Frankenbot CNC Router

[dpgoldberg]

I'm finally getting the build started. I've been delayed by household projects, having promised my wife I would finish building out the master closet so she could hang up her clothes. Now that project is (almost) finished, I can get going on the CNC.

Here is the interior structure of the torsion box base. I had intended to build it out of 3/4" MDF but when I got home from Home Depot I realized I'd grabbed 5/8' by mistake. Doh! Even so, it's more than stiff enough. The framework is held together using biscuit joinery and glue, no screws or nails which don't hold very well in MDF anyway.

[Phife]

IMO the 3rd version is probably the best, but you really should turn the ends of the screw down.

I have the dual bearing design on my machine and its not ideal, unless your machine is dead square, level and perfectly aligned your going to have a slight misalignment in the screw from one end to the other. The thrust bearing design allows for some play but keeps the back and forth motion rigid which is important.

You will only need one thrust bearing on the screw side of the block, not the motor side.

You will want another thrust and regular bearing combo on the other end of the screw.

[dpgoldberg]

Phife - I was leaning towards the third design as well as radial bearings aren't really designed for taking thrust loads. But wouldn't having thrust bearings on the drive block as I've drawn it do the same thing as having one at each end of the lead screw?

[Phife]

Phife - I was leaning towards the third design as well as radial bearings aren't really designed for taking thrust loads. But wouldn't having thrust bearings on the drive block as I've drawn it do the same thing as having one at each end of the lead screw?

Maybe, but now you are relying on the coupler to provide a shoulder for the thrust bearing, If it slips you could have problems. Maybe another locking collar on the motor side of the block instead of relying on the coupler could work, but I think the best would be one on either end of the screw.

Also I dont think you want to sandwich your coupler between the motor and motor mount like that, you should have some space on either end of the coupler so it can flex.

[dpgoldberg]

Phife - Great points to consider, thanks. I can see now another advantage to using the thrust bearings at either end of the lead screw, it divides the back and forth stresses between the two mounting blocks, on direction per block, rather than all on a single block.

What would be the advantage in turning down the end of the screw? The specs on a 12-10 lead screw show the O.D. at the bottom of the thread as .345", which means that turning it down to 3/8" still will not create a continuos surface for clamping collars and couplers to grip. (and I'd really hate to go down to 5/16" or 1/4"). And it weakens the lead screw.

[ger21]

I use your method #2. But you don't need the collar. I use a single bearing to support the screw at the motor end, and the double bearings at the opposite end. I used simple rollerblade bearings.

Haven't had any problems after 1 year of use.

Turning Acme Screw Ends Without a Lathe

[Phife]

Phife - Great points to consider, thanks. I can see now another advantage to using the thrust bearings at either end of the lead screw, it divides the back and forth stresses between the two mounting blocks, on direction per block, rather than all on a single block.

What would be the advantage in turning down the end of the screw? The specs on a 12-10 lead screw show the O.D. at the bottom of the thread as .345", which means that turning it down to 3/8" still will not create a continuos surface for clamping collars and couplers to grip. (and I'd really hate to go down to 5/16" or 1/4"). And it weakens the lead screw.

I also have 1/2" screws, mine are turned down to 10mm and I use regular bearings, 10mmID 35mmOD. I have no threads on my turned section.

I found it beneficial to my machine to turn the screws down so the screw would be captured between its shoulders. I dont use set collars so the shoulders are needed. Also I didnt want any issues with slipping couplings so turning down the screw was the best bet.

You may be able to get away without turning and using collars, make the mounting block bore a standard size OD so if you have issues with the screw without being turned down you can change out the bearings and turn down the screws with little hassle.

[dpgoldberg]

The torsion base is together. It weighs a ton! The first picture is before the top sheet was glued down. Both the top and bottom are sheeted. Very rigid.

[dpgoldberg]

Here are the linear rails and movers I've been gathering for this project. The X axis will ride on two 20mm support rails that I found on Ebay. The Y axis mover is a Dual-Vee assembly that's been used on many old film projects, hence all the holes from the various models that have been mounted to it over the years. The short Z axis mover is another recycled mechanism. It's odd looking, but pretty solid.

[dpgoldberg]

The gantry is assembled. the MDF risers are held together with dowel (barrel) nuts. It's a pretty good fastening system. Strong, but easy to disassemble. Unfortunately despite my best efforts, the torsion base isn't perfectly flat. It has a slight crown, not more than 1/16" to 3/32" running the length of the X axis. Doh! So I've got to do a little surface planeing to get the rails to sit flat.

[dpgoldberg]

The table is now flat! I made a flattening jig by temporarily mounting the X axis rails to the sides of the torsion base, making sure they were level to each other. Then MDF was used to make a perfectly flat temporary gantry that slides on the rails. The router with a 1 inch diameter bit slid back and forth on top of the gantry. It took two passes, one running each direction to get the table more or less flat, then a little touch up with a jack plane and sander. Lots of MDF dust but the results were pretty good!

[ger21]

If the rails are parallel, you could have just let the machine flatten it.

Nice work so far. I've got a bag of dowel nuts I'll be using on my next build. Do you think you'll need to use Locktite on them? My concern is that they may get loose over time?