The machine spindle, the collet and the machining process all contribute to the workpiece concentricity. Consider these factors when trying to achieve the best workpiece concentricity possible…

Concentricity in the Spindle – three major areas of concern:
1.  TIR of the spindle angle – if the spindle angle does not run concentric, it is impossible for the collet and workpiece to run concentric.
2.  TIR and wear of the spindle back bearing – if the back bearing of the spindle is worn, it will allow the back bearing of the collet to pivot, which will cause excessive runout.
3. Runout of the spindle drawbar – if the drawbar is not concentric and square with the head angle and the gripping area (order hole) of the collet, the pressures will be exerted on one leaf of the collet instead of all three. The parts manufactured with this collet will not be concentric with the chucking diameter. The uneven forces on the collet leaves may cause them to break.

Hardinge, a machine tool builder of super-precision turning centers provides a superior spindle that is capable of maintaining a high degree of concentricity that equates to actual part roundness guaranteed within 15 millionths (.38 micron) on specific machine models. Concentricity is very dependent on spindle design. Concentricity can best be maintained by following a regimented machine maintenance program and by selecting the most appropriate workholding product for each individual job application. For best results use the proper collet gripping (order hole) size, use cutting tools that are properly sharpened and centered, and use proper feed rates and spindle speeds for the tooling and material selected. Consistent use of abrasive materials and/or heavy machining operations can wear the spindle. If this is the case the spindle can be rebuilt if required.

Concentricity of the Collet
The TIR of the back bearing, the head angle and the threads apply to the collet as well as the spindle. Collet concentricity is measured by putting a gauge pin or ground plug in the collet gripping area (order hole) and measuring the Total Indicator Reading (TIR) of the pin’s runout with a .0001" dial indicator at a specific distance from the face of the collet. Hardinge guarantees .0005" (.0127mm) TIR for standard 5C round collets with a gripping area (order hole) up to .1875" (4.76mm), .001" (.0245mm) or better for a gripping area (order hole).188" (4.77mm) and up, and .0002" (.005mm) for all special accuracy round collets.

Use exact collet size for optimum Concentricity
Collets should be ordered to the exact size of your stock. If you have stock variation choose the average diameter. When the collet is the proper size for the workpiece, there is full bearing along the angle and the circumference of each segment of the collet where they mate with the spindle angle (seat). The result will be good concentricity and excellent holding power. If the collet is oversize the part may pivot at the line of contact. The turned diameters may not be concentric with the bar or chucking diameters, and the machined diameters can be out-of-round. There will be line contact at the center of the leaf. If the collet is undersize, the contact will be at the top of the head angle on the spindle angle and the collet. This can help with bar push-back because the more tool pressure exerted on the bar, the tighter the collet becomes. The part may pivot around the point of contact when excessive chucking pressure is NOT applied. The turned diameters may not be concentric with the bar or chucking diameters. The edges of the slots will bite into the workpiece and may cause damage or mark the bar.

For optimum workpiece concentricity purchase collets from a reliable source that use quality inspection and testing procedures. Ask what the concentricity and hardness ratings are since hardened collets have a longer life than collets that are not hardened. The concentricity of the gripping surface (order hole) can be affected by certain types of materials such as hot-rolled stock, cast bars and abrasive materials causing collets and spindles to wear faster.

Certain machining operations can cause collets to wear faster and lose concentricity:
1. Heavy stock removal and heavy drilling may cause push back.
2. Heavy forming with a cross slide tends to open the collet and may cause stock to slip.
3. Forming and drilling at the same time.
4. Intermittent cuts can cause radial slippage.
5. Out of balance parts create uneven pressure on the leaves.

All of these applications are common machining practices. Slippage can be overcome by using a properly designed workholding system which includes a collet that has the right head angle design, the proper serration and hardness, a collet actuating system with adequate draw bar force, a spindle drive system with adequate horsepower and torque, cutting tools that are properly sharpened and centered, and proper feed rates and spindle speeds for the tooling and material selected.

Other causes for wear on multi-spindle machines occur when:
1. The feed tube and the collet are not keyed together causing radial slippage when the collet opens and closes.
2. Crooked or bent bars put stress on one leaf of the collet or feed finger, causing slippage and loss of tension in that leaf – can also bend feed tubes with considerable use.
3. Dirty bars create a very abrasive action between the bar and the feed finger and/or collet.
4. Excessive tension on feed fingers 

Hardinge has over a century of experience in workholding and knows all of the ins and outs involved in workpiece concentricity. Rely on Hardinge for the proper workholding application going in to provide the best possible workpiece output. A good workholding vendor will save you time and money in the long run. For more information contact Hardinge at 800-843-8801