The Three-Jaw Chuck

The Three-Jaw Chuck

Overview

Using a three jaw chuck appears to be the simplest of procedures, but as the adage goes, looks can be deceiving.

Three jaw chucks have become one of the most fundamentallathe accessories, specifically when there is a large productionrun of cylindrical parts.

With the introduction of CNCmachining, three jaw chucks became even more technically advanced, with the addition ofhydraulic and pneumaticchucking systems. Because three jaw chucks can be automatically controlled, their place in the modern manufacturing environment is essential.

Advantages of the Three-Jaw Chuck

One may dispute if there really are advantages of using a three jaw chuck over a four jaw chuck. It may be that there are better times to use each type of chuck. As is discussed in other topics, four jaw chucks are used when the work requires it.

The same thought should apply when discussing three jaw chucks. When the work requires that the part be loaded and unloaded with minimal amount of time and effort, then a three jaw chuck is a good choice.

Three jaw chucks are also a good choice if there is a large quantity of parts to machine, and the operations being performed do not require a high degree of precision or concentricity .

Since the jaws on a three-jaw chuck all move in unison, centering a cylindrical part is not a concern. The part will run concentric with the chuck, providing that there are no foreign particles between the jaws and the part, or that the chuck is installed on the spindle properly and the chuck has not worn irregularly.

The jaw faces can also be reversed to accommodate larger diameters. You will notice in the video that the mechanism that moves/guides the jaws is not removed or reversed. The jaw faces are simply reversed and keyed back into place atop the jaw guide.

Disadvantages of the Three-Jaw Chuck

There are certain disadvantages to using a three jaw chuck. Three jaw chucks get used a great deal resulting  in a lot of wear and tear on the chuck. Because of the wear and tear, there should be an appropriate amount  of maintenance performed. Three jaw chucks are considered or assumed to run perfectly concentric, but often that is not the case. Chips  get inside the faceplate of the chuck and into the scroll, where the jaws are connected to the chuck. These  chips often cause the scroll and the teeth of the jaw bottoms to bind, and eventually these parts will wear  away and become less than perfect. As a result, the chuck does not grip the part perfectly even and run out can occur. Three jaw chucks are limited in the size of parts that they can hold, and generally are not considered the best  choice if heavy duty machining is being performed.. Preparing to Mount the Chuck Sometimes, it may be necessary to disassemble the chuck for a detailed maintenance. Doing this you will  obtain a working knowledge of the chuck When removing and or installing a chuck always use a wooden cradle Removing and Replacing Jaws It may be nessecary to change chuck jaws from time to time. This procedure explains how: Back each jaw out of the slots and note the order they were removed. Each jaw and jaw slot is numbered. This tells you which jaw goes into which slot but it does not tell you in which order they were disassembled. Some chucks disassemble 1,2,3 and reassemble 3,2,1. Other chucks reverse this. Note that after removing the jaws the pattern of teeth is different for each one. This should fortify what you just read about jaw numbers and order. Never put a jaw back in the wrong slot! and always put them back in the right order! If the first jaw to be reassembled is jaw #1 then locate slot #1, turn the scroll until the scroll thread enters the slot #3 and then back it out until it is not visible. Continuing with the assumption that jaw #1 is the first to be assembled, drop the jaw into slot #1 and turn the scroll until the scroll thread engages. Wiggle the jaw in and out until you are sure the scroll is engaged. Repeat the steps for Jaws #2, and #3 in slots #2 and #3. Note the end of this video. It shows the slot number stamped into the chuck next to the appropriate slot and the jaw number stamped into the jaw. Soft Jaws Soft jaws are machinable jaws. Why would you wish to machine  them? Remember earlier in this topic when it was expressed that  3-jaw chucks wear out? that the concentricity of 3-jaw chucking  may not be dependable? Machining your own set of soft jaws remedies this problem. If you  can machine your own diameter into a set of jaws then the part that  is chucked by those jaws is guaranteed to run as concentric as the  bearing of the lathe spindle! That is the purpose of soft jaw machining.  To insure concentricity. Basically, the standard jaws are removed and a set of aluminum or mild steel jaws are bolted onto the chuck where the standard jaws used to fit. It is important to make a plug for the new set of machined jaws. A plug is turned to a diameter close to the diameter you wish hold in the new soft jaws. Prior to machining, the jaws should be tightened around the plug towards the rear of the soft jaw set. This will provide rigidity while the jaws are machined and will insure that clamping pressure and scroll looseness are removed while turning. It is also a common practice to preserve special soft jaws in case the part is often machined. The setup would be made quicker if new jaws did not have to be made. Machinists often will stamp the part number and corresponding chuck onto the soft jaws so that easy identification can be made. Chuck Mounts The chuck is usually connected to the lathe by one of three methods: a camlock (figure 1) , a taper (figure 2),  or a thread (figure3). The back side of the chuck will reveal that it connects to the lathe via one of these  methods. It is important to protect both taper and threads from impact, as any dings or chips in this area  of the chuck will cause misalignment.                                 figure 1                                            Figure 2                               Figure 3 Article resource : www.jjjtrain.com