Stepper or servo?

Hi,

I am in the process of building a machine to cut box (finger) joints in wood.
The process I will be controlling is the movement of a router along a sled, by a number of fixed distances, which will equal the widths of the 'fingers' of the joints. The movement will be achieved by using a motor driven screw.
I have previously built a manual version, where the screw is turned by hand and the rotation is measured by an opto device, triggered by a notched wheel on the end of the screw. I use the output of this to drive a couple of decade counters, connected to seven-segment displays. This gives me a resolution of 0.1 mm, good enough, but it does have limitations. For example; I have to remember to reset the counter between cuts, and the counter only counts upward, so if I'm a bit 'gung-ho' with the handle and overshoot, going backward is a pain.
So, for those of you I haven't bored to death at this point, my question is this; do I go down the path of stepper, or closed loop servo?
I have suitable parts 'liberated' from printers, and I'm guessing that the stepper solution would be simplest to implement (from a software point of view). But I also have the linear encoder strip and its sensor...
Suggestions as to the relative merits and pitfalls of each method would be very welcome.
Thank you.

opto device, triggered by a notched wheel on the end of the screw. I use the output of this to drive a couple of decade counters,

I don’t think a standard servo could do this job and with a modified servo you only get control of the speed so I think thats also out.

The stepper sounds good as out have feed back on the position of the cutting head via the “opto device”,

Mark

use a servo if you plan on makeing hundreds and you want to make them as fast as the machine can spit them out.

use a stepper to cuts costs to less than a quarter of that of a servo.

keep in mind that many hobbiests think a servo is a toy motor that moves an arm back and forth for boat, plane or such. few hobbiests know a servo as a full HP sized motor that spins at 2-6,000 RPM, is controlles by 100 volt 20 amp drivers that move the motor at high speed to fractions of a mm with encoder control.

A gecko G320X is only about $120 USD, you can use an old one HP treadmill motor and an encoder. these are great for laser cutters that move a few hundred inches per minute cutting speed.

for your wood router, a good rule of thumb is that your axis drive is between 10% and 25% of the HP of your spindle. if you have a nice water cooled 10hp spindle then a servo drive would make your machine sing.

if you have a 2-3hp router, then nema 23 stepper will work well.
I would suggest you go to the Gecko G201x for steppers and read up on how to determine the proper voltages. a 48v transformer will offer a nice 68volt power supply IF you need it.

if you are just getting things started, use steppers. shake out the movement of the axis and cut some parts, you may find they work great. if you start loosing steps, read the gecko white papers. more voltage done correcttly will often deliver the power you need.

Are we talking furniture or small stuff like a box to hold some matches?

And how many parts are you making?

If your talking about printer parts, do you mean the little steppers and motors that are found in there? Or encoders or something?

I would say choose the motor and then figure out how to drive it. If you already have position sensing available then you can add closed loop feedback control to whatever motor you choose and don't need to limit your choice to stepper motors/servos that provide direct positional control.

Sorry, I should have been clearer.
The machine will be used for small boxes, for example jewellery boxes.
I will be using recycled printer parts, and driving a threaded rod.
I suppose the real question should have been;
Do I just use a stepper - sending commands from the arduino, which equate to the distance required?
Or, do I use the linear strip and sensor from the printer, to provide a feedback loop?
I'm guessing that just sending step commands would be an easy option, Do you think it's worth having a feedback loop?
Thanks for your patience.

sounds like a trim router ? Dremel ? steppes will work great.

It it were me, I would get some of those cheap stepper drivers from Amazon or e-bay and some NEMA-23 or NEMA-17 motors. or larger motors from printers. those cheap drivers require 4 pins per axis. more expensive ones can take 2.

make the machine with your threaded rods and get it to function.

a microswitch at each end of travel to signal the end of travel.
a note here. do not place that switch as a physical stop. if your machine is wired wrong or something fails. the motor will drive that axis into the switch and damage the switch. rather istall the switch to the side so the axis contacts it on passing.

write a HOME routine. press the HOME button and the axis will run high speed to the end switch, then back up, and then slowly travel until that switch changes state. then the software will set that location as zero.

if you have a display, write to the screen something like
New zero is set. it is 'x' steps from the last zero.

that way, if you lost steps, it would know it and tell you just how far away your machine was from the expected location.

I use NEMA23 steppers on my Mini-Mill, so I can assure you steppers can be used in a metal cutting machine without any problems. cutting wood is less intensive.

Thank you :slight_smile:

How old are the printers? IE, how large are the motors, and are you sure they are steppers? Most inkjet printers now use a servomotor system, hence the encoder strip. I expect you knew that.

The stepper would certainly be easier, as long as you get steppers strong enough that they'll never miss a step.

I'm helping a friend of mine do something nearly the same, only to drill a series of holes. His previous setup had a handle with a magnet to lock it into the same position on each turn, and he'd just count turns.

He'd put a stepper motor from a 5-1/4 floppy drive on it and I was just running it back and forth in this video.

Hi Steve,
Just watched the video. Similar concept to the one I have.
The stepper I have was ripped from an Epson A3 inkjet, and is a similar size to yours, however it is only 48 steps per revolution.
coupled with a M6 x 1 thread should give me a resolution of about 0.02mm - close enough for woodwork XD.
As a driver, I have a 'Sainsmart' L298N dual 'H' bridge.

get a larger stepper. the one you have is designed to move paper, not a lead screw.

http://www.mpja.com/Stepper-Motor-NEMA-23-384V/productinfo/17607%20MS/

the driver will work.

you can connect this directly or add a pulley and a belt drive.

If you want to save some effort you should take a look at an Incra positioner.

That stepper from MPJA looks like it came out of a printer. 0.8A, 3.84V. Not particularly powerful.

The OP doesn't need something really powerful, as long as care is given to not creating a lot of friction. A leadscrew converts speed to torque.

I went through this whole thing of reusing printer motors and stuff. They're not very strong. They are more suitable for stuff like this: inventorArtist » Big Oil Rooster

It's better to by steppers and controllers and stuff. It's not that expensive and you can get to where you want to be faster.