what to do with a free scrap dumpster find ?

I was able to put the head back on the machine to see how things move on these plain split bearings and and things move well but the weight back drives the axis on the ball screw pretty easy, so i need something to solve this? i don't have a brake on the servo so some kind of counter balance? i don't think the servo's will have trouble holding things when powered but once the power is off the head will drift downwards until it hits something which is not good

a counter weight would solve this but is going to be bulky and take up to much space. gas struts don't cycle fast enough and the life would not last very long. i seen some VMC's use a gas over oil counter balance which i started to look into and some of the big routers use air cylinders to balance a heavy axis.

the air over oil method seems like i nice way to keep the system sealed and it would be a stand alone system not requiring to much up keep once it's all sealed. they do loose pressure over time but it's usually slow. the air cylinder counter balance would require constant air supply. air leaks faster than oil so this is a down side.

a down and dirty cheap solution would be some kind of head lock to set when the power is off, it only take a slight bit of pressure on the ball screw to hold things in place.

long story short i'm open to ideas if anyone has any input

i was thinking of a pneumatic cylinder honestly. setting it up like a air over oil system in a closed loop. i have an old air compressor tank around 15 gallons or so that i could use for a reservoirs tank. add a Schrader valve and pressure gauge and some check valves to control flow direction and you would have an adjustable system. i don't expect it would hold air forever but if checked once in a while to maintain the pressure it should work the same.

i seen some sheet metal shears that work like this.

since i had the head all together i can see what kind of real travel i ended up with and see what i needed for an air cylinder. i was able to find one with a 14  inch stroke length with a 2.5 inch bore size for a little over $40. so at that price point i got one to play around with to see what i can come up with. the air cylinder is plenty big enough to only need one of them. i think it will be pretty easy  to do a central mount off the back of the machine. a couple of simple mounting brackets should be simple enough to make.

as for travel things worked out ok. i lost some travel since the table got lifted with all the linear rails but it's rigid and it's working out how i wanted even if the travel is not as much as i was hoping for. i ended up with 12" to the center line of the spindle from the table top and around 2" below the table top to the center line of the spindle. so 14" total travel on the head. i knew the work space was not going to be huge but was after something rigid and able to cut steel well. i think i should end up with that.

7.5 months later and still have a ton of work to do on this project. but here is my starting point for the air cylinder counter balance.

  

The attached component may help. It designed to trigger an air ram when the Z axis is moving up.

sudo halcompile --install liftaxis.comp

You will need to have linuxcnc-uspace-dev installed to get halcompile

i don't think this will need anything that complex but it's still good information to know for other applications.

basically all the ram does is even the forces out between lifting and lowering for what i had in mind. this gets rid of the back driving by finding a neutral state, you can add or relieve pressure from the air tank until you get the weight evened out on the axis. the ball screw still does the lifting and lowering with this kind of set up. it's a closed loop low pressure system. all you are doing is moving air between one side of the ram to the other.
i need to get the air cylinder in hand so i can see first hand what is going on and how things are put together to see if i can recycle the exhaust on it or need to modify it to do what i want? this is not something i just cooked up on my own Haas Automation has done this with an air over oil type design that works like this but their system is costly and designed for heads a lot bigger than what i am dealing with. i think they might even use nitrogen over compressed air?

like Andy said the relieving regulator is important if the system has a constant air supply feeding it but with a tank to recycle air and check valves to control the flow back to the tank you create the constant force in both directions without the need to regulate the system. the regulation is done with your tank pressure based of the weight of your head and the diameter of the air cylinder bore. the air cylinder i got can lift over 600lbs but that's only running an max pressure. if you just ant to lift 200lbs you can adjust by running at a much lower pressure. you can find info on the calculations on-line pretty easy for the pressure and force generated by air cylinders.

people add gas struts and counter weights to help with the lifting because the axis motor is under sized and can't lift the axis on it's own, that's not the case in my situation i just want to avoid back drive if power is lost to avoid a crash. the added bonus is a counter balance takes some stress off the lead screw too.

if it works well i will post pictures and get into more details if anyone wants to do something like this?

