Introduction
Abstract:
This project comes from a need to have tensile specimens made for the MET 351, Metallurgy/Materials and Processes, and 426, Applied Strengths of Materials, labs. This punch is designed to be used with an arbor press to create the desired tensile specimen shape out of plastic blanks. The initial concept was suggested by Dr. Craig Johnson. The designs went through many changes, for example getting rid of the sides originally proposed to hold the specimen in place, and modifications to other parts to make them more efficient. Additional parts were also added to the design with the help and advice of Dr. Johnson and Matt Burvee, like the idea for the box used to support the punch. All of the different pieces needed were then created and constructed here on CWU campus. Resources here on campus also provided the materials to make this completed project. Testing was the next step in the process to further refine the design of the punch. Multiple test cuts were used to get a base for the progress. After adjustments were made to the punch the finished product was complete and ready for use. The punch could repeatedly create tensile specimens with minimal time and effort applied from the operator.
To understand the purpose of this project, it is imperative to know what a tensile specimen is and why it is used. A tensile specimen is a piece of material with a controlled size that is tested until its breaking point. This process is called destructive testing because it causes damage to the material. The purpose of a tensile specimen is to find the ultimate tensile strength, maximum elongation, and reduction of area for the material. These properties are important when designing a product; knowing how much force a material can handle before it breaks allows the creator to select the most effective material for the project.
Tensile specimens can be made from almost all materials and composites, in both round and flat bar. For example, steel and aluminum specimens are used in Professor Johnson's classes. Specimens have a distinct shape with large sections at each end, where it is designed to be held by the tensile machine, and a reduced section in the center, where the failure is designed to occur. The dimensions of the reduced section is recorded before being tested: the length, and the diameter, if round, or the width and thickness, if flat.
The tensile test is accomplished by placing the material specimen in the tensile testing machine. Then an increasing force, or stress, is applied. The specimen is initially in the elastic period for the material -- which is to say if the force was to be released, then no change will have happened to the material. This will continue until the material yields -- also called necking -- in which the material initially starts to change in size at a point along the middle, usually stretching and getting narrower. Note that the force will drop after this point due to the change in the cross-sectional area. The force will continue to increase until it reaches the ultimate strength for the material. The stress will start to decrease due to the shrinking cross sectional area, as cavities are forming on the inside of the material. Then the material will break at the necking area and the data will be collected.
The final length will be taken, by putting the two parts back together and comparing to the initial length, to get the maximum elongation. The reduction of areas will be found by comparing the final diameter, or width and thickness, to the initial diameter, or width and thickness.
Analysis
This project was designed to fulfill the requirement of being able to be stored in the lab, operated quickly and easily, and used for both aluminum and steal. The project had to smaller than a 10 in cube to both be stored in the lab and operated using an arbor press. It had to be able to be operated with one hand controlling to arbor press and the other hand holding the specimen steady. Professor Johnson specified that the punch should be able to be used with 16 gauge steel.
The size of the punch was dependent upon the size of the reduced section from the specimen. An ASTM standard testing methods was referenced for sizing of the specimen.The gage length was specified to be 1 inch for this size of specimen, and the radii of the reduction is .25 inch. The base on the arbor press is 7 inches wide and 5 inched deep; this is where the dimensions for the base where designed around.
This comes from ASTM Designation: E8/E8M-09, Standard Test Methods for Tension Testing of Materials. This is the table that was used to base the dimensions of the desired specimen.
Calculations where done to find the force required to shear 16 gauge steel, which is to say the plate is a 16th of an inch thick. Multiple attempted where made to find the force but only confusing results were found. Sixty pounds of force was chosen as the maximum force desired to be input to complete the operation. Testing would be required to find the actual force required and that would be compared to the desired force. If the actual force required was greater than the desired force then the punch angle would have to be redesigned.
Construction
This is the original idea for this project drown by Dr. Johnson.
-
Base
-
Punch
-
Punch Mate
-
Punch Support Box
-
Screws
-
Springs
All parts were created here on Central Washington University campus.
The original design used walls and a channel to hold the specimen in place, but that was changed in later redesigns.
Parts
Materials
All materials for this project were provided by resources here on campus;
-
Dr. Johnson provided the aluminum for the base, and the tool steel used to make the punch.
-
The punch mate and parts for the punch support were provided by the material in the machine shop.
-
Also the screws and springs were provided by the machine shop.
This is the finished design made in SolidWorks
Fabrication
-
The base was created first, cut from a piece of aluminum. On the vertical mill the ends were squared, the step was added, the relief channel was made and both slots were created for the punch and punch mate.
-
The punch mate was created from a piece of bar steel. It was cut to length then the ends were rounded using a belt sander, and the inside geometry was added using the vertical mill.
-
The punch was then created from a section of S7 tool steel that had to be cut down to shape. The ends were squared on the mill, then the cutting geometry and the cutting angle was added.
-
The punch support was created around the punch. The back mounting piece was created out of angle bar, spacers that are as wide as the punch, and a front plate used to hold the punch in place.
IMG_4681
Testing
Professor Johnson suggested that plastic be used instead of steal for the testing, due to the difficulty with finding the force required to shear steal. Tests were preformed with plastic specimens.
Three tests were preformed on the punch; an initial force test, a friction test, and a final force test. The initial force test was performed by putting a blank specimen in the punch and trying to shear it. this did yield any usable data, but observations where made as to how to better the design and functionality of the punch. During testing the punch would not actuate freely. The second test was to find the force of friction between the punch and its support. From there a way to reduce the friction was found.
The screws holding the punch support together broke during this test and required a rebuild of the part.
After the rebuild, test 3 was performed by putting the punch in the arbor press and shearing several sections off of the specimen.
Specimens used for testing the punch.
The specimen on top was used for the first test.
The specimen on bottom was used for the third test.
Results/Discusion
The results of the first test were inconclusive due to the misalignment in the punch, friction with the punch support, and the lack of a shear angle. The misalignment in the punch caused the punch rotate in its supports and not cause the proper shear. The friction caused a drastic increase in force required to actuate the punch, and prevented the punch from returning to its initial position. The lack of a shear angle increased the force by not having a cutting force but a punching force.
The second test resulted in finding the friction force between the punch and punch support. The force was found to be 28 lbs on average to move the punch. By polishing the outer surface of the punch and the inner surface of the support the friction could be reduced.
At this point the punch broke during testing and had to be rebuild. After the rebuild the punch only required 6 lbs on average to opperate.
The third test was preformed in an arbor press, and the input force required the shear the plastic was between 4 and 8 lbs of force. The initial force was between 6 and 7 lbs to start shearing around the radii of the first curve. The force then drops to being between 4 to 5 lbs as it cuts the straight part of the profile. The force increases to being 8 to 9 lbs around the second curve.
Budget/Schedule
-
The budget for this project was set to $500, given the limited resources of a college student.
-
External sources were proposed to provide resources if needed, but they were not needed so they were not taken up on their offer.
-
Professor Johnson stated that he would be able to provide the material needed so that was able to reduce the costs.
Schedule
