Thread: Alternative to Pneumatic controlled PDB

1. Alternative to Pneumatic controlled PDB

Hi all I've been looking at the option to a non pneumatic PDB and rather than retyping and uploading it all over again here's the post link http://www.cnczone.com/forums/1107101-post299.html (Optimum BF20L CNC conversion using the Smooth Stepper) and I'd appreciate opinions good or bad on the viability of the scenario..... so please feel free to post comments in my thread or here......and thanks in anticipation.....

2. You might want to consider what I'm doing - using a stepper motor and high ratio planetary gear reducer to make an uber-power drawbar "wrench". With a 400 oz-in NEMA34 stepper motor mated to a 50:1 NEMA23 gearbox, I can *easily* generate well over 50 ft-lbs of torque on the drawbar, FAR exceeding the max tightening requirement (about 25 ft-lb), while having plenty of excess torque to loosen it. I use switchable current limiting resistors on the stepper driver to provide reduced torque when tightening, and full torque when loosening. By putting an encoder on the stepper, I can tell when the drawbar is moving, and when it's not, allowing me to back off a single turn for TTS tools, or fully undo the collet to swap in an R8 tool. No belleville washers needed, and no significant force on the spindle, as the only downforce we need is enough to pop the collet free (200-300 pounds max).

Regards,
Ray L.

3. Originally Posted by HimyKabibble
You might want to consider what I'm doing - using a stepper motor and high ratio planetary gear reducer....
Excellent thanks Ray, that’s the direction I was looking for that someone had done this already and I'll have a look at what you're doing and also the equation that I was finding hard to convert was Torque to Linear force and my simple research had discovered a conversion of 16 ozin = 1 PSI = 4.44 Newton therefore I required at least a 532.8 Newton Stepper Motor and if my formula is incorrect I’d appreciate input and I was basing my equation on the assumption that most guy’s seemed to operate their respective pneumatic three stage cylinders at approx 120PSI but I wasn’t sure of the added force of a three stage cylinder as opposed to a single stage cylinder under nominal working loads…..and additionally I was hoping to get advice on the electronics integration (wiring) so as maybe not to use or waste an axis on this control….

4. Are you still talking about a Belleville drawbar? I don't see how you'd make use of a linear actuator otherwise. I think you'd have a hard time getting enough force from any motor-driven linear actuator. For a small mill (like an X2), you'd want at least about 600 pounds to properly retain a TTS tool. The larger the machine, the higher the force required. For a large mill, like an RF45 or knee mill, you'd want about 2500 pounds. You'll only get there with a very large diameter single air cylinder, a multi-piston cylinder, or an air-over-hydraulic system (this is how drawbars on VMCs operate). And, of course, with a Belleville drawbar, it is easy to swap out a TTS tool, but a PITA to swap out an R8 tool.

Calculating the linear force you'll get from any motor-driven linear actuator requires making assumptions about system friction, and it's VERY easy to end up WAY off the mark, as friction is a MAJOR factor. That's what makes the planetary gearbox and stepper motor solution so attractive - its VERY predictable, and also ends up being VERY flexible, mechanically simple, and quite compact.

A stacked air (or hydraulic) cylinder will multiply the force by roughly the number of pistons. So, a double cylinder will provide roughly twice the force of a single cylinder of the same diameter.

Regards,
Ray L.

5. Originally Posted by HimyKabibble
Are you still talking about a Belleville drawbar? I don't see how you'd make use of a linear actuator otherwise....
Yeah I had the Belleville drawbar in mind and reading and rereading your posts it seems my concept doesn't really have any merit and I hadn’t heard of planetary gearboxes or looked into them as an alternative either….and as I said in a previous post in my thread that this is what I like about this forum that I can share an idea and get advice from you guy’s who understand the technical side and suggest whether the idea has any merit and progress to build it or simply drop it and move on to the next idea…..

As you have an excellent understanding of the electronics and I don’t, would you be willing to sell a wiring schematic and or DVD for your wiring of your concept or will you be posting a video link??

6. I'll put up pictures when I get around to building it. But there's not much to it. The stepper and gearbox will be mounted on a floating mount, held up by springs. An air cylinder will push the whole thing down, with light pressure, while the motor rotates slowly, until the socket on the gearbox engages the hex on the drawbar, and drops down fully. A microswitch will notify the controller that the socket and hex have fully engaged. At that point, the motor will turn with full torque to release the tool. An encoder on the motor shaft will be used to count revs, so I can loosen the drawbar only one turn for TTS tools, or fully unscrew for R8 tools. Reduced stepper current will be used for tightening, to limit the torque to 25 ft-lbs. The stepper will simply be controlled by an extra "axis" of the controller (in my case, KFlop, using my custom controller app).

Regards,
Ray L.

