Week 6:- Clamp Unit & Pin-Clamp Unit Design Challenge

OBJECTIVE:

To design 3 different types of swivel clamp unit with different riser, Blade and Clamp Arm for the given car Panel and design a pin Clamp unit for the given Car Panel with optimum use of material and space constraint also considering Panel Thickness as 1mm for the given car panel.

INTRODUCTION:

Clamp Unit:

A clamp unit is a unit designed to clamp/hold the car panel with NC blocks/Mylars/rest by using a pneumatic cylinder. Clamp arm, finger, Rest/backup, L-block, shims, riser, pneumatic cylinders are the typical parts of clamp unit. Clamping force should be considered while designing clamp unit. Usually clamp open position is 90^o.Fingers are attached to clamp Mylar. In Clamp unit shim to one side is more enough because it will restrict translation motion in upward direction and downward direction only. There is another type of clamp unit known as swivel type. There is pivot pin around which clamp arm rotate.

3-2-1 Principle:

A work-piece free in space can move in an infinite number of directions. For analysis, this motion can be broken down into 12 directional movements or "degrees of freedom". All 12 degrees of freedom must be restricted to ensure proper referencing of a work-piece.

The 12 degrees of freedom all relate to the central axis of the work-piece. Notice the 6 axial degrees and 6 radial degrees of freedom. The axial degrees of freedom permit straight-line movement in both directions along the 3-principle axis shown as X, Y & Z.

The radial degrees of freedom permit rotational movement, in both clockwise & counter clockwise radial directions around the same three axis.

For a rigid body in plane has 6 degrees of freedom (i.e.) The motion of a ship at sea has the 6 degrees of freedom.

• X+
• X-
• Y+
• Y-
• ZCW
• ZCCW

Now try to understand 3-2-1 principle of fixture. The purpose of Fixture is

• Resting
• Locating
• Orienting
• Clamping 

Resting:

In a fixture the work piece should rest in such a way that it doesn’t move or vibrate on the resting blocks.

Locating:

The fixture should locate the work piece such that all the operations to be performed on it are performed without any problem and all of them are performed effectively.

Orienting:

The fixture should orient the work piece such that all the operations to be performed on it are within the reach of the work piece.

Clamping:

A clamp is a fastening device used to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure. There are many types of clamps available for many different purposes. Some are temporary, as used to position components while fixing them together, others are intended to be permanent.

CLAMPING GUIDELINES:

Locating the work piece is the first basic function of a jig or fixture. Once located, the work piece must also be held to prevent movement during the operational cycle. The process of holding the position of the work piece in the jig or fixture is called clamping. The primary devices used for holding a work piece are clamps. To perform properly, both the clamping devices and their location on the work holder must be carefully selected.

Factors in Selecting Clamps:

Clamps serve two primary functions.

• They must hold the work piece against its locators.
• The clamps must prevent movement of the work piece.

Procedure of Clamping the work piece to the Panel:

• Place the work piece, supply air at clamp side without installing clamping block, operate the clamp arm to the end of clamp.
• Under the above conditions, adjust shim so that the space between the work piece and the clamping block is about 0 mm. Theoretically there is no clamping force for holding a work piece under this condition.
• In order to generate clamping force from the state described in step B, insert additional shim. The thickness of the shim differs depending on the arm length and pressure, so please refer to the graph on front matter 3 as a guide. About 10% error may occur due to the difference in tolerance of the clamp cylinder body.

Clamping Force:

Clamping Force is the force required to hold a part against the locators. Clamping prevents the part from shifting or being pulled from the jig or fixture during the machining operation. The type and amount of holding force needed to hold a part is usually determined by the tool forces working on the part and how the part is positioned in the tool. Sometimes the operation being performed is a factor. Clamping Pressure, as a general rule, should only be enough to hold the part against the locators. The locators should resist the bulk of the thrust. If a part must be clamped with great deal of pressure, the tool should be redesigned so that the tool thrust is directed at the locators and the locators and the tool body. Clamps must never be expected to hold all of the tool thrust.

Relation between shim thickness and clamping force:

When a shim that exceeds the clamping force peak plotted on the graph is inserted, the self-locking mechanism doesn't work. Insert a shim with appropriate thickness. Arm length “L” indicates the distance between the clamp arm shaft and the clamping position.

For distance “A” between knock positioning pinhole and clamp arm shaft.

Panel section creation:

Take a section at PLP either in clamp opening direction or normal direction Sections should be normal to base. Panel section through X, Y or Z direction passing through PLP. Section will help in finalizing the shape of clamp and rest block. Most importantly used for finalization of gun shape. Finalization of a base plate should consider following things before deciding the base plate. Height of maximum and minimum weld spot from the floor. No gun arm should touch the base plate Operator ergonomic while accessing all the spots.

