©2008 Mate Precision Tooling 1 Basic Punching Theory

©2008 Mate Precision Tooling1

Basic Punching Theory

©2008 Mate Precision Tooling

• Die clearance too small

• Poor or no maintenance on the tool

• Turret alignment requires adjustment

The ‘BIG 3’ ProblemsThe ‘BIG 3’ Problems


Punching a hole Die Clearance Punch Tonnage Choosing the right tool Correct tool maintenance …

Basic Hole Punching Theory


Punching CyclePunching CycleImpact Penetration

Fracture (@ 30% t) Stripping

Material Punch

Die

Slug to go approx. 3 mm into die for slug free dies

Roll-over

Burnish zone

Fracture

Punch force diagram


Cross Section of a Quality Hole

Cross Section of a Quality Hole

Burr height graphsSlug


• Die clearance too smallDie clearance too small• Poor or no maintenance on the tool




Proper vs. Tight ClearanceProper vs. Tight Clearance


Proper vs. Tight ClearanceProper vs. Tight Clearance

Extra energy does not disappear,

but is converted into heat !!

- Punching forces are balanced, - Maximum piece part quality and tool life

Fracture lines meet

Fracture lines do not meet

- Secondary shear cracks, - Extra energy required,- Higher tool wear (dies wear quicker)


Large Burr, not compressed and can be removed easily

More Rollover & Less Burnish zone

Excessive ClearanceExcessive Clearance

Tool lasts longer !


Proper Die ClearanceProper Die ClearanceMaterial Type

Shear Strength Material

Thickness Piercing Total

Die Clearance % Blanking Total Die

Clearance %

Aluminum 22 –25 kp/mm²

Less 2.5mm

2.5mm to 5.0mm

Greater than 5.0mm

15% 20% 25%

15% 15% 20%

Mild Steel 40 kp/mm²

Less than 3.0mm

3.0mm to 6.0mm

Greater than 6.0mm

20% 25% 30%

15% 20% 20%

Stainless Steel 60 kp/mm²

Less than 1.5mm

1.5mm to 2.8mm

2.8mm to 4.0mm

Greater than 4.0mm

20% 25% 30% 35%

15% 20% 20% 25%

Note: 1) In general: shear strength determines clearance (10% is already good for low shear strength aluminium)

2) Minimum die clearance recommended : 0.08 mm


Improper Die ClearanceImproper Die Clearance


Piercing

– The slug is scrap. The interior of the sheet is the product.

– Depending on the tool design, more than one hole can be punched at one time. Cluster tools for example.

– In piercing operations, the die clearance is calculated from the punch size.

Piercing vs. BlankingPiercing vs. Blanking


Blanking

– The punched item is of value. The slug is important.

– Special shapes are used mostly in blanking operations.

– Blanking operations calculate the die clearance from the desired blank dimensions.

Piercing vs. BlankingPiercing vs. Blanking


Punching TechniquesPunching Techniques

Blanking

– When the slug becomes the “good” or the “saved part”


Punching TechniquesPunching TechniquesBlanking rather than punching can save time, tool life and sheet usage


Ordering Punches and DiesOrdering Punches and Dies

Piercing a Hole

Punch Size =

Desired Hole Size

Die Size =

Punch Size +

Total Clearance

Blanking a Part

Die Size =

Desired Blank Size

(no Slug Free dies !)

