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

BJECTIVE:

To design 3 different types of power clamp unit with different riser, blade and clamp arm & to design 2 Different types of rest Unit for the given Car Panel.

ABSTRACT:

In this project I have designed a different types of power clamp unit by validating a power clamp cylinder and by considering a clamping force acts at each end of the mylars and also designed a different types of rest unit by considering a holding force for the given car panel. Here by considering the fundamentals of clamp unit design I have briefly explained about the,

• Colour codes used in the fixture
• Purpose of clamp unit.
• (3-2-1 princple) achieved by clamping.
• PLP points (Principle locating points)
• Clamp plans
• Structured design tree.
• How to validate a power clamp cylinder?
• How to calculate the clamping force acting at the end of each mylars?
• How to calculate the holding force?
• Materials, surface finish & Heat treatment used in the parts of clamp unit.

SOFTWARE USED:

CATIA V5

COLOUR CODES IN BIW FIXTURE:

Figure 1. Colour codes in fixture

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.

REST UNIT:

• A rest unit is used to support the car panel, Also it resists the clamping force of the finger with the help of rest mylars & thus clamp the car panel.
• Rest unit consist of rest mylars, shim packs, riser.
• Holding force should be considered while designing a rest unit.
• Mostly to avoid excess part, to reduce the weight & to reduce the cost of the fixture the rest unit is combined with the clamp unit.

Below figure shows the combination of clamp & rest unit which have been designed in this project by considering the basic principles of clamp & rest unit design.

Figure 2. Clamp & rest unit with Coordinates

TYPES OF CLAMP UNITS:

• Power clamp unit.
• Swivel cylinder clamp unit.

In this project, I have used power clamp unit to design.

PURPOSE OF CLAMP & REST 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. For this reason, clamps are also equipped with mechanical transmission gear units.

(3-2-1 princple) achieved by clamping & resting :

• The best & most cost effective method of part location is referred to as the 3-2-1 method.
• Fixtures works on of 3-2-1 principle.
• (3) Minimum : Three locator blocks to establish part plane. Three locators or supports are placed under the work piece. Three locators are usually positioned at the primary locating surface.
• This restricts two axial movement downward & upward and four radial movements. Together, the three locators restricts six degree of freedom.

Figure 3. showing the 3-2-1 method achieved by claming & resting

As by 3-2-1 method, by introducing minimum of 3 clamping & 3 resting blocks will restrict 6 degree of freedom (DOF) [Two axial movement & four radial movement].

CLAMPING: The process of holding the position of the workpiece in the jig or fixture is called clamping. The primary devices used for holding a workpiece are clamps/Clamp unit.

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 informtion 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.

Figure 7. PLP(Principle locating Points)

The above figure shows the PLP points & the coordinates of clamps which will be mentioned in the clamp plan given by the customers.

RENDER IMAGE OF FIXTURE:

Figure 8. Render image of fixture (Isometric view)

Figure 9. Render image of fixture (Front view)

Figure 10. Fixture with panel

Figure 11. Fixture without panel

Figure 12. Render image of fixture

Figure 13. Render image of fixture

FORMULA & CALCULATION:

1. Validate or selecting a power clamp cylinder.
2. clamping force acts at the end of each mylars.
3. Holding force.

1. Validate or selecting a power clamp cylinder:

• I have designed Selection of clamps on the basis of outreach & sheet thickness. Below i have shown how to choose sheet thickness & how to find out the outreach of clamp arm.

1. How to choose sheet thickness ?

1. The thickness of panel is said to be considered as 1mm for this project.
2. Since, the child parts butt on below the main panel I have considered 2mm of sheet metal thickness from the below graph which will be used to select the cylinder by its bore diameter.

ii. How to find out the outreach of clamp arm ?

By measuring the distance between pivot centre of cylinder and the centre of clamping mylars we can get the outreach of clamp arm as shown in below figure.

