Mechanical Injury (Workpiece Control)/the Couple

            This packet is intended for use in an introductory Statics course when the concept of the couple is introduced.  After a brief introduction, a drill press is used to reinforce instruction on the couple and simultaneously sensitize the students to the need to constrain workpieces during machining operations.

Time for presentation is estimated as 10-15 minutes.

Objectives:

1. To reinforce the concept of the couple, as presented in Statics

2.   To introduce the need to constrain workpieces in order to ensure safety during machining operations.

This packet includes the following items.

·        Lecture material for the instructor (in Frontpage Format)

·        Overheads for use during the lecture(in Frontpage Format)

·        Handouts for the students(in Frontpage Format)

·        A homework problem

Download the Homework Problem in a printable Adobe Acrobat Format (pdf).

Download the PowerPoint Presentation and notes

Homework problem solutions, exam problems, and exam solutions are available to qualified recipients. Send an email with request information to Dr. Donald Bloswick.

Assignment for Mechanical Injury (Workpiece Control)/the Couple

The drill press in Figure 1 can develop a 300 lb-in couple.  Assume two bolts are used to hold a workpiece during drilling.  Neglecting friction between the workpiece and the table and assuming a 300 lb-in moment, what would the forces on the two bolts be if they were located in (a) holes 1 and 2, (b) holes 3 and 4, (c) holes 1 and 3?  Based on the physics of the problem does it make sense to ignore friction?  What is the safe assumption?

Figure 1.  A drill press is shown to the left and the top view of a workpiece on the drill-press table is shown to the right.  The drill can generate a moment (couple C) of 300 lb-in at the point shown.  The four bolt holes (1 through 4) and point of drilling by C are in line and 3 inches apart in each case.  The sketch of the drill press is from DeGarmo, et al, Materials and Processes in Manufacturing, Eighth Edition.

Standard procedure in safe machine shops is to require that workpieces be clamped to the table during operations such as drilling.  The safety risk is that if the drill seizes, the workpiece will start spinning and can hurt the operator, and from a manufacturing perspective it is possible to ruin the workpiece.  What would an additional risk be if the drill were a hand drill instead of a drill press?