[letmefixit]

Ok with the huge size of the head on the IH machine. As anyone ever thought of doing a counter balance on the head so that you don't have to use a bigger motor to drive it????

DJ

[Shev]

The standard IH motor is only like 640oz/in. belted 4:1...

[Runner4404spd]

I ended up swapping my 640 oz/in motor for a 850 oz/in motor. i have thought about counter balancing the head and really think that its a good idea. most larger cnc mills have the counter balance. you would have to remount the motor drive of the z axis so it runs front to back as opposed to off to the side. had i thought about it sooner i would have counter balanced it.

[letmefixit]

So would you balance with a gas cylinder or with pulley and weight? Pulley and weight would be ok because the weight of the head pretty much stays the same. I have seen a couple of bigger Bridgeports mills with hydraulic counter balances. There table weight is always changing. But i think thats a little complex for this small of an aplication. Just curious but what is the reduction on the x and y axis?? if anyone knows.

[ninefinger]

I was looking for a counterbalance too for these mills. One advantage of having a counterbalance is that with the power off or heaven forbid a broken belt on the servo drive the head stays in place instead of crashing into the table or whatever is underneath it at the time.

I kinda ruled out a weight due to the bulk and ...well the weight. Lifting the head into place is enough of a challenge. Also a weight adds to the mass that needs to be accelerated in either direction. I'd like a gas spring but finding one with a 20" un-damped stroke and enough force to use 2 (ie 125lbf) has been fruitless so far. Anybody have a source that won't break the bank? Next in line for consideration is a garage door spring hidden on the floor at the back with cables and pulleys. Over the 20" or so travel the force should be fairly linear. One advantage of the garage door spring is it is adjustable.

Thoughts anybody? Any examples?

Mike

[Runner4404spd]

the reduction on the x and y is 4:1 as well.

in terms of the counter balance i would use a weight and pulleys. and in terms of accelerating mass, the current motor, has to accelerate the mass of the head, the friction of the ways etc and it has no problem. static friction is a very high force, i haven't calculated it but i bet its more than the head so adding a counter balance would help offset that to some extent. i mean really the counter balance weight offsets the weight of the head so that really your only fighting friction, but you can then go easier on the gib tightening and the head should move more freely.

[letmefixit]

Would be nice to keep all of the servo's the same size instead of upsizing on the head. I would think also that the weight would work great. If it is a ballanced load accleration should not be a problem. But im no engineer. Is there room in the colum to install a compression spring? I dont have my mill yet... Hopefully this week.

Thanks for your input
DJ

[Cruiser]

Would be nice to keep all of the servo's the same size instead of upsizing on the head. I would think also that the weight would work great. If it is a ballanced load accleration should not be a problem. But im no engineer. Is there room in the colum to install a compression spring? I dont have my mill yet... Hopefully this week.

Thanks for your input
DJ

All this, and you don't even have your machine yet ! I won't say it ! but I really really want to !
If I were to decide my IH mill needed such a contrivance I would go with a live hydraulic assist. It would be expensive to some extent, but once it worked in unison with the servo it would be the ticket. But, it isn't really needed and the extra expense can be spent somewhere else.

[letmefixit]

what can I say... just trying to plan ahead.

[ksanalytical]

I've been day dreaming about this as well (all I can do since my delivery date is probably a month away at least). Has anyone ever mapped out the extension force over the length of extension on one of these gas shocks? I was concerned that it would vary dramatically over the full 22" of travel. I assume the Z ball screw has zero backlash like the others (CNC version), but it still seems like we'd be best served with a constant balance force.

I thought about running a cable up to a ceiling mounted pulley to leave the sides clear. I also thought about using a pneumatic cylinder with a pressure regulator attached to one side (opposite side open). The pressure would remain constant over the full range of motion and it could even be mounted to a cable/pulley for inverted operation. I happen to have odd things like long cylinders floating around, but the smooth bore tube is the only obstacle I could see to making one pretty easily.


Ken

[blighty]

i have an RF-45 that started to get drag problems on the Z even with an 1290oz stepper. i then fitted 76Kg of steel on the back and the problem went away. Z axis seems to run smother now as a result of the counter balance.

well worth the mod.

[letmefixit]

Got Pictures???? I want to see what you did.

