Quality Scan: Ensuring Grinding Quality
The pressure is on for shops to deliver high-quality parts in the shortest amount of time, while eliminating waste and maintaining quality. This is especially true of rough and finish-grinding applications for injector parts, valves, and shafts.
In-process measurement systems on OD and ID grinding applications locate workpieces, identify and control grindingwheel in-feed rates, compensate for wheel wear, and measure and control part size. Before the advent of these types of measurement systems, grinding parts was like playing roulette: you could never be certain what the finished size was going to be. The introduction of in-process measurement equipment eliminated uncertainty from the grinding process, and allowed faster production of parts, while delivering much better accuracy on the finished product.
Although grinding machines have been around for a long time, in-process measurement systems arrived only in the last 50 years or so. The first systems were analog designs, basically just dial-indicator gages mounted on a swing-down assembly that had a few disadvantages, among them difficult setup, limited range, bulk, operator-intensive designs, and relatively high costs due to the multiple systems needed for each station. Advantages provided by the analog systems included faster grinding times and better accuracy, both of which reduced scrap.
Today, a typical in-process system consists of the following: one or more measurement heads, a gap control device, a hydraulic or pneumatic slide, an industrial controller, and possibly an HMI. Until a few years ago, in-process systems were 100% analog in function and data transmission. Recently, though, new digital systems have been developed that eliminate the disadvantages of the analog systems by transmitting data using a digital signal, which, unlike an analog signal, isn't susceptible to electromagnetic interference.
Because finish grinding is one of the last processes a workpiece goes through, the part is generally considered to be at almost its highest value when it arrives at the grinding station. Consequently, the importance of in-process measurement systems cannot be overstated, as rejects at this stage of the game are costly.
Grinding machines today are quite capable, having the ability to grind parts to within ±0.0002" (0.005 mm). They can't do the job alone, however, because tolerances have gotten quite a bit tougher to meet over the last decade. Adding an in-process measurement system to the mix helps extend the capabilities of the typical grinding machine, creating a system capable of delivering accuracies to ±0.00001" (0.00025 mm). This allows much greater control over the finish grind of critical parts such as fuel injector components, valves, and shafts.
Although in-process gaging reduces the need for direct operator input, other factors like changing ambient conditions require that the operator maintain a certain degree of control over the production environment. Process variables that can be partially compensated for by in-process gaging include coolant temperature (which can affect part size), and workpiece temperature (which should be stabilized before the grinding operation).
Grinding cycle management is another way that in-process gaging can improve quality and productivity. To save time in a typical grinding cycle, the wheel approaches the workpiece rapidly, using a gap-control device to prevent overtravel. At some point in the approach, the in-feed rate changes to the coarse-grind rate, allowing the machine to remove a predetermined amount of stock. The in-process gaging system then switches the wheel to a fine-grind in-feed rate, usually 0.2 to 0.8 mm/sec.
During coarse grinding, pressure between the wheel and the workpiece causes the workspace to flex slightly. The fine-grind in-feed rate allows the part to straighten and cool, while the wheel removes stock more slowly to reach finished size. The slower rate also allows the machine to stop more precisely when the gage communicates that it has reached the on-size condition. When the gage signals the machine that the part is on-size, the machine usually allows for spark-out (wheel dwell time), which allows the part to continue to straighten and cool, and produces a finer surface finish.
Do you need in-process gaging for your grinding operation? That depends. The economies of the technique are realized when you need high precision over large production runs. A 100-piece part run is usually the decision point, although as few as 10 pieces could justify in-process gaging, if you're producing complex parts with multiple diameters. It all depends on how much each part is worth. Carefully evaluate your requirements to be assured that all of the variables common to any in-process application are accounted for in an inprocess gaging system.
This article was first published in the April 2010 edition of Manufacturing Engineering magazine.