GMcMahon_Directed_Study_Final_Report_(5-01-15)

College of Engineering
Department of Mechanical Engineering
____________________________________________ ____________________________________________
Peter A. Zink, Ph.D. Geoffrey McMahon
College of Engineering College of Engineering
Lecturer & Research Assistant Professor Department of Mechanical Engineering
Flexible Machining Vice System
Report Document
To: Peter A. Zink, Ph.D. From: Geoffrey McMahon
Boston University 7 Gardner Terrace
Allston MA, 02134
Phone: (617) 358 - 1631 Phone:
Email: pzink@bu.edu Email: gmcmahon@bu.edu

Department of Mechanical
Engineering
Date: 5/1/15 Class: Manufacturing Directed Study
Author: Geoffrey McMahon
Title: “Flexible Machining Vice System”
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TABLE OF CONTENTS
1 Introduction 3
2 Design Process 3
2.1 Problem Definition 3
2.2 Early Design Iterations 4
3 Final Design 7
4 Design Process Review 8
5 Appendix 10
5.1 Custom Vice Components 10
5.2 Sacrificial Material Components 12
5.3 Complete Assembly 14

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1 Introduction
As part of the Automated Manufacturing (ME 345) course at Boston University, students
are required to complete a project comprising of the design and manufacture of a remote
control car. A common issue that students who are inexperienced in manufacture have is
finding effective ways to fixture work pieces in the CNC mills in the ADML. Currently, the
mills are equipped with simple binary pneumatic vices, which limit machining capability for
many popular contours and cuts. In order to aid students in fixturing stock materials and as
part of the restructuring of the ADML in the EPIC facility, a more versatile machining vice
has been developí which allows students to perform several milling operations with one vice
setup. Working with the prototype vice will also demonstrate the importance of machining
fixtures in design and manufacture. Introducing students to a manufacturing cell in which
fixturing has been simplified will allow students to understand the fixturing problem while
reserving time and energy for the other lessons that the project is intended to provide. That
is, understanding the limitations that fixturing systems place on product design while giving
students time to solve other problems in the manufacturing process.
2 Design Process
2.1 Problem Definition
Through experience with the course, it was deduced that the most critical issue with the
traditional pneumatic vises is that they limited the CNC mill’s ability to do three important
processes: milling around the sides of a part, milling both sides of a part, and milling holes
or other contours through the bottom of a part. In the current system, sacrificial material or
spacers must be manually placed in strategic locations in the fixturing set up for milling
around the sides of a part and milling through a part. Moreover, parts must be manually
flipped from one side to the other if both sides need to undergo processing. Resetting
sacrificial material or spacers before running each part is time consuming and tedious, but
more importantly, it is a desirable end to entirely automate the manufacturing strategy
within the cell. This necessitates a vice system which allows milling along the sides of or
through a part but which can be set up by the mill’s attending robot or which can be used for

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the processing of several parts. These considerations lead to the following problem
statement:
“Design and manufacture a fixturing system that allows CNC mills to mill
around work pieces, mill through work pieces, and mill both sides of work
pieces but which does not require manual operation.”
2.2 Early Design Iterations
This problem can be solved with one of two concepts; a fixture that holds a work piece
with as little surface area as possible, or a fixture that incorporates reusable sacrificial
material. The former appears at face value to be the preferable option, since even reusable
sacrificial material must at some point be replaced. The first conceptual design can be seen
in Figure 2-1. This design makes use of the available Mitee-Bite Pitbull clamp1
(1), which
when coupled with rails (2), acts as a cantilever beam and provides a downward force on the
edge of the work piece when the screw is tightened. The fixture would feature a plate (3),
with a field of threaded holes (4) for flexibility in the location of the rails so that all
necessary stock sizes can be held in as many configurations as possible. The most important
flaw in this preliminary design is that it must be set up manually prior to processing. This
could be done in raw materials inventory, in which case the entire fixture would travel with
the work piece throughout the process, or would have to be done in the CNC mill before
each work piece can be machined.
1 Mitee-Bite product details can be found on their website: www.miteebite.com

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Figure 2-1: First Design Iteration
The second conceptual design can be seen in Figure 2-2. This design incorporates
removable bits (1) and rails (2), which slide in and out of the slots in each clamp (3). The
low profile of the bits allows milling around the sides of a work piece and the rails provide
clearance for through holes. Additionally, there are several different configurations that
could be made with the bits and rails, allowing for great flexibility in terms of how and
where the fixture makes contact with the work piece. However, this fixturing system would
be susceptible to variability due to the vibration of the bits and rails during machining.
Although the approach fits the requirements best, without a way to secure the bits and rails
to the clamps in such a way as to prevent movement, this vice would not be an effective
fixture. For a visualization of the benefits and drawbacks of each design, see Table 2-1 and
Table 2-2.

