Week 4:- Pin Unit Design Challenge

Aim: To design Fixed pin units with different risers and pin retainers, Retractable pin units with different types of risers, blades, pin and blocks and to understand design fundamentals, procedure for pin unit of a 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).while assembling a car several processes have to be performed on it at part level so that it is fit for the roads. The process that is performed at the part level can be summarized in one word as joining processes. And to precisely do this, operations on a car panel requires the panel to be held properly at the same position so that the robot can work upon it. To hold the panel in position we use various fixtures which are an assembly of various units like rest pin units, fixed pin units, retractable pin units, rough locator units, sensor units, base unit etc. This project focuses on the design of fixed pin unit and retractable pin unit with design considerations which are usually taken into account while making a fixture.

3-2-1 principle?

It is also known as a six-pin or six-point location principle. In this, the three adjacent locating surfaces of the blank (workpiece) are resting against 3, 2, and 1 pin respectively, which prevent 9 degrees of freedom.

The 3-2-1 principle is a Component holding principle where three pins are located on the 1st principle plane, i.e. either XY, YZ, ZX. And two pins are located on the 2nd plane which is perpendicular to the 1st plane, and at last one pin on the plane which is mutually perpendicular to the 1st and 2nd planes. By using this method Component can be arrested in all six degrees of freedom.

to follow this principle, one has to completely arrest all DOF of a given part.

Translational & rotational position along the X-axis

Translational & rotational position along the Y-axis

Translational & rotational position along the Z-axis

These six coordinates are known as the six degrees of freedom of a three-dimensional object. The translational and rotational positions can vary in either direction concerning each of the three axes.

To completely prevent movement, all six degrees of freedom must be restricted.

So, to arrest each DOF Fixtures are used to contain the part for working.

FIXED PIN UNIT

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

The parts of fixed pin are:

1. Pin
2. Pin retainer
3. Shim/Spacer
4. Block
5. Blade or Adapter plate
6. Riser

RETRACTABLE PIN UNIT

A retractable pin unit is a unit designed to locate the car panel using pins,but the entire pin package parts will be mounted on the retractable pneumatic cylinder.

1. Locating pin
2. Retainer (optional depending on the cylinder being used)
3. L-block
4. Shim pack
5. Blade (optional depending on the design)
6. Cylinder
7. Riser

Types of retractable pin:

• Single rod
• Double rod

FUNDAMENTAL OF PIN UNIT

• It is necessary to have two pins to locate a car panel.One pin must arrest 4 ways( 4 translational) and the other must arrest 2 ways ( 2 rotational) of a car panel.
• The axis of a car panel hole and 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 respot pin is -0.28mm.

1. Pin should engage 3-5 mm from the car part entry surface.
2. All pins should be accomodated with 2 ways adjustable shims irrespective of direction of arresting.
3. Round pin should be used in case of 4 wats arresting of car part if the hole is round.
4. Round pin should be used in case of 2 way arresting of car part if the holes is slot.
5. Diamond pins can be used for 4 ways arresting of car parts only if the hole is round
6. Diamond pins can be used for 2 way arresting of car part only if the hole is slot.

PIN

• It is used for the location of the panel.
• Typically, the locating pins are used to locate a body part in a position sufficiently accurate to execute a welding process.
• The body part is constrained by a set of appropriately locating pins so total constraint by locating pin should be fully constrained at all times to prevent any movement.
• There are dozens of styles of locating pins that can be utilized in a design.
• A pin can either be a make part or a standard part also. Below I have shown the commonly used pins.
• One of the most common pins is a stepped pin with either a small or large head.

Cylinderical Pin

Diamond Pin

CONSIDERATION

• Pin should have a grinding surface on the shaft which fits on to which the pin retainers will be fitted.
• The mounting surface of the pin should be a fine machined surface.
• A pin should have a step machine surface to lock the rotation, which matches with the pin retainer.
• The mounting surface of the pin retainer to the blocks should be machined.
• All machined planes should be perpendicular to the axis of the pin shaft.
• Pin must have a fastening provision like a tapped hole from the bottom or a v- grove from the side.
• A pin must be hardened ( 55 - 60 HRC) & black oxide finish in general.

PIN RETAINER

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

Pin retainer should have a hole with a fit, on to which the pin will be fixed.

The pin mounting surface of the pin retainer should have a step to lock the rotation of the pin.

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

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

CONSIDERATION

A pin retainer is a part which holds the pin in position, the pin retainer can either be a make part or a stand Pin retainer should have a hole with a fit, on to which the pin will be fixed.

• The pin mounting surface of the pin retainer should be a fine machined surface.
• The pin mounting surface of the pin retainer should have the step to lock the rotation of the pin.
• The pin mounting surface of the pin retainer to the blocks should be a machined surface.
• The pin retainer will be fixed to a block or a L - block to the riser with 2 dowel holes & at least one screw hole to maintain the tolerance and part of the pin retainer is used to hold the locating pins firmly.

BLOCK/ L-BLOCK

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

SHIMS

• Shim is generally used to adjust the design to manufacturing deviation.
• In general, a shim is a thin and often tapered or wedged piece of material, used to fill small gaps or spaces between objects
• Shims are typically used in order to support, adjust for better fit, or provide a level surface.
• Shims may also be used as spacers to fill gaps between parts subject to wear.
• Shims are the small parts which comes with the thickness of 0.1mm,0.2mm, 0.5mm,1.0mm etc.
• These will compensate the stack up tolerance which reflects at the mylars.
• We give tolerance to all parts of the fixture. Due to this tolerance the mylars will either lift up moves down from its defined position.
• To adjust the mylars for getting the defined position we use shims.
• Shims will be added above the mylar for adjusting according to tolerance.

APPLICATION

• In automobiles, shims are commonly used to adjust the clearance or space between two parts. For example, shims are inserted into or under bucket tappets to control valve clearances. Clearance is adjusted by changing the thickness of the shim.
• In assembly and weld fixtures precision metal shims are used between two parts so that the final production parts are created within the product drawing's specified tolerance.

Blade:

1. Blades are the plates which are attached to the riser and other components like cylinder, arm etc are mounted on it.
2. It can either be a standard part or a make part.

RISER

Riser is a manufacturing part used to raise/height the assemblies of fixtures. One end of the raiser will be attached to the base unit while the other end will be attached to the Blade. It is a part on which all the parts of a unit are mounted on the base plate of a tool. In general, the riser can either be making a part or standard part.

Raiser can be of two types. They are,

• "L' type risers.

• Square tube risers.

DESIGN METHODOLOGY

1. The project is t0 be titled with all the parts in the feature tree of the CATIA must follow the same process for easy identification of parts in the assembly.
2. The centre of the hole is taken and the pin outline is moulded as per the standard.
3. If there are any standard parts, it is advisible to use the standard parts as many as possible.
4. Retainer are mainly used to accomodate the shimming directions as per fundamental in general. Also in some cases used for other design feasibility
5. The block are modelled in such a manner that the pin can be adjusted in logitudinal and translational direction.
6. A 5mm space is provided betwwen the block and retainers as the shims or spacer blocks are present.
7. The dowel holes are present in all the parts of the pin unit which are to be assembled with tolerance should be designed with 2 dowels and at least of 1 to 6 threaded /free holes and to be fine machined for the mounting surface with 3 to 5mm step machining.
8. The blade is a plate which connects the riser to the block,usually the blades are standard parts if needed can be designed.
9. The riser is a part on which all the parts of a unit are mounted and is mounted on the base plate of a tool. In general, the riser can be either be make part or a standard part

Conclusion : 4 different type of Pin units have been designed by using CATIA V5