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Precision Sheet Metal WorkshopThe Theory of Hole Punching

The four phases of hole punching

“Plastic DeformationPenetration

Fracture”

Phase one: Plastic deformation

“The die is also penetrating the material from the bottom.”

Plastic deformation occurs as the punch tip begins to push against the material. When the material reaches its elastic limit the punch tip begins to penetrate the material.

Phase two: Punch penetration Cut, Shear, Burnish

The punch penetrates the material cutting a percentage

Phase Three: The FractureFracture lines from both sides meet, the material slug will be pushed free of the blank.

Phase Four: Ejection

Forcing the slug from the blank

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“Clearance is the difference between the outside measurement of the punch body and the inside measurement

of the die”

Hole punching theory

“Clearance properly applied will give you the proper cut or burnish.”

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“Of course the roll-over can be calculated”

“Note the compressive stress and the tensile stressThis is why one edge od the slug ‘rolls’ and one edge is sharp.”

Tooling Selection

The greatest single limiting factor for punch press tooling is tonnage.

“Punch shear angle makes a difference!”

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Review - It’s all about the profile“The three faces of punch sharpening:

shear angle, shear depth and the distance from edge to peak “b”

The Angles, Depths and Dimensions“It’s right angle trigonometry; two sides and a angle or two angles and a side”

Shear angle: shear depth / dim "b" = tangent of angle

Shear depth: dimension "b" x Tangent of shear angle

Dimension "b“: shear depth / tangent of shear angle

“ How do we calculate tonnage?We start with land area formulas”

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Round

Obround

Square

Rectangle

Multifaceted (hexagon, octagon, etc)

Start with the shape

“The surface area of the punch is called the "land distance“and is mathematically referred to as "L". “

"C" is the length

"A" represents the diameter or the diagonal dimension

B" represents the width

Material Multiplication factor (F)

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Finding a value for "S"

The shear factor

A multiple based on shear depth…

Punches that are narrower than the material thickness (Mt) are extremely susceptible lateral force…

Safety note: Bending a punch

…causing the punch tip to bendand/or break.

Hole punching pt2

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“As clearance increases, the lateral force, or the side-thrust, rapidly

increases.”

Side thrust

The punch tip can shave the edges of the die, resulting in a rapid dulling of both the

punch and the die.

Galling is the process in which some of the material being

punched adheres to the punch tip and/or the body

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Galling is best removed through the use of a fine polishing stone.

The stone should be used in a motion that is parallel to the direction of the galling.

By polishing the surface of the punch the chances of galling re-occurring is decreased

Roughing the surface up using harsh abrasives only gives the punched material something to

grab onto.

Don’t do it!

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“Heat build up in the tools is not uncommon (it is one of the main reasons for galling)”

Slow the machine or program down to insure that heat is not built up in the first place.

Full speed ahead?

A programming break in the program will allow the tool to cool while punching continues on a different feature.

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Alternate tools, use multiple tools in the turret and switch between them during the course of the punching operation.

“There are a lot of choices for lubricants, for example, WD40 ®™ Kerosene are great for

aluminum and cold rolled steel”

“Lubrication should always be the foremost consideration when considering the ease of operation and the life of the

tool.”

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Specialty Processes for Tooling

Zinconium titanium nitrate, (TiN coated),

Thermochemical processes (boronizing or nitriding), Galvanic processes ( hard chrome or nickel plating),

Physical Vapor Deposits (PVD) or CVD which stands for chemical vapor deposit.

Cryogenic treatment

This is a process in which the tooling is taken down to absolute zero brought back to room temperature and then a heat treat process… and… repeat..

On the molecular level the individual materials making up the tool are as if by "magic“, better blended.

Cryogenic treatment, What happens

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Cryogenic steel treatment have the following effects on the microstructure of steel.

Alteration of austenite to martensitic properties.

Carbide particles are evenly dispersed within the martensite grains.

Minimal coefficient of friction

Great increase in mechanical toughness

Higher resistance to fatigue and wear

Benefits of Cryogenic treatment

There are dozens of different ways to alleviate slug pulling, each relative to the reason for the slug pulling to begin with.

1) Air flow around the tool and turret

Causes of slug pulling:

4) Compression between slugs, forcing the slug back out.

3) Slug may be attaching itself to the tip of the punch (sticking)

2) One tool is sharper ( die sharper than the punch)

Five solutions

1) Oil the sheet with a light lubricant.

2) Double check the punch and the die height.

3) Clean the punch, die and stripper body.

4) Shim the punch spring (where applicable), pre-load the spring tension.

5) Check die clearance and tool sharpness.

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“Another way to reduce slug pulling is through the use

of a "negative taper"

Negative taper

Positive taper

This adds just enough pinching to hold the slug in the die…

Note: watch for compression!

Negative taper

The application of extra material into the die opening. Only a few .0001 of extra

material at a time.

Rocklinizing

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On punches cut to "shear" the shearing action could be the problem.

Still Slug pulling?

The countersink

This replaceable tip is called a "chip" and mates with the bottom die with a similar angle and size

These "solid" counter sinks are used when the top diameter exceeds .550 (13.97mm) or when the material

thickness is greater than 7ga, .179 or (4.554mm).

Solids and larger diameters

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“This pre-hole needs to be .015 (.39mm) to .035 (.89mm) smaller than the required diameter of the hole after punch and

countersinking.”

The process requiring a pre-hole

Checking diametersJust… slide right in!

Forming Tools

For some forming tools to work correctly they to be bottomed.

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After installing into the turret, try a sample form…

Never make adjustments to the forming of more then.005

Important forming notes:

WARNING: NEVER OVER STROKE FORMING TOOLS !

Over stroking can lead to both fatigue and failure in the tooling.

1) Do not place forming tools next to each other in the turret.

Forming tool best practices

5) Always try to form as far away from the grippers as possible.

4) If at all possible, make your forming operations last.

3) Do use some kind of support on either side of the form tool e.g.: a roller ball die or brush

2) Do not use next to a standard tool.

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Tool Sharpening

“If it takes more than .010 (.25mm) to bring a tool back

to a decent state of sharpness you have waited too

long to do it !”

Right way

Wrong way

NOTE: NEVER TAKE A BIG BITE of the tool at the surface grinder, it can ruin the tool and is a very big safety problem.

Dressing the wheel

Make sure the wheel of the surface grinder is dressed regularly to keep it clean and unplugged.

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Tool steels are heat sensitive and should NEVER be allowed to get hot. When sharpening a tool you should always try to wet grind.

Don’t over heat the tool!

Give in to Templation –theArtofPressBrake.com / media

Conclusion

“Now…Ya.. just gott’a tell it where to put the hole! ”