Servo brake, gas spring advice

[Cadman2500]

Hello Centroid community,
Ok I am deep into a new retrofit.
It is a medium sized bed mill with DC servos and a BP clone head.
This is an Acorn build with step direction drives.
I found that the Z axis has a servo brake and no counterbalance. Almost the hard way...

I am looking for advice from someone who has used gas springs as a counterbalance.
The head is quite heavy 3-400 lbs as a guess, I am going to measure tomorrow with a bathroom scale/floor jack. (don't tell the wife, scale says it goes to 400)
I am thinking about adding gas springs.
My reasoning is that the spring will assist in retracting the head leaving more torque to be used for faster acceleration on rapids when retracting the Z.
The gas springs have little to know mass compared to a counterbalance so that is an advantage.
The gas spring is not constant force, so this is a dis-advantage.
There are gas springs you can bleed to get to where you want to be and maybe I could order them pre-charged to what I want.
If you have any good known sources for these please point me in the right direction.

My question is what is the best way forward using the Acorn on my setup. Just brake, no brake and springs, springs and no brake?
Does the software work well with the brake?
How does the software use the brake?
Is it only on in tool change position or any time the servo stops?
I may try machining some 3D surfaces with CAM, so I don't want the brake to cause any glitches.

Regards,
Chris

[Gary Campbell]

Use an air cylinder or two. You can get "neutral bouyancy" via adjusting a regulator

[fusion]

Hi Chris. I did the exact thing you are describing. I added 2 200 pound gas springs from McMaster Carr. They were $26 each I think. I attached them to the head and column on my Haas mini mill Acorn conversion. To drive the Z axis I use a Nema 42 stepper that has a brake. I used a relay set to z axis brake in Acorn. The software works perfectly for this. It activates the brake at the correct times. I'm really impressed with how it just works.
Without the gas springs I was limited to 115 ipm but with them I'm at 200 ipm. Its fast enough to scare me.

[Cadman2500]

I think the Air cylinders with a regulator may be the best option for performance but I am going to be forced to use the gas springs to start based on price. (not working due to Covid-19) Cost adder will be ~300.00 to use air cylinders over gas springs. Design intent will be to make the failure mode (no brake no servo power) move the head most of the way up but not to end of travel. I'll probably have to make some type of spring compressor tool with U-bolts to a steel tube and threaded rod to compress. I might try and look at putting an angle on the spring to get the force more constant too.

Scary stuff; viewers, please don't try this at home. Wait, I'm doing this at home.


Thank guys for the advice.
Chris

[Muzzer]

5 bar of air pressure and a 2" diameter piston will, develop around 200lbf. You can buy heavy wall (> 1/4") plastic pipe eg gas vent etc and suitable lip seals easily enough. The 4" pipe I used for my power drawbar had an impressive pressure rating marked on the side. It would be an easy enough project when the time comes.

Admittedly, the amount of potential energy stored in such a spring is fairly scary but there again it's of the same order as the potential energy of the milling head in the first place.

[martyscncgarage]

Guden sells gas springs. I used one on a build some time ago:
https://www.guden.com/Control/Gas-Springs

Marty

[Gary Campbell]

A 2" diameter cylinder has a surface area of 3.1416 sqin. That means that 100 psi on the cylinder will produce 314 pounds of lift, less friction