7. Just reading up on planetary gearboxes and I'm sure I've asked this question and may even be in my own thread too but just to clarify a point for me if you don't mind when using gearing ratios and lets say for this example a stepper motor pulley is .5" and the second pulley is 2" so @ 4:1 ratio (or is it reffered to as 1:4 ratio ??), does this quadruple the motor’s rated torque ?? and if the pulley’s were reversed would this decrease the motors torque respectively ??

8. Originally Posted by Mad Welder
Just reading up on planetary gearboxes and I'm sure I've asked this question and may even be in my own thread too but just to clarify a point for me if you don't mind when using gearing ratios and lets say for this example a stepper motor pulley is .5" and the second pulley is 2" so @ 4:1 ratio (or is it reffered to as 1:4 ratio ??), does this quadruple the motor’s rated torque ?? and if the pulley’s were reversed would this decrease the motors torque respectively ??
Yes, that is exactly right. Gearing/belts/whatever multiply torque, and divide speed. Power remains constant, ignoring frictional losses. So, if you have a 1HP 3450 RPM motor, it will have 1.5 ft-lbs of torque. Feed that into a 4:1 gear reducer, and you'll end up with 6 ft-lbs of torque, at 863 RPM, but still 1HP. Flip the reducer around, and you get 13,800 RPM, but only 0.375 ft-lbs of torque, but still 1 HP. All, of course, ignoring frictional losses, which are typically on the order of 10-15%.

So, my 50:1 gearbox gives me 50X the torque of the motor by itself. So, 400 oz-in becomes 20,000 oz-in, or 104 ft-lb. Actual will be slightly less, due to frictional losses, but still FAR more than required. I would have gone with a 25:1 gearbox, had the 50:1 not been such a good deal.

Regards,
Ray L.

9. Originally Posted by HimyKabibble
Yes, that is exactly right. Gearing/belts/whatever multiply torque, and divide speed.....

Ok so at least I'm on the right track and understanding what I'm reading …..

my biggest downfall is on the electronics end as this is new territory for me but I’ve wired my CNC setup I understand the basic concept of step and dir pulses and ports and pins setup and could do it all over again, but PLC’s and Solenoid valves to operate oilers and pneumatics are still alien territory form me as I haven’t covered them yet……you say you have designed your own app….is this another way of saying a custom brain that is specifically suited to your future setup??....and you say you’re using “switchable current limiting resistors on the stepper driver” have you designed and built your own PCB driver??....or are you using a controller like an Arduino or the like and have connected the resistors as a separate ‘add-on’??...

and thanks again for all the explainations…..

10. Originally Posted by Mad Welder
Ok so at least I'm on the right track and understanding what I'm reading …..

my biggest downfall is on the electronics end as this is new territory for me but I’ve wired my CNC setup I understand the basic concept of step and dir pulses and ports and pins setup and could do it all over again, but PLC’s and Solenoid valves to operate oilers and pneumatics are still alien territory form me as I haven’t covered them yet……you say you have designed your own app….is this another way of saying a custom brain that is specifically suited to your future setup??....and you say you’re using “switchable current limiting resistors on the stepper driver” have you designed and built your own PCB driver??....or are you using a controller like an Arduino or the like and have connected the resistors as a separate ‘add-on’??...

and thanks again for all the explainations…..
A pneumatic solenoid gets wired exactly like the outputs you're probably already using to control your spindle CW/CCW or ON/OFF control - a BOB output driving a relay, which switched power to the solenoid valve. Relays can also be used to switch one of several current limit resistors into the stepper controller, rather than having a single hard-wired resistor.

I am running a custom controller app in place of Mach3 or EMC. I am using a KFLop motion controller that I wrote myself, and my app provides the UI for that motion controller in much the same way as Mach3 provides the UI for a SmoothStepper. In my case, all the control logic for the drawbar will be written in C code running on the KFlop DSP, but with Mach3 it could just as easily be done in a VBasic macro. It doesn't need to do anything other than command movement on the drawbar "axis", using G-code commands, control digital outputs (for activating a spindle brake, energizing the solenoid valve for the pneumatic cylinder that drops the PDB assembly onto the drawbar, and controlling the current limit resistor selection), and digital inputs (stepper encoder, "engaged" microswitch, etc.). All pretty basic stuff, no different from coolant/spindle controls, limit switches, pendant controls, etc..

Regards,
Ray L.

11. Originally Posted by HimyKabibble
.....I am running a custom controller app in place of Mach3 or EMC. I am using a KFLop motion controller that I wrote myself....
Yeah I had just kinda figured that out before reading your post, as your electronics knowledge is light-years beyond what I'll ever attain and I looked up your K-Flop motion controller Dynomotion | Motion Control Boards and was watching a Visual Script video and things aren't as vague now but I'll still have to do more reading..... and I suppose with my bob's I could simply add an additional Modbus I/O board, configure the Modbus within Mach write a new Visual Script and control things like that…..

and in the mean time while I'm reading win the lotto and just buy a new Tormach and save myself the torture.....

Thanks again Ray