Challenge

Pin Unit Design

VVinayak Amle27 Aug 2021

Aim: To design different types of Fixed pin units and Retractable pin units with different types of risers, blades, pin and blocks to understand design fundamentals and procedure for pin unit of BIW fixture.

Introduction:

Pin unit is a unit designed to locate car panels using pins. It consists of riser, blade, block, pin retainer, pin, shims or spacers and cylinders (in case of retractable pin unit).

As by the 3-2-1 locating principle, the introduction of two pins helps to arrest the 6 degrees of freedom (4 axial & 2 radial movements) whereas the other 6 degrees of freedom are arrested by introducing a minimum of three supports.

There are two types of Pin units; Fixed pin unit and Retractable pin unit. Fixed pin unit have pin fixed on the unit, whereas in retractable pin units, a retracting cylinder is used which moves the pin up and down.

Different parts of Pin unit:

- Pin:

Pins are used for accurately locating the part to welded in an assembly. The holes in the part of the assembly are inserted on these pins. The pins constrain the degrees of freedom of the part of the fixture. The diameter of the pins has tighter tolerances as compared to the diameter of the hole in the part.

- Pin Retainer:

The pin retainer is a part that holds the pin in position, it can either be a make part or a standard part. Pin retainer should have a hole with a fit, onto which the pin will be fixed. The pin mounting surface of the pin retainer should be a machined surface. The step on the surface is provided to lock the rotation of the pin. It will be fixed to a block or an L-block with 2 dowel pin holes and at least one screw hole to maintain the tolerance.

- Shims:

Shims are small parts of the thickness of 0.1,0.2,0.3 mm. The shims are used to compensate for the difference due to the stack-up tolerance of the assembly, which eventually results in the improper locating of the panel. Shims are used at L blocks, pin retainer, or risers to compensate tolerance in the linear direction of the pin.

- Blocks:

Block is used to provide a 2 directional adjustment of pins with the help of shims. L-Blocks are most commonly used in general. It mainly acts as a connection between pin retainer and blade or riser.

- Risers:

A riser is a part that is used to raise the height of the fixture. The riser is fixed to the base of the fixture and the blade is mounted on the riser.