Punch Size =

Die Size – Total Clearance


Required Punch Tonnage (tons) =

hole perimeter (mm) * material thickness (mm) * shear factor (kpmm²) 1000

Some important shear factors shear factors (): Aluminium 22 -25kp/mm2

Mild Steel 40 kp/mm2

Stainless Steel 60 kp/mm2

Tonnage calculationTonnage calculation

Cluster tools :Cluster tools : multiply by number of punchesTonnage reduction by ‘step’-punching: step = ½ material thickness

F (tons) = P x t x S / 1000

Note Note : : try not to exceed 80% of punch press capacity


Material shear factors ( ) :

Aluminum = 22-25 kp/mm2

Mild steel = 40 kp / mm2

Stainless steel = 60 kp /mm2


18


Formula:Formula: Tonnage =

hole perimeter (mm) * material thickness (mm) * shear factor (kpmm²) 1000

Ø 30mm hole

3mm stainless steel

Material shear factors ( ) :

Aluminum = 22-25 kp/mm2

Mild steel = 40 kp / mm2

Stainless steel = 60 kp /mm2

Example: Ø 30mm hole in 3mm stainless steel

Tonnage = (30 x 3.14) x 3 x 60 = 16,96 t 1000



Required Tonnage + Stripping Force must be Tonnage to be within Machine Capacity !

Keep in mind : Stripping springs !Keep in mind : Stripping springs !


Perim

ete

r Calcu

latio

ns

Calculate diagonals to

determine station size

Calculate perimeters for

tonnage calculations

A = Diagonal Dimension

(Station Size)

L = Hole Perimeter


Calculator program for Clearance / Tonnage / Station Size


Use of Shear on PunchesUse of Shear on Punches• Shear typically 1.5 – 2 mm• Max up to 80% of machine

capacity• Shear punches need sharp

edges (sharpen frequently)

• Best shear is rooftop; use min. 75-80% of length; can be used at high tonnage

• Concave shear: inverts stresses, good alternative for nibbling; max 15 Ton

• One-way: good blanking shear

• Four-way: very good on squares; hard to regrind

• Trumpf ‘Whisper’ very good, but hard to regrind

• Cup shear on rounds (Tr)


Punch Shear ExamplesPunch Shear Examples

One-way & Whisper Cup

Concave RooftopFour-Way


Tonnage reduction

Noise reduction

Slug control

Reduced shock loads on the machine

Flatter sheets

Improved stripping

Increased tool life

Advantages of ShearAdvantages of Shear


Treated PunchesTreated Punches

Please note! Nitrated punches not recommended for diameter or width less than 6 mm !!


Treatment vs. Coating Treatment vs. Coating Nitride Treatment

Nitriding is a heat treatment feature for HSS punches. Punches are more wear resistant, but also more brittle (esp. during stripping).Recommended for materials that cause galling, such as stainless steel, galvanized steel, or aluminum, but not if the material is too thick, as stripping might cause chipping. Can also be used for mild steel, will increase tool life (up to 4x).Also recommended for punching abrasive materials such as fiberglass.It is not recommended for punches smaller than 6mm in diameter or width, as punch is too brittle and may chip off.

Maxima® Coating

Maxima is a hard, wear-resistant, multilayer Zirconium Titanium Nitride (ZrTiN) coating,It puts a hard surface on the punch, adds wear resistance and ‘lubricates’ the process.The coating acts as a barrier between the punch and the sheet metal being punched. Its exceptional lubricity, reduces the friction that occurs during the stripping portion of the punching cycle, thus especially recommended in thicker material.Less friction means less heat build up, less galling and longer tool life. Also recommended upon high use of the punch, or as a problem solver.

Maxima Coating is more expensive than Nitride Treatment.


Treatment vs. Coating Treatment vs. Coating


Mate Slug Free® DieMate Slug Free® DieSlug Free® Die Components

1. Punch

2. Stripper

3. Material

4. Slug Free® Die

5. Slug

6. Grind Life

7. Entry -- Constricting Taper 0.5°

8. Pressure Point (at specific depth (*))

9. Exit -- Relief Taper 5.0°

Hole/Slug Geometry

A. Rollover

B. Burnish

C. Fracture

D. Burr

(*) depth = 1.5 mm + 0.7 * thickness (max = 3.0 mm)


