Re: About to start building a CNC wood router

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Charles Knight <charles@i...>
wrote:

> Well, I knew joining this group would pay off. The recent thread

about

> optically isolating the parallel port, I feel, will be most helpful

in

> my building a CNC wood router for my own workshop. Especially

since I'm

> homebuilding my own board. what can I say? I'm a nerd!
>
> Anyway...like the subject line says, I am about to start building

the

> machine itself. And, I'm trying to do it on the cheap, while
> maintaining some level of precision. Unlike metalworking setups, I
> don't require a level of precision measured out to 10 decimal
> places...the wood will move more than that with changes in

humidity! A

> few thousandths of an inch would be nice, though. My intention is

<snip>

Charles: While building a CNC router out of hardware store parts is
certainly doable the end product may not meet your expectations as to
what you want to cut.

This is kind of like saying you want to build an ATV out of hardware
store parts and not do any welding. IT can be done but I'm not sure
I would get on it and ride!

If you want the experience and pleasure of building your own working
CNC machine as cheaply as possible then the hardware store approach
will meet those goals. Sort of a proof of concept approach. On the
other hand if you want to turn out accurate parts (we don't hold 10
decimal places on any of the machines hobby OR commercial). We can
hold down to .0005 by using high accuracy parts (zero backlash
ballscrews, servo systems, etc) but a reasonable amount of accuracy
(.003) requires some precision parts be introduced in the design.
There are two types of accuracy. First how close will the machine get
to where you tell it to go, and second how well is the repeatability
(i.e. how close will it get to that spot after having made a hundred
other moves). Error on leadcrews will tend to cancel out over time
if the leadscrew maintains the same error over its lenght. If the
pitch varies over the length then it will be difficult to hold any
repeatability. Slop in the mounting bearings: same story. Why do you
need repeatability? Because with small routers you will have to take
several passes in your design to keep from overloading the cutting
tool and if the accuracy is off .010 then you will get stairstep
sides on your cuts. If the sides need to be smooth then you will
spend a lot of time sanding them straight.

Lets talk about the leadscrew. There are two considerations. The
first is the accuracy of the pitch (error) over length of the
leadscrew. I would not began to guess the accuracy of threaded rod
over 24". The second consideration is frictional forces. If it
takes all 100oz of your motor to just turn the screws there is
nothing left to move the table! Screws have efficiencies associated
with them. Ballscrews are about 80 to 90 % efficient. Acme screws
made for leadscrews is a lot less and threaded rod is even lower. To
my knowledge there is not a commercially available antibacklash nut
for all-thread. You could of course fashion one from two nuts
preloaded against a spring but then you increase your frictional
forces even higher!

I can relate to living away from a source of commercial parts; its a
100 mile round trip for me to get into a city big enough to have
motion components, but UPS is an amazing thing. I order stuff off
the WEB and the guy in the Brown truck shows up at my door.

Building your own motor drive components is certainly something that
you can do. I am an EE with several years of experience in the
design of power components and switching power supplies. I
considered building my own. I quickly decided the mechanical and
software challenges were enough to keep me busy and the 400 bucks for
a set of Gecko's saved me weeks of design and frustration. Those
were about the only thing I bought "retail" for my machines. Other
stuff I got from salvaged equipment and surplus sources.

Once again my goals may have been different from yours. I wanted to
make stuff with my machines with reasonable accuracy and the end
result was more important that the experience of getting there.

It's hard to make a silk purse out of a sow's ear (as they say in
Texas).