I’ve learned a lot from the orrery project.  I’ve learned a lot about how gears are made.  But mostly, I’ve learned how things are different when building things with a CNC mill as opposed to the typical woodworker way.  I guess this parallels the changes that were brought about by power tools like table saws and routers when woodworkers had only been using hand tools before that.

This is part of a generic process that I’ll call “fabrication”.  These days there are two types of computer controlled machines that will fab things for you.  There are the so-called “3D Printers” that squirt out some sort of material in the shape you want.  The simplest of these use a string of ABS plastic that the extruder heats up and squirts out onto a substrate. There are also some types that harden a material with a laser and thus build up your ‘thing’ one line at a time.  Then there are the machines that take a solid block of material and cut away whatever you don’t want.  That’s what I was using.

I guess the first thing is that the terms “CNC mill” and “CNC router” mean almost the same thing.  The first comes from the metalworking world and the second from woodworking.

Anyway, when doing things the CNC way, you tend to try to think of ways to keep all your parts flat since creating anything with curves in three dimensions will complicate your design process by at least a factor of 10 and sometimes it can complicate things at the machine as well (some machines only work in 2 dimensions).  By sticking to flat parts, you can just draw things on the typical CAD screen and then the machine will cut along those lines you drew (after you tell the machine how thick your stock is – more on that later.)

You also have to give some thought to the router bits themselves.  You can use typical woodworkers bits but there are specialized sources that most CNC folks use because of a couple of issues.  In the CNC world, they refer to them as “end mills” (as opposed to “ball mills” which woodworkers would call a round-nosed bit).  If you’re cutting plywood, you tend to prefer a ‘downcut’ spiral bit because it will keep the veneer from lifting up and creating splintered surfaces.  But it will also force the sawdust down into the cut where it will build up if the resulting channel is pretty narrow (and doesn’t go all the way through) and cause heat to build up.  This can, in principle, start a fire and I’m told you can see smoking occasionally. I haven’t but it definitely smells like burning wood sometimes.  So ‘upcut’ bits are preferred for cutting narrow channels or anything that is not a cut all the way through the stock.  You can get the best of both worlds with what is called a ‘compression’ bit which has both upcut and downcut flutes; one set on the end, the other set near the shank. I can’t imagine how these are produced but I suppose they are made by CNC.  They look like something from an M.C. Escher print.

The same bit (or “mill”) does everything: cutting and drilling.  Drilling is interesting: frequently you tell the machine to “peck drill” a hole which is where the router lowers, drills a fraction of an inch deep, then raises back up to clear the chips; repeating this process until the desired depth is achieved.  As this goes on, the machine appears to be pecking at the stock.  If the required hole is larger than the bit then it will spiral out from the center until your hole has the required depth and diameter.

You’ve really got to think hard to come up with any sort of joint that you can make with a CNC machine that looks nice like the traditional woodworking joints.  It’s hard to be original and so at the moment, CNC milling seems to work best for cutting out parts from flat stock.  A clever person (or a person with access to expensive software) can use it to carve pictures into any reasonably soft surface so I imagine in the future, clever woodworking joints will be developed.  I’ve seen things that look something like dovetails that seem to work well.  As for my friend, he’s written his own software to design and then carve out guitar necks.

When you’re cutting a part out of a piece of wood, you have to make sure you have the software cut on the correct side of your line.  Similarly, if you have something that needs to be cut out that will end up as scrap, you can just have the machine mill away all of it but that takes lots more time.  I’ve heard this referred to as “eating the scrap” but I don’t know if that’s a generic term or not. It’s easier to just leave tabs there to hold the cutoff piece.  That way, the waste pieces are held in place until you come back and cut the tabs with a knife.

