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

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.

"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

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.

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

Does inertia have a direction? Is it a vectorable quantity? :D
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 :D

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.

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. (http://www.cnczone.com/forums/showthread.php?t=18047)

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.

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

http://www.cnczone.com/forums/showpost.php?p=77346&postcount=16

Joe,

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

http://www.cnczone.com/forums/showthread.php?p=151377&highlight=spring#post151377

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

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

Thomas, your quote got me thinking. On these RF-45 square column mills it seems like there might even be enough room to put the gas springs inside the column and hide them completely. There isn't a lot inside there but a leadscrew, and the springs are compact. I'm thinking especially of my IH Mill, similar to what JRouche pictured. You could also hide a counterweight system in there. Heck, I'm not sure but what you couldn't use a regular spring steel spring of some kind too.

One advantage would be that the springs would be more nearly vertical. The system like that shown on the blue IH mill above has them at an angle, which means their effect will change with height because that angle is changing.

One disadvantage is that it seems inevitable that some vertical travel will be lost. I suppose you could run them out the top of the column if that was an issue.

I do quite like the idea of a "hidden" system. I also like the counterweight idea in general. Those IH mill heads are HEAVY (I know having lifted one). I have visions of the thing needing to bounce up and down really fast for a steep Z-Axis contour. Ballscrews and some 600 oz in servos (like the Keller servos Mariss F mentioned on the Gecko site for cheap) ought to work really well with a little counterweight system of some kind.

Heck, my right arm has gotten bigger than my left cranking the head up and down, maybe I should schedule this mod sooner than later! LOL

Best,

BW

I think a counterweight is really the best answer. A gas strut is great and looks good, but limits how close to the table the mill head can get. If I had a 1/2" piece of stock on my mill table with a smaller diameter collet and had a gas strut, I would not be able to reach.

Has anyone used auto-balancing hydraulic cylinder's?

Thomas, your quote got me thinking. On these RF-45 square column mills it seems like there might even be enough room to put the gas springs inside the column and hide them completely. There isn't a lot inside there but a leadscrew, and the springs are compact. I'm thinking especially of my IH Mill, similar to what JRouche pictured. You could also hide a counterweight system in there. Heck, I'm not sure but what you couldn't use a regular spring steel spring of some kind too.

One advantage would be that the springs would be more nearly vertical. The system like that shown on the blue IH mill above has them at an angle, which means their effect will change with height because that angle is changing.

One disadvantage is that it seems inevitable that some vertical travel will be lost. I suppose you could run them out the top of the column if that was an issue.

I do quite like the idea of a "hidden" system. I also like the counterweight idea in general. Those IH mill heads are HEAVY (I know having lifted one). I have visions of the thing needing to bounce up and down really fast for a steep Z-Axis contour. Ballscrews and some 600 oz in servos (like the Keller servos Mariss F mentioned on the Gecko site for cheap) ought to work really well with a little counterweight system of some kind.

Heck, my right arm has gotten bigger than my left cranking the head up and down, maybe I should schedule this mod sooner than later! LOL

Best,

BW

BW,

I could easily hide the gas strut inside. The reason I am not going to do it that way is because the tension will be acting on the gibs in the same direction as the weight of the head. I want to try and counter some of the load on the gibs at the same time as the head.

Thomas

I think a counterweight is really the best answer. A gas strut is great and looks good, but limits how close to the table the mill head can get. If I had a 1/2" piece of stock on my mill table with a smaller diameter collet and had a gas strut, I would not be able to reach.


I am confident that if I do a combination gas strut and pulley system like a counterweight I will be able to retain full movement. I guarantee that I will be able to reach the table. If I loose anything it will be at the top.

Thomas

Here is what I have come up with:

http://ImageEvent.com/tppjr/rf45dovetailmill/zaxiscounterweight

Seems to work perfect so far. Only thing is that I think I will change the strut to a 150 lb instead of the 200. Full counterbalancing without the added mass of actual weights.

Let me know what you think.

Thomas

BTW, I also added a folder for spindle cooling utilizing my coolant flow.

Here is what I have come up with:

Thomas
Looks good Thomas.

One question what cable did you use.

Wouldn't want stainless as it will work harden...... Won't it :confused:

or make your own gas cylinder like I did.

The only problem I had was pressure build up when the cylinder is in full compression.

2 ways to overcome this 1: use a resivoir to let the pressure build in, just the fact that the volume is alot higher will prevent any major build up. or 2: use a airpressure regulator to regulate the air, any time the pressure exceeds the regulator it will blow off into the air.

The downs side of this is you must have compressed air, probably in the form of an air compressor.

here are some decidedly unclear pictures.

Looks good Thomas.

One question what cable did you use.

Wouldn't want stainless as it will work harden...... Won't it :confused:

I used regular 3/32 cable, the stuff made for garage doors.

Thomas

3/32 denotes the strands, not the diameter, is that right Thomas?

I don't know, must be. The wire as well as the ferrells were both listed as 3/32. The wire itself measures about 1/8" in diameter.

Thomas