How NOT to make a CNC Mill (part 1)

PCBMill

Computer Numeric Controlled Mills (CNC for short) are great tools to have around.  Wouldn’t it be great if we could make one from “scrap” parts?!

This series was originally intended to be a tutorial on how to make a CNC mill “on the cheap”.  This build was pieced together mainly from different bits and pieces gathered from some old projects, including an original controller PC that was over ten years old.  The total mechanical parts cost was $60-$90 to start (not including electronics or the re-purposed parts).  It’s been an on-off project for quite a while, many things have been learned – but, currently, it just doesn’t work as required.  That’s what these articles are about – detailing mistakes made so others can learn from them!

 

The Beginning

mantis9

CNC mills have been popping up all over the place the last several years.  One that was especially of interest was a Mantis 9 Mill, created by David Carr over at the MIT Media Lab (definitely check out the media lab – they provide a constant stream of really cool projects).  The reason for looking in detail at the Mantis 9 was because it was inexpensive and shouldn’t require any serious machining to create (more on this later).

Mantis 9 Gotcha’s

First, it should be noted that this is in no way griping about the Manits 9, but merely observations – many of which are specific to this implementation.  The mantis 9 project is all about a cheap CNC Mill.  The project page says this was accomplished in around $90 per mill for the mechanical portion.  While this was true for raw mechanical BOM, when amortized over several mills (and not including shipping charges), you’d be quite hard-pressed to actually buy all of the materials needed for under $100 after shipping charges.  There were also some details that were left out – like the ACME threaded rods that require a fairly precise bore through the end (which should be done by a machinist), the delrin lead screw nuts that were tapped with an ACME tap (definitely not included in the $100).

What’s On Hand?

The main reason for getting into this project to begin with was a few precision rods – a dusty bag of precision rods (5/8″ 24″ long) – enough to make an entire mill!  Looking around a bit more uncovered an assortment of discarded NEMA 17 and 23 stepper motors – sounds like the making of a mill!  So – that’s how/why this entire project got started – because some old “junk” turned up in a bin and sparked a memory of “that cheap Mill MIT put together”.

First, the mill was modeled in Google Sketchup.  Since the linear rods were significantly longer, an entirely new model needed to be created.  This mill was going to be fairly large (well, large for a “small” mill ;-)), having an original working envelop of around 20″ x 10″ x 3″.

pcbMill model 1

The material chosen was 1/2″ MDF because it should eliminate concerns with warped plywood.  You’ll see this used quite a bit for things that need to be cut cleanly, it has a uniform consistency that doesn’t contain any voids like plywood and doesn’t splinter.  MDF is quite strong along it’s length, but is surprisingly worse than plywood for holding on its “end-grain”.  This created quite a few issues during a few stages.

What’s Still Needed?

So, we’ve got the precision rods for linear motion, some motors, and some MDF.  We still need some bushings (to ride along the precision rod), threaded ACME rod, and lead-screw nuts – McMaster Carr, here we come! Here’s what was ordered:

2868T104 12 Each Sae 863 Bronze Sleeve Bearing, For 1/2″ Shaft Diameter, 5/8″ Od, 5/8″ Length
98935A817 2 Each Plain Steel General Purpose Acme Threaded Rod, Right Hand, 3/8″-12 Acme Size, 3′ Length
95155A111 3 Each Brass General Purpose Acme Hex Nut, Right-hand, 3/8″-12 Acme Size
3201T61 1 Each Black-oxide Steel U-bolt, W/plate,5/16″-18 X 1″l Thrd,for 2-1/2″od,610# Wll
92105A660 1 Pack Zinc-alloy Knife-thread Insert For Wood, Hex Drive, W/out Flange, 8-32 Int Thrd, 25/64″ L
91770A197 1 Pack 18-8 Ss Truss Head Phillips Machine Screw, 8-32 Thread, 3/4″ Length
6061K23 2 Each Hardened Precision Steel Shaft, 1/2″ Diameter, 9″ Length

Now all that is needed are the control electronics.

In How Not to make a CNC Mill (part 2), we’ll take a look at what went wrong with the first build.

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