Week 9 - Attachment Feature Creation - Challenge 2

DESIGNING SCREW BOSS AND DOG HOUSE FOR THE COIN HOLDER

OBJECTIVE

To create the Screw boss and dog house in the coin holder by following design rules.

INTRODUCTION

SCREW BOSS

The boss is one of the basic design elements of a plastics moulding. Bosses are usually cylindrical because that is the easiest form to machine in the mould and it is also the strongest shape to have in the moulded part. The boss is used whenever we need a mounting point, a location point, a reinforcement around a hole, or a spacer. The boss may receive an insert, a screw, or a plain shaft either as a slide or a press fit. In other words, the boss is not as simple as it looks. Depending on its use, it may have to stand up to a whole combination of forces – tension, compression, torsion, flexing, shear and bursting - so it must be designed accordingly. We can start with some general design rules, using the principles we have already developed for ribs and walls. The boss can be thought of as a special case of a rib; one that is wrapped round in the form of a tube. An 'ideal' boss, designed according to rib rules, would not produce sink marks or stick in the mould but unfortunately the tubular form of the boss would not be strong enough in most cases. In real life, most bosses break some rib design rules by necessity. This means that boss design is a compromise between sink marks and functionality. Rigidity is the simplest aspect of boss design. This can be achieved by supporting the boss with buttress ribs, and often by linking the boss to a side wall. The support ribs can be designed to normal rib rules so that sink marks and stress points are avoided.

Points to be considered while designing ribs

• The draft angle should be minimum 0.5° on the walls of the screw boss.
• The screw boss thickness should be minimum 40% to maximum 75% of the wall thickness
• The height to diameter ratio of the screw boss should be 3:1
• The edge fillet ratio must be maximum 0.25mm

Procedure for creating screw boss

• Extract the bottom surface of the coin holder solid part
• Create the positioned sketch (circles) on the extracted surface. Make sure the sketch is iso constrained
• Here, for M6 screw is taken for the screw boss. For M6 screw the pitch Diameter is 5.2
• Thickness of the screw boss should be 0.6x2.5 (component wall thickness)
• Extrude the sketch considering the height to diameter ratio (3:1) for the height
• Create draft on wall with 0.5° draft angle, pulling direction as tooling axis and neutral element as extracted surface
• For the stiffness of the screw boss create the ribs
• Create the sketch for the ribs Pad the sketch along the tooling direction
• Thickness of the ribs is 1mm, apply draft of 0.5° on all the surface
• Add the ribs into the part body using union trim option remove the inner surface of the ribs
• Apply Edge Fillet R 0.2mm to all the edges of the ribs
• If we need another ribs as same as the previous one we can simply copy paste and translate it where ever we need

DOG HOUSE

Dog house is an engineering feature used in plastic design. Dog houses are used as support feature. Dog houses are subjected to draft analysis to prevent breakage of the component during ejection from mould. Dog house is used to elevate another features

Procedure for creating dog house:

• Create a plane using XY plane and the tooling axis point
• Create the sketch of the dog house on it
• In surface modeling use sweep option and sweep the sketch along tooling axis with 1.5°
• Extract the base surface of the class A surface offset it to 2.5mm and again offset it to 20mm
• Trim the swept surface and the 20mm offset surface together
• Create a new body, switch to part modeling and thicken the surface from outer side for 1mm
• Apply draft of 0.5° along the side core axis
• The intersection part where the part and the dog house meets should have 40% of thickness the main component. Height of the intersection should be 3 to 6mm.
• Offset the base surface at a distance of 4mm and extrapolate it. Offset the inner body of the of the dog house and offset in outer direction extract the offset surface.
• Trim both the extrapolated surface.
• Split the trimmed surface from the thickened body.
• Apply draft 0.5° along the tooling axis.
• Apply edge fillet of R1mm inside and R3mm to the outer surface and R0.5mm at splitted surface
• Translate the screw boss upon the dog house.

Draft analysis

main tooling axis

side tooling axis

feature tree

CONCLUSION

Thus the Screw boss and dog house are created in coin holder