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Workholding – Introduction

      What makes the milling machine so interesting is the large number of different things that can be done with it. This is largely because of the many different ways that a workpiece can be held. This part of this website covers what can be done not by giving examples of different type jobs, but by different ways of holding the workpiece.

      For example, dovetails can be made using a dovetail cutter with the workpiece clamped to the milling table but they can also be made using an endmill and holding the workpiece in a tilting vice. This is covered here but in two different places. If the reader is just interested in making dovetails this is covered under “making dovetails”.

      A particular effort is made to show how jobs that are best done on a vertical machine can also be done on a horizontal machine and vice versa. Similarly, there is often more than one way of doing a job depending on what equipment is available. One way might be preferable but it might depend upon the user having a certain piece of equipment. So other ways of doing it are of interest.

      In general we will assume the user has a vertical milling machine. But when this is not the case it will be pointed out. Since, as will be seen, vertical type tools can be usefully used on the horizontal spindle and horizontal type tools can be used in the vertical spindle, the words become meaningless. More important is which spindle is being used.

      Vertical mode – the  vertical spindle being used

      Horizontal mode – the horizontal spindle being used

The order of workholding arrangements is:

 Group A     Clamps and clamping.

Workpiece clamped to milling table

                  Clamping through holes

                  Profile milling

            Cylindrical squares

            Angle plates

            Swivel table

            Tilting devices but not tilting vices

                  Tilting table

                  Sine plate

                  Tilting table II – for dividing head and tailstock

                    Jigs and fixtures

            cross drilling jig

Group B     use of vices

Group C     “rotary devices”

                  use of chucks 1 – non-rotary

                        Collet

                        3 jaw

                        4 jaw

                  use of rotary holding devices

                        indexing chucks

                        Rotary table

                        Dividing head 1

                              Spur gears

                              Bevel gears

                        driven

                  helical and spiral milling

                              Helical gears

                              Spiral milling

                              Cam milling

Group D     other workholding methods

Workholding – general principles

      Any workpiece that is free to move has the possibility of moving in the x, y or z directions or any combination of these. It also can rotate about any of these axes or any combination of these.

      In order to be able to machine it on the milling machine it has to be held still in all of these directions.

      The simplest way of doing this is to clamp the workpiece to the milling table but the same principles apply to when it is held in a vice, chuck or any other way.

       Sometimes the forces being applied to the workpiece are sufficiently directional that the clamping forces in some directions need not be as strong as those operating in other directions.

      Regardless of anything else, it is essential that whatever holding arrangement is used, the holding forces applied to the workpiece must not distort the workpiece.

      Where some device is used to hold a workpiece then these rules also apply to this holding device.

      In general, it is highly desirable to be able to do as much of a job as possible without moving it, relative to the milling table, during the machining processes. Each time the position is changed it is a chance for an error to be introduced.

      Similarly, where possible it is preferred to use a method where an angle or distance has to be right because it is physically impossible (hopefully) for it to be wrong rather than rely on measuring it again.

      For example, a dovetail cut with a dovetail cutter has to come out with the right angle, whereas if relying on something being tilted at the right angle, there is a real chance that it will not be as accurate.

      That is a very simple example. The more common cases are where two sides have to be parallel to each other. In this case, if the job is set up properly, both sides can be machined. One cut is done, the workpiece is shifted over and then the other cut is done. Similarly cuts can be done at right angles simply by doing one, using the x or y feed, and then by doing the other using the y or x feed.

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