Posted by Eric Weinhoffer ● Oct 31, 2017 8:24:00 AM
An Introduction to Workholding
For those unfamiliar with subtractive processes like milling or turning, the necessity of workholding may surprise you. It isn’t needed in other types of processes, where material can be printed directly onto a bare surface. In milling, the term “workholding” encompasses all the components required to keep the work secure while milling is underway. 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 milling, when other processes like additive manufacturing don’t? The answer lies in the forces that are placed on the workpiece. In additive manufacturing, there is very little to no force imparted on the work by the printing nozzle. However, a cutting tool in a milling spindle will impart high forces on the work, and any unintended movement of the work requires starting the job over. Loosening grip on the work also causes incredible damage, as parts can fly at high speeds, break a tool, a window or, worse, damage the spindle itself.
The most basic type of workholding in milling 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.
A machinist vise. (Source)
Soft jaws are a critical addition to the classic Machinist’s Vise, especially when holding non-square stock is required. The term ecompasses 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 easily. Take the below example into consideration: a part with a complex profile can be more rigidly held by soft jaws that mimic its profile.
A machinist vise viewed from above, with a simple part (in blue) held on the left and a more complex parts held on the right, with soft jaws (in green).
You can generate soft jaws easily in CAD, and they will quickly become a critical piece of your workholding repertoire as the complexity of your parts increase. They can also include locating features for easy part swaps, and make 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, we use this often in our own manufacturing.
There are many other workholding options past machinists’ 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 will require a lot of upfront work to create, but will pay for itself quickly if you’re doing production. They typically don’t have anywhere close to the holding power of a mechanical vise, either.
A vacuum chuck in a vertical machining center. (Source)
Other issues vacuum chucks can have include 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 the milling bed and hold parts on each side.
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. Just like tombstones and pallets in milling, 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. While using a mill for mass production or the milling of parts with complex profiles, selecting the right workholding method becomes imperative. Thankfully, though, the rapid advancement in machine tool and workholding technologies constantly brings new opportunities to hold onto many different parts in new, safer ways.