Myself Soumitra; I am a B.E. student from the University of Pune and had undergone an In-plant training in Mungi Brothers PVT.,
a tool manufacturing plant at Nashik for my 3rd year Mechanical(Sandwich) training program.
Here I am posting some of my views regarding press tool engineering.
INTRODUCTION
THE FIELD OF PRESS TOOLING
The press stamping (pressing) is one of the important field of technology in the manufacturing processes. This was first developed in the earlier 19th century (around 1890s for manufacturing of symmetrical parts of bicycles (Iron age, 1 June 1892)).
It is particularly employed for mass production. Besides it has great advantage of being chip less production. Also the parts produced have better surface finish, durability and enough strength. The parts can also be (rather always) aerodynamically designed.
It has wide applications in automobile and aero plane assembly industries. Most of the outer shell (body) parts of a car or a jet are nothing but pressed product. A body in white is the best example.
A press working operation, generally completed by a single application of pressure often results in the production of a finished part in less than one second. Press working forces are setup, guided and controlled in a machine referred to as PRESS. Metal is formed in two different stages such as 1.Cold working 2.Hot working.
Cold working:
In this process metal is formed in the cold condition. In cold working the metal is pressed or cut, to get the required shape. The metal is stretched beyond its elastic limit .cold working can be done only on ductile metals. The machine used for pressing the metal in cold working is called a “cold forming press”.
Hot working:
In this process metal can also be formed in hot condition. Here the metal is heated to a temperature, so that recrystalysation takes place. At this hot condition the metal is pressed to get the required shape. The machine used for pressing the metal in hot working is called “hot working press”
PRESS TOOLS:
The tool which is used on press machines with punch and die is known as press tool.
Punch is the male part of the tool, which is fastened to the ram and forced into the die.
Die is the female part of the tool, which is rigidly held on the bed of the press machine. The die has an opening in perfect alignment with the punch.
MATERIALS:
Most metals may manufacture using the stamped process these include:
Machines used in stamped manufacturing range from a simple hand press or hydraulic press, to the much larger machines referred to as drop hammers. All stamping machines incorporate a form or die to give the stamped metal the desired shape.
General Parts of a Press tool
1. Bolster plate :
It`s actually a part of the press. A press has two bolster plates i) Upper and ii) Lower. Each plate has inverted T-slots for accommodating T bolts for clamping purpose. The lower half assembly of the die rests on the lower bolster plate while the upper half is clamped to the lower side of the upper bolster plate.
The lower bolster plate has some holes to accommodate cushion pins.
Cushions are located below the lower bolster plate.
The ram of the press is connected to the upper bolster plate. The upper bolster plate slides with the movement of ram along the guides provided in the press while the lower bolster plate remains at rest on the bed of the press.
2. Heighter or raiser :
They are simple blocks or plates made up of mild steel (MS) material and which are sometimes bolted to the lower or upper plate of the die. They are used to adjust shut height of the press. Sometimes in case of dies used for cutting operations, the heighters are used to higher the lower plate for collecting the cut material, either scrap or blank. They are generally placed in between lower plate of die and lower bolster plate and or between upper plate of die and upper bolster plate of the press. Or sometimes they are used to higher the steel over the lower plate.
3. Cushion or Cushion pins :
Cushions are also actually part of the press and are operated by the press itself. Cushion pins are inserted inside the slots provided on the lower plate. The knock-out plate rests on the cushion pins and the cushion pins rests on the cushions. During the downward travel of the ram, the knock-out plate lowers with the cushion. The main function of cushions is to uplift the knock-out plate during the upward travel of the ram after completion of the operation. And cushion pins transmit the upward movement of the cushion to the knock-out plate which ejects the job.
4. Lower plate :
The lower plate forms the base of the die. It rests (clamped) directly or indirectly (through heighter) on the lower bolster plate. The whole lower half assembly rests on the lower plate. The lower plate is provided with the clamping slots and some holes to accommodate cushion pins. In case of cutting operation dies, holes or slots are provided on the lower plate for removing cut parts, either scrap or blank.
5. Bolts and Dowel Pins :
Bolts and dowel pins both are used for fixing purpose but the difference between them is , dowel pins are used to fix the lateral movement (along X axis &/or Y axis) while bolts are used to fix the axial movement (along Z axis). Bolts are available in different standards like M6, M8, M10, M12, etc and dowel pins are also available with some standard diameters like 8,10, etc. They are generally with p6 tolerance for a H7 hole.
