Need ideas/advice to kickstart me.

[SteveJ]

Hi, I have been lurking for a long time now and am following the message I keep getting which is to make a posting, ask a question or just have a conversation.
So here goes.

I have an Electrovert machine that was built to trim the excess leads off printed circuit boards.
It has 3 axis.
X = about 340mm travel and is on 25mm round rails with 1 Thomson TWN 16 bearing on each side.
Y = about 340mm travel and is on 19mm round rails with 2 Thomson SPB 12 bearings on each side.
Z = about 25mm travel and is on a slide with a winding handle.
The work area is a solid 610mm x 610mm 25mm alloy plate with quite a few locating/mounting holes in it. It is a very solid machine with no (I think) flex in it at all.
I want to convert it to a CNC Router/Engraver capable of working with anything up to brass. My problem is I don’t know where to start despite spending hours reading dozens of posts. Stepper or Servo?, Direct drive or timing belt? Specs of ballscrews (or leadscrews?)?, Sizes of motors? I have spent years working in horsepower sizes and everyone here talks about “oz/inch”. And even if I could convert I wouldn’t have a clue what strength I will need to drive each axis.
Any help or ideas will be very much appreciated.
Regards,
Steve

[jhowelb]

Gecko is coming out with a new and inexpensive and very small drive that would serve you well. Read about them here http://www.cnczone.com/forums/showthread.php?t=51083

By just looking I think you could use nema 23 size steppers, you will find a selection from 150 oz/in to 495 oz/in by going here.http://www.kelinginc.net/SMotorstock.html

I wouldn't presume to size motors for your project but I tend to over-engineer things as too little power won't work at all while too muck results in a small waste of a little cash. Any way, too much can later be used on a later/larger project as you cannabalize one preoject to build the next!

Good luck!

[motion guru]

everyone has a favorite approach to a problem like this - me, I like to model the machine in Excel and go from there.

Upper left hand part of the worksheet put everything you know about what you want to do. Of primary interest in designing a router . . . feedrate, rapid rate, expected feed forces, etc.

Lets say for grins you want to be able to push with 100 lbs of force.
Lets also say that you want to have a rapid speed of 300 in/minute. (convert to 5 inches / second) Is this reasonable?

What is a reasonable speed to run a stepper motor? How about 1500 rpm max? (15 revs / second) . . . is this reasonable for a stepper? Look at motor specifications to find that answer.

Hmmm . . . 15 revs / second x ??? = 5 inches / second.

Do they make a ballscrew with 0.3 inches / rev? NOT LIKELY . . .

Common pitches are 0.2 in/rev and 0.5 in/rev . . . and several metric versions as well.

How about you go with a 0.5 in/rev and use a belt drive to slow the screw down a little from the stepper motor? Based on this approach you could direct drive a 0.5 in/rev screw at 10 revs/sec (600 rpm) and get 300 in/minute rapid speeds. Hmmmm . . . what about ballscrew whipping around? how long is it? what kind of bearing supports at each end? What is the critical speed? If really long, can you clamp it down and spin the nut instead?

Lots of questions need to be answered before you can nail down the exact components for each axis.

Then you do a force calculation - again, sizing instructions are easily garnered from websites for ballscrew manufacturers.

Based on your process loads, drag and acceleration requirements you can get a good idea of how much torque it will take to make your move. For a stepper, you would be well advised to triple this number when selecting a motor and then once you have the motor selected - then you pick the amplifier (stepper drive).

Once you have the motors and drives picked out - then you select your controller - and lots of options out there for that as well.

have fun - post your assumptions and progress in calculations and we will pick your design apart for you so that you wind up with something that will work well.

[SteveJ]

Thanks heaps guys, at least no one told me I was dreaming.....yet so I now have some ideas to work with. Really appreciate your inputs.
Steve