Posted June 11, 2020
An Introduction to Workholding
In CNC milling, the term “workholding” encompasses all the components required to keep the work secure while milling the block of material. It starts simply, when holding parts with square edges, but becomes more difficult for parts with complex profiles and changes entirely when mass production is concerned. Let’s dive into just a selection of workholding styles and explore their common applications.
Back to Basics
First, why do we even need to worry about workholding for machining, when other processes like additive manufacturing don’t? The answer lies in the forces that are placed on the workpiece, or finished part.
Additive manufacturing applies very little force on the work by the printing nozzle. A 3D printing nozzle, for example, gently and precisely adds material to the workpiece in incredibly small increments.
A cutting tool in a milling spindle imparts high forces on the workpiece. Unintended movement of the workpiece requires starting the job over. Loosening grip on the workpiece also causes incredible damage, as parts can fly at high speeds, break a tool, a window or, worse, damage the CNC mill itself.
The most basic type of workholding in machining is the machinist vise. Both jaws have great parallelism and perpendicularity, along with great strength, so they’re ideal for gripping stock and not letting go. When used with parallels to hold the work at a height off the bottom of the vise and an endstop to repeatedly align pieces of work, the humble machinist vise can become a great tool for low-run production.
Soft jaws are a critical addition to the classic machinist’s vise, especially when holding non-square stock is required. The term "soft jaws" encompasses all custom clamping faces that act like the two sides of a vise, but have a particular shape that’s meant to grip the part more accurately. In this example of a soft jaws within a machinists vice, a part with a complex profile can be more rigidly held by soft jaws that mimic its profile.
You can generate soft jaws easily in CAD software, and they will quickly become a critical piece of your workholding repertoire as the complexity of your parts increases. Soft jaws can also include locating features for easy part swaps, making it more difficult to insert a part incorrectly before pressing “go” on the milling machine. Due to the inherent great balance between ease of fabrication and results from using soft jaws, Plethora often uses soft jaws in our own manufacturing.
There are many other workholding options past machinist vises and the soft jaws they often employ. Vacuum fixtures are popular in both the wood and metal shop, often used to hold down large sheets on wood routers and small pieces in metal machining centers. Like soft jaws, vacuum chucks are great for holding parts that have strange outer profiles.
However, these fixtures fall under the “Advanced” category due to challenges in setup and getting them to work reliably with your part. They require a vacuum pump to operate and typically aren’t well suited for milling different parts. A good vacuum fixture requires a lot of upfront work to create, but can pay for itself quickly over the course of longer production runs. Vacuum chucks typically don’t have anywhere close to the holding power of a mechanical vise, either.
Vacuum chucks may develop leaks in the system, which scale in complexity and number as the part size goes up. You also have to ensure that your fixture still holds the part flat, and if multiple operations are necessary, it can be tricky to get good side-to-side registration between operations where the part is flipped.
You’ll also come across companies that specialize in workholding technologies, such as Mitee Bite. They create clamps and other products that make fixturing easy, even for very large or complex parts. These products are typically meant to work with t-slots, which are found in almost all milling machine tables. There are many other specialty workholding systems that meet very specific needs. Take this freeze-clamping system, which uses a very cold plate to freeze a delicate part to a surface, so it can be milled without using a potentially-damaging clamping device like a machinist vise.
Workholding for Mass Production
When Mass Production is the end goal, the workholding strategy changes. One primary goal during production is to keep the machines running as much as possible. In order to do this, the “takt time” or time between the beginning of each operation, should be minimized. There are a few solutions for this in milling.
A pallet holding many pieces of stock for milling. (Source)
Pallets and tombstones fall under one category of workholding, where multiple pieces of stock are prepared outside of the machine while it’s cutting another pallet or tombstone. They hold many pieces of stock and can be loaded into and out of a machine in seconds, often by loading onto a vacuum or other quick-change fixture. A pallet is a flat plate that holds many parts, while tombstones are typically large structures that rise off of the milling bed and hold parts on multiple sidessa.
A large array of modular tombstones ready to be loaded. (Source)
Tombstones are common in 5-axis machines where the mill can easily access all sides of the stone. They also lend themselves well to horizontal milling machines, where the spindle comes in from the side and mills each part in a 3-axis configuration, before the tombstone is spun around for the spindle to access another side. Horizontal mills and tombstones also lend themselves to mass production due to their advantage in chip control: instead of having to deal with chips falling all over the work in a vertical machine, chips fall right past the work and spindle in a horizontal mill.
Workholding in Lathes
Since lathes typically turn cylindrical parts, they can often be held with the same style of workholding, including collets and hydraulic chucks. Mass produced turned parts requires additional equipment. This is where bar feeders come in: as long as the stock size stays the same, many bars can be loaded into the feeder and are fed, bit by bit, into the lathe until they’re used up.
A bar feeder set up to the left of a lathe. (Source)
Lathes with production capabilities typically come with a parts catcher as well: usually a small bin that that swings out on an arm to catch each part as they’re cut off. With short cycle times, a bar feeder, and a parts catcher, a powerful lathe can crank out hundreds or thousands of parts in a single “lights-off” period, without any human intervention.
Workholding is often a complex, but necessary, problem to solve in manufacturing. CNC milling production quantities, or the milling of parts with complex profiles, requires careful consideration of the workholding method.
Talk to your production vendor about what type of workholding is right for your project.