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Figure 2-2: Second Design Iteration
Table 2-1: Pugh Chart
Criteria Weight Current Vice Concept 1 Concept 2 Concept 3
Automatable 3 0 -1 +1 +1
Manufacturability 1 0 +1 -1 +1
Effectiveness In
Fixing Part
2 0 +1 -1 +1
Total 0 0 0 6
Table 2-2: Morphological Chart
Function Solution
Mill Sides Pitbull Clamp Square Bits Reusable Sacrificial Material
Mill Through Rails Rail Bits Sliding Sacrificial Base
Mill Top/Bottom Flip Part w/ Robot Arm

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3 Final Design
The final design prototype in Figure 3-1 borrows from the second conceptual design
(Figure 2-2) in that it features removable pieces (1) which lock into the permanent left and
right clamps (2 & 3) of the vice. However, in order to secure the removable pieces, they
were redesigned to slide over the face of each clamp and onto two hex-head screws that are
then given a quarter turn to tighten the pieces onto the clamps. Since these removable
pieces are much larger than the bits proposed by the second conceptual design, they do not
reduce the surface area of the work piece that is held by the vice. Instead, these removable
pieces are made from sacrificial material (in this case polyethylene plastic). This allows
work pieces to be held very tightly due to the large contact area but at also allows the mill to
cut around the edge of the work piece. In order to allow through holes, an additional snap
fitted, sliding piece of sacrificial material (4) rides along the guiderails (5) of the vice to
provide clearance between the work piece and the bottom of the vice. This sliding piece
makes it easy for the attending robot to place the work piece perpendicularly to the cutting
tool and hold it in the correct position while the vice closes. Additionally, the sliding piece
features a positive stop (6) to ensure that the work piece is oriented properly in regards to
the origin. This design is further advantageous as it allows re-use of most of the current
vice’s components. Only two major components must be replaced: the left and right clamps
of the vice. For detailed drawings of each custom component, please see Section 5. For a
list of components and instructions on the operation of the vice, please see the User Manual.

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Figure 3-1: Final Design Iteration
4 Design Process Review
Although the final prototype design was successful in achieving the major goals set by
the project, the design and manufacturing process could be done more efficiently in the
future. Working through the development of the new fixturing system emphasized the
importance of taking time, cost, and most importantly capability constraints into
consideration during the design phase. A more thorough understanding of the operation of
the vertical manual mills would have driven the design of the individual components
towards doing most machining in the x and y plane of the mill. On the mills available, the
vertical axis has no digital readout thus making precise and repeatable cuts in the z direction
more difficult. One of the most critical cuts on the left clamp (Appendix Section 5.1) is the
recess into which the vice base is bolted. By orienting the stock piece such that cuts are
measured using the x and y coordinate system, the recess would have been easier to machine
within tolerance. This would have prevented scrapping several pieces of stock and several
reworks of the final part.

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Using drawings that are dimensioned clearly, fully, and accurately is absolutely critical
for machining parts. Even with digital readouts, it can be difficult to properly locate cuts
and holes unless the drawings are dimensioned in a logical way. For example, the most
critical dimensions of the sacrificial material are the relative locations and size of the sleeves
into which the hex head screws slide. By dimensioning the drawings so that all
measurements were taken from bottom center of the stock material, even slightly oversized
or undersized parts fit properly.
The development of the fixture reiterates how tolerances can affect the fit of two mating
parts. In many past school project assignments, students were given the opportunity to go
no further than CAD modeling and assembly, so often tolerances were ignored or given a
low priority. With the progression from theory to practice comes a significant increase in
complexity, and the importance of how components will fit together becomes paramount.
In this case, tolerances should have been a fairly easy obstacle to overcome, as almost all of
the vice components are borrowed from the original configuration. The only two new parts
(the left and right clamps) are intended to fit with existing parts, which can be measured
directly for nominal dimensions. Time, material, and frustration would have been saved if
the tolerance considerations were made early and were initially included in the part
drawings.
All of these lessons seem straightforward and perhaps even common engineering
knowledge. However, the value of lessons learned in practice far outweighs the value of
lessons learned in theory. Even limited experience with working machinery vastly
improved the understanding of the key manufacturing operations. This led to a much better
ability to make effective design decisions and gave unique perspective into the issues which
machinists face on a regular basis; particularly fixturing and tolerancing. Making better
design decisions early on saves time, energy, and money in the design phase and will lead to
a higher quality machined product. Additionally, the positive experience of working
together with machining staff and drawing on machinist knowledge at the conceptual,
design, and manufacturing level is critical for engineers. Having the ability to effectively
communicate and work with people from different educational and experiential backgrounds
will make the application of the lessons learned in this project far more effective in industry.

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5 Appendix
5.1 Custom Vice Components

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5.2 Sacrificial Material Components

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5.3 Complete Assembly

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