Week 5:- Rest Unit & Clamp Unit Design Challenge

REST UNIT AND CLAMP UNIT DESIGN

OBJECTIVE: To design two Rest units and three Power Clamp units with different Riser, Blade and Clamp arm.

ABSTRACT: While a car is in its production stage, several joining processes and other sheet metal processes are carried out on each panel before assembling them to get an entire vehicle fit for market. These processes are carried out in an automation line where different robots would perform different operations on the panel. Hence it is crucial for the panel to be located without any kind of movement for the precision operation. This purpose is achieved by use various fixtures which is 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 and study of Rest units and Clamp units with few brought out parts and few make parts.

SOFTWARE USED: CATIA V5

FUNDAMENTALS:

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:

1. Translational motion along x-axis.
2. Translational motion along y-axis.
3. Translational motion along z-axis.
4. Rotational motion along x-axis.
5. Rotational motion along y-axis.
6. Rotational motion along y-axis.

FIG. DEGREE OF FREEDOM

Degrees of freedom can be simply understood as the direction in which any component can 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 other 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 the 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 pins in the given order to fix it on a fixture:

1. 3 pins are located on the bottom side of the component. These pins constrain the translational 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 translational 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.
3. 1 pin located on the adjacent side of the 2 pins we can constrain the last degree that is translation along y-axis.

FIG. LOCATING PINS

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: https://www.youtube.com/watch?v=NVTEKYFMHLU

When we use clamps we restrict following motions:

1. Translational motion along z-axis.
2. Rotational motion along x-axis.
3. Rotational motion along y-axis.

REST UNIT:

• A rest unit is a unit which is designed to support or rest the car panel on it.
• Its sole purpose is to rest the panel on to it and thus no force or clamping is done.
• But as it is simple design, it can be combined with other units to reduce the cost of overall project.
• For example it can be used along with locating unit to locate and rest the panel or with clamp unit to clamp and rest the panel.
• Or the rest unit, locating unit and clamp unit can be used together in one single project.

Components of Rest unit:

1. NC block
2. Spacer/ Shim pack
3. Blade
4. Riser

CLAMP UNIT:

• Clamps are versatile tools that serve to temporarily hold work securely in place.
• A Clamp unit is a unit designed to clamp/ hold the car panel at a certain location.
• Its purpose is to hold the panel at that place with the application of force.

Components of Clamp unit:

1. Clamp Mylar
2. Rest Mylar
3. Shim pack
4. Blade
5. Riser
6. Pneumatic clamp cylinder
7. Clamp arm

CLAMP PLAN:

• The customer provides the clamp plan which consists of the position of the clamp, pins, rests and dump etc.
• It has the exact coordinated of these components and in addition it also has the hole’s dimensions and tolerances.
• As this is a critical input data, it must not be modified.
• In case the spot for weld and the clamp position is nearby and it may create problem later and there is need to change the position of the clamp, the same must be done so with the permission of the customer as it will be easy for the customer to make changes from his side also.

NC block/Mylar:

1. NC blocks or popularly also known as Mylar is the ones which hold the panel.
2. It prevents the panel from deforming while welding.
3. They always come in pairs.
4. Top Mylar is known as clamping Mylar and bottom Mylar is known as resting Mylar.

Few points to remember about Mylar:

• Generally Mylar clamping block clamp cross section (the area which in contact panel) is considered as 20mmx20mm.
• Most of the OEM's /Tier 1 prefer Mylar to have 5mm allowance material for machining. Most of Mylar are CNC machined if it is clamping 3D profile.
• Depending on the angle of cross section, Mylar will be having shimming either in one direction or in 2 directions. General Thumb rule is if the section is inclined more than 15 deg or clamping surface is 3D profile then 2 direction shimming is provided. The reason being simple mathematics. Equation of inclined line/ plane will be having 2 parameters to controls and we control with shimming in 2 directions. Less than 15 deg does not mean that there are not 2 parameters. These are their but they are neglected if angle is less than 15 deg.
• Clamping or holding of Mylar depend upon various OEM or Tier 1 stand. Various options are available. 2 screws and 2 dowel OR 1 screw and 2 dowels. Size of dowel or screw selected as per the customer input or existing experience. Mostly M10 screw, Diameter 8mm dowel or M8 and Diameter 6mm dowels are selected.
• Tolerance of dowel is made as per the customer or Tier 1 guidelines. Keep simple rule in mind, if you are removing Mylar then should have loose /sliding fit. D10 or F8 depending upon guidelines.
• Mylar’s material can be C45, Teflon, Aluminum or S-Grun as per the customer requirement. Generally, S-Grun Mylar are made of more thickness

 Shim:

