1 Basic Punching Theory Tt 2010
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The document discusses basic punching theory and techniques. It addresses the key issues of die clearance being too small, lack of tool maintenance, and improper turret alignment. Proper die clearance, regular tool sharpening, and ensuring turret alignment are essential for maximizing tool life and part quality during punching operations.
1 Basic Punching Theory Tt 2010
- 2. The ‘BIG 3’ Problems Die clearance too small Poor or no maintenance on the tool Turret alignment requires adjustment
- 3. Punching a hole Die Clearance Punch Tonnage Choosing the right tool Correct tool maintenance … Basic Hole Punching Theory
- 4. Punching Cycle Impact 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
- 5. Cross Section of a Quality Hole Burr height graphs Slug
- 6. The ‘BIG 3’ Problems Die clearance too small Poor or no maintenance on the tool Turret alignment requires adjustment
- 7. Proper vs. Tight Clearance
- 8. Proper 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)
- 9. Excessive Clearance Tool lasts longer ! Large Burr, not compressed and can be removed easily More Rollover & Less Burnish zone
- 10. Proper Die Clearance 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
- 12. 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. Blanking
- 13. 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. Blanking
- 14. Punching Techniques Blanking When the slug becomes the “good” or the “saved part”
- 15. Punching Techniques Blanking rather than punching can save time, tool life and sheet usage
- 16. Ordering 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
- 17. Tonnage calculation Required Punch Tonnage (tons) = hole perimeter (mm) * material thickness (mm) * shear factor (kpmm²) 1000 Some important shear factors ( ): Aluminium 22 -25kp/mm 2 Mild Steel 40 kp/mm 2 Stainless Steel 60 kp/mm 2 Cluster tools : multiply by number of punches Tonnage reduction by ‘step’-punching: step = ½ material thickness F (tons) = P x t x S / 1000 Note : try not to exceed 80% of punch press capacity
- 18. Material shear factors ( ) : Aluminum = 22-25 kp/mm 2 Mild steel = 40 kp / mm 2 Stainless steel = 60 kp /mm 2 Tonnage calculation
- 19. 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/mm 2 Mild steel = 40 kp / mm 2 Stainless steel = 60 kp /mm 2 Example: Ø 30mm hole in 3mm stainless steel Tonnage = ( 30 x 3.14) x 3 x 60 = 16,96 t 1000 Tonnage calculation
- 20. Required Tonnage + Stripping Force must be Tonnage to be within Machine Capacity ! Keep in mind : Stripping springs !
- 21. Perimeter Calculations Calculate diagonals to determine station size Calculate perimeters for tonnage calculations A = Diagonal Dimension (Station Size) L = Hole Perimeter
- 22. Calculator program for Clearance / Tonnage / Station Size
- 23. Use 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)
- 24. Punch Shear Examples One-way & Whisper Cup Concave Rooftop Four-Way
- 25. Tonnage reduction Noise reduction Slug control Reduced shock loads on the machine Flatter sheets Improved stripping Increased tool life Advantages of Shear
- 26. Treated Punches Please note! Nitrated punches not recommended for diameter or width less than 6 mm !!
- 27. 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.
- 29. Mate Slug Free ® Die Slug 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)
- 30. Slug Free ® Die Detail 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
- 31. Uniform Clearance Dies No corner burrs with Constant Clearance in shaped die corners Stress relief in die corners
- 32. Uniform Clearance Dies Corner burrs when non-uniform clearance dies are used
- 33. 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
- 34. Standard Back-taper 1/8 degree per side (1/4 degree TOTAL) Punch Size
- 35. Heavy 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
- 36. 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
- 37. Punching Thick Material (>4mm) Common Poblems / Solutions Stripping problems HD configuration – Lubrication – Coating on punch – Use bigger station – Gas canister Excessive tonnage / High tool wear / High vibrations Make 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/stretching Change punch pattern (bridge hitting) – Slow down machine – Use Fully Guided – Sharpen tools
- 38. Slitting Tip Bridge Hitting As consecutive nibbling may cause unbalanced forces, apply bridge hitting to avoid side loading during slitting operations:
- 39. Large Holes Without Exceeding Press 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
- 40. 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
- 41. Punching Thin Material (<1mm) Common Poblems / Solutions Sheet marking Urethane stripper pads – Ultra Light Slug pulling Maximize die penetration – Demagnetise tools – Slug Free Light – Correct die clearance – Slug ejectors – Shear on punch High tool wear due to small clearance Maintain turret alignment Work piece distortion/stretching Change punch pattern – Slow down machine – Sharpen tools – Use Fully Guided
- 42. Punching 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
- 43. ‘ 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 .
