Views: 915 Author: Site Editor Publish Time: 2017-09-29 Origin: Site
ABSTRACT:-This project work deals with the design of pneumatically controlled small scale punching machine to carry out piercing operation on thin sheets (1-2 mm) of different material (aluminium and plastic). Reduction in punching force requirement being the main aim of this project work is obtained by modification in punch tool design i.e. by provision of shear on punch face. Subsequently it results in reduction in amount of punching force requirement. And further a CATIA model of the machine is developed on the basis of calculations with respect to punching force requirement.
KEYWORDS:-Punching Force, Stripping Force, Punch, Single and double shear, Percentage Penetration, and Pneumatic cylinder.
A pneumatic punching machine is always a better choice than a hydraulic punching machine for the production of similar products if it is suited for the method. It is comparatively more economical for production of large quantities of products as it uses compressed air rather than some hydraulic fluid which is rather expensive. A pneumatic punching machine uses compressed air to generate high pressure to be applied on the piston. A solenoid valve controls the directional flow of air into and out of the cylinder. Polyurethane tubes are used for pressure transmission from the pneumatic cylinder to the punch assembly. The high pressure air fed to the punch, forces it on the material and as the punch descends upon the sheet, the pressure exerted by the punch first cause the plastic deformation of the sheet. Since the clearance between the punch and the die is very small, the plastic deformation takes place in a localized area and the sheet material adjacent to the cutting edges of the punch & die edges becomes highly stressed, which causes the fracture to start on both sides of the sheet as the deformation progresses.
The Pneumatic punching machine is developed using various components. The components are pneumatic cylinder, pressure regulator, Solenoid/direction control valve, flow control valve, compressor, mounting table. The cylinder is used for up and down motion of the punch tool which performs the punching operation on the sheet of aluminium/plastic material. The compressor provides compressed air to the cylinder, which causes movement of the piston rod. Pneumatic automation components extensively use sealing material made out of rubber compounds. For efficient and trouble free working of these seals, they need to be oiled or lubricated to reduce friction and corrosion. To lubricate compressed air actuated equipment, the most efficient and economical method is to inject the lubricant in to the compressed air that powers this equipment. Solenoid/Direction control valve is used to control the direction of the air.
The compressed air from the compressor at the pressure of 8 to 12 bar is passed through a pipe connected to the Solenoid valve with one input. The Solenoid Valve is actuated with Control Timing Unit. The Solenoid valve has two outputs and one input. The air entering into the input goes out through the two outputs when the timing control unit is actuated. Due to the high air pressure at the bottom of the piston, the air pressure below the piston is more than the pressure above the piston. This moves the piston rod upwards which further moves up the effort arm, pivoted by control unit. This force acting is passed on to punch which also moves downwards. The punch is guided by a punch guide which is fixed such that the punch is clearly guided to the die. The materials are in between the punch and die. So as the punch descends down, the material is sheared to the required profile of the punch and the blank is moved downwards through the die clearance.
⒈Material Selection: To prepare any machine part, the type of material should be properly selected considering design and safety .The selection of material for engineering application is given by the following factors:
⑴ Availability of material
⑵ Suitability of the material for the application of the product.
⑶ Suitability of the material for the desired working conditions,
⑷ Cost of the materials.
The machine is basically made up of mild steel. The reasons for the selection are:
① Mild steel is readily available in market,
② It is economical to use,
③ It is available in standard sizes,
④ It has good mechanical properties i.e. it has good machine ability.
⑤ It has moderate factor of safety, because high factor of safety results in unnecessary wastage of material and heavy selection. Low factor of safety results in unnecessary risk of failure,
⑥ It has high tensile strength,
⑦ Low coefficient of thermal expansion.
The materials of the sheets to be punched are taken as aluminium and plastic as they are replacing many metals in the present scenario because of their distinguished properties and features.
⒉ Force Calculation for existing punch design:
Terms and formulae used:
• Cutting force: - The force which has to act on the stock material in order to cut the blank or slug.
