Building A Z Axis Head On A Universal Mill.

Just between us, i don't see a way that you can have "too much power".

You may have paid more money. You need a bigger controller. You may use more electricity. Other than that, what have you lost? It's arguably more solid. This may not be a place to shoot for "minimum size", eh?

If, OTOH, you bought too small, then you have to buy something else that's larger. So don't worry.

Braking a screw is one way to do things, but I know that some machines would brake elsewhere, like the ways. The non-bearing surface, or back, of the ways are what was used on the first NC machine i ever ran back in the 70s. A 3.5 axis Cincinnati CIM-X.

But don't confuse "servo braking" with "positioning brakes". Okay, maybe i'm doing the confusing ;-) Servo braking, AIUI, is when the motor tend to stay put by the application of power and not move until it's "told" by the controller card. There's a considerable amount in of braking force in some good sized motors and your setup with a planetary box would tend to multiply that force. Positioning brakes are very much like your car brakes and are activated when the resolver tells the controller that the machine has arrived on its assigned point. Not all machines have these, especially not small machines.

Like i said, maybe I'm the confused one.

Wrat
 
Adding a brake to the rails may be difficult, braking the screw might be the way to go, idk
I'm in agreement, can't have too much power, my knee mill has 2.8kw servos on the scews, and with a 3:1 drive it's around 12lb-ft of torque and I think a little under 9hp driving the screws. The problem with too much power is that you can break stuff in a hurry.
 
Thanks for the reply guys. I have been doing the numbers on the screws and my various options. Does any of this look reasonable???

Servo to screw direct drive. 5.6lb-ft torque (7.5lb-ft peak)

Servo RPM 3000.
Screw lead. to IPM travel / rapid speed
5mm - 590IPM

Servo, planetary box 7:1 ratio 36lb-ft torque:eek: (52lb-ft peak:eek::eek::eek:)
Servo RPM (through gearbox) 428RPM
Screw lead. to IPM travel / rapid speed
5mm - 84IPM
10mm - 168IPM
16mm - 269IPM
20mm - 337IPM

Looking at some other VMC type machines Z axis's, which I suppose is the closest to what I am trying to build they are quoting anything from 300IPM to 600IPM rapids. What is actually usable? being that I will be looking to use it in its most basic form initially as a sort of electronic lead screw and mostly manually controlled. Eventually I think I will build the daughter table for 3 axis but not yet.

I am leaning toward removing the reduction box and running it direct drive with the a 5mm lead screw, my reasoning is this- gives me the widest range of speeds, I have more options on available screws, and they wont need to be as large and expensive. ( 20mm lead screws mostly seem to be 2in diam range)
With the 5mm lead I wont need as much torque as I have available with the servo I have. If I have done the sums right.

I think I would need a fairly good fusible link (so to speak) if I stay with using the planetary box.

As far are travel I think I am going to shoot for the 500mm range if it is going to evolve into a 3 axis franken-minimill. There seems to be some nice linear roller and or ball guides available on ebay if you look hard enough.
 
The faster it goes the more stable everything must be, acceleration and deceleration will play a major role with all that speed, my Rapids at 100ipm have very fast acceleration and deceleration, if I increase to say 250ipm I would have to slow those two down, your going to have to think about the coupler you use and what kind of power it's rated for. I have to tweak some things on mine to get the speed up, I need a smooth stepper to handle the fast pulses needed by the servo drive to spin the motors fast, right now I have to use electronic gearing to get 100ipm.
Another thing is safeties, your gonna want really good limit switches if your gonna have fast rapids, because at 3 or 400 ipm rapids it will probably over travel before you have time to process in your mind to hit the estop...
 
I really dont think i will get to the stage where 400IPM rapids are required, but if the project gets to that stage i dont want to have to buy new screws etc because of a bad choice now.
If I do use direct drive and 5mm lead screws i can just not spin it flat out to maintain a manageable speed? The servo looks like it is a flat torque output from zero up. I have tried a few calculators and havent found a combo that needs more than 3Nm with 5mm lead. Well inside it capabilities.
Having massive torque above that is just going to mean o have to over engineer everything.
My servo drive has a built in motion controller, it looks pretty capable of managing a high degree of electronic gearing. I also have a module which is more complex again. Need to research more on which set up to use. Which is my main aim, slave the feed to the spindle rpm and have the option of a more rapid jog style function for moving the head for set up etc.
 
Ok, i have been reading up on motion controllers, and i am confused. For my mill to start i will have 1.5 axis on the head or a basic electronic gear box for z axis feed. I think i can handle that set up. If further down the track i choose to build a daughter table how do integrate control ove the axis's? With my existing setup?
I have fairly complex drives already they can accept the usual analog signals and also ethercat or ethernet CANopen type communication. Can a mach3 or similar communicate directly to the motion control on board or is another motion controller inbetween required.
Control techniques digitax servo drive is what i have 2 of, plus a unidrive sp for the spindle. These seem to be more for industrial applications cant fond many using them on cnc? Anyone have ideas?
 
You must log in or register to reply here.
Back
Top