lit00954 sb reducing sheet distortion · try using a cluster tool (photo, below). a cluster tool...

SOLUTION BULLETIN

1295 Lund Boulevard ● Anoka ● MN ● 55303 Phone 763.421.0230 ● 800.328.4492 ● Fax ● 763.421.0285 ● 800.541.0285

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REDUCING SHEET DISTORTION IN A PUNCH PRESS

THE PROBLEM: Punching thousands of holes in a single part in a punch press will create sheet distortion, no doubt about it. The process involves a punch tip pushing material down through a hole in a die, stretching and fracturing the material through the hole in the die, while creating stress in the area around the punch tip. While there is no way to completely eliminate distortion, there are some ways to reduce it, making life easier on the operator and reducing the quantity of scrapped parts. MATE TIPS AND TECHNIQUES: This Solution Bulletin provides you with tips and techniques that Mate has uncovered over the years to minimize sheet distortion in a punch press that we have found useful. PUNCHING TOOLS: • Hole Quantity. If possible, reduce the amount of holes to be punched by spacing the holes

further apart. This will not only speed up the punching process, it will also reduce the stress put on the sheet by reducing the number of holes to be punched in it.

• Hole Size and Spacing. Increase the hole size and the spacing between holes. This will allow

the same amount of material to be removed while reducing the number of holes required. • Cluster Tools. If increasing the hole size is still not an option,

try using a cluster tool (photo, below). A cluster tool allows a number of holes to be punched at the same time.

o Using cluster tools (a single tool with multiple punches installed) will reduce the number of hits required to complete a hole or perforated pattern.

o Cluster tools are typically designed to punch a specific hole pattern, with adjacent punches removed from the cluster. They are ideal to guarantee center-to-center tolerances. For greater hole uniformity and flatter sheets, spread the punches to avoid punching adjacent holes in the same hit.

o Complete the desired pattern with the technique known as bridge hitting. Cluster tool use may require additional programming to place hit locations

correctly. To help minimize sheet distortion, a staggered hit pattern is recommended. For example, creating a 4 hit pattern (hit #1, hit #2, hit #3 and hit #4) to punch an area of the perforation that can be repeated equally to complete

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all (or most) of the hole pattern. Punch order procession: punch all of the hits #1, then punch all of the hits #2, etc., until the complete pattern has been punched.

• This method may require an additional single punch to complete the hole pattern. Do not re-punch through a previously punched hole to complete the pattern.

Use staggered punch lengths in a cluster assembly. This will reduce the tonnage necessary to create the holes. When using staggered tool lengths, be sure to compensate die penetration for the shorter punches to push their slugs into the die far enough to prevent slug pulling.

o Other constraints depend upon station size, punch size and shape, press tonnage capacity and distance between the border of the holes (web).

• Die Clearance. Correct die clearance is very important to

help reduce sheet distortion. Too much clearance results in the material stretching more before the fracture of the slugs occur. This adds stress to the sheet and increases distortion. Too tight of clearance makes the tooling work harder and increases the heat transferred to the punch tip and the sheet, adding to distortion.

• Die Height. Be sure that all dies inserted into a turret press

are the same height to prevent unwanted marking and help reduce the possibility of sheet deformation. Uneven die heights, especially forming dies (they sit higher than a punching die in a normal turret configuration) can cause extra flex in the sheet metal when installed next to punching dies.

• Brush Inserts. Brush inserts (photo, left) placed in unused die holders

can help reduce sheet marking and improve the quality of the part by minimizing the amount of contact the underside of the sheet makes with tool steel.

• Sharp Tooling. Keeping the tooling sharp will help keep holes accurate

and reduce sheet distortion. Sharp tooling with proper die clearances will produce the best holes with the least amount of force required to create those holes. More frequent sharpening while removing less material from the tooling will result in greater hole quality and accuracy while greatly increasing tool life.

• Fully-Guided Tooling. Fully guided tooling helps to insure accuracy and improve tool life, while

minimizing sheet distortion. Fully guided tooling also increases hole accuracy by guiding the punch tip through the stripper with greater accuracy, reducing the chance for punch deflection and distorted holes, and reducing the chance for damaged or worn tooling.

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• Punching Direction. Always punch perforated and large hole patterns starting away from the clamps. The material will stretch as it is punched, and by punching towards the clamps, the pitch of the hole pattern will remain consistent.

• Lubrication. Lubrication is recommended, especially when using cluster tools. Lubrication helps with

hole punching, prolongs tool life, and reduces heat build up in the tooling and the sheet. Mate Eliminator pads can be utilized in many punching applications. These pads add lubricant to the punch tip with each stroke of the machine.

• Coining. Coining of sheets may be necessary to help reduce

sheet distortion when punching a large hole pattern consisting of many holes. A combination of angles added to a die and stripper may help reduce sheet distortion by coining the sheet as it is being punched. (See image, right.) This is a difficult process to fine tune, as it may require some trial and error punching to determine stripper pressure settings for best results.

