Week 4:- Pin Unit Design Challenge

AIM:-

To design the fixed pin unit and the retracting pin unit for locating a biw panel in a fixture.

pin unit is a unit designed to locate the car panel using pin. Pin unit consist of riser, blade, block, shims or spacer, pin retainer & pin. pin unit is the part of the 3-2-1 locating principle of fixture design. It restricts 6 degrees of freedom of the part.

INTRODUCTION:-

FIXED PIN UNIT:-

Fixed pin units are used in a fixture to locating the car panel during any operation.

RETRACTABLE PIN UNIT:-

Retractable pin unit is opposite to fixed unit which can be moved up and down up to a certain distance.

Its used for unloading complex panels after welding.

THE PURPOSE OF THE PIN UNIT:-

The fixed pin unit is used to arrest the 6 degrees of freedom of the assembly of a fixture.The other six degrees of freedom.

Pins help to locate the part of an assembly very accurately for welding operation.

The basic parts of the pin unit are:-

• Pin
• Pin Retainer
• Shims
• Blade
• Riser

Fundamentals of pin unit design

it is necessary to have 2 pins to locate a car panel & here one pin must arrest 4 way(4 translational) & other must arrest 2 way(i.e. 2 rotational) of a car panel.

The axis of the car panel hole & the axis of the pin should be same (in general)

The pin tolerance for a geo pin will be of closer tolerance i.e. -0.15mm & for re-spot pin is -0.28mm

1)Pin should engage 3 to 5 mm from car panel entry surface.

2) All pins should be accommodated with 2 way adjustable shims irrespective of direction of arresting.

3)Round pins should be used in case of 4 way arresting of car part if the hole is round.

4) Round pins can be used for 2 way arresting of car part only if the hole is slot.

5)Diamond pins should be used in case of 2 way arresting of car part if hole is round

6)Diamond pins should be used in case of 4 way arresting of car part if hole is slotted.

There are 2 types of pin units

1) Fixed pin unit :In this type of unit the pin is fixed by means of pin retainer & riser.

2) Retractable pin unit- In this type of unit there is the provision of moving of pin in up-down direction.

1)Pin:

Pin is the part which locates the car panel. It can be a make part or standard part

1. a) Pin should be grinding surface on shaft with fits on to which the pin retainer will be fitted.
2. b) The mounting surface of the pin should be a fine machined surface.
3. c) The pin should have a step machine surface to lock the rotation, which matches with the pin retainer.
4. d) All machined planes should be perpendicular to the axis of the pin shaft.
5. e) Pin must have a provision like tapped hole from the bottom or a v-groove from the side.
6. f) A pin must be hardened (Heat treatment) & black oxide finished in general.

2) Pin Retainer:

A pin retainer is a part which hold the pin in position, the pin retainer can either be a make part or a standard part.

1. a) Pin retainer should have a hole with a fit, on to which the pin will be fixed.
2. b) The pin mounting surface of the pin retainer should be a machined surface.
3. c) The pin mounting surface of the pin retainer should have a step to lock the rotation of the pin
4. d) The pin mounting surface of the pin retainer to the blocks should be a machined surface.
5. e) The pin retainer will be fixed to a block or a L block to the riser with 2 dowel holes (pins) & at least with one screw hole to maintain the tolerance.

3) Block/L Block:

These parts are mainly used to accommodate the shimming directions as per fundamental in general. Also in some cases used for other design feasibility.

4) Shims/Spacers:

Shims & Spacers are some standard parts in general,which are used for adjustments of locators(NC Blocks & pin retainers) for desired values for the precise working of a unit with respect to car part. In general to adjust the design to manufacturing deviation.

5) Blade:

Blade is part like a plate, which connects the parts on a unit to a riser. In general the blade can be either a make part or a standard part.

6) Riser:

It is a part on which all the parts of a unit are mounted & is mounted on the base plate of a tool.

7) Dowel holes & machined surface:

All make parts which are to be assembled should be designed with 2 dowel holes & at least of 1 to 6 threaded/ free hole(depends on size) 7 to be fine machined for the mounting surface with 3 to 5 mm step machining.

Fundamental:-

3-2-1 principle:-

Our ultimate intention for using pin unit IS to fix the work piece so that it does not deviate from the position while the operation is being performed on it For this reason we use 3-2-1 principle.

Every component we know has 6 degrees of freedom they are listed below:

Translational motion along x-axis,

Translational motion along y-axis.

Translational motion along z-axis.

Rotational motion along x-axis.

Rotational motion along y-axis.

Rotational motion along y-axis.

Degrees of freedom can be simply understood as the direction in which any component cam move .These 6-degrees becomes 12 if we consider translation along negative and rotation in anti-clockwise direction too. That means, every component can move in all the direction when it is free But processes like welding, stamping or others and in fact every ether cannot be carried out on such free to move body for obvious reasons. Thus the body must be constrained in all the major direction in order for workman to work upon it. This constrain on motion is achieved by 3-2-1 principle.

In 3-2-1 principle we make use of locating pin sin the given order to fix if on a fixture

1.3 pins are located on the bottom side of the component. These pins constrain the translation motion of the body in Z direction and rotational motion along x-axis and y-axis. Thus 3 degrees are restricted. But the body still can rotate along z-axis and translate in x axis and y-axis.

2.2 pins are located on the back side of the -Component. These pins constrain the translation motion along x-axis and rotational motion along Z-axis Thus we have constrained 5 degrees in total. But translation along Y axis is still possible.

1 pin located on the adjacent side of the 2 pins we can constrain the last degree that is translation along Y axis.

Thus we can constrain all the degrees of freedom of a body using 3-2-1 principle which used 6 pins for the same. The video explains the 3-2-1 principle