April 24, 2003
Cutting tubing with a circular cutoff saw is a common metal fabrication operation. This type of saw can produce a smooth finish that requires little secondary finishing.
But sometimes this common process can be puzzling, especially when the fabricator is cutting titanium or other alloys. Tubing may be cut successfully in one production application, but the next time the result is galling and extremely short blade life. If an operator understands why this happens, he can set up guidelines to ensure consistently high-quality cuts.
An aircraft manufacturer facing just such a problem approached TW Metals Inc., Savannah, Ga., to supply cut-to-length titanium, stainless, and aluminum tubing for assemblies used in hydraulic and oxygen systems in its business jets.
The aircraft manufacturer had its own fabrication department, but it was looking for a better way to produce a high-quality end finish. It was using a secondary finishing operation to remove the burrs created when tubing was cut with an abrasive saw.
Knowing that a circular saw can produce a high-quality cut on titanium stock, Dennis Reardon and Keith Turner, operations managers at TW Metals, began investigating the factors affecting the quality of a cut.
"Any metal can be cut successfully if the correct blade speed and feed rate are used, and they are especially critical for titanium. Depending on the stock, wall thickness, and bundle size, a variation in blade speed as little as 1 RPM can make a significant difference [in quality]," Reardon said.
To understand why such a small change makes such a big difference, it is necessary to examine the effect of blade speed and chip removal on the cutting process.
"When the blade cuts through a piece of tubing, the teeth generate heat as they remove the material," Reardon said. "Slowing the blade speed reduces the heat, but the cutting rate is too slow to be economical in a production application. If the blade speed is too fast, the heat generated leads to premature dulling of the tool."
The chips produced during the cutting process must be extracted at the correct rate or they might stick in the gullets of the teeth. This results in a loss of cutting efficiency, which in turn increases heat buildup, causing galling as well as shortened blade life.
While cutting fluids and the correct tooth pitch are important factors, finding the correct blade speed is essential for difficult metals such as titanium.
Even if the machine downfeed rate is variable, typical two- or four-speed spindle speeds often do not allow the blade to operate at the precise speed for optimal heat control and chip extraction.
Reardon and Turner came to the conclusion that a circular saw with an infinitely variable speed was the answer. Their search for such a machine led them to Kalamazoo Machine Tool's (KMT) C370 SA-V cold saw.
"We sent a sample of tubing to KMT, and upon successful testing, concluded that the ability to match a saw's blade speed and downfeed to the specific stock was in fact the answer to achieving a high-quality cut," Reardon said.
The test also solved the puzzle of why some cuts are successful, while others are not: variables.
"If the blade speed on a fixed-speed saw is exactly right for the stock being cut, the result is successful," Reardon said. "But vary the dimensions, profile, or wall thickness, and the cut will be less than optimal."
It is impossible to provide a chart covering every conceivable combination of tubing diameter, wall thickness, and alloy-much of that knowledge is acquired by experience-but it is possible to establish general guidelines that will provide a starting point.
"The tubing we were cutting was titanium, aluminum, and 21-6-9-stainless steel in sizes ranging from 1/4 inch OD to 1 1/4 in. OD, with walls from 0.020 in. to 0.052 in.," Reardon said. "The KMT saw we used has an infinitely variable feed speed rate of 0 to15 in. a minute, and an infinitely variable blade speed range of 13 to 76 RPM. This gave us the capability to cut a wide range of materials besides the titanium."
For titanium, TW Metals' operator set the head feed rate between 2 1/2 in. and 3 in. a minute and the blade speed at 13 RPM and slowly increased the speed to 15 to 17 RPM while monitoring the chip extraction. For stainless steel, the operator used the same head feed rate and set the blade speed between 25 and 28 RPM, to accommodate the amount of chrome in the alloy. For aluminum, the operator increased the blade speed to 72 to 74 RPM, again monitoring the chip extraction for that particular aluminum alloy.
Setting up the tubing bundle is an important factor in making quality cuts, Reardon said. "Since we deal with so many possible combinations, it's impossible to define an optimal size and configuration for a bundle."
This is why operators should have experience with many types of stock already and be able to monitor the cut to prevent excessive burr formation or tube rotation during the cutting process. If either occurs, the operator should reduce the number of pieces or vary the shape of the bundle.
Understanding the factors that affect a cut can give a fabricator the information he needs to determine a starting point and achieve consistently good cuts.
John Manchester is a freelance writer based in Kalamazoo, Mich., and can be reached at firstname.lastname@example.org.
TW Metals Inc., 175 Tubeway, Forest Park, GA 30297, 404-361-5050, www.twmetals.com.
Kalamazoo Machine Tool, 6700 Quality Way, Portage, MI 49002, 269-321-8860, www.kmtsaw.com.
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