Coming from ordinary woodworking, it was hard for me to think ahead and take advantage of everything.  You need to have all your dimensions handy before you start instead of the old “cutting to fit” routine that I used to use.  In other words, it requires you to completely design everything down to the last dimension before you start cutting. You also need to forget the fractional measurement system and switch to decimal.  It was interesting to start a woodworking project with a calculator and a caliper.  You can also do some cool things that were not easy before, for example, my project called for half inch thick plywood but I only had quarter inch material.  No matter, although I could have gone out and bought some, we simply cut twice as many parts and glued them together.  It was plywood so it didn’t look any different after it had been laminated together that it otherwise would have.  We included some alignment holes in the parts so that I could stick in a 1/8 inch dowel to keep things aligned while the glue set up.  This dowel was just cut off and sanded flush – it was barely noticeable.  That’s not the sort of thing you do in traditional woodworking.  If you’re willing to change bits (which is no small task by the way since you have to recalibrate the machine after a bit change) you can switch to an engraving bit (v-groove bit) and engrave the parts with labels.  That would be handy if you were producing large numbers of parts. We didn’t do that because we were still prototyping but that leads us to another issue.

You have to tell the machine before you start exactly how large your stock is (so that you or the software can place your parts on it) and where your stock is (so that the machine will cut where you think it should).  This latter is something you do after you clamp your stock to the bed (by any of several means).  First, you use the arrow keys (in our case) to move the router around until you are at the lower left corner of the stock which is probably where you defined your zero point to be in your original drawings.  (A very Cartesian way of looking at things. It doesn’t have to be this way – you just want to align the part you’re making with the stock you have.)  Then you place a metal plate on the stock and tell the machine to find it by lowering the router and bit until it just touches the metal.  It does this twice just to make sure things stay consistent.  In this way it knows where the surface of your stock it (less the thickness of that metal plate which is well known from the factory.)  At this point you can start cutting.  Noobs will usually make the mistake of designing a part and then not noticing where the origin of their system is and cut a part out of the middle of a sheet of stock.  This can screw up a lot of material depending on how extensive your toolpath is.  If we had wanted to change bits (for example to label the parts with a v-groove bit, we’d have to have gone through the calibration process again.  If you’re using a laser cutter then presumably you would not have this problem.)

Dust collection is an issue.  If you’re doing a lot of cutting, the router can be in continuous operation for an hour or more so a lot of dust can get created.  Dust collection is built in to most of these things and that’s fortunate.  Perhaps it’s even necessary depending on your allergy situation.  I suppose you could just stand there with a shopvac hose; it can get pretty boring when the machine is doing its thing for an hour.  You might feel that you may as well be doing something.  Still, I prefer the automated dust collection.  I was taking photographs during my milling session.

Holding down your stock is another issue.  The bed of most of these machines is made of sacrificial MDF which you replace when it gets too chewed up.   So you have some options in how you stick down the pieces you’re milling but screwing your stock to the bed is the most obvious option.  You just have to make sure that no screws are anywhere that the bit will be cutting.  You can also use holddown clamps as long as the router doesn’t get too close to them and knock them off.  A little forethought will keep that from happening.  People who use CNC mills for a lot of production will commonly use a vacuum clamp to hold down the stock.  This complicates the process in that you need a vacuum system first and a shopvac will not do, nor will a dust collector.  You need something dedicated but once you have your fixture built, it becomes a simple matter to put your stock onto the vacuum fixture and turn on the vacuum.  This holds things down admirably and there are no obstructions to the router when you’re cutting.

This is all just what I learned from playing around with this for a few days.  There’s much more to learn but the biggest steps are: designing your parts with a CAD program, importing them into your CAM software (which usually comes with the machine) to convert your parts into a toolpath (usually saved in a language called ‘g-code’), and the actual machine control software.  To get from the CAD to the CAM phase, you need to save your files in a common format; “DXF” is the most common and this may limit the CAD software you use.   Google Sketchup for example will not save in this format unless you purchase the professional version or purchase one of several add-ons that will do the same thing.

Once you go through the process, you begin to see that if you can draw it on the computer, the machine will cut it out for you and it will do so perfectly and as many times as you want.  With all this capability it’s surprising to me that what most CNC router owners (Shopbot specifically) end up doing is making carved wooden signs.  You can literally make anything; and a few hardy souls do just that.  My friend is using his to carve guitar parts (necks, all the internal braces, fretboard, etc).

There are also other ways to have flat parts cut which might work better in some cases.  There are laser cutters that work much the same way.  Probably the water jet cutter has been around the longest and is advertised most often as the way to get many different materials cut.  Places that do this are all over the country and often advertise themselves as being willing to do only one article.  The possibilities are endless it seems.

It’s fun. And it can change the way you see the building process.