Pick up of HSM reflects name
HIGH speed machining (HSM) is advancing as rapidly as its name might suggest.
With constant improvements in computing power and advances in hardware such as tooling and clamping systems, HSM is one of the manufacturing industry’s most rapidly evolving areas.
According to John Hart senior applications engineer Jamie Woolaston, Australian manufacturers in sectors like aerospace are world-class in their application of the technology, but in other industries we lag the competition.
Woolaston told Manufacturers’ Monthly Australian manufacturers have access to all the latest tools and machinery.
“If a machine’s available in Europe it’s not very long before it’s available here,” he said.
He admitted cost was a barrier for some manufacturers, with HSM requiring substantial up front investments in machine tools and tooling, but added it could provide long term benefits.
Woolaston said very hard materials including tool steels up to 60 Rockwell hardness can now be processed using HSM. This is beneficial to toolmakers who previously had to machine materials prior to hardening.
“There’s a lot of heat involved in hardening a part so dimensionally it could go anywhere. By having the material pre-hardened and then machining it, you can retain the dimensional accuracy.”
Woolaston said HSM was suitable for most materials although plastics that would not short chip, or that produced long stringy swarf, and highly volatile materials such as magnesium could cause problems.
Cool tools
Sutton Tools chief engineering executive for the southern group, Phillip Xuereb, told Manufacturers’ Monthly tooling developments had helped eliminate some common problems associated with HSM, such as “cold welding” which occurs when the material being cut sticks to the cutting tool.
“The tool ends up with built up edges and so instead of machining with the surface of the cutting tool, you are machining with the built up edges and the performance is deteriorated,” he said.
New surface coatings can help resist this material build up, Xuereb said, enhancing the life and performance of the cutting tool.
“Surface coatings are a thin film layer on the surface of the cutting tool which is much harder than the base material. It provides the ability for the tool to sustain a longer life.”
He added base materials are also stronger now.
“There’s two types of materials used. The powder metallurgy and high speed steels, or fine grained carbides and the carbide tool and over recent years there have been advances in both of those areas,” he said.
Woolaston said equipment developments have also helped improved tool life with new cooling mechanisms preventing the tools overheating.
Traditionally, the cutting zone would be flooded with coolant to keep temperatures down, but Woolaston said HSM operations could use air blowers or air/oil mist combinations to keep tooling cool.
Clamping down
New clamping systems have also helped reduce tolerances on machined parts.
“Some of the machine tools have hydraulic clamping systems, but there’s also what’s called a heat shrink clamping system,” he said.
Heat shrink clamping systems use a chuck that is heated in a special unit so the tool holder expands. The tool is then inserted into the warm chuck and when the chuck cools, it shrinks down onto the tool and holds it in place.
“That provides excellent run out capabilities where the tool will spin on centre within a couple of microns, and that can effect the surface finish,” Woolaston said.
Taking control
Machine controllers have also been moving forward in leaps and bounds, with computer processing speed regularly increasing.
Woolaston explained these developments have been critical in allowing HSM processes to operate more rapidly as the faster the tool rotates and travels, the faster the controller has to process the information it is receiving.
“If you try and do HSM using a controller that’s too slow, the tool will jump. It will move to a point then wait for the buffer to empty out then go to the next point. If the controller is fast enough to read all the information you will have a smoother transition and a smooth cutter path.”
Woolaston said every time the tool stops on a point it rubs the metal at that point, damaging the surface finish.
Controllers are also better equipped to compensate for the needs for high speed five-axis machining.
“As you pivot the tool around a point, the machine has to compensate for the pivot length and there are some advancements in machining in that respect,” Woolaston said.
Controllers can also influence acceleration and deceleration rates, which Woolaston says helps provide greater dimensional accuracy to a job.
“If you program a cutter to go at a high speed in a straight line and it then has to make a 90 degree turn for argument’s sake, you can actually get the controller to decelerate into the corner and accelerate out of it,” he said.
15-Sep-2005