Week 9 - Attachment Feature Creation - Challenge 2

Design and Draft Analysis of Screw Boss and Dog House for Centre console Coin Holder

Screw Boss:

Bosses are used for the purpose of registration of mating parts or for attaching fasteners such as screws or accepting threaded inserts (molded-in, press-fitted, ultrasonically or thermally inserted).
The wall thicknesses should be less than 60 % of nominal wall to minimize sinking. However, if the boss is not in a visible area, then the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws.
The base radius should be a minimum of 0.25 x thickness, The boss can be strengthened by gussets at the base, and by attaching it to nearby walls with connecting ribs.

Dog House:

Dog house is an engineering feature used in plastic trim design. Dog houses are used as supporting feature. Sometimes other engineering features like snaps locators etc. are mounded on them to increase their strength. Dog houses are subjected to draft analysis to prevent breakage of the component during ejection from mould cavity.

Dog house and other engineering components are built on B-surface. mounting screw boss and other components on dog house prevent formation of sink marks on the class A surface which in turn provides a good aesthetic finish to the surface.

Design Guideline for Screw Boss:

• Draft - 0.5 degrees on walls of boss
• Thickness - Typically 40%* of the nominal wall at the base.
• Height to Diameter Ratio - 3:1
• Radii - 0.25mm maximum radii.

Design Guideline for Doghouse:

• 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

Screw boss and Doghouse Creation:

Extract is created from the class B surface, A sketch is created on the surface for screw boss

Once the iso constrained sketch is done they are extruded in part body

Boss is extruded to a height of 15mm taking 3:1 ratio

Boss is then drafted with respect to tooling axis with an angle of 0.5 deg.

The Boss ribs are extruded seperately to a height of 12mm , and they are drafted to 0.5 deg with respect to tooling axis and the extract is taken as neutral element.

Once the part are created they are assembled and union trimmed to obtain the below part body.

Edge fillet value of 0.25mm is provided. After creating the screwboss another copy is created and they are translated to a required distance on the class B surface of coin holder.

For Doghouse creation an extract is created from the classB and a sketch is created.

The sketch is swept with 21mm with tooling axis

A 20mm offset is created from the extract.

The offset is then trimmed with the sweep

Thicken the part to 2mm and a 0.5 mm filllet is given.

A 4mm offset is created from the extract

The trim is then offet to value of 1mm

They are trimmed together to form the C wall of Doghouse

Once the trim is created they are split with the part body to obtain the dohhouse part.

Fillet value of 0.2mm is provided, For clearing the doghouse a seperate tooling is created and 0.5mm draft is provided. Once the doghouse is created the second screw boss is translated above the doghouse part and they are assembled into the coin holder part body.

After part body creation draft analysis is done on the doghouse and screw boss

Below Image shows the draft analysis with respect to the doghouse tooling axis. AS per the draft analysis colour code we can see that surface have a positive draft

The draft anlaysis done with respect to the main tooling axis and according to the draft analysis colour code surface is clearing the tooling axis with positive draft.

Tree Structure: