8.5 Work Setup
Micky R. Jennings
The work setup is the way the operator holds the material in order to perform the cutting operations. One of the main aspects of setting up a job is establishing the datums that are referenced on a print (see Chapter 3). A datum is a surface, line, or point that the dimensions of the print use as a reference. The dimensions of the print need to be represented in the work machinists perform. These datums may be intentionally called out as in a GD&T style print, or just implied by having one or more dimensions linked to a side of the part, as in a traditional print. Sometimes machinists will establish datums without even realizing it. When the operator puts a part in a vise, they establish one datum where the part sits at the bottom of the vise. If the table is square to the machine, the bottom of the vise is parallel to the top, so the bottom of the part is also parallel to the top. Any hole type features that are put in the top surface will be perpendicular to that surface. A good example of establishing datums for drill press work is the layout process. When making intersecting layout lines on a part, the machinist must make the line parallel to the edge or feature that is referenced in the print. That edge or feature is an implied datum.
After the layout process is complete, the operator must set the job up on the drill press. Depending on the size and shape of the work, this process will vary. The illustration below shows a machinist putting a basic square-shaped part into a vise and establishing the third datum in the process. The part should be put in the vise so that it is secure and that the top of the part sits slightly above the top of the jaws. This is done by resting the part on parallels. When a part is installed on parallels, the machinist must make sure both the vise and the parallels are clean and free of burrs. Once the part is in the vise and the vise is tightened, parallels need to be seated. This is accomplished by wiggling the parallels side to side underneath the clamped part while tapping the top of the part with a dead blow hammer until both of the parallels become tight. If the part isn’t square, it is possible for the side of the part to conform to the solid jaw of the vise rather than the bottom of the part conforming to the parallels, making only one parallel become tight. If that happens, and it is critical that the bottom surface is the primary datum, loosen the part in the vise a little bit until the part conforms to the parallels.
If a part can be allowed to float on the table to machine all of the hole features, then the machinist could go ahead and start the process of center/spot drilling and so on. However, if the hole feature that is to be machined is going to create a great deal of torque, the machinist will need to put some sort of rotational stop in place to keep the vise from twisting uncontrollably.
On occasion, just retaining the rotational force of the part isn’t enough.