Workholding Collets And Factors That Affect Grip Force
August 2004 - Drawbars
Daniel P. Soroka, ProductionMachining.com
In today's complex manufacturing environment, maintaining operations at optimum levels will require a significant degree of attention, effort and priority. This is particularly true of the higher tech elements such as machine controls, hard working mechanical structures, programming and general equipment reliability. Fortunately, on the other hand, there are certain manufacturing components that are robust and consistent, and that will yield a long lifetime of usage. Workholding collets fall into this category, having been historically incorporated in a wide variety of applications and industries for more than 100 years.
It is not certain when the first collets were employed, but it has been established that workholding collets were available before the turn of the last century. At its facility, workholding manufacturer Hardinge (Elmira, New York) displays collet drawings and complete line catalogues that date from 1901. The production of collets at Hardinge occurred in the 1890s, with many of the applications at that time focusing on the watchmaking and lens industries. Collets have proven to be as useful on today's CNC equipment, with state-of-the-art control systems, as they were on the early engine lathes and the cam operated multi-spindle automatics from the 1920s. As strange as it might seem, when we look back at what has evolved with machining and equipment technology, it is as though the basic collet was suspended in design time and space, while everything around it was required to adapt to productivity improvements.
This phenomenon of the staying power of the collet, in light of the rapidly changing technology in machine tool design, is attributed to the utility and the elegant simplicity of the device. The collet is a small but powerful component for the machine tool industry, incorporating all of the following features:
- The capability to accurately grip a workpiece or a tool, resisting both rotational forces and multi-directional cutting loads.
- The capability to amplify the actuation force, converting it into workpiece or tool gripping.
- The inherent ability to rapidly release the workpiece or the tool.
- The ability to operate at high repetition levels without loss of accuracy or material failure.
- The ability to operate at a wide range of rotational speeds with minimal loss of gripping force.
- The ability to accomplish all of the above with a minimum of rotational inertia.
Grip Force
The overall design and use of the workholding collet is a tremendously broad subject, considering the multiple collet families for the huge array of material-cutting machines and the variety of individual styles and features. The total count of workholding collets would probably number in the tens of thousands. There is a common misconception that collets are limited to grip round, square or hex shaped materials. This could not be further from the truth; in fact, nearly any shape or part form that fits within the envelope of the collet can be engineered to grip the part piece for processing. This article, however, is limited to a discussion regarding the factors that can and do affect gripping force. It is important to note that the principles described below can be associated with both internal and external gripping operations.
Factors That Affect Grip Force
Gripping force is simply the amount of force that is exerted upon the workpiece. Figure 1 depicts a lathe collet with a drawbar, although a similar scenario will exist with collets used for workholding, grinding or a host of other applications. The drawbar, which is connected to the rear of the collet, will apply an axial force from a cylinder located at the rear of the machine spindle. This force direction is converted from axial to a direction perpendicular to the closing angle or head angle of the collet. At the same time that the direction is altered, the system has also amplified the gripping force through the closing angle. It is not unusual for some collet systems to operate with gripping force amplifications in the range of 3:1 or 4:1.
The collet must hold the tool or the workpiece within the same reference as the spindle. Relative movements will result in incorrectly machined parts. Part or tool spinning and part or tool pushback are absolutely detrimental to workpiece consistency and accuracy. The collet is a simple device but there are several primary factors that can affect the gripping force. A fundamental understanding of these factors may help in both job setup and troubleshooting activities. The factors that affect gripping force include:
The level of axially applied force. In the illustration, this is the drawbar force. In toolholder collets, the axial force may be applied by different means, but the principle is the same. Obviously, high axial forces will result in higher gripping forces and visa-versa. Typically the draw bar force can be adjusted by the operator.
The collet system closing angle (or head angle) will dictate the level of amplification that can be expected. This is generally determined by the machine tool builder and the collet manufacturer. When new machine designs are being pursued, the designer is encouraged to use the designs of existing collet families for reasons of economy and the use of proven concepts. Standard tapers or head angles have been established to accommodate the needs for the different requirements for lathe collets, stationary collets and toolholder collets.
The resultant friction of the tool or workpiece material with the gripping surface of the collet will have a direct impact on the gripping force. Lower friction values will result in lower gripping forces and visa-versa. The workholding supplier can implement features into the collet design that will overcome slippage or pushback. These include serrations or carbide impregnation of the gripping surfaces.
The resultant friction of the collet closing angle with the spindle or toolholder. The sensitivity of this effect can be quite noticeable in terms of lost workpiece grip force, and it may also result in an accelerated long-term wear condition on the collet and its receptacle. Workholding applications that are subject to a large number of open and close cycles, such as in a turning center, are best when operated with some amount of lubrication. For this reason, flood coolant is sometimes preferred because it provides a small but definite level of lubricity. Otherwise, a small amount of lubrication applied to the closing angle on a periodic basis will reduce long-term wear and increase gripping force. Some of the more effective materials to use include greases with EP (Extreme Pressure) properties or any material that has some type of wax base. As strange as it might seem, some clever operators have been known to apply women's lipstick when facing a difficult application in which all else has failed to provide acceptable results.
Full bearing of the collet to the workpiece is necessary for high gripping force and is maintained when the collet order hole is properly ordered to fit the workpiece. If the order hole is too large for the workpiece, only the outer face of the collet will grip the workpiece and a geometric mismatch will occur around the circumference, resulting in a lower gripping force. If the order hole is too small for the workpiece, only the inner head angle will make contact and the tendency will be to increase the gripping force, but it can cause concentricity problems. Full bearing will be achieved when the order hole is ordered to the same size as the workpiece. Hardinge recommends targeting within 0.001 range when selecting the collet order hole size.
As further evidence of the utility and staying power of the workholding collet, consider the constant changing technology for the need for speed. A hundred years ago, the spindle speeds were measured in hundreds of RPM. Today, spindle speeds are measured in multiple thousands of RPM, and they are trending higher each year for faster material removal rates. Surprisingly, after 100 years of successful applications, there is still no better workholding element for the new high-tech, high speed spindles than the workholding collet.
(Original Source: http://www.productionmachining.com/articles/workholding-collets-and-factors-that-affect-grip-force)
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