Need Help!- Too many decisions to make for 1 peson :D

[orsay]

Hi, everyone.

I did some research on the subject.. and then some ..and again..
Right now I'm still undecided on some things - mostly because if I change something,
I'll have to consider other things, too. A dilemma.

So I'll just cut to the chase..

I want to build a router with (tops) 300x200x75mm (~12"x8"x3") working area;
mostly for aluminum and such (maybe stainless steel once in a while);
speed is not important (for now - might consider improving it later);
fair precision (once again, might improve it later on).

It's my first router and I kinda wish I already had one to make some parts for it.
Unfortunately, I don't even have a vertical drill, so my resources are limited
to normal drills, taps and such.. -I'll have to improvise, I guess.

For the frame I'm considering square alu profiles - something like 40x40mm.
As for design, I think (70%/30%) I'm gonna go with mobile bed and stationary gantry.
Seems easier to start with and since I don't have too much
room/space around, it might be easier to remove the gantry from the 'platform'
(less height) and put it away (flat) in a box or sth, if needed.
To make it portable, if you like.
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Right now, the biggest decision to make is whether I should go for linear bearings
-something like that:

-and then again, shall I take one 'double', as above, or two single ones per rail?

or if it would suffice to use precision sintered bronze bushings(?)
(at least for now, until I'm able to make some parts myself)
And yeah - the most important question - what diameter?
Right now I'm torn between 12 and 16mm.
If bronze bushings, 16mm (or even more) is no problem; for linear bearings, however,
the step from 12 to 16mm is a much more costly one (which I thus try to avoid).

------------------------------

The 'normal' threaded rods (low pitch - 1.00 or 1.25mm) are something I consider using for now and
switch to ACME (or the like) rods later on.

------------------------------

Considering all of the above, what motorisation would be sufficient (and 'future-proof')?
Right now, the best bang for the buck seem to be NEMA 17 motors with ~0.4nm holding torque.
(2 phase, 4 wires, 1.8° step)
Will this be enough?
These steppers, however, seem to have some 'strange' voltage ratings - 2.8V per coil.
I often hear people using steppers with sth. like 5-12V.
-------------------------------
>I #do# have enough knowledge and resources to build a controller myself
using popular chips like L297/L298, buying one (for that price)
just seems easier and maybe even more reasonable.

So, since I'd like to use a pre-built 3(4?) axis controller board (see below), will the board
'take care' of that (low voltage) by driving the steppers by current?
(my knowledge is somewhat limited here..)

Again - the board itself needs 12-35V, so I guess it should do so.
3 Axis TB6560 CNC Stepper Motor Driver Board Controller bei eBay.de: Car Video (endet 29.01.11 23:13:54 MEZ)

^^They also sell 3xstepper+controller bundles (same steppers as above),
so it seems logical.
--------------------

..And the last question - in case I'll want to use the cnc as a reprap,
shall I get the 4-axis controller board (for the extruder feed stepper)
or can I just go with a 3-axis one for now and build one later and mix it in?
(will it work that way?)


Thanks in advance,
I hope some of you will bother reading the long text and pointing me
in the right direction


-orsay

[voltsandbolts]

Not to turn you off on building your own, but it sounds like you need a Mini-Mill to me.

[rmarchjr]

"...(and 'future-proof')?"
"...build a controller myself"

Dude, slow your roll.

Are you looking to play with a machine or use one to make stuff?
I think you are working a bit too hard - lots of what your trying to do has already been done - likely better than any one man can. Siemans, Heidenhain, Mitsu, etc. let them do that stuff, that and reliability is what you pay for.

Hope your not offended but what's wrong with buying what you want used - there you have an "engineered system" - play, modify, improve - you can find some really nice used stuff cheeeeeap.

Good Luck!

[leaveme]

@orsay:

Bearing: I would suggest using two single 16mm for each rail.

Stepper: The stepper you've mentioned is not sufficient. I suggest NEMA 23 something like 1.5A to 2A for your application. Voltage is not a problem. You can happily drive them at 20 times rated voltage. (i.e. 2.7V x 20 = 54V).

reagrds

[Phife]

Dont use unsupported rods and bearings for CNC machines, you will want to use the supported rod and open bearings.

Unsupported is no good, it will flex. Also the thicker the rod the better.

[orsay]

Thanks.

Regarding the motors - yeah, I think even those with ~0.5Nm #would#
suffice in the beginning (using low-pitch threaded rods), later on I'd have
to switch to sth. a lil more powerful.
So I'm gonna look for something in the 1Nm range.

As for the bearings, I don't think I'll need that much rigidity since I don't
plan on having heavy loads on it (doesn't need to be fast);
on the other hand, open bearings are not #that# much more expensive.

- Dammit, now I've got one more thing to consider
(Dammit #2: If I go with open bearings & supported rods,
a mobile gantry setup becomes a viable alternative)

Anyhow, I think it's gonna be 16 or 20mm diameter - depends on what I
can get (and/or afford)



BR