• Stripping force: - The force developed due to the spring back (or resiliency) of the punched material that grips the punch.
• Cutting force = L x t x Tmax
• Stripping force =10% -20% of cutting force
• L= Length of periphery to be cut in mm
• t= Sheet thickness in mm
• Tmax= Shear strength in N/mm2
• The formula to calculate the press force is as follows-
• Press force = cutting force + stripping force
Sample Calculation for Aluminium Sheet
Here is a sample calculation to calculate the punching force required for different thickness of aluminium sheet.
• Total length of cut, L =50 mm.
• If Sheet thickness, t = 1mm.
• Maximum tensile strength of aluminium, Tmax = 180 N/mm2
• Total cutting force = L x t x Tmax
• Total cutting force = 50 × 1 × 180
• Total cutting force = 9000 N
• Stripping force = 15% of the cutting force= 1350 N
• Press force = Cutting force + Stripping force= 9000 N + 1350 N= 10350 N
Sample Calculation for Plastic Sheet
Here is a sample calculation to calculate the punching force required for different thickness of plastic sheet.
• Total length of cut L = 50 mm.
• If Sheet thickness, t = 1mm.
• Maximum tensile strength of plastic, Tmax = 90 N/mm2
• Total cutting force= 4500 N
• Stripping force = 675 N
• Press force = Cutting force + Stripping force= 4500 + 675 N= 5175 N
⒊Modification in Punch Design:
Shearing of Punch: If the face of the punch is normal to the axis of motion, the entire perimeter is cut simultaneously. By tilting the punch face on angle, a feature known as shear, the cutting force can be reduced substantially. The periphery is now cut in a progressive fashion, similar to the action of a pair of scissors or the opening of a beverage can.
Fmax= Maximum force required to punch the sheet of thickness t in Newton (N)
K= Percentage Penetration
t= Thickness of sheet in mm
I= Amount of shear given to the tool (in terms of t) in mm
i) Aluminium Sheet
1) For I=t/5 & K=0.6
F=0.75Fmax
2) For I=t/4 & K=0.6
F=0.705Fmax
3) For I=t/3 & K=0.6
F=0.643Fmax
4) For I=t/2 & K=0.6
F=0.545Fmax
5) For I=t/1 & K=0.6
F=0.375Fmax
⒋ Force comparison for Aluminium and Plastic Sheet:
• For Aluminium, Punching Force (F) = 11643.75 N
• For Plastic, Punching Force(F) = 5796 N
• As the cylinder will be designed for maximum punching force (in this case aluminium), so the thickness of the plastic sheet can be varied further.
• Therefore, the maximum thickness of sheet of plastic that can be punched is calculated as
Faluminium = 1.15 x (L x Tmax x t) plastic
11643.75 = 1.15 x 50 x 90 x t
t = 2.25 mm
Maximum thickness of Plastic sheet that can be punched = 2.25 mm
⒌ Designing of cylinder:
• Force required = 12000 N (rounding off 11643.75 to 12000 N)
• Working pressure = 10 bar
• To find the Bore diameter of the cylinder we use the following formula:-
• According to the formula bore diameter of the cylinder is = 123.6 mm
• As per the standards bore diameter = 125 mm
According to the bore diameter,
• Piston rod diameter is = 32 mm
• Stroke length = 200 mm
A CATIA model of pneumatic punching machine is developed on the basis of calculations done as per the punching force requirement.
Pneumatically operated punching machine is suitable for small scale and medium size industries. Based on the shear provided on the punch face the punching force reduction of 25% to 60% thereby increasing tool life and reducing tool machining cost. Therefore with this force reduction we are able to easily punch sheets of thickness upto 2.25 mm for plastic sheet having tensile strength 90 N/mm2 and upto 1.5 mm of aluminium sheet having tensile strength 180 N/mm2.
In this machine, compressed air is used to move the punch tool for carrying out punching operation. After the completion of the cycle the air moves out through the out port of Solenoid valve. This air is released to the atmosphere. In future the mechanism can be developed to use this air again for the working of cylinder.
English
Pусский
