Understanding Material Properties before Making the First Cut

The CNC machines we work with at the desktop level have limitations both in terms of how big a piece of raw material they can process and what the material itself can be made of. Every material has specific properties that require attention when being milled. These properties include hazards from cuttings/dust, viable milling speeds, and heat dissipation, among others.

By the end of this chapter, you will understand the following:

• The fundamental properties of certain materials to help inform the process of material selection in your projects
• How to tailor your cutting methods, settings, and preferences to your specific materials, machine and parts you are fabricating

You will also be able to select materials that are viable for your specific project (for example, a generic suggestion to use wood is not enough since you may need material that is light, or durable, or flexible). An important benefit...

Cutting hardwoods, plywood, and balsa

Wood is the easiest material to mill with desktop machines. The material is relatively plentiful and much of what we do with wood can also be applied to engineered wood such as MDF, pressed/particle board, and manufactured board. I have recycled materials from scrapped IKEA furniture for years and cut it with a CNC machine and laser for all sorts of projects. You can even make some interesting engraved art with the laminated stuff. A lot of what I do extensively recycles materials like this to minimize waste (I recently built a full flight-simulator panel using engineered wood repurposed from an old foosball table).

Softer woods, such as balsa, can’t be cut effectively with a high-speed end mill if they’re too thin. The fibers will quickly shred and destroy the workpiece. This is also true for any wood, but really soft wood is most susceptible. If I am cutting thin bulkheads for a model airplane it is far more efficient to use...

Cutting foam and composites

Cutting foam is a very interesting way of using a CNC machine. You can shape a block of dense foam (available at most DIY stores) with a CNC machine and then lay-up fiberglass on it to create a very strong and light object. I have built parts of a full-sized airplane this way. The wings have a foam core that was then layered in fiberglass, but the shape was cut by machine (either hot wire or CNC). When cutting foam, you must have a high-speed spindle (10,000 RPM comes to mind) and very sharp bits to prevent shredding.

Foam also makes a lot of mess and it is always a good idea to mount a dust shoe/brush and hook the other end to a shop vacuum to suck up as much of the dust as possible. Wear a mask when cutting foam as you also don’t want any of that dust in your lungs. One use case for foam is to cut inserts into packing cases and enclosures (for example, a camera case). Make sure your cutting lines overlap in your toolpath files so that you don...

Cutting plastics and PVC

When cutting plastics such as styrene and acrylic you must consider the effects of having a bit or a laser penetrating the material and marring surfaces. Cutting plastics requires some aspects that don’t apply to metals or wood. Many plastics including acrylic and plexiglass are brittle and will crack under stress. Therefore, clamping the stock material so that it cannot move will be important especially as loads are applied during cutting. Also important is the use of lower spindle speeds. High RPMs will heat up the surrounding material and cause the plastic to deform, which can add stress to it and ultimately result in cracks and breaks when least expected.

Cutting thicker stock material is much easier than thin. Thicker stock will hold its shape better and is less likely to get damaged by the bit. When I am cutting thin plastics (2 mm thick or less), I usually resort to the laser. By the same token, cutting softer plastics presents similar issues...

Cutting aluminum and other soft metals

Cutting through metals requires strong end-mill bits as well as the need to keep the material cool. If you have ever used a drill press on a block or thick piece of metal, you will likely have made the mistake of scorching the bit (where it gets black and starts to squeal as it cuts). This is when the bit gets very hot and binds against the metal it is cutting. This can be addressed with some cutting oil or similar coolant. There are tons of videos on YouTube showing industrial CNC machines cutting into blocks to make things such as engine blocks, or you might have seen lathes milling metal as well. In both cases, you will see the workpiece being drenched in a liquid that has two purposes. First is to keep the end mill and material cool and the second is to carry away the chips, dust, and other debris away from the work area so those don’t interfere with the machining work.

For our desktop mills we have to rely on our dust shoes to collect...

Selecting the right end mill

A common theme so far has been that the end mill should be as sharp as possible. While they certainly wear differently depending on what and how they are made as well as what materials they have been used with and at what speeds, you should still consider your bits as consumables. Most kit-built 3018 machines will come with a handful of end mills (some as few as two and at least one kit I built had none). You should also consider different collets to accommodate the various end mills. A collet is the flange or sleeve that you insert into the spindle end that holds the end mill in place. On a common hand drill, this translates to the chuck, which you tighten by hand with a key. Other machines that use collets include Dremel tools. Here’s a picture of some collets that came with my spindle; note how the hole diameters differ. This allows you to use different bits in your machine:

Figure 3.2 – Sample collets

Different...

Summary

We have discussed the handling of various materials in CNC in this chapter, and we have also touched on the various types of end mills. We will get deeper into a number of materials and end mills later in the book to give you some examples of specific tasks and materials. The number of permutations will boggle the mind so we can’t cover every single one here, but I can detail a few and then give you the skillset you need to make your own decisions for your own projects.

As with everything else, all tools, including end mills, are not created equal. I cannot endorse any particular vendor in this book, but you only need to choose your end mill type and read the reviews from other machinists to make an informed decision.

In the next chapter, you will make inroads with your machine and start making some cuts, applying what you have learned so far before jumping into a number of projects that will build your skills as well as offer you the ability to upgrade your machine...