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Case Study: Machining tolerances
DMG Mori explains how its machines are helping Kent Cams produce camshafts with tighter tolerances, quicker and more accurately
Kent Cams has been able to cut machining times by two thirds and increase its accuracy through the installation of a DMG MORI NLX 2500SY|1250 machine, which is allowing for 'one hit machining' to be possible.
Based in Folkestone, Kent Cams specialises in the manufacture of camshafts for motorsport and performance cars. Its experience means the company can give advice on profiles while achieving the tolerances demanded for high performance engines. Cam profiles are delivered as a dataset of points, and Kent Cams uses its skills to reverse engineer this information into the cam profile, delivering the desired acceleration and synchronisation of valve opening and closing with piston movement to deliver maximum performance.
Initially, Kent Cams needed to increase capacity, which drove it to look at new solutions to its machining requirements. The existing process for a camshaft 900mm long, was to turn one end with 450mm out of the chuck, reverse it to turn the other end, and then locate it on a vertical machining centre to drill oil holes and machine the drive end of the shaft, which may include features such as keyways. Finally, grinding, heat treatment and finish grinding operations complete the part. Not only was this a slow process but, with three resetting operations, the stacking of the tolerances could lead to inaccuracies and out of true running.
When selecting a new machine, Kent Cams looked in detail at the construction of the shortlisted machines and their ability to drill deep holes. “Firstly we needed a machine capable of turning shafts up to 1m long,” explains Andy Burns, managing director of Kent Cams says. “We also needed C and Y-axis and live tooling to cut the machined details and additionally, the ability to drill deep holes. Just a visual examination of the NLX 2500 made it clear, from an engineer’s point of view, that it had the strength in the castings and the attention to detail in its construction which we were looking for. Specifically, the NLX 2500 had an option for 70-bar coolant, which we needed for swarf clearance during deep hole drilling. Other machines we looked at needed uprated seals to cope with the increased pressure, the NLX 2500 seals could manage this as standard, giving us much more confidence.
“The rigidity of the turret and its built in motor combined with the high pressure coolant has enabled us to drill far deeper than we expected and, at the same time, achieve increased tool life,” added Burns.
In addition to the high-pressure coolant, the programmable steady makes it easy to turn a long cam in one operation. The sequence is to rough turn, position the steady, continue rough turning and then finish turn. Next the steady is moved towards the end of the shaft and the tailstock is withdrawn so that drilling, tapping, counterbore and keyway operations can be added to the drive end of the shaft. On occasion, the diameter of a cam can go down to 19mm, so a further advantage is the fully programmable tailstock. With this, pressure can be backed off under CNC control to avoid distortion of thin shafts.
For cutting the machined elements on the camshaft, the NLX 2500 has ±50mm stroke in the Y-axis, which is more than enough for the largest cam profiles.
“Cutting the complete cam in one operation gives us much greater accuracy as everything comes off one datum,” explains Burns. “We know that with this process we can easily achieve angular tolerances of 6 seconds. We also get advantages in our grinding operations with increased accuracy and repeatability in the turned and machined part. Not only has the NLX 2500 cut camshaft machining times from three hours to one hour, but the automated nature of the process has enabled us to utilise our workforce more efficiently, allowing them to complete additional tasks while the machine is running.”
Camshaft designs are much more demanding with the evolution of variable valve timing and new emission levels. “We have taken an incredible step forward in our cam machining,” concludes Burns. “With the NLX 2500 we have the most up to date equipment in Europe outside an F1 manufacturer, which is a major asset.”