Week 6:- Clamp Unit & Pin-Clamp Unit Design Challenge
OBJECTIVE: To design 3 types of Swivel Clamp unit and 1 Pin-Clamp unit with different Riser, Blade and Clamp arm while maintaining the stroke of cylinder as 100mm.
INPUT:
FUNDAMENTALS:
3-2-1 principle: Our ultimate intention for using the pin unit is to fix the workpiece so that it does not deviate from the position while the operation is being performed on it. For this reason, we use the 3-2-1 principle.
Every component we know has 6 degrees of freedom. They are listed below:
1. Translational motion along the x-axis.
2. Translational motion along the y-axis.
3. Translational motion along the z-axis.
4. Rotational motion along the x-axis.
5. Rotational motion along the y-axis.
6. Rotational motion along the 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 become 12 if we consider translation along negative and rotation in anti-clockwise direction too. That means, every component can move in all directions 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 the body for obvious reasons. Thus the body must be constrained in all the major directions in order for the workman to work upon it. This constrains on motion is achieved by the 3-2-1 principle.
In the 3-2-1 principle we make use of locating pins in the given order to fix them 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 the z-direction and rotational motion along the x-axis and y-axis. Thus 3 degrees are restricted. But the body still can rotate along the z-axis and translate in the x-axis and y-axis.
2. 2 pins are located on the backside of the component. These pins constrain the translational motion along the x-axis and rotational motion along the 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 the y-axis.
FIG. LOCATING PINS
Thus we can constrain all the degrees of freedom of a body using the 3-2-1 principle which used 6 pins for the same.
When we use clamps we restrict the following motions:
1. Translational motion along the z-axis.
2. Rotational motion along the x-axis.
3. Rotational motion along the y-axis.
sign two Rest units and three Power Clamp units with different Riser, Blade, and Clamp arms.
Clamp unit: Clamp unit is a unit designed to clamp/hold the car panel with NC blocks/ mylar/ rest by using a pneumatic clamp cylinder & below are different
parts of a clamp unit in general. Usually clamp open position is 90°. Fingers are attached to clamp mylar. In clamp unit shim to one side is more enough
because it will restrict translation motion in upward & downward direction only. There is another type of clamp unit known as the swivel type. There is a pivot pin around which the clamp arm rotates.
Factors that should be considered while designing a clamp unit:
There should be adequate space between the cylinder & other parts like the riser, car panel because the cylinder moves or swing while opening & closing Pin clamp unit is used where two panels or component are located for welding.
Clamping make in such a way that it clamps two-car panels at once
All parts should snap & butt properly to each other.
Make sure that parts of the clamping unit are 10 to 12 mm away from the car panel so they do not damage the car panel.
Avoid any clash. It will not arise an issue at the assembly.
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, thesame 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.
Swivel Clamps :
The primary components of the clevis clamp include clamp arm, blade, cylinder, angle riser, locator block, and finger. Swivel clamps shall typically be designed with the center of the clamp cylinder, the center of the locator, and the center of the clamp arm on a common line. However, it is acceptable to offset the clamp point from the clamp cylinder to allow for improved access for welding and part load and unload.
The clamp arm shall be welded construction including a 20.0mm thick arm with an integral pivot to accommodate a 19.50mm wide Swivel. The female portion of the clevis shall always be part of the arm. The male portion of the Swivel shall always be part of the blade.
Below are general parts of the swivel clamp unit.
Clamp Cylinder:
Cylinder designation: In this unit, we used Clevis cylinder for clamping designated as CKG_1B_63_100Y
Nomenclature of cylinder as belows:
CKG1: Series of cylinder
B: Clevis width as 19.5 mm.
63: Bore size
100: Stroke of cylinder
Y: End bracket i.e. double knuckle joint (M6 without tap)
FIG:Clevis cylinder
Clamp arm: It is part to which clamp mylar mounted & also clamp cylinder is attached to the end of the arm with the use of clevis pin. It has two plates of 8 mm thickness welded & helps in rotating of clamp arm about pivoting point of the blade.
It shows 100 mm stroke achieved & clamp arm end in line with cylinder rod. See Y & Z values.
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.
CLAMP &REST MYLAR
1mm is the component thickness.so the distance between the clamp and rest mylar is 1mm.
Stopper:
The function of stopper is to restrict motion of clamp arm. It avoids clamp arm rotating beyond the limit. It has two parts male & female.
Blade: Blade is part to which all other units are attached for e.g. mylar, stopper, clamp cylinder, riser. Blade is designed according to position of all other parts.
Bush: Bush is friction-type bearing. It is lubricated with grease, oil. It permits relative motion between two mating parts. It is made of hard material also its ID & OD are precisely machined or grinded. It press fitted & after worn out it is replaced with new one instead of replacing other part which save cost. Mostly phosphor bronze or gunmetal material used for making bush. In this unit three bushes are used of same internal (12 mm) & external diameter (18 mm) but different length.
Pin
It locates the car panel . it can either be a make part or a standard part .
A pin should have a grinding surface on shaft with fits onto which the pin retainer is fixed .
The mounting surface of pin should be fine machined surface .
It should have step machine surface to lock rotation which matches with pin retainer .
The mounting surface of pin retainer to the block should be machined surface .
