Guide to Machining Fixtures: Workholding Devices and Jigs for Milling, CNC, and More

Machining refers to any subtractive process where material is removed from a large part in order to create the desired shape. Machining has been the primary means of creating distinct designs out of raw materials like wood, metal, ceramic, and other materials for hundreds of years, though ‘machining’ typically refers to an industrialized version while earlier methods were mostly carving by hand. There are many different types of machining, as determined by how — and with what kind of tool — the material is removed.

1. Milling: Involves high-speed rotational cutting tools. During a milling operation, the product materials are kept still while a cutting tool moves around it, cutting material away to reveal the intended shape.

2. Turning: This method uses a stationary tool that removes material as it touches against a quickly rotating workspace. CNC fixtures for turning need to be able to hold the workpiece very firmly and securely during this high-speed rotation. 

3. Drilling: The most common machining method, drilling creates small holes (smaller than boring) and can be done with a machine or manually with a handheld drill found in everyday households. 

4. Boring: Commonly thought of as the method to dig holes in the earth, boring is also the technique used to create hollow cylindrical parts like gun barrels. 

5. Broaching: The broach, or broaching tool has a series of ‘teeth’ that remove material from the part in order to create the shape, and is the main machining method for creating gears. Linear broaching moves the broach through the material while rotary broaching rotates both the tool and the workpiece together — necessitating machining fixtures to hold the workpiece in place.

6. Grinding: This method is typically used as a finishing method rather than the primary way to remove large amounts of material, and is often employed as a secondary process to clean up the surface of a machined part. 

7. Reaming: Used to enlarge holes made by drilling, reaming uses a multi-edge tool to remove material from drill holes.

8. Planing: Much like creating planks of wood, planing involves a stationary tool and a workpiece being moved across it to remove material in a desired geometry. Planing creates flat, smooth surfaces well, or can also be used to create dovetail joints, and curved or concave surfaces. 

9. Sawing: Sawing is a well-understood method of machining, and involves using a teethed or serrated edge tool to split or separate a workpiece into multiple parts.

10. Waterjet cutting: Waterjet cutting is a type of machining that uses high-pressure water streams to split material or remove material from a workpiece. 

11. Burning machining methods: These methods of machining (e.g. plasma cutter, laser cutter, oxy-fuel cutting) are used in highly precise applications to separate material using highly concentrated amounts of heat. 

Machining is used to create highly precise parts used in industrial environments, and these parts are often made from materials, like metal, that are difficult or expensive to form precisely in other ways (like casting or molding). 

Furthermore, machining produces parts individually and generates waste from the material removed during the process. These characteristics make it less suitable for mass production. Consequently, people choose machining due to specific and essential requirements for precision and accuracy.

This accuracy is partially ensured by the fixtures and jigs, like milling jigs, CNC workholding devices, or drilling fixtures, used during the process. These fixtures are extremely important to overall accuracy — they need to precise, hold their shape (no creep), and often customized to each product and each machine. 

It’s a common adage in manufacturing that to make something you need to make four other ‘somethings’ just to get started, and this is true of machining fixtures and jigs. 

Perhaps the most well-understood benefit of in-house 3D printing is the improvement in speed: more iterations allow for a more thorough design and testing cycle, creating products (whether end-use or manufacturing aids) that have been more rigorously tested and developed. 

The benefit of speed is perhaps especially impactful for manufacturing aids — which, if they’re not fabricated quickly, present an obstacle to any other prototyping or manufacturing even starting. Perhaps, considering the downstream impact of their efficiency and its effects on creating a better end use product, CNC fixtures, like CNC workholding clamps, should be given even more attention in the design phase. 3D printing prototypes of milling or machining jigs and fixtures would allow manufacturers to test their fit and efficacy — an ability often deemed unnecessary given the accessory nature of the part. 

This can lead to better-designed parts with more efficient use of material — the operator may note that the part could be redesigned for ergonomics and ease of installation, removal, and replacement. It can also lead to opportunities for customization; machining tools like CNC turning tables are mass-produced while their programming and the products they shape are unique to the manufacturer. CNC jigs and fixtures can bridge that divide, with some simple streamlining of the CNC fixture design process. 

The design flexibility of 3D printing — especially technologies that require no supports like selective laser sintering (SLS) — can open up new possibilities for fixtures and jigs to become more intricate, customized, and ergonomic. 

The digital nature of 3D printing also allows for more efficient storage of files. With a 3D printer in multiple factories, one large manufacturer can affect change at each of their locations by sharing a CNC fixture design file that improves efficiency on a common machine. 3D printing makes the sharing of knowledge more effective and impactful and gives big companies a way to improve at each level of their network quickly and easily. 

The range of materials available with different 3D printing technologies also makes 3D printing for machining jigs and fixtures much more powerful and useful in more situations. For example, 3D printed welding fixtures need to be heat-resistant, and certain stereolithography (SLA) materials, like Formlabs’ High Temp Resin or Rigid 10K Resin provide an affordable, on-demand, fast alternative to metal welding fixtures. 3D printed nylons, like Formlabs’ SLS Nylon 12 Powder or Nylon 11 Powder, are excellent for machining fixtures like chucks, which need a combination of durability and rigidity to hold parts tightly during a high-speed turning process.

When choosing whether to 3D print or use CNC machining for fixtures, the main factors to consider are: material properties, the degree of complexity the part requires, and lead time. 

In terms of material properties, machining fixtures and jigs don’t often need to be technically advanced beyond a certain level of strength and durability. Most industry-familiar plastics, like nylon, are sufficient. There are, of course, situations in which a specific material property, like conductivity, or heat-resistance are necessary, and at these times, a 3D printing technology like stereolithography (SLA) can offer the widest variety of advanced material properties purpose-built for different environments. 

Once you’ve determined the base levels of force and friction these parts will undergo, you can consult 3D printing manufacturers’ data sheets to find a material that will work best. Machine shops like The Factory Amsterdam in New York have found that Nylon 12 Powder printed on the Fuse SLS Series can handle the high-speed CNC turning processes they perform. 

Another consideration is the level of complexity the part requires. For certain machining fixtures, like collet pads, the pads themselves are simple shapes, and the manufacturer of the CNC turning machine often supplies original parts made out of steel. For applications where you’re making thousand or hundreds of thousand of the same thing, metal CNC machined fixtures will unquestionably stand the test of time. But when the fixtures and jigs start to become more complex, like a test fixture for a welding machine that needs to switch in between multiple programs, it becomes more costly and complicated to machine them. 

The single biggest consideration for deciding whether or not to 3D print or machine your machining fixtures is time — how soon do you need these parts? In most manufacturing scenarios, the worst outcome is to have a production stoppage because of a missing fixture or jig. If you rely on machining for these parts — especially if you’re using metal to prototype them as well as produce the final design — you’ll add to your machine shop’s backlog and can cause a bottleneck that affects the entire output of the business. 3D printing machining fixtures instead can deliver them in a matter of hours, without taking attention away from other projects, as the machinists would be forced to do. 

There is no one ‘best’ 3D printing technology for creating machining fixtures; the best tool for the job relies on multiple factors, and many businesses employ different types of 3D printers. The three most common 3D printing technologies are fused deposition modeling (FDM), stereolithography (SLA), and selective laser sintering (SLS) also sometimes referred to as powder bed fusion. 

When considering which printer to choose for 3D printing your milling jig, CNC fixture, drilling fixture or other machining aid, consider the following.