If you want to make a complicated project multiple times, you may want to make a custom fixture. This is especially true if your project requires machining more than one side of the object. A fixture that is specially made for your project can securely hold your part, while precisely positioning it so that multiple machining operations all align with each other. It can also greatly simplify the process of attaching your material to the bed. Many industrial manufacturers make custom fixtures to speed up the process of making their parts.
A custom fixture is generally comprised of a piece of material bolted to the bed, with a shape cut out of the middle to hold the object that will be milled. Many custom fixtures have locking mechanisms that can be tightened to secure the material being milled.
This guide shows you the basics of making custom fixtures, and it assumes that you already have some experience with milling.
Step 1: Determine if a custom fixture is right for your project.
A custom fixture is most often needed in the following circumstances:
- To make a project multiple times, from a material of known size and shape
- To mill an existing object
- To mill an object with a bottom that isn’t flat
- A combination of the above
Making a project multiple times, from a material of a known shape and size: The custom fixture shown below is made from white HDPE for a yo-yo project that has to be milled from both sides, which requires very precise alignment. This fixture holds the material during both machining operations. It has one pocket for each operation, and it places the material in precise locations on the bed, which correspond to where the machine will mill.
Milling an existing object: An example is engraving a lot of dog tags. (This is actually a combination of the first and second cases.) We made a custom fixture with a recess that was just the right size for the tag we wanted to use, which took care of positioning and aligning the material. We also added a tapped hole so we could secure the tags with a screw instead of tape or other methods, which sped up the process of attaching the material.
Milling an object with a bottom that isn’t flat: If you wanted to engrave the underside of a computer mouse, you’d need to have a way of securing the rounded top to the bed. (This is actually a combination of the second and third cases.) You could make a custom fixture that held the mouse by the sides, with the underside facing up, perfectly flat. Then you could mill the underside easily.
Step 2: Plan your fixture.
Start by looking at the shape of the material you’ll be milling, and imagine how it'll fit into the fixture. For example, is it round or square? Does it come in uniform blocks, as a sheet, or as a rod? The answers to these questions will inform the shape of the fixture. In the image below, a Delrin rod is held in place by a Delrin custom fixture with a locking mechanism.
If you need to mill both sides of an object, it'll often be a different shape after the first side is milled. If you want to secure it so you can machine the second side, you’ll need to have that same shape cut out of your fixture so the object can fit into it.
Decide if you need a locking mechanism or if the material can be held in with just a press-fit. Taller materials generally need locking mechanisms, whereas thin, flat materials can often get away with a press-fit. The precision you need also affects this. If your project only needs to be precise to 0.010", it just needs to be kept from flying completely out of the fixture. In contrast, the yo-yo project mentioned earlier needed to be within 0.002", ideally 0.001", so a locking mechanism was necessary.
Step 3: Choose a material for your fixture.
What material will your fixture be made of? There are many choices, but we’ve found that plastics such as Delrin and HDPE are both great options. They’re easy to mill and flexible enough to make locking devices. However, you can make fixtures out of anything you want. The dog tag fixture mentioned earlier was made from a scrap piece of aluminum, and it worked great.
Step 4: Design your fixture either in CAD or as an SVG file.
This mostly comes down to how complicated your part is and how much precision it needs. If it needs to be machined on both sides and/or to within a few thousandths of an inch, you’ll want to draw up your fixture in a CAD program. If you need that much precision, you’re probably already making your project in a CAD program, so it makes sense to incorporate your fixture into the same CAD project. If your part doesn’t need to be machined on both sides and/or doesn’t require high precision, a simple SVG file may be faster and easier.
Step 5: Mill your fixture.
Mill your fixture. And send us a picture. Hey, that rhymes!