Mind The Mechanicals Before Upgrading Your CNC

Don’t plan a control retrofit project too narrowly. Proper mechanical alignment is a key part of a comprehensive approach to upgrading a machine’s CNC.

Article From: 11/6/2009 Modern Machine Shop, ,

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Retrofitting a machine tool with a new control is a cost-effective way to improve the speed and productivity of a dated piece of equipment. However, Jim Leigh believes that control retrofit projects often are planned too narrowly.

Mr. Leigh is a co-owner of Pyramid Rebuild & Machine. He says it’s important to take a comprehensive approach to every control retrofit project. Proper mechanical alignment, he contends, is a key part of that approach that maximizes the benefits of a control upgrade. Similarly, repairs or improvements made to a machine’s mechanical systems ensure that the retrofit project not only meets a shop’s expectations, but also won’t require additional money and time getting a machine to run effectively.

Although a typical machine tool has a number of mechanical systems, Mr. Leigh says the slideway system is the most vital one. The slideway system largely determines a machine’s accuracy, precision and rigidity. That’s why it’s essential that this system is carefully evaluated and properly aligned during a retrofit. While some shops outsource machine evaluation and alignment, others have the necessary equipment to do this in-house. Mr. Leigh suggests that shops choosing to perform their own machine alignments take the following steps.

Look for evidence of wearing ways. Begin an alignment evaluation by removing all the way covers and examining the ways for evidence of wear. Move the slides around to try to expose as much of the way surface as possible. Sometimes the wear is evident to the eye, but other times it’s not. If the machine is equipped with box ways, use a pair of micrometers to measure the thickness and width of the way bars at various points along their length. On a hardened and ground surface, variations of more than 0.0005 inch indicate wear. On a hand-scraped surface, measure over a precision puck in order to span the minor deviations produced by the scraper.

Determine that the machine is level. If using a precision spirit level to verify that a machine is level, a series of level readings should be taken along the length of each horizontal axis and also across each horizontal axis. This cross-leveling technique helps identify twist that may exist in the machine slides. If discrepancies are noted, the errors may be corrected by adjusting the machine’s levelers or jack screws. If a shop’s maintenance staff is unfamiliar with the leveling process, then it might be prudent to consult a maintenance specialist.

Check the squareness of perpendicular axes. Once the machine is level, check the squareness of perpendicular axes. This can be done using a cylinder square or square level for vertical axes and a high-precision cast iron or granite square for horizontal axes. Oftentimes this aspect of machine tool alignment can be adjusted only through realignment or re-scraping.

Measure spindle tram. Spindle tram is one of the most critical aspects of alignment on a milling machine. Simply stated, the machine’s spindle must be square to the workpiece plane. Several factors can affect this measurement, and the spindle bearings are prime players. Check the spindle for axial and radial looseness. Excessive play in the spindle bearings must be remedied before any tram reading can be considered reliable. If the bearings check out, proceed with the tram readings. Typical tram specifications for a boring mill are 0.0012 inch total indicated runout (TIR) over a 36-inch sweep. If a tram error is noted, it must be addressed or it can cause poor machining blends in which one cut overlaps another. On boring operations, spindle tram affects the alignment and the location of a bored hole. Tram error also can produce bored holes that are out of round.

Mr. Leigh says that regardless of whether an evaluation is performed in-house or with contracted assistance, make sure the results are preserved in writing.

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