Around this time, CTMC installed several Blohm high-precision surface grinders to increase our production capacity and meet the demand for our products. Two of these machines were programmable CNC versions with the capability to grind parallel curved surfaces to sub-micron accuracy. This got me thinking about the possibility of compensating for the angular deflection by grinding a compensating curve in the linear guide rail mounting surfaces.
With the rails mounted on the bottom of the Y-axis casting, a 13 micron high arc in the center of the rail mounting surface would compensate for the total angular deflection, and provide a correction to the angular error in proportion to the axis travel. One of the CNC Blohm grinders was programmed to produce parallel surfaces for the linear rail mounting with the 13 micron high arc in the middle of the ground surfaces. We produced a test casting to this specification and built it into one of the machines in our normal production run. We tested on the machine and it showed a reduction from the expected 8 arc-seconds of pitch error to a deflection of less than 2 arc-seconds.
We implemented an engineering change to modify all 40 series Y-axis rail-mounting surfaces to this specification. All subsequent machines showed an average pitch error of less than 2 arc-seconds. This eliminated the machine rework, and yielded an accuracy improvement for this important component of overall geometric accuracy of 400 percent.
This entry was submitted by Frank Samples and edited by Rob Spiegel.
Frank Samples led development on a variety of manufacturing equipment, including the FI 230 wire EDM, FO41 drop tank EDM, and a laser welding system with more than 90 percent weld productivity. He currently works as an engineering consultant at Steelcase Inc.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Very interesting. I guess this is one way to do that.
A few years ago I also needed to compensate for single arcsec angles, and used a tiltsensor to actually detect these small deflections. The part number was SX-C29.
It looks like a great sensor, but I am not sure how you would compensate for the angular error in this example. The angular machine deflection is referenced to the machine table, and a mechanical compensation is necessary to maintali Z-axis squareness to the machine table (and workpiece).
If I understand it correctly, you have one reference surface and then the other that changes its orientation. You can use a unit called EZ-TILT-5000, that reads two sensors and can provide you with individual angular orientation of each sensor and also the differencial. This way you can always see the actual deviation of the required surface in reference to the standard.
The workpiece is attached to the table, the diesinking form tool is attached to the bottom of the Z-axis. Angular errors, positioning errors, straightness and squareness errors will all produce unwanted deviations from "perfect" in the finished workpiece. Improving the machine accuracy will improve the accuracy of the workpiece, measuring the errors will only tell you how much error you are introducing into the workpiece.
I agree with you, but inorder to make corrections and ajustments you need to make measurements. This is a real time sensor, so while you see what are the errors in position, you can make correction.
If you were using an angular sensor with a reference to the work surface you could make linear corrections to the position to compensate for the positioning error introduced by the angular motion of the structure. The linear correction of 5 ppm per arc second would be dependant on the the distance from the linear measurement reference for the Y-axis. Angular motions would still be introduced at the tool, possibly creating inaccuracy when large die sinking tools are used.
The machine in question is a relatively rigid structure, the angular deflections were extremely repeatable under a range of operation conditions. Real time measurements would be more expensive (and less effective) than mechanically compensating for the underlying structural deflections.
I appreciate your input. With the wide range of sensor technology available today in conjunction with inexpensive computational power a self-correction machine tool could obtain incredible levels of accuracy.
As a control engineer, I am fascinated by the measurement approaches suggested. As a mechanical/project engineer, I am on the side that says if you can grind in the compensation on each part and you get what you want in terms of accuracy, you should. Any kind of sensor can eventually fail, connections fail, and sensors always needs to communicate with the controller. If you don't have those components in your design, you won't have the failure mode, you won't have to put them on the BOM, inventory or buy them. I think this is a great solution.
Thank you for the compliment. It did work out as almost the ideal engineering solution, solving a long-standing product issue with no additional cost other than the initial programming of the Blohm grinder. The sensor based approach would be a fascinating project to try for an order of magnitude improvement over current best practice for "standard" machine tools.
Jeff, I totally agree with you. A one time permanent fix as opposed to constant tweaking is always preferable. By the way tolerances are given for a reason. Once you have solved the problem to consistantly be, not only within tolerance, but exceeding by a factor of 4, any further messing around is a waste of time and money. Well done kyoshi.
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