         

i spent some time working on the servo motor mount and pinion shaft to drive everything. the back driving seems to have stopped with the drive gear engaged and i can actually drive the head up with a cordless drill so a 1HP servo should have not trouble. i still want to play around with the air cylinder idea plus i have one coming tomorrow anyways. it's still good to have some kind of aid for lifting a heavy axis so i will try to make something of it .

the only down side i encountered was my mount for the servo motor. i can only get 3 connection points. not the end of the world just something i don't typically do but i have seen it done quite often so it is what it is. but now i know how long to make my stand offs and i will have to buy some longer cap screws for that part as well.

i had a extra short 20mm ball screw that i bought just for the nut for another project. i figured half the work was done so i decided to find a use for it since i had the bearing block for it. it was cheaper for me to buy the whole assembly rather than just the nut so it was just extra parts on hand.

i made some new wipers for the linear shafts the old ones were in pretty sad shape and the one came off in pieces so they were ready to retire after almost 50 years

after looking at my old air compressor tank i think it is actually to big for this. more volume is good but i think it might be a 30 gallon plus tank which is to big to fit well anywhere. so i think i will just buy a portable air tank and use that instead. i found a 11 gallon tank for $60 which will fit much better behind the machine. a 5 gallon tank is about half that in cost so it's a toss up between the two.

i stared working on the mounts for the air cylinder but stopped at the base mount. i was just going to attach the threaded rod of the air cylinder to a mount attached to the head but decided to get a clevis pin so that had has a pivot point. i was worried about putting stress on the shaft if things were not perfectly straight.

fittings and everything else add up fast so it is not as cheap as i thought i figured a regulator with a oiler would not be a bad idea either. i'm not sure what kind of oil would be best for an air cylinder so will have to look into that part some more.

so how i think things will work is the portable tank will act as an accumulator and the regulator will control my set point. i can get a pressure relief vale and add that on the accumulator side to blead off any pressure if it should go above my set point. you can get those in 5 psi increments so easy enough once the set point is determined.

i'm thinking you get constant force going up and down but you probably only reduce the head weight by maybe 60 percent. the part i am curious about the the amount of air loss? the return action is not used so that part either vents to or pulls a vacuum from atmosphere.

so as of now the cost is around $275 in case anyone is curious

i'm also going to take a guess and say having some force in both directions would help in the backlash department as well on this axis. tracking down the right fittings to seal everything up has been a treat too. everything thing here in the Sates is NPT but a lot to the cheap stuff you pick up on line is not so finding fittings is a pain in that respect.

I feel your pain re air fittings. I do remember thinking the tank was so cheap but spent about 4x its value on air fittings. Its another example of where the  non-ISO US standards catch you out. You use NPT and the rest of the world uses BSP I have encountered this myself when fitting a similar air tank to my 4WD. I think you are looking at the pressure switch the wrong way. It should be there to ensure that the design pressure of the tank is not exceeded. Say its 100, 120, 150 psi whatever. Set the pressure you wish to use with a regulator. In my case, the tank was good for 180 psi so I used a 150 psi pressure switch so I had maximum air to seat a tyre and ran my air lockers from a regulator set to 90 psi.

To regulate different pressures for the up and down stroke, you can use a needle valve.
 

the tank has to be able to handle the pressure no doubt and in a compressor situation what you are saying holds true 100 percent. but with this kind of set up it's different than what you would find on a compressor. the accumulator tank is just the holding place for the working pressure of the air cylinder. air cylinders and the force they create are directly related to the bore of the air cylinder and the amount of pressure you apply to them. this particular cylinder can produce over 600lbs of force if you apply enough pressure to it. i don't need to operate at that to do what i am doing. i only need somewhere in the ball park of around 40 psi to generate the amount of force needed to offset the head weight of the machine, the basic idea is to to equalize the force going up and down so both directions of travel need the same amount of force to move.

when going up the air cylinder pushes the head up and coming down it pushes the air back into the accumulator tank. the pressure relief valve just is a safety valve to make sure the pressure build up in the tank never gets to far away from the set point being the working pressure.

the compressor supplying the air is it's own system like your thinking .the regulator and oiler will only output say 40 psi to the accumulator tank. i may need to add a check valve after the regulated 40 psi? so my quest for fittings might carry on