Attachment Feature Creation (Screw Boss & Dog House)

Aim: Creating Screw Boss and Dog house on the Coin Holder plastic component and understanding its design rules.

Workbench and its Toolbar used:

Generative Shape Design Workbench

• Surface Toolbar
• Wireframe Toolbar
• Operation Toolbar

Part Workbench

• Surface Based Feature Toolbar

Theory:

Tooling Axis - Tooling Axis is the direction in which the component will be ejecting after the molding process is completed. The rules of selecting an optimum direction of tooling axis for any particular component is as follows :-

• The direction of the tooling axis should be clearing the maximum number of portions of the component once the molding process is completed.
• The direction of the tooling axis should be selected such that the number of side cores required should be minimum, as inserting side cores causes a rise in production cost.

Drafting - Draft is the inclination given to any particular plastic component’s cavity walls in order to achieve easy ejection, once the process is completed.

• Consider that the draft angle between the plastic component and the core is zero.
• When the molten plastic inside the cavity cools, it sticks to the walls of the core.
• During such zero draft condition, the ejection of the plastic component causes friction between them which might bring damage to both the core and the component.
• This damage further leads to inefficiency in the production & also affects the product’s surface finish.
• Hence, a minimum of ½ to 1 deg draft is provided to help in releasing the component.
• However 3 deg draft is widely accepted as a norm in the industry.

Prerequisites before creating the tooling axis and performing draft analysis-

1. Ensure that the Class A surface imported should be a single surface.
2. This can be checked using the Boundary tool in the Operation Toolbar.
3. If the surfaces are disassembled, join them using Join command which is also found in the Operation toolbar.

Creating any component from Class A Surface ( Overview ) -

1. Decide a suitable tooling axis direction and create the Tooling Axis.
2. Perform Draft analysis for analyzing whether the drafts on the component follow the industry standards.
3. Create another surface ( B Surface ) offset to the Class-A Surface. The offset distance should be equal to the thickness of the component.
4. Create a C- Surface ‘Sweeping’ the boundary of Class-A -Surface along the Tooling Axis.
5. Trim Class- A surface with B-Surface and C Surface to create a closed surface.
6. Finally create the solid part from this surface to obtain the desired component.

In previous challenges we designed an incorrect tooling axis so we corrected it in this model and new draft analysis results are attached below.

Rules for Design of Screw Boss:

1. Outside Diameter = 2.0 to 2.4 times the outer diameter of hole. (D)
2. Step distance = 1.5 mm
3. Distance of Screw penetrated into the component = 0.3 * thickness of the component
4. Thickness of the Screw Boss = 0.6 * thickness of the component = outside Diameter of screw boss - Hole Diameter
5. Draft = 0.5 deg
6. Fillet Radius = 0.25 mm Max
7. Height to Diameter ratio = 3: 1

Rules for Design of Dog House:

1. C is the portion which is in contact with the b surface and its Thickness of C = 0.4*thickness of the component.
2. Length of C (B) = 3mm to 6mm
3. Width of the Dog House = 12 mm Min.
4. Draft on the Dog house = 1.5 mm Min.

Procedure:

I] Creation of Tooling Axis:

1. We create a centre point on the base surface and assign an axis system there.
2. Further we intersect the Class-A with the respective planes.
3. Then we create a mean line between the faces which might create problems during ejection.
4. Lastly we bisect these two lines to generate the main tooling axis.
5. This line can be modified for generating optimum results during draft analysis.
6. Also we have introduced a second tooling axis for clearing the hole.
7. The dog house created also has its seperate tooling axis.

II] Draft Analysis Of Class-A Surface:

• Draft Analysis is performed on the component in order to check whether the component can be manufactured or not.
• This Draft Analysis tool in Catia allows us to analyze the drafts provided on the surface.
• We can check whether they follow industry standards and if not we can modify it accordingly.

The procedure of Draft Analysis is as follows-

1. Before starting the draft analysis ensure that the ‘Materials’ option is selected under Customize View Parameters in order to view the results of the analysis.
2. Click on Insert >> Analysis >> Feature Draft Analysis.
3. Draft analysis window will be opened having three sections - Mode , Display and Direction.
4. In the Display section select the compass and drag it onto your component. Place it along the tooling axis.
5. Lock the direction of the draft.
6. Another window will be displaying ranges of draft on the component. The ranges are - Green for more than zero , Red for zero and Blue for negative draft value.
7. Assign the lower limit as 3 deg to the Green range from the value edition dialogue box.
8. Once you do it, click on the component to see the results.
9. You can also check the draft value at any point using the ‘Analysis under running point' option in the Display Section.

Here we performed the draft analysis with respect to two tooling directions separately, such that each and every section of the component should get cleared.

III] Draft Analysis of Solid:

Completed Design:

Front View:

Top View:

Side View:

Isometric View:

Conclusion:

Hence we have successfully understood the design rules for creating the screw boss and the dog house.