All machined planes should be perpendicular to the axis of shaft .
Pin must have fastening provision like tapped hole from bottom or a V groove from side .
A pin must be hardened ( heat treatment ) (55 to 60 HRC) and black oxide finished in general .
Material - Hardened material like C45 , 16MNCR5 , 20MNCR5 , EN31 etc .
Surface finish - black oxide , Zinc coating , Chrome plating .
Typically, locating pins are used for controlled, fine tolerance positioning of a, For example, a pallet that is moved along one axis, where the drive mechanism is not accurate and stable enough to place it in a position sufficiently accurate to execute a particular process, the use of locating pins may be the best solution.
The Diamond Shape is a critical feature that helps with machining inaccuracies and smoother locating When two round headpins are installed onto one plate, the distance between two mounting holes must be extremely precise and even then, the workpiece will not be placed on the base as easily as it would with the use of a diamond shape pin.
There are several styles of Pin Head They serve different purposes and can be utilized in a variety of applications.
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.
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.
Hardware: These are standard parts. Parts are assembled together with help of washer and bolt. for better assembly should use two dowels and four bolts for riser and blade. Where for bolting one part should have tapping hole and on other part simple hole (1 to 2 mm larger from bolt size.)
Length of the bolt is decided by width of part + (1.5*D)
For ex. where
L=Length of Bolt
D=diameter of bolt to be used
Let’s, take M8 Bolt
L= 25 + (1.5 * 8)
L=25+12
L=37 mm.
Dowel pins: These are standard part. There are various types of dowel pin like split, parallel, taper, clevis, and spring, grooved. Dowel pins are used to locate part precisely & to restrict movements. For mylar dia. 6 or 8 dowel pins should be used. For blade, shimming block dia.8 dowel pin should be used & for riser base dia. 10 or 12 pins are used.
Washer: Standard steel grade A plain normal series washer used.
Types of Clamps: Various methods of clamping are common to both jigs and fixtures. The type of clamp the tool designer chooses is determined by the shape and size of the part, the type of jig or fixture being used, and the work to be done. The tool designer should choose the clamp that is the simplest, easiest to use, and most efficient.
Strap clamps: Strap clamps are the simplest clamps used for jigs and fixtures Their basic operation is the same as that of a lever. Strap clamps can be grouped into three classes, each representing a form of lever.
Screw Clamps: Screw clamps are widely used for jigs and fixtures. They offer the tool designer almost unlimited application potential, lower costs, and, in many cases, less complex designs. The only disadvantage in using screw clamps is their relatively slow operating speeds. The basic screw clamp uses the torque developed by a screw thread to hold a part in place—either by direct pressure or by its action on another clamp.
Swing Clamps: Swing clamps combine the screw clamp with a swinging arm that pivots on its mounting stud. The holding power with this clamp is generated by the screw. The rapid action needed is achieved by the swinging arm.
Hook Clamps: Hook clamps are similar to swing clamps but they are much smaller (Figure 4–17). They are useful in tight places or where several small clamps rather than one large clamp must be used. A variation of the hook clamp is shown in Figure 4–18. The modified hook clamp is made to be operated from the opposite side of the tool. It is useful in tight places or where a safety hazard may be present if the tool is operated from the front side.
Cam-action clamps: Cam-action clamps, when properly selected and used, provide a fast, efficient, and simple way to hold work (Figure 4–20). Because of their construction and basic operating principles, the use of cam-action clamps is limited in some types of tools.
Wedge Clamps: Wedge clamps apply the basic principle of the inclined plane to hold work in a manner similar to a cam. These clamps are normally found in two general forms, flat wedges and conical wedges.
Toggle-Action Clamps: Toggle-action clamps, shown in Figure 4–29, are made with four basic clamping actions: hold down, squeeze, pull, and straight-line. Toggle clamps are fast-acting. Because of how they are made, they have the natural ability to move completely free of the work, thus allowing for faster part changes. Another advantage is their high ratio of holding force to application force.
Power Clamps: Power-activated clamps are an alternative to manually operated clamping devices. Power clamping systems normally operate under hydraulic power or pneumatic power, or with an air-to-hydraulic booster. The system used is determined by the type of power supply available. The air-to-hydraulic system is preferred because it can operate from the regular shop line pressure and no pumps or high-pressure valves are needed. The advantages of power clamps are better control of clamping pressures, less wear on moving parts of the clamp, and faster operating cycles.
Purpose of Clamp unit: The purpose of clamping tools is to securely hold the workpiece 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 (Principle 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 15deg 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, turn table 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 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 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) then select the proper blank size as per the requirement; consider 5 mm above the panel.
Select the clamping cylinder as per the pivot point distance from the PLP.Once clamp and rest block is build, start construction of mounting plate or fabrication structure as per the company standard to make the connection with the base plate. shimming is 5mm solid +5mm shim pack, In few companies 3mm shims are used. Make connection plate between rise and Mylar. Material can be C45, Teflon, Aluminium or S-Grun as per the customer requirement. Generally S-Grun mylars are made of more thickness.
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-ordinates in the particular position.
Clevis clamp Cylinders:
Clevis 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 cylinder are mechanical devices which use the power of compressed gas to produce a force in a reciprocating linear motion. Like hydraulic cylinders.