Figure 14. Length of clamp arm

Below given details are the outreach of the clamp arm taken from the respected clampunit used in the project,

1. P001_ST02_Z20_04_00_Powerclamp unit - Outreach of the clamp arm is 183 mm.
2. P001_ST02_Z20_05_00_Powerclamp unit - Outreach of the clamp arm is 208 mm.
3. P001_ST02_Z20_06_00_Powerclamp unit - Outreach of the clamp arm is 120 mm.

After choosing the sheet thickness and the outreach as shown above, now we can refer the graph shown below for selecting a suitable power clamp cylinder as per our requirement.

Figure 15. Graph - Sheet thickness Vs Outreach

By relating the sheet thickness & outreach from the above graph, we can find the bore capacity of clamp cylinders. Based on the bore capacity the cylinders will be selected for the clamp unit.

I have compared my clamp unit design with the above graph, as per the relation between the graph & the design I have found the bore capicity of cylinders which is been used to design the clamp unit.

As from graph, The selected bore capacity of clamp cylinders for three different types of clamp unit used in this project are shown in the below figure,

Figure 16. Table - Bore capacity

2. Clamping force acts at the end of each mylars:

i. Clamping force - Analytical Method:

General procedure when choosing a clamp:

A. Definition of the required clamping force a the particular component
points:

• Sheet thickness (s)
• Quality of metal sheet (e.g. ST quality)

clamping force (Fs) = 48 x s³ = Force in Newton

Figure 17. Shows the sheet thickness (S)

In this project, we consider the sheet thickness as 2mm so analytically the clamping force will be,

clamping force (Fs) = 48 x 2³ = 48 x 8 = 384 =Force in Newton

clamping force (Fs) = 384 N (Analytically)

B. Determination of the clamping torque required “Ms“:

• Outreach component / clamping arm length “l”

clamping torque (Ms) = Fsx l

Figure 18. Shows the length of the clamp arm (l)

In this project,

1. For P001_ST02_Z20_04_00_Powerclamp unit,Ms= Fsx l = 384 x 183 = 70272 Nmm = 70.272 Nm= 70 Nm

Clamping torque at P001_ST02_Z20_04_00_Powerclamp unit is 70 Nm (Analytically).

2. For P001_ST02_Z20_05_00_Powerclamp unit,Ms= Fsx l = 384 x 208 = 79872 Nmm = 79.872 Nm = 80 Nm

Clamping torque at P001_ST02_Z20_05_00_Powerclamp unit is 80 Nm (Analytically).

3. For P001_ST02_Z20_06_00_Powerclamp unit,Ms= Fsx l = 384 x 120 = 46080 Nmm = 46.080 Nm = 46 Nm

Clamping torque at P001_ST02_Z20_06_00_Powerclamp unit is 46 Nm (Analytically).

Figure 19. Table - clamping force (Analytical method) & clamping torque.

C. Selection of clamping tool that generates this clamping torque with an assumed reliability of 30 %.

ii. Clamping force - Graphical method:

• Clamping force (Fmax) produced by the cylinders:

Already we have found the bore capacity of the clamp cylinder & length of the clamp arm (outreach) above. With the help of this details we will get maximum clamping force by relating the inputs with the below shown graph.

Figure 20. Graph - Clamping force against length of the clamp arm

Below given details are the clamping force taken from the respected clampunit used in the project,

1. P001_ST02_Z20_04_00_Powerclamp unit - Maximum Clamping force 800 N.
2. P001_ST02_Z20_05_00_Powerclamp unit - Maximum Clamping force 700 N.
3. P001_ST02_Z20_06_00_Powerclamp unit - Maximum Clamping force 1000 N.

