Design Process:
Concept: My concept is a simple rectangular aluminum handle and interference fitted jaw. The handle completely encloses the Jaw to ensure maximum strength under torsional load. The handle is designed to be easily machined and features cavities for light weight. The design uses the same 3/8 end mill size for most fillets and only requires squaring off once to machine the entire wrench. The fillets, a result of being billet machined, also serve to decrease stress concentration at the joints, a relatively large fillet size was chosen for this reason, and to make machining easier with less chance of snapping an endmill. The design also features a bottle opener, placed at the rear of the wrench to avoid thinning the supports for the jaw which are under high stress. The bar/ edge for the bottle opener also acts as an extra support for where a hand would apply load to the wrench for use. The wrench is of the maximum allowed dimensions, 6” x 1.5” x 0.25” exactly. The wrench handle weighs in at 50 grams, and the total weight of the assembly is 52 grams including the jaw.
CAD Images |
ABS Jaw with flange (interference fit)
Wrench assembly with Handle
Wrench assembly exploded view
Wrench assembly underside view |
Final Drawing and dimensions of wrench :
We performed two functional tests to evaluate the strength and performance of the wrench. Both tests involved using the wrench to loosen a 5/16”-18 bolt torqued to the assigned 13.5 N·m. The assembled tool included the aluminum handle and an ABS jaw printed with 100% infill.
In both tests, the aluminum handle remained fully intact and showed no signs of permanent deformation. However, in each case, the ABS jaw failed at the hex cavity during torque application. The failure appeared as stripping or tearing along the internal hex corners, suggesting that the ABS was not able to withstand the full torque applied.
The failures were consistent across tests with two sample jaws, confirming that the issue was not due to printing defects or a one-off flaw. The stress was concentrated in the unsupported portion of the jaw near the hex socket, which aligned with the results from FEA simulations. However; those results did not predict the jaw would exceed its 40MPs yeild strenght during loading. The friction-fit jaw stayed securely in place throughout the test, and the jaw stop geometry inside the handle successfully prevented it from sliding through. The spare jaw holder functioned as expected, securely storing the second jaw in the side cutout. The bottle opener also functioned as expected, capable of opening standard bottle caps with minimal surface wear.
Final Weight:
Assembled wrench and jaw: 52.7 grams
Test Images:
Figure 1. Assembled wrench before testing, with ABS jaw inserted into the aluminum handle.
Figure 2. Top-down view of the wrench after both tests, showing stripped ABS jaw cavity
Design process:
Final Drawing and dimensions of wrench
I began by-hand analysis by calculating the required torque to yield for a medium strength bolt of the specified size. Bolt material data was taken from McMaster Car.
Once I had my neccesary value for torque, I chose a factor of saftey of 2.0, this was chosen as a wrench designed for this factor of saftey is unlikely to fail within thousands of cylces of use, and light weight is a strong consideration so any more robustness would be uneccessary. All material data for all optimizations was taken from Matweb for AISI 6061 T6 aluminum.
To begin, I set all the dimension necessary for the wrench to meet size requirements, and fit the jaw, and based all other dimension around the thickness of the members, which I set to be a uniform value, t. The calculations solved for this value to complete the design of the wrench. The height was left at 0.25 in. and the length at maximum length 6 in. The first calculation optimized t under constraints of bucking for a fixed end beam, the beam being the longest member under compression during torque application. Once I had a value from this optimization, I checked that value for shear stress at the cross memeber holding in the jaw. This t value failed the test, so I optimized it using the shear stress at the jaw cross memebr to find a new value.
Following this, I analized the longest memeber again, this time for principle stresses evaluated using Von Misces stress critera, for the tensile side, as the compressive side should fail via buckling first due to its length, and the principle stresses should be the same either way, just invereted which one is stress one and which one is stress two.
The matlab analysis for this criteria found that the length optimized previously is ok. Just to make absulutelty sure that the wrench handle will never fail, that you could even hit it with a hammer to break a seized nut if need be, I did one final analysis for axial tensile stress in the longest beam, assuming force was applied to it in the worst way possible, i.e. an impact to the weakest point of the wrench that would provide 45 ft*lbs of torque. I analized this at a lower factor of saftey than the other criteria, due to how unlikely it is in regular use.
The thickness from the shear analysis at the jaw member failed this test, so I again used matlab to optimize for the new critera, and rounded up the answer. Conviniently this resulted in the maximum possible thickness that would still meet the size constraints, making the machining process much more simple.
The weight of the assembly came out well within the weight limit.
4: Finite Element Analysis Set-up
Finite element analysis (FEA) was used to evaluate the wrench’s structural performance under torque and bending. The goal was to ensure that both the aluminum handle and ABS jaw could withstand the expected loading without failure. Simulations were done in ANSYS Workbench using the Static Structural module.
Materials Used in Simulation
Handle (Aluminum 6061):
Young’s Modulus: 68.9 GPa
Poisson’s Ratio: 0.33
Yield Strength: 276 MPa
Jaw (ABS Plastic):
Young’s Modulus: 2.1 GPa
Poisson’s Ratio: 0.35
Yield Strength: 40 MPa (approximate)
Model Overview
The FEA model included the final geometry of the wrench handle and jaw. The bottle opener and jaw cavity were modeled as designed. The jaw was inserted using a friction fit. Three different load cases were simulated to reflect expected usage and testing.
Load Case 1: Bolt Torque Only
A torque of 13.5 N·m was applied at the end of the handle. The jaw was fully fixed at the hex cavity to simulate gripping a bolt. This case tested the normal use condition of the wrench.-
Above are results shown as heat maps for strain (Von Mesces), stress (Von Mesces), and displacement (total).
Load Case 2: Torque with Off-Axis Force
A lateral force of 25 N was added at the handle tip, in addition to the 13.5 N·m torque. This simulated a real-world case where the user may pull sideways while applying torque. The goal was to see how the wrench reacts to combined torsion and bending.
Above are results shown as heat maps for strain (Von Mesces), stress (Von Mesces), and displacement (total).
Load Case 3: 3-Point Bend Test
This model recreated the shop’s physical test setup. The jaw was removed and the handle was supported 5.5 inches apart with a 25 N downward force at the center. The purpose was to confirm that the handle would return to shape after bending. For a more accurate representation, the middle was fixed and the equivalent load was instead applied to either side of the wrench, to more accurately simulate the ability of the wrench to slide down into the supports like in a real three point bend test.
Above are results shown as heat maps for strain (Von Mesces), stress (Von Mesces), and displacement (total).
Mesh and Setup Details
Mesh type: SOLID185
Mesh refinement applied to jaw connection, bottle opener, and critical edges
Contact: Frictional contact between jaw and handle
All other parts were treated as bonded
Assumptions
All materials assumed to behave elastically
No manufacturing defects or print layer inconsistencies were included
Geometry was based directly on the final SolidWorks design
No fatigue or thermal effects were considered