Week 9 - Attachment Feature Creation - Challenge 2
Screw Boss Design
Bosses find use in many parts designs as points for attachment & assembly . The most common varity consists of cylindrical projections with holes designed to recive screws, threaded inserts or other types of fastening hardware. As a rule of thumb, the outside diameter of bosses should remain within 2 to 2.4 times the outer diameter of the screw or insert.
To reduce stress concentration and potential breakage, bosses should have a blended radius rather than a sharp edge at the base.
Avoid bosses that merge into sidewalls because they can form thick sections that lead to sink.
Consider using open bosses designs for bosses near a standing wall.
Design Rules for Screw Bosses
Boss Thickness (w) :- [w=0.6*T]
-Measure base thickness of rib at tangent of radius if radii are modeled in
-Larger the are at the base of the rib, the more retained heat.
-The retained heat causes more shrinkage, which causes sink.
Fillet Radius :-
-Optional Rib fillet radius = 0.25mm max
-Must not violate 40% thickness requirement.
Draft :-
-Maximum 0.5 degrees on both sides of boss.
-Different rules if sleeve ejection is used.
Height (H) :-
Height to Diameter Ratio - 3:1
Procedure to make the Rib :-
1) Create a Plane and a point where we want the base of the Screw Boss.
2) Create a Sketch and Draw 2 Circle with the thickness of screw boss.
3) Extude them .
4) Now give Draft and edge fillet to the rib.
5) Added the rib to the Screw boss for stiffness.
6) While making screw boss and ribs always give the proper dimensions as provided in the design rules.
Dog House Design
Dog house is a feature which helps you to avoid sink marks. Solves length issue. Also uses when we have perpendicular screw bosses.
Dog House Width (X) :-
-Minimum width is 12mm
-3:1 ratio of steel width to height
-2:1 is preffered
Dog House Draft :-
-Minimum Draft (D) for doghouse is 1.5 degree
-If a slide is being considered, this requirement can change to 3 degree minimum
Dog House Height (H) :-
-Minimum Height of Dog house is 6mm less than 6mm causes weak steel condition.
- 3:1 ratio of steel width to height , 2:1 is preffered.
-"C" is where the doghouse contacts the backside of the surface
-Doghouse wall thickness at the intersection of the part wall ( C ) should be 40% or less than the main wall stock ( T ).
-Height of ( B ) should be between 3.0 – 6.0 mm
Procedure to make the Dog House :-
1) Create a Plane and a point where we want the base of the Screw Boss.
2) Create a Sketch and Draw line in the shape of dog house.
3) Extude them in surface workbench and fill the top part of dog house.
4) The go to Part workbench and thicken the surface.
5) Now give Draft and edge fillet to the Dog House.
6) While making Dog house always give the proper dimensions as provided in the design rules.
Steps to do draft analysis:
1) Use Generative shape Design, Open Feature draft analysis option from Insert > Analysis > Feature draft analysis.
2) A new window pops up by default it will be in quick analysis mode. Change the range to .49deg.
3) Next you have to position the compass on the tooling axis by dragging the compass from the top right corner of the working area. The W axis of the compass should be inline with the axis.
4) Then you have to define the compass by click on the compass icon on the draft analysis window. Then lock the position of the compass by clicking the lock icon.
5) After that select the surface and the whole surface will show a color range. Here the screw bosses and Dog houses is fully in green color, which means all faces are having a minimum draft angle and it’s good for manufacturing. If any face is showing blue or red color we have to work on that face to give a good draft.
Dog House inner Draft analysis in Z direction:-
