Re: Newbi needs advise on torch table.

I will take a stab at some answers:

>
> * I would like to build a table that can
> handle a 4x8' sheet at a time so I think I would
> probably need to build the system slightly larger then
> this some where around 4 1/3 x 8 1/3'

Give yourself a foot of overtravel (5' X 9'). It provides for a lot
more flexiability in setting up your material and defining a
machine "home" of 0,0 rather than working at the machine limits.

>
> * I plan on powering the system with two
> stepper motors on the y axis one on each side driven
> with two lead screws. From what I have seen most
> people are using motors in the 175-200 oz/in range.
> On the x axis I would like to use a single motor and
> screw of about the same size.

That works real well but you need either logic or software that will
home the two drives separatly so the gantry will automatically square
itself on every home (reference) sequence. You may find that with
the smaller steppers the problem will be getting enough speed. Here
is the issue: Stepper torque drops off rapidly with RPM so that at
the high end of the range you start to have problems driving a load.
The way to get more torque is through gearing (leadscrew pitch,
toothed belts, etc) BUT that causes a reduction in RPM (speed). You
always have to play the balancing act between speed and torque. The
heavier your load the more complex it gets. See the answer about
Router VS Torch below.

>
> * I seen a kit for a table on the net where
> the gantry rode on a couple of strips of 1/2" cold
> roll and then bearings supported the vertical and
> horizontal. This seems like a good cheap way of doing
> things to me. Though I also consider buying some
> linear bearings but I think that would be cost
> prohibitive.

That will probably work okay for torch work expecially if you use a
THC on the head but it will not give you the accuracy for router
work. Since router cuts have to be done in stages you have to have
pretty precise positioning for the repeat cuts.

>
> * For the z axis I would like 4-6" of travel,
> so that I could put a small router or die grinder type
> tool for wood routing and engraving, though this is
> not terribly important. I was planing on using a
> smaller motor to drive this probably via a linear
> gear.

Designing a table for both router and torch takes some compromises.
Torches need to move in the upper range of the IPM scale (200 +) but
need lower torque because there is no cutting resistance. Routers
need less speed and more torque. i designed and built a Router/torch
table with interchangable cutting surfaces and a gantry big enough to
handle the forces of a fullsized router. If I were to do it again
(and I probably will) I would build just the torch. Once I switched
to cutting steel with a plasma my router just gathers dust!

>
> That pretty well sums up what the tentative plan is so
> far, now for some of my questions that I am not sure
> about.
>
> * Where is a cheap honest place to buy motors,
> controllers and lead screws on the net?

Depends on your definition of "cheap". EBay has become a good source
for CNC parts but the pro's know what they can get and the really
good deals are often snatched up by the commercial guys. Be careful
because there is a lot of stuff that while it is high dollar
commercial gear it will not work with hobby (PC based) software.

>
> * Will I need to provide some sort of gear
> reduction for the drives or will the reduction through
> the screws be enough? Are all lead screws cut at the
> same pitch?

You need to work backwards from the needed travel speeds for the
application and then design your drive system to meet them.

>
> * For the z axis I would like to eventually
> hook up a programmable
> micro controller to keep the torch at the correct
> height but for the time being I would like to put just
> a up down switch to manually control
> the torch height. Can I hook up a three way
> controller in this manner and have the computer
> control the x,y and me or a second circuit handle the
> z axis?

You can but you will not be fast enough and an external THC control
will be expensive (if purchased commercially) or a LOT of work to do
yourself...read on.

>
> * I am not a fan of M.S products and would
> like to power this thing
> with a Linux based system is there any software out
> there that can control a torch system?

Too bad about your aversion to MS stuff. MACH2 is a Windows based
controller that does realtime pulses up to 45KHZ and we have designed
an integrated low cost THC for it. The schematic is free for anyone
and there is a low cost product about to be introduced (assembled and
tested) based on the design. The MACH2 THC has to work with that
software since the "smarts" for the torch are in the software.

> * How does the on off on the torch work? I
> understand that I will need a machine torch head for
> either plasma or oxy/acetylene is the switch
> build into the torch?

Actually I designed and built a "floating head" assembly that mounts
a standard hand torch (modeled after the PlasmaCam design). The
spring mounted head allows me to do an automatic material "touch" and
zero the Z axis. It has also saved my grits several times.

>
> * How much would one expect to spend on a
> system like this?

It is directly proportional to your scrounging ability for linear
components and willingness to learn from other peoples mistakes.

>
> I think that about does it for now. I am sure that
> there is many more issues that I am not aware of, but
> any help would be much appreciated.
>
> Will
>

Come over to the "Dark Side" and visit the group at MACH1MACH2CNC
group on Yahoo. There are several guys that have built or are
building CNC torch units. Who knows, we might have you facing
Redmond and doing the chant to Uncle Bill before this is all over! ;-)