Z axis counterweight

[TPPJR]

I am thinking about doing a counterweight for my RF-45 CNC conversion and am trying to decide on the route to go. Either a true counterweight or a gas spring. If I use a counterweight won't the total mass of the system be felt on the servo's. Meaning if the system is balanced then the servo will not have to push against the weight of the head to move up but it still has to get the mass of both the head and the counterweight moving and once moving it has to stop it. Will this be a problem?

Thanks for any info.

Thomas

[sdantonio]

Hi Thomas,

I enquired into that a while ago and everyone who got back to me said they didn't think it was necessary. The majority were running their z axis with 270 in-oz steppers and had not seen any problems due mass of the head.

[S_J_H]

"counter weight'. The 2 weights oppose one another and act to cancel each out.
This argument seems to stem from guys who have never used such a system and are thinking to much.
I have not done any actual mathematics on this and I can see how one might consider this a issue.
But I have done back to back testing with my mini mill and a 2:1 ratio 22lb compounded counterweight system(44lbs acting against the head).

With a 269 in/oz stepper I can see much faster rapids and change of direction WITH the counter weight than without.
In fact I NEVER lost a step with the z axis Until I removed the counterweight system. Nope, big difference for the better with the counter weight.
I added such a system to my x3. Incredible difference in cranking force. I can move the ~ 100lb head with one finger at the hand wheel.
Steve

[nicad]

TPPJR-

I understand what you are getting at. I wondered about the same thing. It is a question of head inertia. Both gas springs and counterweights will result in the same thing ("weightless" head), but the counterweights will add twice the moment of inertia. I personally would go with the gas springs.

[bill south]

With all the mechanical gizmo's required for a counterweight system and the cost of gas springs from mcmaster, it was a no brainer for me. I purshased 2, 1 for each side of the head. This give a balanced push/pull on the heads (in theory) and doesn't bind the gibs. Looks better too.
Springs for me.
Bill

[HuFlungDung]

Does inertia have a direction? Is it a vectorable quantity?
The head and the counterweight move in opposite directions, so they should cancel.

However, if the rapid speed is high enough that moving the head up puts the counterweight in free fall, I guess you'd want to avoid that

[sdantonio]

Hey Hu.

Inertia is a scalar, which means, by itself, it is not a vector. The problem is, once it is set in motion it is momentum which is a vector and the forces requited to change the direction of the momentum vector (and this is what most people are actually dealing with in a situation like this).

Inertia is the resistance the change in the state of motion, momentum is the actual measurent of the mass in motion (mass and inertia are interchangable out to the current limit of detection which I think is somewhat like 38 decimal places or some huge number like that). What we should be calculating here is the forces acting on the head

Highly simplified:

f=ma where a is the acceleration of the motor not forgetting
w=mg, gravity acting on the head i.e. weight
and cw=-kx counter weight, assuming the gas shocks can be simplified to a spring equation.

But, with all this being said. if it works, draw a picture of it and post it here please.

[JRouche]

You guys have prolly seen this one but incase not. Looks good to me, the best I have seen yet. JRouche

The original thread. Max711 is the author of the gas strut adaptation.

[sdantonio]

I hadn't seen that one, I was at one time thinking something simple like a spring rather than a gas strut. Set so that the spring, at full stretch, takes almost 100% of the head weight. But this one looks much more high tech and impressive.

[joecnc2006]

This is what i did on my 2nd machine when i had 100oz motors on it.

http://www.cnczone.com/forums/showpo...6&postcount=16

[sdantonio]

Joe,

That's basically the same thing I was looking at (hopefully this link will work right)

http://www.cnczone.com/forums/showth...ing#post151377

[TPPJR]

Originally Posted by nicad TPPJR- I understand what you are getting at. I wondered about the same thing. It is a question of head inertia. Both gas springs and counterweights will result in the same thing ("weightless" head), but the counterweights will add twice the moment of inertia. I personally would go with the gas springs.

I think that is what I am going to do but think it will be a combination of the two systems. I am looking at using a cable system just like I would if using a hanging counterweight but use the gas spring in place of the counterweight. This way everything will be in the back and out of the way. With the Z axis DRO, limit switch and the geartrain lube cooling hoses I have enough stuff around the head already.

Thanks for all the help.

Thomas