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Hold it right to machine it right

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These days workholding systems have to accommodate increasing demands for high-speed, lower-volume operation.<[etk]>

In addition to this, new demands are being placed on traditional workholding methods with the increase in popularity of multi-tasking CNC machines such as the mill-turn lathe.

One aspect to the need for high speed is high-speed production. Higher investment multi-function multi-axis machines have to keep operating to be cost effective, so the ability to rapidly position and grip a workpiece is critical.

High speed in the sense of high-speed machining often means greater loads on the part during the machining processes. Workholding devices have to be able to maintain precise part positions under the increased loads and force vectors without damaging or distorting the part.

But workholding is not just a one-dimensional issue. The two biggest facets of workholding, milling and turning, are complex and require an approach that is both commonsense and economical.

According to Dimac Tooling general manager Paul Fowler, there is a lot of scope for workholding-based gains in the metal cutting industry.

“There are three key areas of focus in regards to workholding - safety to the operator and machine tool, the gripping method in terms of rigidity and machining access, and then the cost effectiveness to the company,” he said.

“There are frequent examples where neglect or poor maintenance of a workholding device has caused customers considerable grief trying to maintain tolerances and surface finishes.

“For example, the regular greasing of a power chuck can have a significant impact on workholding performance.”

In the case of turning centres the power chuck is one the hardest working elements of the machine, and a worn chuck can cost a company many times the replacement price when total productivity is assessed.

Wear can manifest in several ways such as poor concentricity when second operations are performed, loss of grip force due to internal friction in the chuck, difficulty machining soft jaws or having to machine a taper to compensate for the jaw flaring and lower quality surface finishes are just some of the issues encountered.

Chucks are a critical part of a lathe setup, but like most workholding devices, they do not always get immediate consideration in process planning or design.

Yet current trends in turning operations are amplifying the role of the chuck. Typical spindle speeds are considerably higher than ever before, for example. These higher speeds increase the forces acting on the chuck's clamping power.

Furthermore, tolerances on workpieces are tighter and workpieces are more likely to have thin walls, delicate features or be “near net shape”, making distortion from clamping forces in the chuck more troublesome.

Move to Lean

The move to lean manufacturing has heightened interest in part loading and unloading, drawing attention to ways in which a chuck lends itself to streamlined, error-proof operation.

According to Fowler, in job shop environments it is also typical to try and grip every job that comes along in the standard 3-jaw power chuck.

There are many variations of top jaws available today to support using a 3-jaw chuck ranging from custom jaws that have been fitted with carbide gripper pads to ensure a reliable and effective grip of castings, forgings and black scaly bar through to custom jaws that allow small volume production of square and rectangle bar (or billets) in a 3-jaw chuck.

There are many occasions however where the tried and tested power chuck may not provide the ideal solution, such as when machining shafts.

High levels of accuracy and productivity can be realised by machining with a face driver. This working device allows a shaft to be machined between centres with the driving force coming from drive pins engaged with the headstock end of the workpiece.

Face drivers have generally been thought of as a finishing device and not very capable at roughing, however Fowler says nothing could be further from reality.

“Having witnessed cuts up to 28mm deep with a face driver quickly changes your perception of what these workholding devices can accomplish,” he said.

The most notable advantage is the ability to completely machine a shaft in one set-up, thus removing concentricity issues associated with “end for ending”. As face drivers can be mounted in 3-jaw chuck with soft jaws, set-up times are minimal.

Careful examination of the clamping application may lead to either a stock solution or a customised workholding device for difficult or troublesome work pieces. This is certainly the case when it comes to static workholding.

Fowler says that when using machine vices, there has been an increasing trend for companies to purchase lower-quality vices.

“Pulldown quality is imperative in a vice,” he said.

“Quality is an indicator of the pulldown ability. The better the quality of vice, the more rigidity it offers and the less chance that the part being machined will move about.

“This comes back to production costs and time. The better the quality of the vice, the higher the overall productivity.”

Magnetic chucks

The approach for milling is to generally use a machine vice. While these very universal devices play a significant role in most machine shops, there are other solutions that can yield greater benefits and profitability depending on the type of work.

Fowler says that while it is common practice to use magnetic chucks in European machine shops, this is quite uncommon in Australia. Magnetic tables are generally considered to be the domain of grinding.

The significant evolution and development of magnetic materials such as Neodymium-ion-Bohr has changed this situation. Heavy milling applications through to thin workpieces are candidates for magnetic chucks, and also often allow a much higher density of parts under the spindle when compared to conventional vices.

Another area of workholding for machining centres that can go unexplored is that of the modular clamp. Available in many forms and function the basic concept is to make a simple fixture which, when fitted with modular clamps, can provide a very high density of workpieces, low cost-per-part investment and great flexibility and tooling access.

Fowler offers the following key points for getting the most out of workholding solutions:

• Avoid the well-trodden path and take time to explore better ways to hold workpieces to gain higher ROI. Customisation should be an important part of achieving the best possible fit;

• canvass the full range of options available;.

• increase part density, and consider how to double or triple the number of parts under the spindle, per cycle;

• don’t compromise on quality. Low cost rarely translates to the best investment when acquiring workholding;

• pay attention to the maintenance of an investment in workholding; and

• always factor in safety and ergonomics to the solution.

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