• Shims are washers or thin strips of material used to align parts, make them fit or reduce wear. 
• Shims are designed to fit in between blocks and fixtures to provide support or reach a dimension required in the assembly. 
• Typically designed in a U shape matter so that they can fit between mounting screws. This shape helps to make them easy to install and remove or replace. 
• Designers will use shims individually or as packs of 5, 6, or 7 pieces. Thicknesses range from 0.25mm to 2mm.
• Many assemblies require a tolerance to be hit in relation to a length or height; shims allow designers to hit these tolerances. They can be stacked between NAAMS NC blocks, L blocks, or pin retainers.
• They come in a variety of sizes and styles. The material is typically very thin so that they can accommodate smaller to larger tolerances. 
• NAAMS specification for the material is an SAE cold rolled steel. 
• Different widths of the shims can have different surface treatments either yellow zinc dichromate, black oxide, or none. Yellow zinc dichromate surface treatment is used for corrosive resistance and acts as an anti-rust primer. Black oxide coating is also for corrosion resistance.
• Generally shim is used as a pack with 2*0.5mm, 2*1mm, 2mm shims.

Riser:

• Riser is a structure generally used to mount the toggle clamp and rest, clamp Mylar and location pin and to give height to the assembly.
• It is not necessary to have one riser where all the parts of clamp or rest or location units are mounted.
• Risers can either be a standard part or make part.

Blade:

• Blade is a plate which is attached to the riser and other components like cylinder, arm etc are mounted on it.
• It can either be a standard part or a make part

Standard parts/Bought out parts:

1. Fasteners
2. Clamp Cylinder
3. Clamp arm

Fasteners: For this project ISO 4762 Hexagonal socket head cap screw were used with varying diameter and length. Mostly M8 screws with varying lengths are used but to fasten the riser to the base plate M10 or M12 are used. The length varies with respect to the thickness of the part that is to be fastened. This length is calculated by the formula:

Length of the screw= 1.5d+ distance between the start of surfaces to be fastened

For example in the below image, we are to fasten riser to the L-block. Distance between their surfaces is 25mm and we are using M8 screw.

So the length of the screw will be= 25+1.5*8 i.e. 37mm. But standard available size is 40mm so we used that. And similar process was followed everywhere.

FIG. STANDARD FASTENER (ISO 4762 Hexagonal socket head cap screw)

Clamp cylinder: A clamp is a pneumatic device which works with pressurized air. It is used to open and close the parts fixed to the clamp arm fixed to the clamp body.

• They are in general of 40, 50, 63 and 80 bore capacity.
• Horizontal clamp mounting opens up to 135deg.
• Vertical clamp mounting opens up to 105deg.

V 50.1, 63.1, 80.1 BR3 Vario Clamp: Vario clamp with toggle joint mechanism and continuously adjustable opening angle, Over-centre-lock, Enclosed aluminum housing, Oval-shaped cylinder in the sizes 50, 63, 80, Forked clamping arm, standard hole pattern for adapting contour blocks, Continuously adjustable opening angle 5°-135°, optional: Weld immune inductive sensing of closed and opened position (T12).

FIG. STANDARD CLAMP CYLINDER

VU_63_BR2_A05_T12_135_GRAD_TUENKER: Vario clamp, Naams version, for clamping of medium and large body panels with stepless adjustable opening angle, integrated toggle lever mechanism, self-holding type. Enclosed aluminum housing, flat oval cylinder in sizes 50, 63, 80. Fork shaped clamp arm with standard hole pattern for receiving contour pieces. Opening angle variable from 5 ° -135 °. Options: - Inductive sensing, weld immune for open and closed position (T12) - Hand applied version.

FIG. STANDARD CLAMP CYLINDER

U_63_BR5_Z_A00_T24_3_TUENKERS: Variable high-performance clamp for clamping of medium and large-sized car body components with continuously adjustable opening angle and new force optimized knee lever mechanic. With the same force as a Vario Clamp, smaller cylinder diameters are used and so the air consumption is reduced by 30%. Enclosed aluminum housing, oval-shaped cylinder in the sizes 50, 63 and 80. Fork shaped clamp arm with standard hole pattern for adapting contour blocks. Continuously adjustable opening angle in the range of 5° to 135°. New two-parts switch for easy replacement.

FIG. STANDARD CLAMP CYLINDER

Clamp arm: Clamp arm is a bought out part, to which all the movable parts are fixed. In general a clamp arm is a part of clamp cylinder.

• Clamp arms are different for different bore of cylinder
• Clamp arms are available in different hole pitch
• Clamp arms are available as:

Middle arm

FIG. STANDARD CLAMP ARM (MIDDLE)

Right arm

FIG. STANDARD CLAMP ARM (RIGHT)

Left arm

FIG. STANDARD CLAMP ARM (LEFT)

FINAL MODEL

CONCLUSION: Designed models of rest unit and clamp unit with different Mylar, Riser and Clamp cylinder.