[ksubenny]

So after lurking and watching for awhile I've decided to build my 1st cnc. It's going to be a steel frame with a roughly 2' x 4' envelope. It will be used mostly for wood and plexiglass, however I'd like to be able to do light passes on aluminum in the future. Looking forward to any comments and suggestions as I start.

X-Axis:
5/8" Thompson Rails, supported in 3 sections with 2 frelon bearing trucks per side

Y-Axis:
THK 15 Series, 2 bearing trucks per rail

Z-Axis:
Purchased Vexta ballscrew/stepper stage I got off eBay. I don't have the ballscrew modeled in the screenshots below...but its there in real life.

Frame:
2" square tubing, 1/8" wall thickness

Gantry:
2" square tubing, the two horizontals are 1/4" thick to make it easier to tap for the THK rails

Steppers:
HobbyCNC Nema 23 305 oz/in unipolar steppers on X and Y, Z came with a stepper

Belts/Pulleys:
X and Y Axis main drive-open ended L series timing belt, .375 pitch, .75" wide
The endless belts off the stepper pulleys will be XL series
All pulley's off the steppers will be 20 tooth, all other pulleys are going to be 10 tooth.

Router:
Porter Cable, not sure which model yet but I will be using the ER precision collets.

Drivers/Breakouts: HobbyCNC Pro Chopper Driver Board.

Initially workpieces will be clamped on to the table, however after I get the CNC part figured out I plan in doing vacuum clamping, but I'll start with 1 step at a time.

Got the pictures to work....so let me know you guys think and any questions/advice you have on it.

[LeeWay]

That's looking pretty good. Going to use some nice materials. I look forward to seeing how this one goes. I know it's a little early and these could be added at a later date, but I think the little gusset you are using on the gantry is insufficient. Consider using a tepee shaped gusset that goes front to back on your X trucks and at least 3/4 of the height of the gantry. What you have when the router comes down and meets the material is a form of lever. Since it's connected up top, the first force will be backward on the gantry.
You may think, well it's going to be cutting the material and not putting upward pressure on that. The fact is when it start moving around, it will transmit upward force as well as side to side. That's a pretty tall gantry, so it will have a pretty good lever effect. Good luck with this one and I will stay tuned.

[drafterman]

I agree with LeeWay. I tried something like this initially with my steel frame router and it had enough of a lever effect to "rock" the gantry over on a cut of any depth. I used wider steel plates on both sides of my gantry to spread the load over the bearings better and now I get pretty good results; even on aluminum. Sounds like you are using good quality items in this build, best of luck to you.

drafterman

[ksubenny]

Well I have take LeeWay and drafterman's advice on the gussets as well as added and cleaned up a few more things.

Router: Added a router to show roughly where it will go, I'm thinking I will want to fab a new mounting plate that will allow the router to sit a bit lower on the z axis.

X-Axis: In order to give some adjustment side to side on the x-axis rails for parallel, I've modeled the plate setup shown below. The red plate is drilled and tapped to attach to the support rail of the x-axis rails. The slotted, light yellow color plate will be attached to the red plate(likely welded) to give the rails adjustment, and then the purple plate on the bottom will be welded to the frame with a standard clearance hole for the 3/8-16 socket head cap screws.

I also have the x-axis rails centered on the frame and I may move them closer to the right side end (end opposite the router side of the gantry) in order to get a bit more working envelope out of the machine.

[supertechster]

Nice Design! Nice drawing too (what program?)

OK... Hate to be a party pooper, but if all you are going to be routing, cutting, engraving and shaping is relatively 'flat' stock, why have such a TALL Z? Seems to me all you'd need is max 4" of travel or so, which would alleviate 90% of the 'flex' when your bit touches down and starts moving around. The Z I think I see is ~15in or so which is great for making scallops on the tops of table lamps, but not so good for low-altitude projects ;-)

[LeeWay]

I tend to agree. There is not much reason to have the router bit rise much above the lowest part of your Z axis. That is all it would need to clear everything.
Your router mount also needs to hold the router at a higher point so that the router can pass the bottom Z plate down to the material.
Just as aferinstance, most Z axes here at the Zone are 8 " travel or less. Much easier to confine the forces on a shorter Z.
I have just over 6" from my router collet to my lowest table surface. I then have a 1/2" plastic sub table and use 3/4" plywood as fixtures on top of that. This gives me 4" travel from the tip of my bit to my material, which is inset in the fixtures. I use toggle clamps to hold my material down on these fixtures and use a safe Z height of 2.5". This is plenty of clearance. If I need a little extra for something at some point, I can always remove the sub table and mount the job at the 6" clearance point.

I do have 9" travel on my mill, but mills tend to need a little more travel to clear vises and rotary tables and things.


Oh yeah.
Those X rail adjustments look great. You will certainly be able to use that to your advantage.

[ksubenny]

The modeling is done in Solidworks '06, and I do agree that my Z is pretty big considering most of what I'm planning on doing is going to be flat....however I got it for so cheap off eBay I couldn't pass it up, and if I decide there's too much leverage on the top side of the gantry I'll pull it off and shorten it up after I get everything working. Thankfully I have access to a pretty well equipped machine shop(boring mill, surface grinder, Bridgeport, lathe ect..) so shortening it up isn't that big of an undertaking if I'm not happy with the length. Once again thanks for all the comments.

[LeeWay]

Ahhh. I didn't realize it was a premade unit. Those do save some time and work.
The usable area on it is still going to be at the bottom of the slide. Your router will need to drop down below that bottom plate a little. Hard to see the actual distance on the drawing, but looks like it would need a really long bit to hit the table when bottomed out.

I think you would get a lot more real estate if you actually turned that slide around. Mount the router at the bottom. Then you would have a lot more clearance. You would need to narrow up the gantry rails quite a bit. It's the bottom plate on the slide that will really shorten the usable length the way you have it unless you offset the router quite a bit.

That might get pretty wiggly though.

[ksubenny]

Well now that I'm back from traveling for work I can fine tune the design a bit. I like the idea of turning the slide 180 degrees but I'm not a big fan of what I would have to do to get the movable slide to mount to the Y axis thk rails. So I'm going to put a drop plate mount as seen in the bottom screenshot. The plate will be made out of 3/4" aluminum. I'm not a huge fan of having the router hung out and cantilevering the load, however if I think I'm getting some deflection due to the cantilevered load I will but another pair of bearing blocks and that should fix it. The pictures below show current design, there is .30" between the bottom of the PC router collet and the top of the work surface and 4.25" under the gantry. I will be using the Precision ER collets from PreciseBits, does anyone have any experience with how that will affect the clearance height on the router?

[bhupen]

hi
once you complte machine pl. send the video.

[LeeWay]

That is looking better. Thats about all the clearance I have on mine when I have my tooling plates mounted. I even use some toggle clamps (small) to hold down parts and still have clearance. I run safe Z at 2.5" on the router to clear these clamps.

Just something else I am noticing. Even though you have a tall Z axis ready made, you will only be using the bottom 4 or 5 inches. I think you would see less flexing and racking if you were to shorten up the height of the gantry. This would help in both X and Y directions and maybe even Z. In this case, I don't think it's a benefit to have the gantry rails spread so far apart. Closer rails are far less prone to rack.