Global Reflection of Tolerance Analysis:
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Based on our calculation, the minimum plastic hole radius must be 0.1095 in. which corresponds to a diameter of 0.219 in. In order to make sure that the holes line up in every scenario, we must adjust the plastic hole diameter, originally designed as 0.200 in. to 0.220 in. We originally designed the hole to fit a flat-head 8-32 screw. The head diameter for this kind of screw is 0.312 in. With the modified hole diameter, 8-32 screw head will not fall through the plastic piece.
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Based on these calculations, the expected value for the length of the slots cut into the metal piece would have to be 0.986 in with the tolerance of +/- 0.002 in. Since there is a decent difference between 1 in and this calculated value, we decided to restrict the machining in a different way to remove the extra wiggle room. With this calculated value, the pieces would not fit together snug, which would defeat the purpose of the slots. Instead, we plan to define the slots on the metal piece to be 1 in + 0.000 in - 0.006in. With this value, it allows the slot to still fit into the plastic part reliably, but it does not create extra space that is not needed since it is more specific to the already existing slots.
Tolerance Analysis 1: Screw Holes Location
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Global Statement:
Our goal with this tolerance analysis is to find out the relevant hole diameter of the plastic piece.
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Relevant Physical Restrictions:
Plastic Injection Molding Tolerance: +/- 0.003in
Aluminum Extrusion Tolerance: +/-0.04in
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Assumptions:
We picked one hole on the base piece to be in a set location, and calculated all our tolerances around the location of that point. We calculated based on the worst-case scenario, which would be the aluminum holes being as far apart as possible, while the plastic holes are as close together as possible. The aluminum hole diameter was set to be a big as possible and we used that to calculate the smallest possible hole size for the plastic.
Sketches
Top view drawing of the aluminum extruded part
Top view drawing of bottom plastic part
Hole location and hole size sketch for tolerance analysis
Calculation:
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Solving the length of l
l in inches
Solving for r
r in inches
Discussion:
Based on our calculation, the minimum plastic hole radius must be 0.1095 in. which corresponds to a diameter of 0.219 in. In order to make sure that the holes line up in every scenario, we must adjust the plastic hole diameter, originally designed as 0.200 in. to 0.220 in. We originally designed the hole to fit a flat-head 8-32 screw. The head diameter for this kind of screw is 0.312 in. With the modified hole diameter, 8-32 screw head will not fall through the plastic piece.
Tolerance Analysis 2: Finger Joint Tab Size
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Global Statement:
Our goal is to find the necessary machining tolerance that still allows the plastic piece to mesh with the aluminum piece.
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Relevant Physical Restrictions:
Injection Molding Tolerance: +/- 0.003 in
Slot with End Mill Tolerance: +/- 0.002 in
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Assumptions:
For this worst-case scenario, we assumed that the slot in the plastic piece was as small as possible, while also shifting the center-point of the slot as far to one side as the tolerance allows.
Sketches
Calculation
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Calculating the length of a
Length of a in inches.
Calculating the length of x
Length of x in inches.
Discussion:
Based on these calculations, the expected value for the length of the slots cut into the metal piece would have to be 0.986 in with the tolerance of +/- 0.002 in. Since there is a decent difference between 1 in and this calculated value, we decided to restrict the machining in a different way to remove the extra wiggle room. With this calculated value, the pieces would not fit together snug, which would defeat the purpose of the slots. Instead, we plan to define the slots on the metal piece to be 1 in + 0.000 in - 0.006in. With this value, it allows the slot to still fit into the plastic part reliably, but it does not create extra space that is not needed since it is more specific to the already existing slots.