6. Lifters :
A lifter is an arrangement used for lifting the die assembly. Lifters are provided on both the assemblies (upper and lower) separately. Generally they are four in number for each assembly. For casting type of dies, they are integral with the die shoe while for plated type dies they are separate products bolted to the die shoes.
7. T-Bolts and Clamping slots :
T-bolts are straight bolts with heads having T cross-section used for clamping purpose. They are inserted in between the slots made on the die shoes called clamping slots. Sometimes some metal blocks are inserted in between the die shoe and the bolster plate for increasing the shut height for the die and then the die shoe is clamped through these clamping slots with T-bolts and nut-washer fixing elements.
8. Pillar guide posts :
These are metal block fixed to the die shoes with bolts and dowel pins. A pillar guide post is having a hole for housing guide pillar bushings.
9. Guide bush bearing :
Guide bush bearings are made-up of some wear resistant material like bronze. They are available in different types e.g. oil-less bushings, ball-cauge bushings, etc. Their main function is to slide the guide pillar smoothly. They are fixed (generally press fitted) inside the pillar guide posts. Guide bush bearings are nothing but housings for guide pillars. They are fixed to the upper and lower plates of the tool. The guide bush bearing fixed on lower plate houses the guide pillar while guide pillars slides on guide bush bearing fixed on the upper plate. It is fixed within guide bush retaining plates inside guide post. Most bushings are made up of tool steel they are also available in bronze.
There are two types:
1. Plain bushing :
These are simple sleeves, pressed into the top plate.
2. Shouldered bushings
These are turned down at one end.
10. Guide bush retainer plate :
The guide bush retainer plate is fixed on the pillar guide post. Its function is to retain the guide bush bearing inside the guide post under the guide bush retainer plate itself. It is simply bolted with the guide post.
11. Guide Pillars :
These are solid cylindrical bars with specific surface finish (generally N6). They are used to guide the travel of whole upper half assembly over the lower half assembly during the press strokes. They are generally mounted and fixed to the lower plate while their opposite ends slides in the bushes called as guide bushes fixed to the upper plate. Generally any two guide pillars do not have same lateral dimensions i.e. they have different diameters. This is to ensure perfect alignment of the upper half assembly over the lower half assembly. Guide pillars used for precision die sets are chromium plated to provide high degree of accuracy of resistance to wear. The addition of chromium reduces wear up to 50%. They are specified at least ¼ inch shorter than the shut height. These are made out of case hardened tool steel (17Mn1Cr95) or sometimes O.H.N.S. (T110W2Cr1) and brushes are hardened and tempered to 56-60 HRC.
12. Stacking ring :
Stacking rings are used simple circular rings inserted inside guide pillars. They are used when the tool is not in use. Their main function is to maintain a gap between the two assemblies. This gap is necessary protect strength the die elements which could have reduced because of continuous loading under self weight of the tool. They are attached with the die shoes with chains.
13. Stroke end blocks :
These are thick walled circular rings attached to lower shoe and upper shoe co-axially. Their function is to limit the travel of upper half assembly during lowering stroke of the ram. At this condition, both the stroke end blocks on upper and lower shoes mate their outer faces.
14. Heel Plates :
These are solid rectangular plates attached generally to upper shoe. Their function is to align the upper half assembly with the lower half assembly by sliding with the wear plates fixed inside the C-guides on lower shoe. They are suitably lubricated and maintain perfect alignment. They are also available in oil-less type.
A proper clearance is maintained in between heel plates and wear plates.
15. Wear Plates :
Wear plates are also solid rectangular chamfered plates generally three in numbers fixed in three sides of each C-guide assembly. Their walls form sliding faces for heel plates. They are well lubricated and proper clearance is maintained in between them. They are also available in oil-less type.
16. Die plate :
A die block is defined as the block or plate from which the die profile is cut. It is usually lower member of the tool. It is usually made from T215 Cr 12 W90 (D3) material and is hardened to 60-62HRC. The die opening has different designs and the design is selected after taking into considerations the requirements and facilities available. The most common die section has straight line and then angular clearance is given in order to allow easy fall of components and slugs. Bottom inserts can be used conveniently for circular holes. Large die are made from many segments, which are secured in a sturdy die holder (bottom bolster) by Allen screw and dowels.
