Small CNC mill/router concept

[HOM3R j4y]

i've been viewing these forums for a good while now, and have had a go at designing my own CNC miller. its only a small one (around 12" x 12" with about 3" of z travel) but its mainly a practice build so i can get a better understanding of how to build one.

it will be made out of mild steel, a combination of 12mm and 25mm plate, and the base is constructed primarily of 50mm box section.

i am using ground rails as they are cheaper and because of the small machining area was hoping this would be sufficient? i originally wanted linear rails but wanted to make this as cheap as possible. the ballscrews are precision rolled C5.

please let me know what you think. i am only planning on cutting plastics and maybe some aluminium if possible.

[hanermo]

You have overhangs in many places.

The x- screw is far too far below the table. This makes the nut bend and chatter. You want the screw as close to the table as possible.

The lateral diagonal supports need to go to the top, not halfway up the sides.
Likewise, a plate along with the diagonal, would greatly improve it.

The y-axis frame member is too small in cross section, in all directions.
Even then, it needs to be solid.

And, despite desires, the round rails will bend unacceptably.
Profiled rails are the correct solution.

For milling, with round rails, you need twice the cross-section.

The y-axis crosswise plate is undesirable. Remove it completely.
This will
-drop the distance to the screw (very good)
-reduce length of vertical supports at corners of table (very good)
Mount the table to the nuts by having a single solid at teach end, below the table, aligned with the x-axis, at the bottom edge of the table. The nuts mount to the solid.


Here, x axis is meant as in the direction of the visible ballscrew.

[HOM3R j4y]

thanks for your comments, the axis you are referring to with the visible ballscrew is the y axis, but i understand what you mean.

however i am unsure as to what you are referring to as the y-axis cross-wise plate? is this the plate which the lateral diagonal supports are attached to? if i was to remove this then what would i attach the ballnut to?

the ground rails are only 500mm long at 20mm dia, would this still flex badly? what diameter rail would be sufficient? i would like to use proper linear slides but simply cannot spend the money on them at this stage.

the t-nut bed is only for visual purposes btw, i am more likely to have a flat plate with tapped holes for workholding.

thanks again

[TarHeelTom]

i've been viewing these forums for a good while now, and have had a go at designing my own CNC miller. its only a small one (around 12" x 12" with about 3" of z travel)

If you're only going to have 3" of Z travel, you need to design in some method of moving the spindle up and down, independent of the travel.

For instance, what will you do when you need to rout the lid of a box, and the box is, say, 5" thick?

For instance, a typical Bridgeport has 5" of Z travel, plus another foot or more of knee movement. I can tell you from experience that 5" of Z travel is not much. Might be doing a simple job with a milling cutter, and the work is very close to the spindle. Next operation is to drill a hole, and the tool holder, chuck, and drill bit might be 10" long. On the Bridgeport (or clone) you simply lower the knee to get the clearance you need to insert the chuck and bit.

[HOM3R j4y]

i know what you mean, i operate bridgeports regularly at work so can fully understand how frustrating it is having to wind the bed up and down due to using longer tooling.
with my miller i was primarily focusing on being able to machine only small items which is why i only accounted for 3inches of travel, my design should allow me to have maybe 4inches max.
if i need more room, that may be the time when i build a bigger machine! like i said before this is mainly just a practice build to learn from, although i would like to achieve some level of accuracy.

[TarHeelTom]

With some planning, you should be able to design the spindle holder separate from the Z bed. Maybe have a set of index holes and pins so you could attach the spindle holder at different elevations.

Also make sure you make two or more different spindle holders, so you can switch between routers, engraving spindles, etc.

Tom

[HOM3R j4y]

cheers for the input, i may have a little tinker with inventor tomorrow to see what i can come up with!

[HOM3R j4y]

would the 50mm box section be sufficient for the base? i thought it would because people use 80/20 extrusions successfully and surely thats not as strong as welded steel? the base dimensions are only 500mm x 500mm.