Re: Choosing the right design for woodworking

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "wayne_cordrey"
<wayne_cordrey@y...> wrote:

> I'm a woodworker new to CNC interested in building my own CNC

router

> to make furniture components and cabinets. After immersing myself

in

> the wealth of info here an on the web, I still can't determine how
> much machine I need to do the job.
> 1) Servo or stepper motors?

That's more a matter of choice rather than cold hard reasons in the
hobby space. It ususally boils down to cost. With the advent of the
semi-closed loop systems like the Gecko 320/340 or Rutex 9X series
the cost is less of a factor. You can argue all day about which is
best but bottom line is that servo's will offer torque where you need
it (at moderate to high feedrates) and have positional feedback to
make lost steps a non-issue. All that being said a well designed
stepper will not loose steps and will work well for the speeds
normally seen in home type router applications. If you can live with
the 25 to 30% price differential on the motors and drivers then
building a servo based system is no harder than a stepper. You can
use smaller servo motors than steppers in a given design.

> 2) how powerful of a router motor?

Well, depends on if you are cutting decorative plaques using an 1/8"
bit or hogging out sheets of plywood for cabinets. Under most
circumstances the mediuum 1 1/2 HP to 3HP (one has the scratch their
head to figure out those ratings since it defies physics) are
sufficient to do the things most hobby routers are used for.

> 3) what about tool changers?

There are some aftermarket collet systems but unless you are going
the go with a commercial router head ($$$$$) or a custom spindle
setup then its better to just setup a jog that allows you to change
bits and have the end at the right spot each time. You could instead
choose to mount your cutting tool/head on a 'floating' mechanism and
do an automatic surface touch and rezero of the tool. If you are
talking a fully automatic tool changer then you need a collet system
that is not normamly available on smaller routers.

> 4) Is three axis enough? - I'd like to turn legs and some carving

To turn legs you need to either CNC a wood lathe or build a 4th axis
rotary system. Your router table design needs to take into
consideration the added vertical distance you would need to cut work
on a rotary axis. One added design element is that to longer the Z
vertical axis travel the more twisting forces on the gantry with the
axis at long extension and the greater the mass of the Z axis
carriage itself so the gantry must be designed with that in mind

> 5) Why do hobbiest level machines communicate with the PC via
> serial/parallel ports, but commercial units use ethernet? Is there

a

> need for faster data transfer?

Most commercial units have a proprietary dedicated controller inside
the box the machine runs from. The ethernet is not used to send step
and direction signals but merely to allow you to dump a g-code
program from your CAM system to the controller. They used (and some
still do) to use serial interfaces. Not being able to afford
commercial closed systems most of us us the parallel port on the PC
as the solution. Over the last three years the cost of software,
hardware and expertise has been lowered by using the low cost PC
platform. (that's not to say the expertise is worth less, there is
just more of it because of the large number of people involved in the
hobby)

The need for speed comes into play when you have a system that uses
step and direction and especially micro stepping that demands that
the pulse rate be 8 or 10 times faster that a non-microstepping
setup. Based on gearing, belt reduction, leadscrew pitch and other
factors you could find youself having to supply 100,000 pulses per
second to get the linear speed you want (IPM) from your system.
Software only conrollers that run on a PC to be affordable by us
humans top out at about 45,000 pulses. Several people offer external
pulsing engines controlled by the PC that will output more (and
smoother pulse trains) but each has to run with its brand of
controller software.

>
> I know, I ask alot of questions for someone from the Northwest.

BTW,

> I did search the archives, and read the faq.
>
> Wayne from White Salmon

Wayne, all of us started out asking a lot of questions and when we
sometimes didn't get the answers we liked we kept asking the question
until somebody offered a product that DID answer the question!

I am working with some guys up in the Northwest (don't know how close
to White Salmon) on some CNC related projects. While my largest
number of hours is with CNC plasma I have also designed and built two
routers, a rotarty/flat engraver and have my large vertical mill
about 70% converted to servo control. Don't hesitate to drop me a
line and I'll help as much as time permits.

Nice thing about the internet and e-mail is that I live in North
Texas but have CNC buddies all over the country and that other place
up North of us called Canada ;-o