Material held securely by stripper before punch makes contact

Penetration begins and metal deforms into the entry taper

Material begins to fracture at stress points

Slug fractures away from sheet

Pressure point constricts slug

Punch stroke bottoms out as slug squeezes past pressure point

Punch retracts and slug is free to fall down and away through exit taper

Punch

Stripper

Material

Slug Free® Die

Slug Free® Die DetailSlug Free® Die Detail


Uniform Clearance DiesUniform Clearance Dies

-No corner burrs with Constant Clearance in shaped die corners

-Stress relief in die corners


Uniform Clearance DiesUniform Clearance Dies

Corner burrs when non-uniform clearance dies are used


Slug Free Light ®Slug Free Light ®

Optional Die Land to prevent slug-pulling in problem materials or extremely thin material.

Blips 0,05mm deep are cut into the die wall to allow the slug to expand into the slots. Blips act as mechanical locks to trap the slug.

Especially for bigger diameters and thin material (max. 1,5 mm).

Material Type Recommended Material Maximum

Stainless Steel 0.80mm

Mild Steel 1.00mm

Aluminum 1.20mm


Standard Back-taperStandard Back-taper

1/8 degree per side(1/4 degree TOTAL)

Punch Size


Heavy Duty Back-taperHeavy Duty Back-taper

1 degree per side(2 degrees TOTAL)

Punch Size

Recommended when punching force over 18 tons and/or thickness above 4 mm


Punching Thick Material (>4mm)Punching Thick Material (>4mm)

When ordering tools:

• HD tool configuration (Rooftop punch with extra Backtaper)

• Clearance of 25-30% of Material Thickness

• 0.5mm radius on all punch corners

• Punch to material thickness ratio of 1 minimum

When punching:

• Lubricate the sheet / punch / guide

• Run Machine on slow cycle

• Inspect tools frequently for wear / Use sharp Punches & Dies


Punching Thick Material (>4mm)Punching Thick Material (>4mm)

Common Poblems / Solutions

•Stripping problemsHD configuration – Lubrication – Coating on

punch – Use bigger station – Gas canister

•Excessive tonnage / High tool wear / High vibrationsMake sure tonnage calculation is correct – Use Lubrication – Sharp tools – HD

configuration – Maxima coating – Shear on punch – Correct die clearance – Pre-pierce holes – Bridge hitting

•Work piece distortion/stretchingChange punch pattern (bridge hitting) – Slow

down machine – Use Fully Guided – Sharpen tools


Slitting TipSlitting TipBridge HittingBridge Hitting

As consecutive nibbling may cause unbalanced forces,apply bridge hitting to avoid side loading during slitting

operations:


Large Holes Without ExceedingPress Tonnage

Large Holes Without ExceedingPress Tonnage

The customer wants to pierce a 50.0mm dia. hole in thick mild steel.He does not want to buy a special shape radius tool, but wants to use standard tooling.

Tool 1 Use one Square 10mm and pre-pierce along the 50mm periphery 8 holes with equal pitch.

Tool 2 Finalize the hole with a 50mm flat punch

8 hits 10mm sq..

Final Hit 50mm round


Punching Thin Material (<1mm)Punching Thin Material (<1mm)

When ordering tools:

You may require

• Ultra Light canisters

• Light shear on punches

• Fully Guided guide/stripper solution

• Urethane stripper pads

• Slug Free Light dies

When punching:

• Run Machine on slower cycle

• Inspect tools frequently for wear

• Use sharp punches & dies


Punching Thin Material (<1mm)Punching Thin Material (<1mm)

Common Poblems / Solutions

•Sheet markingUrethane stripper pads – Ultra Light

•Slug pullingMaximize die penetration – Demagnetise tools –

Slug Free Light – Correct die clearance – Slug ejectors – Shear on punch

•High tool wear due to small clearanceMaintain turret alignment

•Work piece distortion/stretchingChange punch pattern – Slow down machine – Sharpen tools – Use Fully Guided


Punching non-metallic materialPunching non-metallic material

Use concave shear punches

Use Maxima or Nitride punches

Reduce die clearance with 5 %

Use sharp punches and dies

Run the machine on a slower cycle

Lubricate hard plastics if possible

Use urethane pads if marking occurs

Support thin material if needed


‘Shaving’ – Make Straight Walled Holes Without Drilling

‘Shaving’ – Make Straight Walled Holes Without Drilling

Finished hole size is the starting point.

