This post is to provide basic terminology and a glossary for communicating about CNC projects. Work-holding is the general term for securing a part against the forces of machining.
Work-holding solutions have to be considered while quoting a CNC project. Fixture costs and CAD + CAM costs are the reason an individual or prototype CNC parts tend to be considered expensive.
Work-holding devices range from simple vise to complex pneumatic systems. Developing work-holding solutions is complex, thus it’s where machinists get to express their creativity and expertise. The starting point for most work on a mill is the machinist vise.
Machinist Vise: A vise uses clamping force to secure a workpiece. They can be used to hold fixtures or other work-holding setups. Vises are fastened to the machine bed with T-slots allowing repositioning when needed. Many vises come with flat or ground-accurate sides which allows for precise alignment with pins and locating plates. A vise can be equipped with a thickness of machinable material called a jaw.
Hard Jaws: These are generally made out of steel and are designed with teeth or edges that can bite into a part.
Soft Jaws: These are machined from softer steel or aluminum and are shaped to the part.
Other Types of Vises:
Auto Vise: Pneumatic or hydraulic motorized vise similar to a regular machinist vise, however, they offer convenience and controlled clamping forces.
Universal Vise: These use complex mechanisms or sliders that contour to the workpiece shape.
Work-holding Fixtures: Mill work-holding fixtures are used for mass production of parts or difficult to secure workpieces. Fixtures are designed to allow access to machinable areas and are designed to reduce part changing times. Usually made from “hard metals” like steel for longer lifespans. As fixtures are designed for specific workpieces they aren’t interchangeable. Even workpieces that look similar at first glance may have differing dimensions making it important to have a mating fixture. Fixtures may contain pneumatic pistons or are designed to facilitate automation.
-A selection of fixtures samples.
Jig: A jig is similar to a fixture where it secures a workpiece, however, its intended purpose is to guide the cutting tool.
Vacuum work-holding: Removes the air under a part causing air to push down on the part at 14.9 pounds per square inch at sea level. Parts can quickly be changed when the vacuum turns off. Great for flat, higher surface area nested parts. Rubber gaskets are used to create air seals.
Turning machine work-holding.
Chucks: A chuck connects to the headstock of the turning machine. Chucks have 3 or more jaws. These jaws, whether Hard or soft, support and locate the workpieces. Through-hole chucks allow the use of bar feeding systems. While chucks are usually used on turning machines they can be used to hold round workpieces on mills.
-Three soft jaws with a installed work piece. A set of soft jaws installed on a lathe. A chuck with hard jaws mounted in a vice on a lathe.
Pneumatic Chucks: Air is used to open or close the chuck. An operator often uses a foot pedal for opening and closing the chuck.
Collets: Collets are used with small dia. lathes and Swiss machines. Suited for parts smaller than 3 inches and used when spindle speeds are required to be higher. When paired with an automatic collet chuck, collets provide automated part changes or bar feeding. Collets need to be near the same diameter as the workpiece they are holding.
Expanding Mandrels and Arbors: These hold round parts from the center. They push outward against the inner diameter of the workpiece thus securing it in place.
Work-holding Accessories
Tab Supports: While tabs aren’t exactly work-holding, they do keep parts located. Tabs are small thin pieces of material left on the exterior of the workpiece that keep it attached to the surrounding or neighboring material. It’s also possible to have small tabs that fully support a workpiece and can be quickly separated.
Tooling plates or grid plates: These are fastened to the machine bed; Tool plates have precisely located holes that enable the use of an assortment of fixtures. The main benefit is that pegs can be used to set up work-holding quickly. Tooling plates are made from steel and are great for shops that need many options. Tool plates are also useful for QC operations and laser engraving fixturing.
Pallets: Pallets are used to work on multiple workpieces at a time known as part nesting. The main reason to use pallets is to increase the walk-away time and decrease machine downtime. Pallets will be preloaded with parts via hand or robot. When a machining cycle finishes, they are removed from the machine and a new pallet with parts is inserted. The operator will then unload the finished parts and place parts to be machined on the pallet.
Pallet Changing Systems: These systems automate the process of loading and unloading pallets using a system of conveyors and lifts. These types of systems enable what could be considered lights out manufacturing. Can be expensive and best for horizontal centers and high-end vertical machining centers.
Quick-Change Systems: Reduce the time spent loading and unloading parts. These are much like pallets but they may only hold a single workpiece. Locating male pin mates with a female ball bearing retaining pneumatic system.
This is the "Zero-Point System" a quick change system from Chan Chao International a local Taichung based company. -Checkout them out at https://www.mitpsb.com/
Tombstones: Generally used for large horizontal machining. Tombstones rotate, allowing access to multiple approach angles. Smaller tombstones can be used on vertical milling machines with trunnions or rotating tables. Having multiple sides multiplies the workable parts.
Toe clamps: Hold from the bottom and give full access to the top of the part and most of the sides.
Step clamps: These have incremental steps that when tightened mesh together. The workpiece is clamped downward towards the tip with a bolt and nut. A T-nut will slide into the bed.
V-blocks: V blocks have a triangular shape cutout that holds round workpieces well.
Tape and glue: Believe it or not quick-drying adhesives can be used to adhere workpieces to a flat piece of material, these is useful for thin bendable work pieces.
1-2-3 or 2-4-6 blocks: These are blocks with holes that are useful for configuring work-holding. Can be used to apply larger surface area pressure. Works well in conjunction with step clamps.
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