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The Design - The Big Idea

I started off the mechanical design part of this project with the motor shaft diameter which is 1/4". Ideally and typically most people use 1/4-20 threaded rod for the leadscrew. A little math shows that the motor has 200 steps per revolution and the screw needs to be revolved 20 times to get 1" of linear travel, so there are 200*20 or 4000 steps per inch which equates to 0.00025" per step. I wanted it to move a little faster so I sacrificed some power and accuracy by deciding on a 1/2-13 threaded rod instead... a little more math shows 200*13 or 2600 steps per inch which equates to 0.00038" per step. A theoretical four ten thousanths of an inch accuracy is fine with me, that is as long as I have no backlash in my axis drive. I'm actually hoping for a few thousandths accuracy when all is said and done. This leaves me a factor of 10 yet for any slop that is added into the design.

The Design - The Linear Axis

One nice thing about this design is that once you have designed an axis, you have designed them all! Each axis mechanism is identical. This cuts down on the different types of parts that need to be fabricated.

502-3005.pdf : PLATE, END - CNC MILL
502-3006.pdf : SCREW, LEAD - CNC MILL
502-3007.pdf : ROD, GUIDE - CNC MILL
502-3008.pdf : BLOCK, GUIDE - CNC MILL
502-3009.pdf : BLOCK, SCREW - CNC MILL
502-3010.pdf : PLATE, BODY - CNC MILL

Steel Ball Bearing -- ABEC-1, Open Bearing No. R6 For 3/8" Shaft Dia, 7/8" Od (Mcmaster ref:60355K14)
Sae 841 Bronze Flanged Bearing, For 3/8" Shaft Diameter, 1/2" Od, 3/4" Length (Mcmaster ref:6338K416)
NOTE: the drawings and parts presented in the body of this document are "initial" design components. The final design may or may not incorporate modifications to these parts. Use this information as reference only.

Above is a 3D rendering (in false color) of an axis and all the part drawings in acrobat format. The purple part is the stepper motor, the green plates(502_3005) are 1/4" aluminum, the tan body(502_3010) is .06 aluminum plate, the orange blocks(502_3008) are delrin 500, the darker orange block(502_3009) is also delrin 500, the light blue rods(502_3007) are hardened and anodized aluminum and the golden rod(502_3009) is a 1/2-13 aluminum threaded rod. The end of the threaded rod rides inside of a standard ball bearing and the guide rods (light blue) have bronze bearings in the guide rod blocks. Just make 3 of these linear axis and 90% of the construction is done.

What's not shown here are the limit and home switches (Omron VX-56-1A3). Microswitch mounting holes are provided on the BLOCK, GUIDE - CNC MILL parts for mounting these switches.

The Design - The Machine Frame

There are several ways to tackle this part of the design, John Kleinbauer's BRUTE mill uses a piece of plywood and some water pipe. All that's needed here is to provide places to mount the X-Y axis stack, the Z axis, the power supply and the stepper controller along with some terminal strips. This part could be made from 3/4" plywood and be more than sturdy enough for this machine however, it would grow and shrink a bit depending on the humidity. I decided to make this part out of 1/4" aluminum plate... less prone to growing and shrinking. The parts for this frame can either be fastened with screws or the whole thing could be welded.

502-4001_1.pdf : BASE, BOTTOM - CNC MILL
502-4001_2.pdf : BASE, BOTTOM - CNC MILL
NOTE: the drawings and parts presented in the body of this document are "initial" design components. The final design may or may not incorporate modifications to these parts. Use this information as reference only.

Additional features such as mounting holes and tie downs need to be added but these are best left to assembly time to determine the optimum location for these features.

The Design - The Tool Holder

Oh yeah, I have not mentioned what drill motor I intend to use, well, it's the old reliable hobby workhorse, the Dremel® Tool. Actually, it's a Model 395 Dremel® MultiPro. This Dremel® tool is fitted with the Dremel® Chuck #4486 which will hold even the smallest drills. (NOTE:10.20.2007 this model has been obsoleted by Dremel, they offer a 75th anniversary version but substitution is called for here.)

I have a drill press(Dremel® #212) for this tool and considered cutting off the mounting bracket and reworking it to fasten to my CNC drills Z-axis but that would be too easy (maybe). I decided instead to design some parts that mimmic the drill press mount, saving my drill press from destruction.

502-3020.pdf : PLATE, DREMEL MOUNT
502-3021.pdf : BASE, DREMEL MOUNT
502-3022.pdf : SUPPORT, DREMEL MOUNT
NOTE: the drawings and parts presented in the body of this document are "initial" design components. The final design may or may not incorporate modifications to these parts. Use this information as reference only.

The yellow bent rod is a #4-40 threaded rod bent in a horseshoe shape. I will probably be a good idea to heat up this rod before bending it around a mandril to minimize snapping or kinking it.

The tool has 10 speed positions ranging from 5,000 to 35,000 RPM which roughly lays out like this:

15,000 RPM 28,000 RPM 311,000 RPM 414,000 RPM 517,000 RPM
621,000 RPM 724,000 RPM 827,000 RPM 931,000 RPM 1035,000 RPM

It would be nice to set the speed via software control but this would necessitate that I disassemble and modify the Dremel® tool with some sort of switching electronics or yet another servo. Seeing that I will not be able to automatically change the drill bits (tool change), I see no reason to go to these lengths to fully automate the drill motor. I will have to start-set-stop the tool manually... small price to pay.

The Design - The Table

This part of the design is the least critical of all the parts of the cnc drilling machine. I will start off with a piece of 1/2" MDF 8-1/2" square fastened dead center on the X-axis. To be more precise, it will actually be a piece of 3/8" MDF with a 1/2" border of 1/8" spruce fastened to it to hold a 1/8" thick piece of spruce as the table top backing material. This way, as I drill holes I will gradually chew up this backing material and I can easily replace this part of the table as need dictates.

The machine is designed to accomodate many tables from this bare bones "setup" table to a yet to be designed vacuum base table.

Download the entire 3D layout (DWG - 2.36Mb)