Punch #1 = finished hole size – clearance (20%)Die #1 = finished hole size + 0.1 mm

Punch #2 = finished hole size (full mat’l contact : Maxima !)Die #2 = die #1

.


Preventing Slug PullingPreventing Slug PullingUse Slug Free® Dies

Eliminate Magnetism In Tools

Lightly Dull Recently Sharpened Tools (with oil stone)

Decrease Die Clearance

Maximize Die Penetration

Use Shear on Punches

Use Urethane Slug Ejectors (need replacement !)


Overcoming Stripping ProblemsOvercoming Stripping Problems

Use Sharp Punches & Dies

Lubricate Sheets

Remove Galling

Check Stripping Springs For Fatigue

Use Additional Back-Taper on Punches

Increase Die Clearance

Use HD Tool Configurations

With Spring Tooling Use Larger Station


GallingGallingGalling is an adhesion to the punch tip by the metal being punched, caused by pressure and heat.

The best technique for removing galling is to rub it off with a fine stone. The rubbing should be done parallel to the direction of the punching motion. This will polish the surface that contacts the material, decreasing any chance of future galling. Do not sandblast, belt sand or use other harsh abrasive methods. These create a coarse surface finish to which material adheres more easily.


Reduce GallingReduce GallingUse sharp Punches & Dies

Lubricate Work Piece

Adjust Machine Hit Rate (Slower)

Use ABS If Available (Tool Lubrication)

Increase Die Clearance

Use Maxima or Nitride Treated Punch


Small Diameter or Narrow Holes


When punching small diameter or narrow holes, maintain the following ratio of punch size (minimum) to material thickness:

Material

Aluminum

Mild Steel

Stainless Steel

Punch to Material Ratio

0.75 to 1

1 to 1

2 to 1


Material

Aluminum

Mild Steel

Stainless Steel

Punch to Material Ratio

0.75 to 1 (Fully Guided 0.5 to 1)

1 to 1 (Fully Guided 0.75 to 1)

2 to 1 (Fully Guided 1 to 1)



With Fully Guided


Minimum Spacing Between Holes and Forms


Minimum = 2 x Material Thickness between holes

Minimum = 2 1/2 x Material Thickness to sheet edgeTo

p v

iew

of

sh

ee

t

To avoid hole distortion:




Spacing between forms

Minimum = 2 1/2 x Material Thickness to sheet edge


NibblingNibblingNibbling

– Minimum feed rate is equal to material thickness.


NibblingNibbling

Nibbling is not always recommended …


Excessive NibblingExcessive Nibbling

Nibbling patterns can be done with much less hits with a banana tool or multi-radius tool (with tool rotation)


Punching Tip Punching Tip Combating WarpageCombating Warpage

To avoid sheet warpage when punching multiple holes,

punch every other hole first, then come back


In order to reduce noise , vibrations and tonnage on cluster tools it is recommended to shorten 50 % of the cluster punches with about 50 – 60% of the sheet – thickness.

Example:

Cluster of 16 x square 10mm

Perforating in 1mm mild steel

Insert Punch length 37mm

Shorten 8 Punches to length 36,5mm ( Balanced Shortening)

Cluster ToolsCluster Tools


Blanking --

When the slug

becomes the

“good” or the

“saved part”

Blanking OperationsBlanking Operations



•Blanking rather than punching can save time, tool life and sheet usage …

•Special shapes can be done with a combination of standard tooling



Extremely sharp punches and dies

Reduce die clearance by 5%

Determine which blank dimensions & tolerances are critical

Notify tooling provider that tooling required will be used for blanking

Use non-slug free or straight taper dies

Punches should be flat faced or with slight one-way shear

Frequently inspect tools for wear

Note: Be aware of the changing of burr side when piercing and blanking is combined.