• Symmetrical Sheet Patterns. If a sheet has a symmetrical hole

pattern, punch the sheet with half of the holes then flip it over and punch the other half. This will help equalize the sheet stretching of both halves (this method may have limitations due to cosmetic finish and appearance of the punched part).

• Gripper Strippers or Gripper Stencils. These are usually used on down forms that use standard

punch holders where a V-line stencil is placed around the stripper opening and to dig into the material. Doing so forces the material to stretch mostly from inside the form and not the surrounding material.

• Additional Programming Settings. Program settings such as dwell commands and reduction in sheet

or table acceleration may not reduce distortion, but they can help insure accuracy with large perforated hole patterns. Assuring that the tools have cleared the sheet and the sheet has come to a full stop before punching will help with the accuracy of the hole locations.

ROLLING TOOLS: Rolling tools, such as Mate’s Rollerball®, might be the most difficult application to keep a sheet flat when adding features to a part in a punch press. Here are some tips to help reduce sheet distortion when using roller tools. • Direction of travel. If possible, program the rolling tool to start near the clamps and form away from

the clamps. This allows the punch press to pull the sheet through the forming process rather than push the sheet towards the form. Pushing the sheet toward the forming process may cause the sheet to flex, resulting in extra distortion and—possibly—scrapped parts.

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• Using a “ramping or lead-in” command* will help reduce sheet distortion and prolong the life of your roller inserts. This command will ease the tool down into the form as the sheet is moving, reducing the initial stress of the material and creating a nice looking transition into the form. You also eliminate the “pinch” mark that appears when a ramping or lead-in command is not used.

• Use multiple pass programming when possible. When creating higher forms or forms in thicker material, using multiple pass programming helps reduce sheet distortion and prolongs the life of roller inserts. With this technique, multiple passes of the formed contour are made, each at a slightly lower ram/tool position than the previous pass. This allows the machine to create the forms with less material stress and load on the tooling during the forming process. (See photo above.)

• Other considerations:

o When forming an open contour (the start and end of the contour are not joined or closed), use a “ramping or lead-out” command. While this command won’t necessarily reduce sheet distortion, it will give the start and end of the contour a uniform look, while also avoiding the “pinch” mark from the tool raising away from the sheet after the form is complete.

o When forming a closed contour (where the start and finish points are the same), add a step for an overlap of the “ramping or lead-in” area to complete the uniform look of the contour. This won’t reduce sheet distortion, but it will create a nice looking form.

Photo at left shows single pass programming. Photo at right shows multiple pass programming.

* Please note the specific name for this type of command varies by the type of machine. For example, PrimaPower uses “Wheel_Soft”; while Murata Wiedemann uses “ramping”. Others may refer to it as “lead-in.” The concept is the same. Refer to your machine documentation for specifics.

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MACHINE SPECIFIC TIPS MURATA WIEDEMANN PRESSES • Use of Wheel Type Programming with Ramping function eases the Rollerball into and out of (if

desired) the material until the desired form height is reached. This reduces the initial load placed on the inserts as well as the stress put on the material. A smooth transition into the form helps reduce distortion and prolong life of the roller inserts.

PRIMAPOWER PRESSES Soft Punch Feature. The soft punch feature brings the punch tips down at a slower speed, until a certain amount of resistance is applied to the punch head. The ram is then driven downward with the punching force required to penetrate the material, reducing the initial impact to the punch tips and the material. This can also help control sheet distortion as it softens the impact of the punch tips to the sheet. TRUMPF PRESSES • Use the presser foot function.

o The presser foot function is designed to apply pressure to the sheet from the bottom of the stripper before the punch tip reaches the material. The stripper holds the sheet with the programmed (or manually set) amount of force while the ram pushes the punch tip through the sheet. The stripper continues to apply pressure to the sheet until the punch tip clears the sheet top; the stripper then releases from the sheet. While this is a slower punching process, the presser foot feature helps spread the punching force applied to the full area of the stripper, helping to reduce sheet distortion.

• Use the soft punch feature. o The soft punch feature brings the punch tip down at a slower speed, until a certain amount of

resistance is applied to the punch head. The ram is then driven downward with the punching force required to penetrate the material, reducing the initial impact to the punch tip and the material. This can also help control sheet distortion as it softens the impact of the punch tips to the sheet.

• Use the built in vacuum system. o Stroke the punch tip to a maximum of 1mm die penetration and the default value of 1mm

stripper height. The shorter die penetration reduces the heat transferred to the punch tip during punching/stripping, reduces the chance of galling and increases the speed of the punching process.

OTHER MATE PRODUCTS TO CONSIDER: • Brush dies. • Cluster tools. • Mate fully-guided tooling (Ultra TEC®, Marathon® and Marathon Plus™)

lit00954 sb reducing sheet distortion · try using a cluster tool (photo, below). a cluster tool...

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