Week 4:- Pin Unit Design Challenge

WEEK 4: PIN UNIT DESIGN

Objective:

1. Design two different types of fixed pin units with different pins, pin retainers, blocks, blades, risers.
2. Design two different types of retractable pin units with different pins, pin retainers, blocks, blades, risers.

3-2-1 Principle:

During the metal working process, for example welding, the workpiece must be held properly in the correct position for exact weld to take place.Any variation of the part location on the fixture adds to dimensional tolerance that must be assigned to finished parts. The method of supporting and securing the part in the fixture affects not only dimensional tolerances, but surface finishing as well.Techniques for supporting and clamping must be considered together with the method of locating in order to assure repeatability from part to part. The basic principle of the fixture is to restrict all the degrees of freedom in the desired location and orientation.

Pins A, B, C in the above image on the base plane( a plane parallel to the plane which contains X and Y axis) restrict the rotation of the component about X axis and Y axis. It also limits the downward movement of the component along the z-axis. 1,2,3,4 and 5 degrees of freedom is restricted. Pins D, E is in the plane parallel to the plane containing X and zaxes. It prevents the rotation of the component about the Z-axis(6, 7 degrees of freedom) and the movement of the body along the Y-axis towards one direction( 8 degrees of freedom). It facilitates the loading of the component in the fixture. This three degree of freedom may be restricted after loading of component by using clamping devices.

Introduction:

• It is necessary to locate the car panel with two pins. One pin to restrict 4 way motion( 4 translational) and one pin to restrict 2 way motion(2 rotational).
• Pin should engage 3-5mm from the car panel entry surface.
• All pins should be accommodated with 2 way adjustable shims irrespective of direction of arrest.
• Round pin should be used in arresting 4 ways in the car panel if the hole is round.
• Round pin should be used in arresting 2 ways in the car panel if the hole is a slot.
• Diamond pin should be used in arresting 4 ways in a car panel if the hole is a slot.
• Diamond pin should be used in arresting 2 ways in the car panel if the hole is round.
• The axis of the car panel hole and the axis of the pin should be the same.
• The pin tolerance for geo pin will be -0.15mm and for respot pin will be -0.28mm.

There are two types of pin units:

1. Fixed pin unit: Fixed pin unit is a unit designed to locate the car panel using pins.

• Retractable pin unit: A retractable pin unit is designed to locate the car panel using pins, but the entire pin package is mounted in the retractable cylinder. The cylinder will move up and down.

The parts included in a pin unit are:

Pin:

• It locates the car panel . It can either be a make part or standard part.
• A pin should have a grinding surface on the shaft which fits into the pin retainer.
• The mounting surface of a pin should be a fine machined surface.
• All machined planes should be perpendicular to the axis of the shaft.
• Pin must have fastening provision like tapped hole from bottom or a V groove from side.
• A pin must be hardened ( heat treatment) (55 to 60 HRC) and black oxide finished in general.
• Material- Hardened material like C45, 16MNCR5, 20MNCR5, EN31 etc.
• Surface finish- Black oxide, zinc coating, chrome plating.

Pin Retainer:

It is a part which holds the pin in position, it can either be a make part or a standard part.

It should have a hole with a fit into which the pin is fixed.

The pin mounting surface should be fine machined.

It should have a step to lock rotation of the pin.

The mounting surface of the pin retainer to block should be a machined surface.

The pin retainer will be fixed to block or a L block to riser with 2 dowel holes(pins) and at least one screw hole to maintain tolerance.

L-BLOCK:

These parts are used to accommodate the shimming direction as per fundamental in general, also in some cases used for design feasibility.

Shims:

It is a standard part in general which are used for adjustment of locator( NC block and pin retainer) for obtaining the desired value for precise working of the unit wrt car panel.

Riser:

It is a part on which all the parts are mounted, this riser is mounted on the base plate.

In general riser is mostly a standard part, but it can be a make part also.

Fixed pin (round):

Fixed pin unit ( Diamond):

Conclusion:

Hence we have designed four different types of pin units

• Fixed pin - round
• Fixed pin- diamond
• Retractable round pin
• Retractable diamond pin