Punch 3.0-5.0mm flat

1.5-3.0 degree one-way shear can be used

Blanking Punch Configuration

Blanking Punch Configuration


Note: Shear punches give flatter workpieces



• Poor or no maintenance on the Poor or no maintenance on the tooltool




Fracture betweenplate and s lug

Burr

Burr

Punch sectionw ith du ll edge

D ie sectionw ith du ll edge

S lug :

S heet :

S traight wall

S traight wall

Roll- over

Roll- over

Close-up of a burrClose-up of a burr


R = 0.25mm

Sharpen when a

0.25 mm radius

forms on the punch or the die

Sharpen when a

0.25 mm radius

forms on the punch or the die

- When the cutting edge shines- When hole quality changes- When the sound of punching changes

Recognize dull toolsRecognize dull tools

When to Grind ?When to Grind ?


Grind Life = SBR - (Stripper Thickness + Material Thickness + Die Penetration)

Stripper Land - Ultra A-B = 4 mm- Ultra C-D-E = 8 mm

Material Thickness

Die Penetration

Usable Grind Life

Punches have a bigger grind life in smaller thickness

Max.

ThicknessSBRUltra

A 3,8 18,9

B 6,4 18,9

C-D-E 6,4 25,5

Calculating grind lifeCalculating grind life

How much to Grind ?How much to Grind ?


Improper Sharpening Proper SharpeningTotal HolesProduced

RadiusFormed

Total HolesProduced

RadiusFormed

100,000 0.25mm 100,000 0.25mm50,000 0.50mm 100,000 0.25mm25,000 0.75mm 100,000 0.25mm10,000 1.00mm 100,000 0.25mm

185,000Total Hits

1.0mmTotal Removed

400,000Total Hits

1.0mmTotal Removed

More than DOUBLE the tool life when sharpened frequently!

Importance of frequent grinding Importance of frequent grinding

How frequent to Grind ?How frequent to Grind ?


Sharpen frequently Shear punches to be reground more often than flat punches

Use coolant

Correct wheel parameters

Demagnetize all tooling after regrinding

Break edges with oil stone

Sharpening Rules Sharpening Rules How to Grind ?How to Grind ?


SharpeningSharpening

Excellent sharpening results


IMPORTANCE OF CORRECT MAINTENANCE : bad sharpening practices

IMPORTANCE OF CORRECT MAINTENANCE : bad sharpening practices


Benefits of ProperTool MaintenanceBenefits of ProperTool Maintenance

Flatter sheets (Note: Shear punches give flatter sheets)

Cleaner holes

Less stress on tool and machine

Longer tool life

Maxima coating not affected


Sharpen frequently

Use proper die clearance

Use proper shear configuration

Lubricate punch, guide and sheet

Inspect tool holders for wear

Use treated tools for special applications

Use radii on all sharp corners

Demagnetize all tooling after regrinding

Check turret alignment regularly

Maximizing Tool LifeMaximizing Tool Life



• Poor or no maintenance on the tool

• Turret alignment requires Turret alignment requires adjustmentadjustment



Thick Turret Alignment ToolsMate PILOT Alignment System

Standard Alignment tool


PILOT Alignment ToolPILOT Alignment Tool


PILOT Alignment ToolPILOT Alignment Tool


Mate PILOTMate PILOT

Indicator light color :

– Red – not aligned

– Yellow – aligned within 0.0012 (0.030)

– Green - aligned within 0.0003 (0.008)


Loosen upper and lower tool holder retaining screws



Station Alignment to machine

– alignment bar and dial-indicator



Questions?Questions?


Thank you !

Documents

©2008 Mate Precision Tooling 1 Basic Punching Theory