[thebodger@roger]

We have a smallish desktop mill here. The head is not heavy at all, but as we started to add items to the head, the weight started to add up. My partner, who was a bit concerned about equal load on the servo motor, decided that a counter-balance was in order. He went to work with pulleys and chains and all the other bits, but had no idea what to use for the mass. So on my way home from work, I stopped at an old abandoned farm house, and discovered that the waste pipe plumbing was lead pipe. I battered it all down with a 5 pound hammer and ended up with about 25 kilos of lead. Smelted it into a 17 kilo block. Counter-weight problem solved. What I have discovered on industrial sized machines is that a set weight is the best counter-weight. Machines that use hydraulics, or air cylinders, can't provide a linear counter-balance. They may, for the first short while, but as these systems begin to wear in, the degree of counter-balance changes. This makes it nessessary to re-adjust, or replace valves and cylinder seals. Gas filled dampers or spring systems can't be linear or may create other issues. Another source of lead is dead car batteries, but be very careful with these. Work outdoors, and wear protective gear.

[blighty]

letmefixit....

sorry, can't give you detailed pics at the mo. i have just moved house and the mill is in bits round a friends house. wont be getting it back for a week or two.

i have posted up a pic of what i have, should give you some idea of what i did.

[Cruiser]

The key to the IH mill is in the Power supply, I believe that they just went with the big one and stopped offering the smaller ones. The people who are having problems probably don't have the power available to make the servo do its job. I have the standard drive motor on my IH and the big power supply and it will take the gearbox up at 110 IPM. All this depends on how good you are at tuning your dirvers and how good you are with tweaking your gibs without going overboard ! The only mod that I did to my Z axis was to put rails on the column to take the tension off of the gib and insure the slideway was in contact with minumum of gib tension. My machine is one of the older machines, pre grind, and it had some binding in the Z Gibs. The new machines won't have this problem as long as the .....
Gib is not too tight
the driver is tuned to perfection
the gib and slideway have good lube
and contact areas have a moly rub before oiling
The new machines have more iron in the Z Gib area and saddle and so I am told the problem I had with mine has gone away ! So, if the above is taken into account then all should be fine as frog fur !

[letmefixit]

So.. Im no engineer here.. but my thinking was to have all my servo's matched. 600 oz. all the way around. Thus the thinking on the counter balance. Now the issue that was said earlier. What about acceleration of the head. Are you moving twice the mass? or since the load is ballanced are you only moving friction?

[Runner4404spd]

Put it this way, if the counter balance didn't help professional cncs wouldn't use it. it really can't hurt to add it, its only going to help. if the head and the counter balance are not matched then you could possibly have some issue moving, but even then even some weight has to be better than no weight. i do disagree with using some kind of spring force like a pneumatic cylinder to counter balance. the only way a pneumatic cylinder would work is if the cylinder has such a large volume and probably a surge tank that it really doesn't change the pressure inside the cylinder.

[ninefinger]

F=m*a
neglecting everything else for a second: if you add a counter balance equal to the mass of the head you will need 2 times the force to accelerate it at the same rate as without the counter balance. This is just acceleration - not top speed.

However I agree that the calculation is more complex than that as the force to move the head is something like:
F=( m*a ) + Friction on ballscrew (20% of mass being accelerated) + Friction on gibbs (???) + Friction of 'counterbalance' mechanism

where does that leave us? Ideally something that adds no mass or friction could magically increase the heads acceleration by a factor of 2 over a counter weight - for a given force (servo). I really like the pneumatic idea I found in this forum posted way back:

http://www.cnczone.com/forums/showthread.php?t=38165

Seeing as I would need to buy something anyway this looks like a real nice solution to speed up the heads acceleration. Plus the head won't come crashing down on loss of a belt or some other silly thing I forgot to tighten:withstupi(loose set screw, etc)

Mike

[letmefixit]

So if you went with the air cylinder you would remove the double mass.. I think i said that right.

I work alot with air cylinders and there is one problem that they all have.... Bleed by. The air will leak past the piston after a while. I would put some nitrogen in the tank. and maybe some kind of light oil that wont screw up the seals. That would make a mighty fine shock absorber.

DJ

[ksanalytical]

A pneumatic cylinder with a regulated pressure supply would provide constant force, ease of tuning and even a modest diameter would allow for single piston solutions. Multiple pistons could be installed with no fear of imbalanced pressure by connecting the supply lines in parallel as well. No additional mass and the only trade off is that you'll use substantially more shop air volume than most of us are used to. Sounds like a great excuse to buy a new compressor.

I've never tried to use a pressure regulator in a bi-directional mode, but I'm sure a solution exists to allow it to regulate in both directions.

Ken