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: [email protected] Email: [email protected]


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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GMcMahon_Directed_Study_Final_Report_(5-01-15)