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Mechanical Engineering Technology

MEC 1011

Assignments

MEC 1011 - Design Communication I
T2&T3
Assignment 11
Screws and Position Tolerances

Due at the end of our First meeting in Week 10

As usual, create a new folder for your work using your full_name and located in V:\MEC\1011\Fall_10\Assignment11/Tx (where Tx is T2 or T3).

Objectives:

Produce complete documentation of two simple assemblies, that just barely fit together (zero clearance at Maximum Material Condition and worst case location), where one assembly has two versions of part drawings.

Both assemblies include two plates that fasten with screws (2 minimum at any size except .25) while a side and an end are aligned. One assembly has a base plate with tapped holes and the other uses nuts in a counter bored base plate. The top plate should be common. One assembly will have a version of part drawings that use ‘rectangular’ tolerance zones to locate holes and another version using positional tolerances.

Clearance hole basic sizes should be based efunda’s free fit clearance hole standards found on their tap drill chart (try a SolidWork’s ‘normal’ fit screw hole. i.e. Counterbore, Countersink or Hole/Screw Clearances).

Model and document a ‘cartoon gage’ for one part.

Where good parts are ones that fit with mating parts (even if just barely, i.e. ‘line to line’) and presuming that nominal fastener sizes equal MMC sizes, all tolerancing should seek to:

1.    Accept as few bad parts as possible.

2.    Reject as few good parts as possible.

3.    Minimize inspection cost

Bonus challenge: your positionally toleranced assemblies accept zero bad parts and reject zero good parts.

Document (with a table) a comparison of your rectangular vs. your positional tolerancing. State your presumed process capabilities (in terms of minimum reasonable tolerances) for hole location and hole size. Assess which tolerancing scheme rejects fewer good parts. Show your calculations (by hand if you like).

Here’s a checklist:

1.    Two assembly models and drawings: One with a tapped base plate and the other with a counter bored base plate and nuts.

2.    Three part models and five part drawings: Two base plates, one top plate, two drawings with rectangular tolerances (based on one of the assemblies) and three drawings with positional tolerances.

3.    One cartoon gage model (like fig 9-10 on page 190) with informal drawing (without tolerances and including only pin diameters and locations).

4.    One document (doc or htm) comparing tolerancing schemes including calculations (possibly by hand).

References:

Handout of CH 8 and CH 9 from Bruce A. Wilson’s Design Dimensioning and Tolerancing.

 

Advice:

1.    Keep it simple!