PDA

View Full Version : Ideal? HI speed stepping air motor for a cheap cnc.



ibewgypsie
08-21-2005, 06:18 PM
I thought I invented it, a chap in England has already patented it. *the air motor not the application.

A air motor with 3 cylinders like a radial piston RC motor. Using solonoids to fire each in sequence it steps around the revolution.

Using high speed solonoids such as automotive injectors it would speed around the revoloution probably as fast as a stepper. Using 2cycle style cylinder venting it will exhaust itself at the bottom of the stroke. (do not continuous power a auto injector, the duty cycle will burn out in a second, it must be duty-cycle freq chopped)

Using Turbocnc, you can set a User waveform pattern to step around without expensive drives.
example rotary motion pattern on a 3cyl engine.
X00
XX0
0X0
0XX
00X
X0X
reverse to change directions just like a stepper..

Using encoders and some more fancy programming you could speed as fast as it reacts.

I have to build some minature pistons, I will use first the Mac solonoids I have here.

It would position a mill table
in 1/6th revolution steps. Close enough to set tooling by.

Where do you buy cheap cast iron rings in 1" size?

Power? 1"xR2*pi @ 50psi=150lbs force.. I think that'll move the table even with the angular deflection reduction on the crankshaft.

Any suggestions before I start cutting up the aluminum bar I have here into pistons?

------------------
David Cofer, Of:
Tunnel Hill, North Georgia

[This message has been edited by ibewgypsie (edited 08-21-2005).]

John Stevenson
08-21-2005, 06:31 PM
Yes forget it.
Air is a compressable medium and trying to get repeatability will drive you mad.
This was all done in the 1970's with Hydraulic actuators but trying to get any amount of accuracy was hard.
Dare I mention Moog ? without someone shuddering.

Last place I worked at we had some old Victorian cam driven machines, ran all day on flat belt leather drives and did 19 pieces per minute.

We then got some air operated machins to do the same job, flat out they did 18 pieces per minute but the repeatability was crap, all over the place, we were working to plus or minus one thou.
With the old victorian machines the cams pushed to work to a dead stop and held it in a vise whilst it was worked on.
same for the air machine but used cylinders.

In the end I hired some high speed camera's and filmed the machines working.
The air machines were throwing to the stop and the vise was closing but they were also bouncing off the stop and then closing short.
We tried higher pressures, too much bounce, lower pressures, not enough stroke then I found the answer.
I cut the bloody things up for scrap.

John S.

Rustybolt
08-21-2005, 06:40 PM
<font face="Verdana, Arial" size="2">Originally posted by John Stevenson:
Yes forget it.
Air is a compressable medium and trying to get repeatability will drive you mad.
This was all done in the 1970's with Hydraulic actuators but trying to get any amount of accuracy was hard.
Dare I mention Moog ? without someone shuddering.

Last place I worked at we had some old Victorian cam driven machines, ran all day on flat belt leather drives and did 19 pieces per minute.

We then got some air operated machins to do the same job, flat out they did 18 pieces per minute but the repeatability was crap, all over the place, we were working to plus or minus one thou.
With the old victorian machines the cams pushed to work to a dead stop and held it in a vise whilst it was worked on.
same for the air machine but used cylinders.

In the end I hired some high speed camera's and filmed the machines working.
The air machines were throwing to the stop and the vise was closing but they were also bouncing off the stop and then closing short.
We tried higher pressures, too much bounce, lower pressures, not enough stroke then I found the answer.
I cut the bloody things up for scrap.

John S.</font>

John. I've owned several moogs. They held plus or minus .001 all day every day. The two main problems with moog accuracy was dirty oil and water in the air supply.


The trick to controlling an air cylander motion is to regulate the exhaust not the intake.

John Stevenson
08-21-2005, 06:57 PM
Regulation only causes delays.
If there were any future in this type of technology we would already have it.
Pneumatics isn't new and it's also one of the most wasteful forms of transmitting energy.

It does have a place but replacing stepper and servo motors isn't one of them.
Can you imagine a heavy machine table moving at 200 inches per minute then stopping at a given point accurate to 0.0001"

John s.

John S.

ibewgypsie
08-21-2005, 07:34 PM
John, I have saw different brands of robots moving items all day long, 24hrs a day. The bases where they got thier air feeds through now, they made me crazy with leaks.. them robots would go 360's and not wind up thier cables.

Servos are better. Quieter, easier to work with. You don't have to pay for a pump to build variable air pressure. Cost lots more money.

Yeah Air is not for every application. Air over oil solved a cylinder bouncing for one of my projects. I used I think three gallons of vegetable oil. Yes it gets hot enough to boil being forced through a orifice.

A piston engine is a positive movement, not a varible linear movement like a cylinder.
I'm going to play, you know how my air-motored shaper turned out don't you? I got them air motors scheduled for something else now the shaper hit the junkmans truck.

David

Stanko
08-22-2005, 04:28 AM
Ibew, now that the airmotor idea has been shot to pieces why not use 3 solenoids to replace the cylinders. When they are energised they go clunk and stay put. with a sensor on the crank you could determine when it had reached the end of the stroke and sequence the next coil. Or you could buy a F&P smartdrive motor (an enormous stepper that powers a washing machine) and mess with that. Cheers Mike

Swarf&Sparks
08-22-2005, 06:51 AM
Stanko, do you know where I can find any tech info on those F&P motors? Fisher & Paykel are pretty common here, plenty of dead'uns available. My first question would be, how many pulses per rev? Drivers and control logic I can figure out (I think) http://bbs.homeshopmachinist.net//biggrin.gif
Rgds, Lin

ibewgypsie
08-22-2005, 10:10 AM
http://www.engineair.com.au/images/crossectiona.gif
http://www.engineair.com.au/
http://www.airoil.com/airmotor.jpg
http://www.airoil.com/ai02002.htm

Stanko
08-22-2005, 03:03 PM
<font face="Verdana, Arial" size="2">Originally posted by Swarf&Sparks:
Stanko, do you know where I can find any tech info on those F&P motors? Fisher & Paykel are pretty common here, plenty of dead'uns available. My first question would be, how many pulses per rev? Drivers and control logic I can figure out (I think) http://bbs.homeshopmachinist.net//biggrin.gif
Rgds, Lin</font>

Here is a start http://www.thebackshed.com/windmill/assembly.asp
I havnt had one of these motors (there is one in the washing machine, but we use that for clothes washing) there is a link to another site with more info. Alot of these are made into generators for country folk