Design and Manufacture of Axial Testing Machine

Design

This page explains the design of various components and their functions, and where they are generally sourced from. Some components can be used straight after they are bought, so there is no “design” process to explain for those.

Frame and fasteners

T-Slot extrusion that acts as the chassis/frame. The extrusion used to build this machine is made of 6560-T6 Aluminum, and is 40mm high (this dimension is also known as rail height), 20mm wide, and about 6” long.

• Since the steel plates are to be clamped upright on the frame, T-Slotted framing brackets and fasteners that are appropriately sized for the rail height are needed. Each plate is inserted between two brackets and fastened to the frame. The fasteners are M5 nuts. The brackets are 3/4” long, have an M5 mounting fastener sizeand can fit the rail height of this extrusion.

All these components can be purchased on McMaster-Carr. On the website, it is easy to find the right brackets and fasteners once the T-Slot of the desired dimensions is chosen.

Open

T-Slot Framing Extrusion

 

Open

T-Slot Framing Bracket

 

 

Rectangular Plates

3 rectangular metal plates, hereby called the First, Middle and End plates, are fixed onto the frame using brackets and fasteners. Each plate has dimensions of 72mm x 38mm x 6.21mm. The holes or slots designed on each plate are all at the exact centre of the plate. At the bottom of each plate, there are two through-holes (5mm diameter that allow the plates to be fastened to the brackets along the thickness of the plate.

• The First plate has a through-hole in the middle to accommodate the M10 hex bolt.

• The Middle plate has a hex slot in the middle to hold the coupling nut.

• The End plate has a thin angled slot the runs vertically down from the top face to the middle. The slot allows space for the wire to be inserted, and is at about a 1deg angle from the vertical to prevent the wire from jumping straight out of the slot.

U-Plates and Pins

There are two U-Plates in the machine. Both U-Plates are made of cold rolled steel, and both have circular slots along their sides to hold a steel pin each. On one side of each U-Plate is a small hole of diameter 2mm, through which one end of the wire is threaded through and then tied.

• One U-Plate is welded to the Coupling nut and the other is welded to the End Plate. Both U-Plates therefore have different widths.

• The U-Plate welded to the Coupling nut has a few differences in its design. It has an angled slot on its front face to allow the wire to pass through freely. The front face of this U-Plate is extruded about 1.5mm higher than the height of the two side faces. This is due to the placement of the circular slots, as well as the pin that is to be inserted in them.

It is highly important that the pins do not rotate, as this causes unnecessary friction to the wire that will be wrapped around them. In early prototypes of the machine, (a version of) these pins rotated about their own axes, which meant that the wire wrapped around them took longer to go into tension. This meant that once the wire did go into tension, there weren’t enough turns left in the coupling nut to pull the wire till it snapped. Another solution to this could have been lengthening either the Hex Bolt or the Coupling nut by about an inch, but this would also mean increasing the length of the frame to accomodate a wire that was long enough to show the effects of stretching clearly. To avoid investing in a completely new structure and have the coupling nuts made again, it was decided that the pin would have to be fixed.

Hex Bolt

A fully threaded, medium-strength Class 8.8 Steel, Hex Head Screw that is M10 x 1mm, 60mm long, is needed. This is perhaps the most important driving part of the machine, since this is part that is tightened (using a torque wrench) and ultimately stretches the wire. The bolt can be purchased on McMaster-Carr.

Hex Coupling Nut

A 5/8” Hex coupling nut (Threaded M10 x 1.5mm), one end of which is welded onto a U-Plate. The other end of the coupling nut is inserted into a hex slot on the Middle plate, and it is at this end that the hex bolt is driven in.

Metal Wire

A metal wire of a fixed length that is constrained between the two U-Plates. The wire is stretched by the mechanism and ultimately breaks. The wire can be made of any material, with the obvious fact being that different materials will break at different strain rates and show slightly different types of failure.

Throughout the whole process of developing a working prototype, a 0.2mm diameter Nichrome wire from the E3 Machine Shop was used for testing. This wire was taken from a spool of wire lying around in the machine shop for about three decades, so there wasn’t a lot of documentation to provide the exact composition of the wire. This is not a problem, because the general properties of Nichrome can be used for any calculations.

