Narrow niche, small parts, tight tolerances
March 13, 2003
Precision abrasive wheel cutting is a small but important niche in the abrasive cutting field. It can be used to cut many types of parts, including metal rods, tubes, extruded shapes, and even wire. It is most useful in operations characterized by small parts, hard-to-cut materials, and tight tolerances.
Precision abrasive wheels can cut solid parts as small as 0.001 inch in diameter and tubular parts from 0.004 to 3 in. OD. Length tolerance for short, small-diameter parts can be as tight as ±0.001 in. Tolerance for parts up to 6 in. long can be held to ±0.005 in. Among the smallest tubular components that have been cut off with a precision abrasive wheel is a length of stainless steel tube stock with a 0.002-in. ID and 0.004-in. OD, and cut to 0.020 (±0.001) in. long for a hinge in a heart device.
For parts smaller than 3.00 in. OD and 6 in. long, the process produces finished parts with no external burr. Secondary operations are not required. However, longer parts may require a secondary finishing operation to hold close tolerances.
Precision abrasive cutting should not be confused with rough cutoff operations used in most machine shops and production plants to make raw blanks for further operations.
Using precision abrasive cutting for heavy-wall tubing in softer metals, such as mild steel and brass, usually is not economical compared with cold sawing and other methods, because precision abrasive wheels are more expensive and wear faster than blades for cold and other types of saws. Abrasive cutting usually is more effective for difficult-to-work metals. However, abrasive cutting can be useful on softer metals when precision or a higher-quality end finish is necessary.
In general, the harder the material, the better the cut finish.
Precision abrasive cutoff relies primarily on abrasive wheels that are manufactured to a precise thickness. It also requires fixtures that hold the work with no linear movement during cutting and that can also guide the wheel. For most industrial tube cutting, these wheels generally are 8 in. or less in diameter and range in thickness from 0.014 to 0.025 in.
Typical precision cutoff wheels used to cut tubing are made of pure rubber that has abrasive grains of aluminum oxide, silicon carbide, or both rolled into it until the mix looks like a giant stick of gum. It is rolled to the desired thickness, and a die punches wheels out of it. The wheels are cured, or vulcanized, under pressure in a large furnace.
When made properly, all the wheels in a batch have a thickness tolerance of ±0.001 in. However, any single wheel normally has a maximum total variation of 0.0005 in.
For some applications, rubber-bonded wheels can be made as thin as 0.006 in. Rubber-bonded wheels thicker than 0.030 in. rarely are used for cutoff applications.
Hand sorting—carefully measuring and selecting several wheels for a particular job—allows a cutoff operator to hold the overall tolerance of the wheels for the job to ±0.0005 in. Such a tolerance permits the fixturing to contain guides that control the wheel as it cuts with as little as 0.0005 in. of clearance on each side. A proper setup can result in mass-produced parts with ±0.001 in. of tolerance.
Hundreds of wheel choices are available. Wheel variables include the type of abrasive, the size of the abrasive grains, and the hardness of the bond. No two wheel manufacturers produce wheels that perform identically.
Because the wheel continuously sharpens itself by releasing worn grains and revealing new ones, wheel wear is often a major factor in unit cost.
Tube is either held stationary for fixed cutting or rotated for rotary cutting. In a fixed cutting operation, the abrasive wheel moves through the diameter of the tube to make the cut. Rigid fixturing allows 0.5 degree of squareness.
If the tube is rotated, the blade stays in one place to make the cut. Fixtures designed to rotate the work must have precision rollers and tightly align the tube with the abrasive wheel. Rotary cutting can achieve 0.25 degree of squareness.
On tubes larger than 0.500 in. OD, rotating the tube reduces cutting time because the abrasive wheel cuts through the wall and not through the entire diameter. Depending on wall thickness, this process can cut square and parallel parts in just a few seconds.
However, achieving these parameters requires extremely straight tubes. Even slightly bent tubes affect the precision of the cut.
Because both the tube stock and the cut-off part must be held securely until the part is cut entirely from the stock, it is customary to bundle smaller diameters so that many tubes are cut in a single downstroke of the wheel. Bundling produces more finished parts per cut, increasing productivity, but the increased productivity usually is at the expense of accuracy.
The best wheel and the best machine do not necessarily guarantee the best result. Experience is necessary to make the most of the art of precision cutting. The optimum combination of wheel and material, peripheral speed, in-feed speed, and pressure usually is learned by experience with assistance from the technical staff of the wheel manufacturer.
In the same way that a great piano and a great score do not guarantee a great performance, the ability to set up a job with many variables is based on the skill and experience of the setup mechanic.