Figure 21. Table - Bore capacity & clamping moment

• Clamping force at each end of mylars:

1. P001_ST02_Z20_04_00_Powerclamp unit:

• From the above graph, we get the clamping force of 800 N for 183 mm clamp arm length & bore capacity 50 at P001_ST02_Z20_04_00_Powerclamp unit.
• Since, I have used two clamping mylars parallelly of equal distance, the clamping force acts at the end of each mylar in this unit will be,

Clamping force acts at the end of each mylar = Clamping force by the cylinder (refered from graph) / No. of mylars used per unit

P001_ST02_Z20_04_00_Powerclamp unit - Clamping force 800 N. (As refered in graph)

No. of mylars used per unit = 2

800/2 = 400 N

Clamping force acts at the end of each mylar at P001_ST02_Z20_04_00_Powerclamp unit = 400 N.

2. P001_ST02_Z20_05_00_Powerclamp unit:

• From the above graph, we get the clamping force of 700 N for 208 mm clamp arm length & bore capacity 50 at P001_ST02_Z20_05_00_Powerclamp unit.
• Since, I have used U - shaped clamping mylars in straight, the clamping force acts at the end of each mylar in this unit will be,

Clamping force acts at the end of each mylar = Clamping force by the cylinder (refered from graph) / No. of mylars used per unit

P001_ST02_Z20_05_00_Powerclamp unit - Clamping force 700 N. (As refered in graph)

No. of mylars used per unit = 2 (Since, it is U-shaped)

700/2 = 350 N

Clamping force acts at the end of each mylar at P001_ST02_Z20_05_00_Powerclamp unit = 350 N.

3. P001_ST02_Z20_06_00_Powerclamp unit:

• From the above graph, we get the clamping force of 1000 N for 120 mm clamp arm length & bore capacity 40 at P001_ST02_Z20_06_00_Powerclamp unit.
• Since, I have used two clamping mylars parallelly of equal distance, the clamping force acts at the end of each mylar in this unit will be,

Clamping force acts at the end of each mylar = Clamping force by the cylinder (refered from graph) / No. of mylars used per unit

P001_ST02_Z20_06_00_Powerclamp unit - Clamping force 1000 N. (As refered in graph)

No. of mylars used per unit = 2

1000/2 = 500 N

Clamping force acts at the end of each mylar at P001_ST02_Z20_06_00_Powerclamp unit = 500 N.

Figure 22. Table - Clamping force acts at each end of the mylar

3. Holding force:

• Holding force depending on load offset :

Figure 23. Graph - Holding force depending on load offset

From the above graph, by relating the load offset in the respected bore size i have found the holding force.

Figure 24. Load offset

MATERIALS USED:

In general, the materials used for Mylars, Clamp arm, shims, shim block, blades & risers are:

1. Mylars : UHMW, Nylon66, PP, Teflon (polytetrafluroethylene (PTFE). Materials of clamp block and rest block to be decided as per the A class panel or B class.
   Most of exterior panel holding Teflon material is used.
   Other wise steel is used with heat treatment.
   Sometimes Aluminium blocks are also used.
2. Clamp arm : SAE 4140 or SAE 1045.
3. Shims/spacer : S.A.E. 1010 CRS FULL HARD, COLD ROLLED STRIP STEEL S.A.E. 1010 FULL HARD, S.A.E. 1020 HRS, etc.
4. Shim block : STEEL ASTM A-36, NM Stainless, STEEL ASTM A-36, SS type 303 or 304, etc.
5. Blade : STEEL ASTM A-36, etc.
6. Riser : STEEL ASTM A-36, etc.

HARDNESS:

In general, the hardness of the materials for clamp arms, shims, shim block, blades & risers are:

• 55 - 60 HRC / 58 - 62 HRC to a depth of 0.5 - 0.7 mm after grinding.

SURFACE FINISH:

In general, the hardness of the materials for pins, pin retainers, shims, shim block, blades & risers are:

1. Clamp arm : Munsell No.3.75YR6/14 safety orange.
2. Shims/spacer : BLACK OXIDE FINISH (SURFACES TO BE FLAT AND PARALLEL TO
   WITHIN 0.015 mm T.I.R).
3. Riser : SURFACES TO BE FLAT, PARALLEL & PERPENDICULAR TO WITHIN 0.015 T.I.R.

Figure 25. Shows the surface finish specification of clamp arm