Construction of Rest Block:

If standard Mylar has to be used then select the proper blank size as per the requirement consider 0.5 mm above the panel. As per OEM, Tier 1 standard use the different standards mounting risers. Do not forgot to add the 5mm shim. If you are combining rough locator or part sensor bracket along with the rest unit. Take necessary care means distance of rough locator from panel. In the process of manufacturing, we use Steel Aluminium, S-gurn, and Plastics. And hardening is necessary in manufacturing Mylar and Pin.

PLP Location & Clamp plan:

PLP (Principle Locating Points): Principle locating points are the points which are used for the accurate location of the panels sub-assemblies to maintain the geometry of the assembly. There are lots of holes on the panel which are not accurate due to stamping operations. So, the PLP are defined in the drawings. The PLP holes must be very much precise.

Clamp Plan: Clamp plan provides the information of location of pins, Rest units, clamp unit, Dumps etc. It is the important input from customer. The clamp plan/position should not be changed without the knowledge of customers it gives the detail of the exact position of the clamping units by mentioning a co-ordinate in the particular position.

Clevis Clamp Cylinder:

Clevis Clamp Cylinder has been designed to operate as an actuator with a clevis clamp at the end to position the final control elements in accordance with the controller signal. Clevis Cylinders are mechanical devices which use the power of compressed gas to produce a force in a reciprocating linear motion.

PARTS OF A CLAMP CYLINDER:

1) Clamp Cylinder:

In clevis clamp unit, swivel type cylinder is used. Pneumatic Cylinder are mechanical devices which use the power of compressed gas to produce a force in a reciprocating linear motion. Like hydraulic cylinders, compressed gas or a fluid forces a piston to move in the desired direction. The piston is a disc or a cylinder, and the piston rod transfers the force it develops to the object to be moved. Engineers sometimes prefer to use pneumatic because they are quieter, cleaner and don't require large amounts of space for Fluid storage. Because operating fluid is a gas and its leakage from the pneumatic cylinder will not only drip out but, contaminate the entire surroundings. Making pneumatic more desirable where cleanliness is a requirement.

Once actuated, Compressed air enters into the tube at one end of the piston and imparts force on thepiston in other direction. Consequently, the piston gets displaced.    

2) Mylars:

A Mylar is an NC block which is mounted on the clamp arm of the cylinder and it presses the car panel to the Rest Mylar, so therefore holding the car panel.

 3)Clamp Arm:

A Clamp arm is a make part, it is made as per the positioning of the clamping cylinder, Stopper and shims. Clamp arm is made from Hard material.

The distance between the pivot point and the cylinder mount should be 50mm as shown in the figure. This is necessary because cylinder stroke is 100 mm and it must be achieved when it is at 90⁰ position and clamp arm mounting hole must be in line with the cylinder rod axis as shown in figure.

Below figure shows 100mm stroke achieved and clamp arm end in line with cylinder rod.

4) Stoppers:

The function of the stopper is to restrict the movement of the clamp from going beyond the limit. Stopper is mounted between Clamp arm and Blade. Stopper has two parts, Male stopper and a Female stopper. The Male stopper is attached to the clamp arm and the Female Stopper is attached to the Blade. When these two Parts meet, they restrict the motion.

5) Blade:

A blade is a make part designed according to the position of the riser, clamp cylinder, Clamp arm, Rest Mylar and stopper.

6) Riser:

A Riser is a standard part or a make part which provides height to the pin unit, Clamp unit as per the requirements. Riser should have adequate strength so it can sustain the load of the car panel, Clamping cylinder, Blades.

7) Bush:

Bush is friction type bearing. It is lubricated with grease, oil. It permits relative motion between mating parts. It is made of hard material also ID and OD are precisely machined or grinded. It is Press fitted and after worn out it is replaced with new one instead of replacing other part which save cost. Mostly Phosphor bronze or gun metal is used for manufacturing of bushes.

Below Figure Shows the Press fitting of bushes into the Blade and Clamp arm.

8) Shim Plates:

In Fixture, we are providing tolerance to each part mostly manufactured. Those tolerances tend to stack up and Reflect together. In order to avoid that we use shims so we can easily adjust after assembly so whatever stack up there we will rectify it through shims. Shims come in thickness of 0.1,0.2,0.5, etc.

We provide shim to accommodate clamp as well as rest mylar in two directions. Clamp & rest mylar can be adjusted in one direction only i.e., up or down. Also, two-way shim provided for locating pin to accommodate adjustment.              

Purpose of a Clamp Unit:

The purpose of clamping tools is to securely hold the work piece in position without obstructing the production process. High Clamping forces combined with compact dimensions are therefore the most important requirements for a clamping tool.

