. I have also seen a vid where someone put a brake on the zaxis motor but I don't think he was completely satisfied with that solutions release/engage timing.

I have an air cylinder I was going to fit to provide an air spring counterbalance but before going to that complexity I need to ask if anyone has solved this more simply.

Well there is a even simpler way seeing you servo motor works normally when energized, if you have a plain section on the Ballscrew then with a piece of leather strap you can fasten one end and have an adjustable spring attached to the other end this wraps around the Ballscrew 180 degree and will stop it from moving when the servo motor is deenergized, it takes very little to hold the Ballscrew from turning, this is simple to add and will solve your problem

It can even be a piece of HDPE just touching the Ballscrew, it takes very little to stop the Ballscrew from back driving

[Old_Phart]

There is very little area to get to the ballscrew on this Avid stage, but I might be able to reach the coupling... I'll check that out.

[mactec54]

There is very little area to get to the ballscrew on this Avid stage, but I might be able to reach the coupling... I'll check that out.

Yes the side of the Coupling would work either on the motor side or at the ballscrew screw side you may be able to drill and tap a hole in the housing and have a plastic cap PTFE or HDPE on the end of a threaded Bolt /capscrew about 3/8" dia should do the job the plastic cap should have a small spring inside, this is just a simple friction pad there are manufactured machines that using this on there Z axes for the same as what you want to do

[Old_Phart]

Apologize for the delay in reply. something happened to the forums database and my logon got mangled - support kept sending instructions on how to reset a password and that never worked. Finally someone *actually read my request* and found some problem and fixed it.

I opened the top and found that it takes considerable friction to stop the Z axis coasting down. Far more than I thought it would. Because of that I dont think a passive friction stop on the rotating member would be a good solution. It would fry during normal operation.
Pity, I liked that idea.

[mactec54]

Apologize for the delay in reply. something happened to the forums database and my logon got mangled - support kept sending instructions on how to reset a password and that never worked. Finally someone *actually read my request* and found some problem and fixed it.

I opened the top and found that it takes considerable friction to stop the Z axis coasting down. Far more than I thought it would. Because of that I dont think a passive friction stop on the rotating member would be a good solution. It would fry during normal operation.
Pity, I liked that idea.

You could make a clamp that is solenoid operated or use a spring loaded air cylinder to clamp it, the spring load would clamp the shaft and the air cylinder would open it on power up

The Emco Mill head is heavier than your head, and a friction brake works fine

So if this is not going to work for you then you will have to add a brake to the motor or buy a motor with a built in brake

[Old_Phart]

You could make a clamp that is solenoid operated or use a spring loaded air cylinder to clamp it, the spring load would clamp the shaft and the air cylinder would open it on power up

Yea, I'm thinking about that. A vertical bar with a caliper type of brake is another idea. Comes down to a Brake or a counterweight So many options . . .. . .thanks for the feedback.

[wmgeorge]

Any sort of friction setup is going to be present when your running also. Either a motor with a brake as been suggested (do they make?) or counter weight.

[wmgeorge]

Stepper motor with brake > https://www.automationtechnologiesin...2-brake-motor/

[Old_Phart]

I am not running steppers, CP doesn't make a motor with a brake, there is no room to add a friction component to the shaft on the stage and a third party Nema 34 sized brake is $500!

I'm going to have to go with a counterweight of some design. Air Cylinder is my current favorite but I'll keep thinking. Its not an imperative yet, this is a hobby machine.

[FoxCNC1]

Hi guys, I was looking at the clear path servos myself. So I am understanding is that when the machine is off, the Z drops because the servo motor just rotates because of the weight of the Z ?

I wanted to ask, what happens when you hit the E-stop button during operation so the axis keep coasting?

[joeavaerage]

Hi,
I imagine the servo would crash stop but thereafter be uncontrolled. If the weight of the machine causes an axis to sag...then that's what it will do.
Either fit a brake as described above or get a braked servo.

I recently bought a braked 750W Delta B2 series servo, it cost about $100 more than an unbraked servo of the same size.

Clearpath servos are good quality but are expensive for their power output. Other brands like Delta and DMM are also good quality
and are more powerful for the same or less money.

Craig

[Old_Phart]

That depends on what you do with the enable signal to the CP motor. You can wire the enable high and they wont drop (or coast) till power is removed. But if you do that, an E-stop will stop the train of step signals sent to the motor but the motor will continue to complete the steps is has been issued.n Not exactly coasting because it will use all its power to complete the steps it has been given.

If you remove the enable signal it could probably coast but not very far. There is a lot of reluctance in these so they don't like to be pushed or to coast.

[Rogdc]

Which model of Clearpath Servo are you using?

Does this happen when the motors are energized or only powered off?