MEC 1011 - Design Communication I
Screws and Positional Tolerances
the end of our ‘Lecture’ meeting in Week 10
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:
Accept only good parts (i.e. reject all bad ones).
Reject as few good parts as possible.
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).
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).
Lockhart CH 12 p.536-p.538
Handout of CH 8 and CH 9 from Bruce A.
Wilson’s Design Dimensioning and Tolerancing.
1. Keep it simple!