Design of Swivel Clamp unit:

• Study the PLP (Principal location Points) data along with the nearby spot data.
• Depending on the angle of cross section. Mylars will be having shimming either in one direction or in 2 directions. General thumb rule if the section is inclined more than 15⁰ or clamping surface is 3D profile then 2 direction shimming are provided
• Take an overview of the working height if you're having manual welding. For robotic welding also it's important to see the working height in consideration of the robotic gun, turntable height if manual loading is done.
• Put the proper gun at the spot location. If process engineer has gun shape stick to X, C or Y type of gun. Do the proper gun study and place the gun as per the Geo or re-spot decision and operator approach. Once basic gun is placed considering the throat depth and throat gap start building the clamp and rest block at the PLP.
• Panel section creation: Take a section at PLP either in clamp opening direction or normal direction Sections should be normal to base. Panel section through X, Y or Z direction passing through PLR Section will help it finalizing the shape of clamp and rest block. Most importantly used for finalization of gun shape.
• Finalization of a base plate plane: Consider following things before deciding the base plate Height of maximum and minimum weld spot from the floor. No gun arm should touch the base plate. Operator ergonomic while accessing all the spots.
• Construction of Rest and Clamp block: If standard Mylar has to be used (Few companies make standard height and length of steel blank with few holes and dowels for clamping) they select the proper blank size as per the requirement consider 0.5 mm above the panel.
• Select the clamping cylinder as per the pivot point distance from the PLP Once clamp and rest block are built, start construction of mounting plate or lubrication structure as per the company standard to make the connection with the base plate, shimming is 2mm solid+3mm shim pack. In few companies 3 shims are used. Make connection plate between rise and Mylar Material can be C45, Teflon, Aluminium or S-Gurn as per the customer requirement Generally S Gurn Mylars are made of more thickness.

Factors to be considered while designing a clamp unit:

• There should be adequate space between the cylinder and another part like riser, car panel because cylinder moves or swing while opening and closing.
• Pin clam unit is used where two panels or components are located and clamp for welding.
• Clamp make is in such a way that it clamps two car panel at once.
• Every clamp Mylar should have rest Mylar.
• Clamp unit has shims in only in one direction.
• All parts should snap and butt properly to each other.
• Make sure that parts of the clamping unit are min 10 to 12 mm away from the car panel, so they don't damage the car panel.
• Design tree should be structured in a way that makes a part on the top and followed the standard part.
• Avoid any clash, it will not rise issue at assembly.
• The product design should be in such a way that it will be easy to make, assemble and quality check.
• We should preferably use quick acting clamps, the clamping method will not damage the work piece. The clamp should be open in such a way that clamp arm moves away from the operator.
• There must be adequate clearance for loading and unloading. So, the process becomes fast and easy. The Size variation must be accepted.
• Avoid sharp edges and corners, they can harm the operators and cause a serious injury. Provide chamfer or fillet to the corner.
• The fixture must possess sufficient rigidity and robustness. It should not vibrate as it can lead to unwanted movement of the work piece and tools.
• Must ensure security in the operation.

 Clamp Cylinders Used:

DESIGN OF SWIVEL CLAMP UNITS AND PIN CLAMP UNIT:

Design of Swivel clamp Unit1:

Design of Swivel Clamp Unit 2:                   

Design of Swivel Clamp Unit 3:

Structure of Design tree:

• A process where all the assemblies and parts are named in a specific standard Different customer have different set of standards and the structure of design tree.
• Every company, every customer has different standards of their own structural design tree.
• So, they will be giving a template on how the structuring and detailing of the design is done. And you have to follow that particular designing or structuring of the tree.
• In the design tree itself if we mentioned that particular name, wherever that part is we can easily know that this part belongs to this one or this station or this assembly on particular unit. So, this helps us to avoid errors during the design study, manufacturing assembly and inspections.
• It is an important process which helps us to identify a specific part or assembly as to which project, tool, it belongs to.
• Structuring a design tree is actually to avoid the errors, which might come during manufacturing assembly, or either in house inspections as well.
• If the part, or assembly are not named properly, or design tree is not structured properly, it will create confusion to understand the design and may create major possibilities of making an error.
• Every part, every component should be named according to their procedures.
• If we did a mistake or renamed the entire standard parts from the customer as a design part. Now the customer is going to manufacture the entire part again from the scratch, but he already has that product in his itinerary. But extra cost extra time and is feasible. This is also known as an error which is actually going to increase the cost of the entire assembly as well
• Naming a fixture or a unit or a part gives the information as to which project zone or assembly do, they belong.
• If we name the part properly or name the fixture properly ill actually tell which unit or which assembly or which zone or in which station it actually belongs
• It can help us to identify the quantity of items of same types or different types that are being used in an assembly.
• While starting of designing a fixture, the first unit will be the base unit.
• In a Design tree, all the manufacturing parts should be first and the standard parts should be at the last.
• Design tree helps to study or understand the parts in the assembly easily.

Assembly of Swivel Cylinders:

CONCLUSION:

Hence as per the objective, the three different swivel clamp units and Pin Clamp unit have been designed by considering the Clamping forces acting at each end of Mylars. Also, the parts of clamp unit, purpose of clamp unit,3-2-1 principle achieved by clamping points and Clamp plans are briefly explained in detail.