Acoustic emission sensing helps position control
Machine tool maker Max-Tek prolongs grinding wheel life by using acoustic emission sensing and controls for precise wheel shape, improving quality while saving time and cost. One grinding wheel dressing pass is said to be enough 80% of the time, avoiding unneeded second dressings, often performed in high-precision applications. An automotive component manufacturer provides an example.
In fabricating precision metal parts such as automotive components, it is important to maintain precise grinding tolerances throughout a production run, so grinding wheel dressing is a necessary operation. Ideally, a wheel should be dressed only when its shape falls out of specification, and no more material should be removed from the wheel than is necessary to restore it to its desired shape.
Monitoring the profile of a grinding wheel during the dressing operation can be difficult because the operation typically takes place beneath a high-volume stream of cooling oil. That makes visual techniques for observing wheel profile ineffective. Therefore, to ensure quality, machine builders typically run an extra dressing cycle to ensure that the desired shape is attained.
This used to be the strategy of Max-Tek LLC, a manufacturer of superabrasive grinding machines based in Newington, Conn. Max-Tek Machines (Figure 1) produces metal components for the automotive and aerospace industries.
"For our customers, ensuring that production parts meet very precise tolerances is a make-or-break issue," said Ed Elie, Max-Tek president. "Because we knew that one dressing pass over the grinding wheel might not be enough to bring it to the correct shape, we ensured that, even though it might result in some unnecessary wheel wear, our machines always did a second dressing pass."
To minimize waste, the company sought advice from its manufacturer of acoustic sensing technologies, which solves the problem of how to "view" the profile of the grinding wheel through the stream of cooling oil. The acoustic sensor monitors the high-frequency vibration of the dressing wheel as it contacts the grinding wheel and "listens" for the sound of the two coming together. The sensitive instrument requires only a small number of grains of the wheels to make contact to indicate a touch.
Wheel shape confirmation
Software controls the dressing process by measuring the live acoustic emission signature of the dressing operation and comparing it with the profile of a known successful wheel dress. When there is a match, the system knows that the wheel is dressed correctly.
"Now, we know that a single dressing pass is sufficient more than 80% of the time," said Elie. "This saves our customers both time and cost. They don't need to replace grinding wheels nearly as often."
To set up the software controls, a learn cycle is first initiated whereby the system gain and overall measurement scale, measurement window, and crash and gap sensitivity parameters are set for the particular wheel to be dressed. Then a successful dressing cycle is performed and the acoustic signature is recorded. Once the reference data set is finished and stored as a known good dress, the data can be used to evaluate subsequent dresses of the same process.
Automotive peel grinding
A manufacturer of precision automotive components uses Max-Tek's peel grinding machines. Peel grinding uses a narrow grinding wheel to grind a part in a process that is much like cutting with a conventional lathe. The tool is plunged into the part (a casting in this case) and is moved along the surface, taking off between 35 and 38 1000ths of an inch with each pass, to create the desired finished shape, accurate to within 8 microns. The high-precision process enables the machine to make complex, small features. Because it concentrates grinding force over a relatively small area, peel grinding works well on hard materials that conventional machining cannot handle.
Prior to installing the control software that uses acoustic sensing on the machine, the customer would need to dress the grinding wheel every few parts, said Elie.
"We never knew if the wheel was being dressed adequately the first pass, so we typically over-dressed to make sure that the correct profile was being maintained," Elie said. Because a relatively small grinding tool is used, over-dressing can result in frequent replacement of the tool, increasing production costs and downtime unnecessarily.
Acoustic emission technology helps both grinding machine designers and operators save money and improve productivity.
- Mark Astor is application engineer at Schmitt Industries; edited by Mark T. Hoske, content manager, Control Engineering, firstname.lastname@example.org.
- Grinding machine may more dressing cycles than necessary because of inaccurate measurements.
- Acoustic sensors and control software compare grinding wheel size to an ideal to minimize over-dressing, saving time and extending wheel life.
How can tighter measurements, data analysis, and controls save you time and money?
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