Furniture, wall hangings, and custom signage are familiar CNC project categories, but downsizing your ambitions can also open up a world of possibilities. That said, machining miniature parts is not without its own challenges. Small parts are tricky to secure on a CNC, so you may need to get creative with your workholding and look beyond using basic clamps. The importance of using the right materials and tools is also elevated, as your margins for error are smaller.
WORKHOLDING
Holding your material securely is a prerequisite for success in CNC, and tiny projects can be difficult to get a grip on.
A LOW-PROFILE VISE is indispensable where you may not have a lot of z-clearance. They keep the top face of your part exposed. However, if your part is oddly shaped, or you need complete access to the sides, then consider a different method.
ADHESIVES are great for holding thin stock with a lot of surface area. Double-sided tape is often used to secure PCB blanks, plastic, or plywood.
Fixturing wax or hot glue can be a good way to secure small pieces to your table. Melt it to the surface, press your stock into the molten puddle, and maintain pressure while it cools. A nice benefit with these methods is that there’s no sticky residue to gum up your end mill once you cut through your part since wax and hot glue aren’t tacky when cooled.
When machining parts with curved or irregular surfaces, custom fixturing solutions called SOFT JAWS, which are clamp faces that have the negative profile of your part machined into them, can be useful.
You could also cut a shallow pocket in the spoilboard to hold your parts.
MATERIALS
Small projects are a great opportunity to start working with metals. The most common non-ferrous metal to machine is ALUMINUM, but not all aluminum is made equal. Different alloys and tempers can drastically affect how well it machines. Pure aluminum is relatively soft and almost acts like clay at the microscopic level — and it clogs end mills easily.
7075 aluminum, which is alloyed primarily with zinc for strength, is a harder alloy that shears much more cleanly than pure aluminum; it “forms a chip” as they say in the machining world. A cheaper alternative that’s slightly softer but still very CNC-friendly is 6061 aluminum. 6061 is alloyed primarily with magnesium and silicon, and is a common general-purpose grade aluminum. Using the wrong alloy can cause you endless headaches, so do some research before buying. McMaster-Carr’s website is a good reference.
In PLASTICS, it’s important to have a good ratio of feed rate to spindle RPM. Wood will char if your cutter loiters too long in a single spot, but plastic will melt, clog your end mill, and ruin your project. Heed the recommendation of your CNC manufacturer, or better yet, the cutting parameters of your tool vendor.
END MILLS
Speaking of tools, different END MILLS have their own pros and cons but the one truth that holds in any situation is that you should use the shortest tool you can get away with. The more your tool sticks out of a collet, the greater your risk of vibration or “chatter.”
Furthermore, the longer your flutes are, the weaker your end mill is. In a small machine, if you get your cutting parameters wrong, you’ll likely stall your CNC before you break a tool, but in miniature scale machining your end mill will be the weakest link.
If you need exceptionally fine details, an ENGRAVING BIT can be a good alternative to using tiny, fragile end mills to define sharp corners or other tight geometry. Bonus: the engraving bit can also chamfer edges, saving you from having to file or sand off burrs.
FINISHING
Finish passes are essential for accuracy and surface finish. A duplicated cut will clean up excess material that your cutter might have missed on the first pass due to vibrations or other physical phenomena. A quick way to implement finish passes in any CAM software, basic or advanced, is by simply duplicating an existing toolpath.
A LITTLE INSPIRATION
I hope these tips help you understand some of the variables at play when tackling smaller projects, and encourage you to try your hand at making more detailed or precise parts on a CNC.
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