Manufacturing Process:

This section shall discuss in brief the manufacturing processes for some of the above components. All of these parts were made in the E3 Machine Shop, or altered at the E5 WEEF Machine Shop at University of Waterloo. Since this wiki page was written during the prototyping stages of the machine, it was necessary to go back to the design stages and fix some of the problems that arose after manufacturing. This really made me appreciate the beauty of the design process all over again.

Rectangular Plates

Originally, the plates were all made of Aluminum 6061-T6. However, when the decision to weld a cold rolled steel U-Plate to the End Plate was made, the End Plate was made of Stainless Steel. This is because only similar enough materials can be welded together.

When the next set of prototypes are made, all the rectangular plates need to be made of Stainless Steel to aid uniformity. Regardless of the material, the plates and their features (holes and slots) are waterjet cut.

U-Plates

Originally, the U-Plates were supposed to be made of 11 GA (so the thickness was 1/8”) 1010 Cold Rolled steel. It was thought that a strip of the metal could be bent into the shape of a U and then machine the features. However, it was learnt from the Machine Shop that cold rolled steel cannot be bent. So an alternative was to waterjet cut the Us from a sheet of appropriate thickenss. The Machine shop did not stock 1/8” thick cold rolled steel at the time of prototyping, so the U-Plates (all there features included) were waterjet cut from 1/2” thick Cold Rolled 1010 Steel.

After testing the prototype, it was discovered that the top edges of the slot on the U-Plate welded to the coupling nut were too sharp, and caused unwanted stress buildup on the wire where it touched the slot, which skewed the real stretching. To fix this, the top edges of the slot were chamfered slightly using a drill press. This seems to satisfactorily fix the issue, but to make the design perfect, the front face will have to be extruded a bit higher than it already was. This will ensure that the only points that the wire coud accidentally touch are the two inner faces of the slot, which aren’t sharp like the top edges.

The U-Plate can be made of Aluminum too, as long as the rectangular plates are made of Aluminum too. However, the fact that Aluminum is a lot lighter than steel should also be considered.

Hex Nut

The Hex Coupling nut was made from Cold Rolled Steel Hex stock. The stock was cut into the desired length, i.e., 2”, and a hole was then drilled through the middle and threaded. In this case, the hole and thread were M10 x 1.5mm. The coupling nut had to be custom-made in the machine shop because a 2” long coupling nut was not available online or in stores.

Welding

Welding is a type of fabrication process where two or more parts are joined together by heating the materials (usually metals or thermoplastics) until they melt, and then allowing them to cool so that they fuse together. A weld “filler” material is sometimes added to the molten mixture of the parts in order to acheive a better welded joint. [2] To create a good weld, the metals need to have similar enough properties, such as: [3]

• Melting Point: The parts that are to be welded need to have the same (or similar enough) melting point so that the welder can use the same temperature to melt both metals.

• Coefficient of Thermal Expansion (CTE): The CTE of a metal specifies how the object’s size changes as the temperature changes. While welding two vastly dissimilar metals, this means that each metal would expand and contract at different rates as they are exposed to high heat and then allowed to cool. As the temperature changes around the welded joint, there are a lot of stresses acting on the weld, especially around the intermetallic zone (the space where the two metals fuse together with the weld filler material).

• Electrochemical Difference: The two metals have to be as close to each other as possible on the electrochemical scale. This ensures that the intermetallic zone does not corrode.

• Solubility: The solubility of both metals have to be similar too, because this ensures that they are compatible with each other in the molten state.

For more information about welding, please refer to the Weldments wiki page.

For this machine, there are four welds to be done.

• The Coupling Nut and its corresponding U-Plate are welded together.

  • The steel pin is first pressed into the circular slots in the U-Plate, and then welded to it.

• The End Plate is welded to its U-Plate.

  • The other steel pin is pressed into the slots of this U-Plate and then welded.

References

[1]Carr. McMaster. (n.d.). Retrieved December 8, 2021, from https://www.mcmaster.com/47065T139/

[2] “What is welding? - definition, processes and types of Welds,” TWI. [Online]. Available: https://www.twi-global.com/technical-knowledge/faqs/what-is-welding [Accessed: 11-Nov-2021]

[3] A. Mulkerin, “What is dissimilar metal welding?: Metal Welding Techniques,” APX York Sheet Metal, 28-Jan-2021. [Online]. Available: https://www.yorksheet.com/york-sheet-blog/dissimilar-metal-welding . [Accessed: 11-Nov-2021]