17. Die Housing :
Die housing is a part of the tool in which the steel (lower steel) rests.
Its main function is to support the steel and it itself rests on the lower plate directly or indirectly through the raisers.
Its profile is made accurately on V.M.C. so that the steel is butt inside the housing.
18. Steel :
The steel part is one of the most important parts of a press tool. A blank or component is first kept on the steel. It has exact shape or profile that our part should have. Its profile with suitable tolerance is made on V.M.C. and then it is hardened to specific scale. Sometimes in case of piercing type cutting operations, die button being harder material is butt inside the steel and the rest part of the steel is left non-hardened. In some cases there are two parts of the steel. 1) Upper steel and 2) lower steel. The upper steel and lower steel is used almost in all operations. In case of forming operations the upper and lower steel has same profile made on V.M.C. Actually the whole cutting or drawing or forming etc. forces are exerted through the steel. It rests directly or indirectly (through housing and / or raiser) on the backup plate.
19. Die Button :
The die button is generally used for piercing operations.
For cutting operations, it is necessary to have a hard base which is also provided in the form of steel. But if the die mechanism malfunctions, the steel part could damage partially or completely. Instead we use a die button which is a thick walled, hollow cylindrical shaped component made-up of a hard material below the cutting region. So it provides a hard base below the cutting region. It enables us to replace only the hard base below the cutting region when the die mechanism malfunctions. Otherwise we could have to change the whole steel part which could be difficult, costly or time consuming. The punch after piercing travels some distance through the die button.
20. Strip Guide :
Strip guide is generally a metal plate made of a wear resistant material used to guide the strip. It is fixed by bolting to the lower stripper plate or blank holder parallel to the strip length. The strip is fed to for the operations by sliding the strip within the boundaries of strip guide. On prolonged use the strip movements could wear the guiding plates. Hence to eradicate this problem the strip guide plate is made-up of a wear resistant material.
21. Locator pin :
It is a pin which as name implies locates the job for an operation. Sometimes slots, holes, trimmed edges, etc. form regions to lock the movement of the job/strip by locking them with locating pins which generally have the same profile as the slot, hole or trimmed portion have. The locating pin is generally spring loaded.
22. Stoppers :
The stoppers are metal plates use also used to locate the strip for correct execution of operation. The only difference between a strip guide plate and a stopper plate is the stopper plates are fixed with their lengths parallel with the length of the strip while the stopper plates are used with their lengths parallel with that of a strip. The strip feed is actually stopped and not guided at the stopper plate. In some dies movable stoppers are also used. These can be spring loaded and mechanically works to stop and allow the feeding.
23. Stripper plate or Pad :
A stripper plate or a pad is thick rectangular plate generally uppermost major component of an assembly. Its main function is to apply a stripping force on the blank or strip. During cutting operations if we do not use a stripper plate i.e. if we don`t use a stripping force we cannot guarantee that the job only on the basis of clearance provided in between the die and cutting steel could shear. Hence to perform the cutting action neatly, the stripping force is applied with the help of a stripper plate. The stripper plate rests on some proper suspension like springs or rubbers (Poly urethane rubbers) Their travel is maintained with the help of spool retainers or shoulder bolts. It is provided with a slot with clearance maintained for a cutting steel. During the operation, the pad after coming in contact with the job, stops its movement with respect to steel and nothing but allow the steel to move forward and cut the job.
Besides applying the stripping force, it also functions to remove the skeleton (the partially or completely worked strip) after performing the operation.
24. Punch :
The punch being the most important part of a press tool is generally termed as a male part of the tool. It is made up of a very hard material since it is actually acted upon with very high magnitude of force during the working of the tool. It given with an exact profile of the part with completed required operation. It transmits the whole cutting or forming force of the tool to shear or form the job respectively.
25. Punch holder or retainer :
The punch retainer or simply holder is a metal block which as name implies retains or holds the punch. An appropriate slot is made inside the retainer to house the punch tightly inside itself. This block is then bolted directly or indirectly (through back-up plate or raiser) to the die shoe.
26. Punch backup plate or thrust plate :
The punch with punch holder rests on punch backup plate also called as thrust plate. And the thrust plate rests on the die shoe or sometimes on raiser.
