Re: [CAD_CAM_EDM_DRO] Re: About to start building a CNC wood router

>
>
>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!
>

:-) Point well taken.

As my goals are modest, I think hardware store parts (for the most part)
will be sufficient. I'll be aiming for repeatability above all, as per
your suggestion.

>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
>

As I said in another message...version 1, release .01. I'm a software
guy... :-) Undoubtedly I will make improvements as I gain experience.
I expect this to be a learning opportunity, as well as a lot of fun!

>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's a nice cutaway drawing on the McMaster site that explains a
ballscrew -- I finally understand what the fuss is all about. It's a
high precision helical ball bearing, basically. and definitely not
something that can be easily homebuilt.

>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
>
>

Thankfully my entire machine will not be 24" long...pitch errors will
not have a lot of time to add up. However, your point about quality
materials is well taken, and I am shopping around.

>takes all 100oz of your motor to just turn the screws there is
>nothing left to move the table! Screws have efficiencies associated
>

I intend to make it such that I can easily turn the leadscrews with my
fingers -- if that's achievable. I think it is...it'll just take some
precision craftsmanship to make it happen.

>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.
>
>

Allegro has come out with a single chip step translator that even
supports microstepping. A very nice chip. I'm trying to get some
samples, right now. Tell me if this is workable, if you don't mind --
stepper translator outputs to 4 pins -- connect to appropriately sized
transistors, use to switch power supply voltage which is fed directly
into steppers. I explained it a little crudely, but I think the meaning
is clear.

>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.
>

I do too...but since my medium is wood, the level of precision I'm
expecting may be lower than that required for your projects. I don't
know what your requirements are. Of course I want it precise to
ten-thousandths of an inch, but if it costs orders of magnitude more to
build it (first version...before I get "hooked") then I'm willing to
compromise a little. Someone mentioned backlash of 1/16" in some
woodworking machines -- this would be wholly unacceptable to me.

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

Are you from Texas, too? I'm in Bonham...northeast of Dallas, near Paris.

-- Chuck Knight