For the manufacture of turbo shaft and wheel assemblies there are a number of operations to produce the finished component. Typically on car size turbo’s up to 54mm diameter the shafts are finish machined and ground with a location spigot on the end of the shaft. The vane wheel is machined with a counter bore register into which the shaft locates. The two items are then electron beam welded together to make an assembly. Larger turbo’s normally up to 112mm diameter, the shaft is finished turned and friction welded to the vane wheel. After welding the shaft is finished ground between centres.
The next two operations are to grind a groove or a number of grooves and finish grind the vane profile. On the smaller car size components this can be completed in one plunge using conventional aluminium oxide type grinding wheels with full form diamond roll dressing. This method is relatively slow due to the need to dress very frequently typically every 3 components at 0.050mm per dress as wheel break down is significantly high. Another factor to consider is for each different form a full form diamond roll is needed. Wheel changing and roll form changing will significantly increase the overall cycle time of the component together with high tooling costs.
The Curtis method is to use super abrasive vitrified cbn grinding wheels for the vane profile and an electroplated cbn full form wheel for the grooves. Typically for the vane grinding operations a straight wheel with a radius on one corner is used with the radius being smaller by 0.5mm than the smallest radius on the vane profile. The vitrified wheel is dressed on the machine with a single point rotary dressing disc using the x & z axes to generate the form. Each different vane form is then programmed in the cnc control and can be called up automatically from the user interface on the control system. The profile is generated on to the vane wheel using the x & z axes. Wheel speeds are considerably higher up to 100 mtrs/second and with the much harder wheel dressing frequency is reduced to typically 40-50 components at 0.005mm per dress.
For the groove grinding operation an electroplated cbn grinding wheel is used. This can have one form on or multiple forms for groove plunge grinding, for multiple forms this can only be achieved if the spacing between the forms is sufficient to enable wheel manufacture to produce the required forms. With this process there is no dressing, when the wheel is worn it can be returned to the manufacturer for replating.
In high volume production each of the above operations are carried out on separate machines. In low volume or start up volumes then these two operations can be combined on one machine. This means that the components are loaded twice. The first operation is machined as a batch; this can be either groove or profile. This is usually the grooving operation as the components normally have no centres in the wheel end. The grooving wheel has a 60 degree angle on the wheel form that machines a small centre on the end of the wheel for location into a female centre. This centre provides stiffness to the component enabling faster grinding times as there is no push off.
For large quantity machining, Curtis developed a manufacturing line for shaft and wheel，the assembly line integrates all the processes from Electronic beam welding to balancing, it includes：
EB Welding—Laser Marking——DMC reader——Groove Grinding——Profile Grinding——Shaft Polishing——Deburring——Washing and Drying——Optical Measuring——Balancing
Workpiece: Passenger Car Turbo Shaft Diam 46mm
Material: nickel-molybdenum alloy
Machine Type: CMT Automatic Line
Cycle time: 35s