A slacker’s take on CNC fabrication

I don’t in any way claim to be an expert at CNC techniques and methods, but I have cut a lot of things out of a lot of sheets of plywood over the years…thousands for sure. I’ve developed a system that works well for me but freely admit that it’s not the most efficient way to work… there are lots of resources that are focused on production and will help you become hyper-efficient.

Sometimes you just want to get a job done:

Although it’s really nice to watch an efficiently toolpathed file while it’s cutting…no wasted moves and everything cutting in a logical order…in many cases it’s really not worth the time it takes to optimize your toolpaths. Running a ShopBot is MUCH cheaper than what labor costs, so spending the time to perfect your toolpaths might not be a worthwhile use of your time. The ShopBot is almost never the bottleneck in my shop…there’s always something to be done while the tool is cutting. So most of the time I want to limit the decisions I need to make while still cutting parts that turn out OK.

There are situations however where it is definitely worth spending a little extra time at the computer. You don’t want the CNC machine to be a production bottleneck, with people waiting around for parts to come off the tool and more cutting to do than the tool can produce. If that’s the case you really only have two options: add more machines or cut more efficiently.

Sharing is caring:

If I know I’ll be cutting a file just one time…maybe a prototype or a sample…I let the software make most of the decisions and it almost always turns out fine. The defaults are there for a reason…they’re good enough to get things done. There is a situation where I always spend the time to semi-optimize files…when I’m going to be sharing files like the Shelter 2.0 files. If I know my files is going to be used by others I spend a little extra time trying to get it to cut a little more efficiently and elegantly. It’s partly ego I guess but I feel better afterwards, so what follows describes how I work when I’m planning on sharing the files.

Try to be green:

In general I tend to be much fussier about material efficiency that toolpathing efficiency, I guess because I don’t mind wasting time nearly as much as I do wasting stuff. It may have something to do with enjoying the process of nesting parts and optimizing materials…it’s a puzzle. My only advice is to not get too carried away by making material efficiency constrain your design. If you really want a 7’ table, don’t make an 8’ one just because you don’t want to waste that extra foot of plywood! And although good toolpathing software does an amazingly good job of nesting, a person can almost always do a little better! It’s one of the few times that we can consistently defeat our Robot Overlords.

One Bit to rule them all:

I would rather not have to think about what kind of bit to use, or to have to change bits within a file if I can avoid it…I don’t want to have to hang around the tool waiting to change the bit. So when I’m working on a project I try to pick one bit that will do everything I need it to do, and to use the largest size of that type of bit that will cut all the features in that project. If the parts allow it, my go-to bit is a 3/8" diameter 2-flute straight bit…it leaves a reasonable edge and face quality and can make shallow cuts that would be a problem with an up-cut or combination bit. In lots of cases though a 3/8" bit is just too big to fit where it needs to go, but I try not to go smaller than 1/4" if I can help it.

There are lots of other options though. An upspiral bit cuts cleanly, but tends to chip the top face veneer and lift your parts, even with vacuum holddown. A downspiral solves the chipping issue on the top face, and will also help hold parts down because of its geometry, but can also cause the clips to pack in the kerf, causing friction and heat which is the death of bits and potentially a fire hazard. Combination bits solve some of these problems, but the upspiral section at the tip still causes problems with shallow pockets and you really have to cut at full depth all the time to get much utility out of them.

You can certainly also change bits depending on the feature, such as using an 1/8” bit to drill pilot holes for screws instead of having to hand drill afterwards, or a v-bit to mark any screw hole locations with a countersink. If you’re happy changing bits all day long, who am I to say you’re wrong!

HoldDown and Cutting Strategy:

I have a simple Vacuum holddown system that I can’t imagine doing without one. If you’re cutting features that are depth critical like 3d carvings or parts that have to precisely interlock, holding the material firmly to the table is critical. If however you cutting shapes out of sheets, holding DOWN in the Z-axis is only a part of what it does for you. I feel like its most important job is to hold the parts AND THE REST OF THE SHEET in place in the X and Y direction. If your material moves a little in the Z-axis you might be alright, cutting a little too deep or too shallow, but if it moves in X or Y you’re in trouble. At a minimum you will lose that part, but often you will ruin the whole sheet of material and/or break the bit. Not the end of the world, but a pain none the less.

It’s just like slicing bread:

That doesn’t mean that you can’t cut if you don’t have a vacuum holddown system. There are some toolpathing strategies that can help you cut with or without a vacuum system.

On just about every file I create I spend a little extra time picking the order of cut, and the point in each part where it will start cutting. The goal is to have the parts connected to the sheet for as long as possible before finally cutting them free. Imagine slicing a loaf of bread…you want to hold the loaf firmly while cutting off slices. As you slice, the “slice” is firmly held until the point that it’s cut free. So even with a vacuum holddown I try to put clamps at the far corners of the sheets whenever I can. This also helps keep the waste part of the sheet from “ooching” around as parts are cut out…those clamps are the “hand” that holds the loaf of bread!

Clamp at corners if possible
Clamp on corner

I also try to cut small parts first when the holddown is the best. More surface area makes larger parts easier to hold, so do them last.

Tabbing:

Tabs are helpful but can be a pain to remove. I generally only use them with smaller parts, and try to use one tab at the start point (in red circle below, which can be set in NODE EDIT mode). My exception to the one-tab-per-part rule is in long thin parts.. I tab the heck out of them because they will want to chatter, leaving a rough edge and probably breaking a bit.

Single tab at start point
Preview with tab

If you can place the tab so that it’s in a section of grain that’s parallel to the tab, it will be easier to remove. And if you can place the tabs so that they will be hidden in the final assembly, you won’t even have to clean the tabs up after the parts are cut and can just break them out of the sheet and use them just the way they are.

Place tab in area that won’t be seen

Ease in to the cut:

I try to ramp into every part that I cut…it makes bits last longer and leaves a better edge. More importantly, since it’s cutting a thinner and thinner bit of material as it gets to the end of the final pass, the cutting force is reduced and the part will stay connected until cutting is finished. For most parts I use a Smooth ramp, varying between 5–8” long depending on how aggressively I’m cutting. With small parts however I use a Spiral ramp, which along with a tab at the start/end point gives better control of the cut and the part.

Every rule has an exception, and the only exception to my “always ramp” rule is when I’m cutting a shallow feature like a slot or recess for bolt heads…it’s just not needed.

Layers are your friend:

Layers are an easy way to convey details and intent to others, so try to use descriptive layer names to describe the entities contained in those entities. Entities in a “screwhole location marker cut 0.06” deep” layer might cut shallow dimples that indicate where to eventually drill pilot holes for screws. Sometimes features like all the holes in all the parts might be on their own layer if they are to be cut using a different strategy than the other parts, such as when using a spiral plunge. It doesn’t hurt to be long-winded in layer names, so use them to convey useful info.

Be wary of flipping:

Until recently I would have said to avoid two-sided cutting like the plague, and still feel like you need an awfully good reason to ever flip a part over and cut the other side. Fortunately the fine Vectric folks have added some new features to their VCarve and Aspire software that changes my advice on two-sided cutting from “Never ever do it no matter what” to “Make sure you have a good reason”

Just one fool’s opinion:

Understand that although this is the way I work, there are as many reasons to do it differently as there are to do it this way. All modern toolpathing software, especially the fantastic Vectric software, has amazing tools for efficiency and you would be foolish not to use them if you want to. Just don’t let the desire for efficiency get in the way of getting things done and turning your materials into dust and noise (and hopefully something useful!)