Getting a handle on workholders

While a lot of attention is paid to the tooling and the machines themselves, it seems not a lot of focus is given to workholders or ‘chucks’, despite the fact they make up an inherently important part of the machining process.

Manager of business development with Romheld Australia, Vince West, says this attitude was particularly evident until quite recently, but explains with the range of workholding devices now available, manufacturers need to be more discerning.

“In the past, workholding was usually the last thing to be considered and even then, conventional wisdom suggested you could easily just buy a vice, chuck or a couple of clamp straps and away you go,” West explained.

“But these days, with the advent of high speed machining and 5-axis machining centres, it is now necessary to carefully select the type of workholding system that will enable these machines to perform to their full potential,” he told Manufacturers’Monthly.

With the whole point of HSM to increase productivity, workholding devices need to be able to maintain precise part positions under the increased loads and force vectors without damaging or distorting the part.

One of the myths of that surround high speed machining of parts is that cutting forces are a lot weaker in HSM than in conventional machining, therefore requiring less rigid fixtures.

But this is not specifically true. As West points out, a major concern for machinists is rigidity.

“Any vibration or movement of the workpiece during machining impacts on both tool wear and life which usually results in a reduction of cutting conditions to alleviate the problem,” West said.

“Not only is this a costly process in terms of tool cost and job cost, it can also lead to massive problems with the high speed spindle of the machine. A well thought out workholding system will pay for itself many times over,” West said.

Smarter fixturing

The good news however is while manufacturers might have more choice of chuck, as West explains, fixturing devices are also becoming more customised and suited to applications such as multi-axis and high speed machining.

“With 5-axis machining, one of the main concerns is access to the workpiece for the machine spindle nose.

“Conventional machine vices did not take this into account, however new releases in the past couple of years have seen the advent of specialist vices for 5-axis work,” West said.

This is a trend that Dimac Tooling general manager, Paul Fowler has noticed, and says with dropping volumes of parts to be machined and implementation of lean manufacturing principles, more is being asked from workholders.

“Manufacturers want workholders where you can insert or remove the workpiece easily to help avoid operator fatigue, so things like power-assisted devices, or automatic vacuum grips commonly used in the automotive industry, are becoming more popular.

“Also people want more parts under the spindle, so there needs to be a minimal amount of changeover time. Things like quick fixture location systems which aid in accurate and speedy fixturing, helps to reduce dead spindle time,” Fowler said.

West agrees and says with the knowledge that machine idle time can be worth tens of thousands of dollars per annum, investment in technologies aiding quick changeover is getting increasingly popular.

“For instance, zero point mounting systems enable different workholding devices (chucks, vices, mandrels, fixtures etc) to be interchanged precisely and within seconds, rather than minutes or hours,” he explained.

“With the aid of these systems, parts requiring more than one operation can be transferred to each operation without ever being unclamped.”

Stuck on chucks

Another development in workholder design that is enabling manufacturers to have a better grip on their parts is magnetic fixturing devices.

According to West and Fowler, permanent electro-magnetic technology used in workholders has led to a higher level of performance clamping.

“Magnetic clamps require only a short pulse of electricity to clamp or unclamp,” West explained.

“They are available in modular clamp plates for wide area coverage, or smaller magnetic set-up blocks (or pods) which can be positioned quickly and easily via the machine t-slots,” he said.

Fowler notes that even though many still associate magnetic workholders with grinding applications, due to advances in their design and strength, they are now suitable for a range of operations, including mill and turning.

“In Europe we are seeing a strong trend to use them in 5-axis machining as they allow the machinist access to all sides of the component.

“We are also seeing an increased use of modular clamps which have a lot more flexibility and allows for higher density, fitting more parts under the workpiece than with conventional devices,” he said.

When choosing a workholder, West says there are several key considerations for choosing the right workholding system for the job; workholder rigidity, to allow the machine to work at maximum cutting conditions without any vibration or flexing, the ability to complete the job in one setup, and flexibility to support fast changeover times.

Fowler advises manufacturers to avoid going down the ‘tried and true’ path when choosing a workholder, as it might work but there could be limitations.

“Devices like vices can handle most parts, but can compromise on productivity. Think outside the square and take the time to explore better ways to hold workpieces to gain higher ROI.

“Customisation should also be an important part of achieving the best possible fit.

“Increase part density, and consider how to double or triple the number of parts under the spindle, per cycle.”

Fowler also advises manufacturers to invest in quality solutions and to look at the full range of options available.