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Mechanical Engineering Technology

MEC 1011

Assignments

MEC 1011 - Design Communication I
Assignment 11
Screws and Positional Tolerances

Due at the end of our ‘Lecture’ meeting in Week 10

Objectives:

        Produce complete documentation of two simple assemblies, that just barely fit together (zero clearance at MMC and worst case location), where one assembly has two versions of part drawings. Also, model and document a ‘cartoon gage’ for one part.

        Both assemblies include two plates that fasten with screws (2 minimum) 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 can 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 Inventor’s ‘normal’ fit clearance hole).

        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 only good parts (i.e. reject all bad ones).

2.     Reject as few good parts as possible.

3.     Minimize inspection cost

Bonus challenge: your positionally toleranced assemblies 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 (2X iam, 2X idw): One with a tapped base and the other with a counter bored base plate and nuts.

2.      Three part models and five part drawings (3X ipt, 5X idw): Two base plates (consider derived part which adds an ipt), 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 (four feature ipt) with informal drawing (.idw without tolerances and including only pin diameters and locations).

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

References:

1.     Lockhart CH 12 p.536-p.538

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

Advice:

1.      Keep it simple!