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What is Centerless Grinding?

Vita Needle Centerless Grinding Machine

Centerless grinding is a secondary operation used to reduce the outside diameter (OD) of tubing or bar stock to a smaller desired diameter. It’s especially useful for applications where there is a requirement for a tight tolerance OD stainless tube that would be challenging to meet using tube drawing techniques alone. In such situations, tubing can be produced intentionally oversized during the drawing process, and finished to final size.

What are the Benefits of Centerless Grinding?

Such an operation can be tremendously helpful for several reasons. One, if the customer requires a tight tolerance to ensure the tube works in the intended end function (say for example, the SS tube slides within a tightly machined bore or another tube), or the customer desires an improved OD finish (lower Ra) delivered by centerless grinding. Second, enhancing downstream automation---if the tubing will be fabricated in a machine using automatic barfeeders, such barfeeders work more reliably if the OD of the tubing being fed into the machine is highly consistent. Third, efficiency from a tube manufacturing process standpoint and better consistency / tighter CpK’s, etc.

There are a variety of centerless grinding machines built domestically and internationally, whose capabilities vary widely. Some machines are constructed to rapidly grind high volumes of product to what would be considered fairly loose tolerances by centerless grinding standards (+/-.0005”). Other machines are built for smaller volumes / less material removal and are used by manufacturers to hold tight tolerances (+/-.0002” or in some cases +/-.0001”).  Whether a certain tolerance is achievable on a certain stainless steel/nickel alloy tube depends on a number of circumstances, including quality of the incoming tube, wall thickness, straightness, and setup/capabilities of the machine.

The Centerless Grinding Process

The grinding process works by feeding in lengths of tube as short as 6” and as long as 15’ via a tray into the grinding machine, where the tube encounters a grinding wheel and a regulating wheel, flooded by coolant. The grinding wheel turns at a higher RPM than the comparatively slow regulating wheel. The regulating wheel’s role is to turn the tube as it is being ground so no flat spots are formed----keeping the tube as round as possible. The finished tube comes out the other end of the machine, to the new finished tube OD, simply needing a wipe down or ultrasonic clean to remove the water-based lubricant.

Grinding tubing on a centerless machine requires configuring adjustable features of the machine such as guides/trays/blades, dressing the grinding wheel square and ensuring lubricants are clean and pumps are operating at appropriate flows. The operator will normally lose several pieces of tubing during setup to get the machine tuned in and meet the desired OD tolerances on the finished tube. Once the machine is setup, ongoing monitoring and adjustments by the operator to keep tubing in tolerance are required. For tight tolerances, more frequent adjustments are needed to keep the machine performing properly.

Vita Needle is Your Trusted Partner for Small-Diameter Tubing and Needle Manufacturing Excellence.

In summary, centerless grinding is a useful function that allows tubing to be manufactured to tighter tolerances than might normally be possible from drawing operations alone, and is a widely used method for non-tubular applications to achieve precise tolerances and finishes. Contact Vita Needle today to learn more!


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