Friends, we are going to discuss threaded fastener joint means tapped holes in one plate and thru holes in mating plate joined together by screws or bolts. There are two major requirements by design are necessary a) Joint should have required clamping force after applying fastening torque b) To assemble plates without any fouling with bolt. Geometric Dimensioning and Tolerancing (GD&T) focuses on functionality requirement and supports designer for drafting of part drawings with Quality of Event (QoE).
Following non conformities can be eliminated by application of right GD&T controls by design engineer. Let us illustrate it with example.
a) Bolt axis perpendicularity leads to less clamping force at joint after fastening during assembly:
Plate no. 2 is having multiple threaded holes of M10 and corresponding thru holes are on mating plate no. 1
In above example threaded hole is having positional tolerance of ⌀ 0.4. As there is no separate control for axis angularity or perpendicularity, it is controlled by positional tolerance ⌀ 0.4 itself. Illustrated in below figure.
This perpendicularity of hole axis will leads to insufficient bolt head seating after fastening by application of torque. In this situation bolt will not generate required clamping force even if torque is applied as per specification. Subject phenomenon occurs as bolt seating is away from threaded hole by plate thickness of top plate. Below figure will depict this phenomenon.
GD&T standard Y14.5 ASME addressed subject functional issue and minimised tilt of the axis by using projected tolerance zone concept. This is achieved by projecting cylindrical tolerance zone till bolt seating face ie. positional tolerance is to be maintained at height (30mm) bolt seating level not at joint face. Now you will see hole axis is more perpendicular than previous situation without projected tolerance zone. Positive effect of projected tolerance zone is depicted in below figure.
to be replaced by new control frame with projected tolerance zone for threaded hole in plate no. 2
b) Rejection of plates due to fouling of bolt during assembly even if parts 1 & 2 are made as per specified position tolerance zone for holes.
Subject non conformity occurs generally when position tolerance of threaded hole and thru hole is not designed by considering virtual Worst Case Boundary (WCB). GD&T standard recommends simple formula for calculating positional tolerance for holes on both the plates.
In above example.
F= MMC for bolt M10 = ⌀10.0
H= MMC for thru/ clearance hole (⌀ 11.0 +/-0.2) =⌀10.8
T= Position tolerance for hole on each part = (H-F)/2 = (10.8-10.0)/2= 0.4
⌀0.4 is assigned for both holes on plate 1 & 2. This would result in following control frames for threaded hole and clearance hole.
For clearance hole on plate no. 1 it will be
For threaded hole ( assumed as shaft considering bolt is mounted ) on plate no. 2 it will be 
In above example clearance ⌀ 0.8 at MMC condition is distributed 50%-50% for positional tolerance, however you may assign little bit more tolerance for threaded hole than clearance hole considering complexity in manufacturing (e.g. 60%-40%) by keeping worst virtual envelop same for both .i.e ⌀10.4
Off-course final call to be taken by design engineer based on application and expected performance of final product.
If both the above key requirements of fastener joints are adhered in design, then definitely life of manufacturing & Quality personnel will be peaceful. You will enjoy no fouling in assembly and better clamping force after applying torque, means better reliable fastener joint on product.
Author is corporate trainer on Quality (9820424387, nnwalve@gmail.com)
Q-Sanwad : The Blog on Quality https://qsanwad.blogspot.com/
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