- 44. Preventing Slug Pulling Use 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 !)
- 45. Overcoming 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
- 46. Galling Galling 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.
- 47. Reduce Galling Use 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
- 48. Small Diameter or Narrow Holes Material Aluminum Mild Steel Stainless Steel Punch to Material Ratio 0.75 to 1 1 to 1 2 to 1 When punching small diameter or narrow holes, maintain the following ratio of punch size (minimum) to material thickness:
- 49. 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) Small Diameter or Narrow Holes With Fully Guided
- 50. Minimum Spacing Between Holes and Forms Minimum = 2 x Material Thickness between holes Minimum = 2 1/2 x Material Thickness to sheet edge Top view of sheet To avoid hole distortion :
- 51. Minimum Spacing Between Holes and Forms Minimum = 2 1/2 x Material Thickness to sheet edge Spacing between forms
- 52. Nibbling Nibbling Minimum feed rate is equal to material thickness.
- 53. Nibbling Nibbling is not always recommended …
- 54. Excessive Nibbling Nibbling patterns can be done with much less hits with a banana tool or multi-radius tool (with tool rotation)
- 55. Punching Tip Combating Warpage To avoid sheet warpage when punching multiple holes, punch every other hole first, then come back
- 56. Cluster Tools 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)
- 57. Blanking -- When the slug becomes the “good” or the “saved part” Blanking Operations
- 58. Blanking Operations Blanking rather than punching can save time, tool life and sheet usage … Special shapes can be done with a combination of standard tooling
- 59. Blanking Operations 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.
- 60. Blanking Punch Configuration Punch 3.0-5.0mm flat 1.5-3.0 degree one-way shear can be used ©1999 Mate Precision Tooling Note: Shear punches give flatter workpieces
- 61. The ‘BIG 3’ Problems Die clearance too small Poor or no maintenance on the tool Turret alignment requires adjustment
- 62. Close-up of a burr
- 63. R = 0.25mm 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 tools When to Grind ?
- 64. 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 Calculating grind life How much to Grind ? Max. Thickness SBR Ultra A 3,8 18,9 B 6,4 18,9 C-D-E 6,4 25,5
- 65. More than DOUBLE the tool life when sharpened frequently! Importance of frequent grinding How frequent to Grind ?
- 66. 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 How to Grind ?
- 67. Sharpening Excellent sharpening results
- 68. IMPORTANCE OF CORRECT MAINTENANCE : bad sharpening practices
- 69. Benefits of Proper Tool Maintenance Flatter sheets (Note: Shear punches give flatter sheets) Cleaner holes Less stress on tool and machine Longer tool life Maxima coating not affected
- 70. 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 Life
- 71. The ‘BIG 3’ Problems Die clearance too small Poor or no maintenance on the tool Turret alignment requires adjustment
- 72. Thick Turret Alignment Tools Mate PILOT Alignment System NEW! Standard Alignment tool
- 75. Mate PILOT Indicator light color : Red – not aligned Yellow – aligned within 0.0012 (0.030) Green - aligned within 0.0003 (0.008)
- 76. Loosen upper and lower tool holder retaining screws Mate PILOT
- 77. Station Alignment to machine alignment bar and dial-indicator Mate PILOT
- 78. Questions?
- 79. Thank you ! www.mate.com