MEC 1011 - Design Communication I 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! |