Designing/planning my first cnc router

[capncrunch]

Hi all,

I've spent a good amount of time over the past few weeks reading up on homebuilt CNC. I'm still in the planning stages, but i'm pretty sure about what i want.

my requirements:
a)-cheap! Need to keep within a tight budget so i can spend more money on airplanes/woman.
b)-I plan on cutting balsa and lite ply for model airplanes, perhaps other stuff, but I can build a second machine as my needs grow. cutting area should be at least 24" long, maybe 12" or so on Y axis. I have access (and skills to use) 3D cad, so i'd like full 3D cutting capability, not just 2.5D.
c)-has to fit in a storage closet: no wider than 30" to fit through the door. length is not so much of a problem, it can take up most of the closet.
d)-no table saw: can't cut dadoes. might be able to get around this.

design choices:

a) has me leaning towards an MDF frame with 75-100 oz/in. steppers, and either a dremel or a 5.5 amp cutout tool (just saw a DeWalt at lowes for $65). I'm also thinking rollerblade bearings on steel pipe, and allthread for my screws.
b) balsa/ply: dremel will work fine. 24" cutting area to keep the size down and tolerances tight, but I'll want an open-ended workbed so i could feed 36" or 48" stock in for 2 cuts. I'm looking for around ten thousandths (.010") precision.
c) 30" wide should be plenty for 12" usable length on the Y axis. I am building a board-and-mdf table that's 30"x48" for in the closet and will have space for an aging pentium underneath. since space is a concern, a rolling gantry/stationary bed seems like a good choice here.
d) should be able to work MDF and delrin to a reasonable precision, but I'll have to design around what I can do.

---
Given my desired specs, the JGRO machine is pretty close to what I'm looking for. I have a few concerns about the design, and I wonder if anyone can comment on a few of my modification ideas:

1) I don't like the bearing arrangement on the guide pipes. I would prefer to go with 3 points of contact arranged either at 90 or 120 degrees. this will slightly increase the cost, but not hugely. would 120 degrees have a significant advantage over 90 (which would be just contact points on the top and sides), and would this be worth the added complexity of building it to a weird angle? I've seen someone do 120 deg, but I dunno where I could get non-right-angle angle stock. any thoughts?

2) I don't really see the need for two torsion boxes, especially as this machine will sit on a table that is already pretty stable and flat. I think i'll skip the bottom torsion box, add some simple bracing, and keep the torsion box on the cutting bed base. this should also speed up the build, as it looks like one of the most time-consuming parts. convince me that this is a bad idea?

3) If i skip the dadoes designed into a few of the gantry parts, will I have to be more concerned about losing strength or alignment?

-also, have I missed any threads with good additions/omissions/upgrades/mods to an mdf rolling gantry design?



-barrett lawson

[sol]

In my opinion 3 point contact is definitely the way to go, the added cost is negligible. I am surprised that people are even considering 2 point on the X axis.
Your thoughts regarding the torsion box reflect mine as well, they are not necessary on smaller tables.

Dados do add strength, but they are not needed on a small table, long screws and glue will be more than adequate. I believe dados are frequently seen because so many homebrew builders are familiar with cabinet-making where dados do add rigidity and overall quality.
Check out my site, some of the concerns you raise are addressed there.
Good Luck!

[capncrunch]

David,

thanks for the reply - I went back to your site and found exactly what I was thinking about on page 3 of your first design's build. I can't believe I missed that on the first visit, my brain must be getting saturated!

funnily enough, I can weld, but can't run a dado. I should be able to fabricate something like that bearing bracket you show.

still busy reading stuff, I'll start doodling in autocad soon.

-barrett

[Jimmy Southern]

Barrett,

First off let me say welcome. One question, do you have a router, even a laminate trimmer? If you do then for the dadoes it is simple to set up a guide fence. I would say if you can build the JGRO machine without the dadoes then you should easily be able to home brew a small router table. You will wonder how you got by without it.

Good Luck
Jimmy

[capncrunch]

Jimmy - thanks for the welcome!

I'd have a router if i drove up to baltimore (from north carolina) to get the rest of my tools from my dad's shop. (I moved out in june, with a small car, and higher priority stuff to take...). But that's about $150 in gas and food and stuff for the trip. If we drive to my girlfriend's relatives' Thanksgiving celebrations, we could pick up the last of the stuff I left behind on the way. or i could rationalize going to get it instead of spending money on new tools. I think im gonna start frequenting pawn shops and classifieds and scrap heaps for stuff I can use.

---

I have a design related question: what dictates the vertical placement of the x-axis leadscrew in relation to the rails? do i want it in line with them (or as close as possible) so that it takes a minimal torque load if the gantry racks? or do i want it below the plane of the rails so that the force of the leadscrew on the gantry is balanced wrt to the force of the workpiece on the cutter?

I'm reading through Yukonho's design thread and finding it quite useful. Anyone have a favorite thread that covers design?

-barrett