Newbie- First Build: Looking for comments on my design

[brianrumburg]

After drooling over this forum for weeks, I have decided it is time to start my own build. This will be my first CNC project and I'm starting from scratch in
order to learn as much as possible during the process. I'm looking for guidance from folks who have been down this road. Any help or comments (good or bad; I can take it) would be greatly appreciated!

The basics of my design are as follows:

The base frame will be 2" square welded steel tubing. The rest of the structure will be constructed from 3/4" MDF. The rails are 1/4"x2" aluminum sheet. All rails overhang 1/2" from the edge. Sitting on the rails are linear carriages from cncrouterparts.com. The gantry rides on the extended linear carriages.

I will be using 5 start 1/2"-10 precision threaded rod and anti-backlash nuts from cncrouterparts.com. I will be placing thrust washers at each end to bear the axial (correct?) loads. I plan to use 380oz-in steppers with a Gecko G540 and 48V 12.5A power supply. I'm also planning to use a Makita RF1101 2.25HP router due to its built in speed controller and reported low noise.

When all is said and done, I should end up with a cutting area in the neighborhood of a 45.5" x 31" x 8.5".

I have modeled the machine I intend to build in Inventor. I have posted the designs here:

• CNC Router.dwfx
• CNC Router.easm

You can view the model with Autodesk Design Review or eDrawings Viewer (both free).

I have also put together a BOM with price, quantity, and source.

So, what do you all think? Did I make all of the newbie mistakes at once, or could this actually function? Thanks in advance!

-Brian

[marzetti]

I just completed a router using Ahren's parts and I'm completely satisfied with the results. I don't think you can go wrong with his approach.

[marzetti]

I accidently sent that post before I was finished. The only advice I would give you is to use something a bit stiffer than MDF for your gantry. I used a laminated wood structure but if I was doing it again I would definitely go with the 80/20 extrusion for the y-axis beam. I did use the 80/20 for my Z and I was able to assemble it in less than an hour.

[ahren]

Looks good! I would second the comments by Marzetti than an MDF gantry could be problematic. Unless you drive it from both sides, this material just isn't stiff enough to resist racking and torsion problems. I'd get a piece of extrusion or steel in there.

Best regards,

Ahren
www.cncrouterparts.com

[ger21]

Yes, everywhere you have a single layer of MDF, it won't be stiff enough. At a minimum, I'd say either laminate 3 pieces of 1/2" MDF into a solid 1-1/2" piece, or build torsion boxes. 3 pcs of 1/2" will be stronger than 2 pcs of 3/4".

You're going to be spending a lot of money, and you won't be happy with the results from that design.

[marzetti]

I hadn't noticed the other day that you were planning on aluminum for your rails. While this would certainly work short term, it would wear very quickly. Cold rolled steel is cheap, I'd stick with that.

[ahren]

Marzetti, that's a good point on the aluminum rails -- I admit I went straight to the pictures . Another thing to keep in mind is the tolerances held on CRS is much better than aluminum. So if you want a nice flat rail of uniform thickness, you'll be much better off with the cold rolled steel. Additionally, the piece you use for the gantry rail will add substantial stiffness to the system if it's steel.

Ahren
www.cncrouterparts.com

[brianrumburg]

First, I'd like to thank you for the feedback. You have already saved me hours of building and rebuilding.

I've made some updates to the design according to some of your advice. I already have some steel laying around the garage. This seemed like an ideal home for it.

I have reinforced the gantry vertical members with 1" square steel tubing (welded). Also, I have sandwiched a piece of .13" thick steel between two 3/4" sheets of MDF for the main gantry cross member. These reinforcements should make the gantry a bit more rigid. Another option would be to put a 1" square steel tubing frame on the back of the gantry cross member. Do you think these additions will be enough?

On the Z axis, I realized that I had no material to "close the box" around the rails. I added an aluminum back plate to ensure the bearings stay snug against the rails.

I've also decided to use cold rolled steel for the rails. I'd rather spend a little extra and only have to do it once. Thanks for the tip!

I've attached updated drawings. Thanks again for the thoughtful commentary!

-Brian

[brianrumburg]

After some doing some simulation in Inventor, I have decided to to use torsion boxes for the gantry verticals and horizontal to reduce flex.

I've also decided to mount the Y and Z bearings back to back and place rails on the Z axis.

I think the motor pushing the gantry is the biggest weakness with the current design. Ideally I'd go with a rack and pinion drive for the X and Y axis, but budget is starting to get tight and I can't make the numbers work. For now I think I'll stick with one motor driving the gantry and upgrade to 2 if it becomes a problem down the road.

I'm feeling much better about this design. Do you see any new major flaws? Thanks!

-Brian

[mlabruyere]

I don't think you are going to be happy with the Y drive nut up on top like that. You may get binding due to the offset forces.

If you have not re-thought the use of aluminum rails.....you should.

[ahren]

The offset screw will actually work OK, although it's not ideal. These carriages are rolling element, and would take a significant offset moment to bind. I agree on the rails however -- definitely CRS and not aluminum.

Other than that, I think your design is looking pretty good!

Ahren
www.cncrouterparts.com

[CarveOne]

I'm in agreement with Ahren and mlabruyere. I think that your previous design is the better one due to placement of the Y axis lead screw and as Ahren indicates, the CRS rails are more precise will last much longer than aluminum. If this machine is not intended for long term use then aluminum will work ok. The CRS flat stock rail is probably less expensive than the aluminum anyway.

CarveOne