Actually the punch transmits a very high in magnitude and most important force in the tool. During the operation, while transmission of this force by the punch, a reaction force, same in magnitude but opposite in the direction with the applied force could try to shear the die shoe if we had not used the punch back-up plate below the punch. That is a punch back-up plate takes up this high magnitude force and transmits over a wider area of raiser or die shoe with reduced magnitude. The punch back-up plate need not made with a wider area; instead it is made-up of a hard material. Hence it can withstand the high magnitude force from the punch.
27. Shoulder bolt :
A shoulder bolt is a special type of bolt with long shank to provide travel for the pad. It is provide with threading on its end face to screw inside the die shoe. It has a head which rests with the counter head hole provide in the pad. The pad slides or travels over this shoulder bolt long shank through the periphery of the hole provide in the pad. Though it guides the pad, its main function is to control the pad travel. i.e. the head of the shoulder bolt restricts the pad travel beyond it.
28. Spool retainer :
The spool retainer has same function of the shoulder bolt i.e. pad travel control. But the main difference between a spool retainer and a shoulder bolt is, the shoulder bolt has threading as integral part, but for a spool retainer it doesn’t. A spool retainer is a simple cylindrical long shank part with large head resting and sliding in the counter hole provided in the pad. An external bolt is inserted in the spool retainer to fix it to the die shoe tightly. It enables longitudinal but locks the lateral displacement of the pad.
29. PU Rubber :
PU rubber stands for Poly Urethane rubber. It is a special type of rubber used in press tool die making. Its function is to provide a suspension travel for the pad element. They get compressed due to the stripping force and cause the pad to travel over spool retainer or shoulder bolt.
The press force should be larger than the addition of cutting and stripping force. So that the press after expending a part of its force to compressing the rubbers could be able to cut or form, etc. the job. They are placed within the die shoe and pad and held fixed by inserting a spring or rubber retaining pin in its hole. These rubbers can also be nested in parallel and series manure.
30. Springs :
The springs also bear the same function as a PU rubber. But generally springs are used in dies involving relatively stripping forces. Also they have a longer life span but high cost. They are placed in between the die shoe and the pad and held fixed with a spring retainer pin inserted in between. Same as PU rubbers, the springs can also be nested in parallel and series manure with washers inserted in between each pair.
31. Spring retainer pin :
A spring retainer is a pin which holds the spring or rubber. It has external threading on its lower periphery and a shoulder on the other end which supports the spring. It is bolted with the stripper plate.
32. Pad guide blocks :
These are blocks with machined surfaces. They are fixed to the stripper plate and are used to guide it during the travel. Sometimes instead of blocks pad guide pillars can also be used to serve the same purpose.
33. Transfer plate :
Transfer plate is used if there are space restrictions i.e. when longitudinal dimensions stripper plate are not enough for accommodating suspensions like springs or rubbers. In such conditions a plate with enough longitudinal dimensions is used which houses springs or rubbers with retainer pins. The stripper plate is joined to this plate with special longer shoulder bolts or spool retainers. In this way the stripping force can be transferred to the required number of suspensions which house on the transfer plate
34. Upper plate :
This is the upper most part of a press too die assembly. It supports many components of die`s upper half assembly like guide post, upper steel or punch back-up plate or retainer plate, etc. It is provided with clamping slots to clamp it with the upper bolster plate.
35. Shims :
The shims are plates with thickness ranging from 0.5mm to 3mm. In case of mating of parts during assembly, sometime gap formation may occur due to various reasons such as machining error, faulty assembly or dimensions, etc. In such cases to fill the gap between the mating surfaces, thin plate having thickness equal to the gap are introduced or sometimes more than one plates having combined thickness equal to the gap are used. These thin plates are called as shims.
Important terms in press tool design:
STOCK MATERIAL:
General term for any of the various materials from which piece a part is made.
SHUT HEIGHT:
The distance from the bottom surface of the bottom shoe to the top surface of the top bolster when the tool is in closed condition.
CLEARANCE:
Cutting clearance is a gap between a side of punch and the corresponding side of the die opening on one side of the edge when the punch is entered into die opening.
IMPORTANCE OF CUTTING CLEARANCE:
Proper cutting clearance is necessary to aid the life of the die and the quality of the piece part. Excessive cutting clearance result in objectionable piece part correction. Insufficient cutting clearance causes undue stresses and wear on the greater punching effort required.