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By John Stagge Hold on a minuteA look at the ER collet system's benefitsWhen faced with the market's enormous choices in collet systems, finding the right collet system for your CNC lathe or CNC machining center may seem daunting. To simplify your choice, take a moment to look at the advantages and disadvantages of the more popular collet systems used in the US. Several systems are used to hold round shank tools. These include:
All of these systems are good, depending on the application. There are, however,
differences in the systems based on holding power, flexibility, price, and application. A selection of ER collet systems. Ten years ago, the ER collet system was rarely heard of in the US. When European manufacturers started importing machines to the US, they created a market for ER collets. As carbide became more popular, so did the demand for collets that were accurate, short, and had good holding power. Here's a further look at the information we've gathered: Drilling Drilling under 3Ž4创, where accuracy is not important, the double angle collet system is the cheapest system to use. However, when drilling with carbide drills under 3Ž4创, the ER or TG collet systems should be used because of their holding strength and greater accuracy. The more accurate the collet system, the better life experienced with carbide tools. When drilling from 3Ž4创 to 1 1Ž2创, the TG collet system is the best choice because it is a larger collet. More contact on the shank is used because of the larger collet, thus it offers more holding power. Drilling over 3Ž4创 is not common in most shops, however, and represents only a small percentage of holes drilled. Tapping Rigid Tapping Rigid tapping has become popular in the latest generation of CNC machines, and the tooling requirements for this feature have made collet chucks a viable solution for tapping. Both the TG and the ER collet systems make rigid tap collets for their holders. They are manufactured with a broached square in the rear of the collet to lock the square of the tap into the collet. This eliminates any twisting of the tap inside the collet. The ER collet seems to work the best for this application because the collet is short and the tap fits better into this system. Floating Tapping Collet In this area of tapping, the ER collet system has a significant advantage over other systems. This collet consists of three main parts: the shell of the collet, the spring, and the insert that holds the tap. The collet can be used in any standard ER collet chuck by exchanging the collet the same as any other collet. Since this collet is short, it does not take up a lot of Z-axis on the lathe or machining center. This collet is popular on Swiss-type turning centers where toolholder length is important. This makes the ER collet system a contender for CNC lathe applications. Reaming Normally reaming is done with a floating holder, but in certain applications, a standard collet holder can be used. Both the ER and TG collet systems can be used for reaming with a floating holder, but due to the length of the TG collet, the ER collet is a better choice. The ER collet is short enough to provide less interference on the turret of the lathe. Also, ultra-precision collets can be purchased to provide better specifications for concentricity than the TG collets. Coolant applications Increased tool life and increased speeds and feeds are the spoils of coolant through the tool applications. Their popularity led collet and collet holder manufacturers to develop a system to hold these new tools and to seal them to prevent leaking. The first solution was to seal the collet with silicone and try to bond the metal to the silicone. Although this worked, the collet had no clamping range, which ruined the collet's benefits. The other disadvantage was that once the bond broke down between the collet and the silicone, the collet was ruined. All of the pressure was lost at the end of the tool. This led to the development of coolant discs which are used in special coolant nuts to provide the clamping range for a standard collet and sealing the tool up to 1500 psi. The new development was a "metallic sealed collet." However, the major disadvantage to this system was that the customer needed to replace $60 to $70 collets instead of a standard $27 collet, which was used with the coolant disc system. All three collet systems offer coolant through the tool, but the most useful and cost efficient method is the ER and TG collet disc systems. These systems offer a one-time investment in a coolant nut and disc and the customer only replaces the standard collets in the future. High-speed machining High speed machining is another growth area that has included collet chuck manufacturers. The TG and the ER collet systems can be used with a well-balanced holder. The ER system seems to be the better choice though, because of the shorter length of the collet and collet chuck. The closer the collet to the face of the spindle, the less chance of run out at the end of the tool. Length The collet length is important because the farther the distance from the end of the spindle to the end of the tool, the less accurate the cutting tool. This can be illustrated by extending bar stock through a lathe spindle about 8X the diameter and checking the bar stock runout. The TG collet and collet chuck are about 17% longer than the double angle and the ER collet. Since the double angle collet is not normally used in high-precision applications, the ER collet has an advantage in terms of length and accuracy. Milling; holding power In the past, milling with a collet chuck seemed a waste of time. But with innovative accessories, it is now possible to mill with a collet chuck. If the customer is experiencing pull out or shank spinning while milling, one solution is the TG non-pull collet. This collet has a pin or plug in the side of the collet to mesh with the weldon shank on the end mill or drill. This prevents the shank from spinning inside the collet. The drawback to this system is that the collet may slip inside the pocket of the holder. The ER collet system's answer is a bearing nut. The bearing nut eliminates friction between the nut and the collet as you clamp down on the collet, thus pushing the collet farther and straighter down into the collet pocket. With a standard ER collet nut the friction is high between the 30š nose taper on the collet and the taper on the nut; it tends to create friction between the two tapers and may eliminate an extra 1Ž4 or 1Ž2 turn on the collet nut. Another main difference between holding power of the TG and the ER collet systems is the taper on the front of each collet. The ER collet front nose taper is 30š and has more surface area for the nut to clamp down on. There is more contact area between the nut and the collet, pushing the collet in the collet pocket more firmly. For more information from Techniks Inc, Indianapolis, IN, http://www.techniksusa.com |
Departments Smart Shop WebSightings Fundametls: Dollars & Sense Workholding
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