Will the form fit?

Tackle feed clearance challenges on the turret press in-house

Wilson forming tooling picture.

Sheet metal fabricators can determine if a form can be created on a punch press machine by taking into account the functionality of the equipment and using some basic math.

It’s one of the most common questions among manufacturers: Will the form fit in the machine? While one of the easiest and sure-fire ways to answer this question is calling the tooling manufacturer and asking for help, sheet metal fabricators can easily answer this question themselves by taking into consideration specific application factors, assessing the functionality of the punch press, and implementing some simple math.

Determining if the form will fit in-house can save fabricators hours of time in quoting jobs. Bypassing an extra call to the toolmaker to find out if their machine can do the job helps fabricators send out quotes and bring in new jobs much quicker, keeping production up to speed and increasing revenues.

The first step is defining and understanding feed clearance and the challenges this space can present. Simply stated, feed clearance is the machine’s allowable workspace, which is generally 0.787 in., depending on the type of turret press. This is the space where operators can move the sheet around to create the form unencumbered. Incorrectly measuring feed clearance or making assumptions about available space can lead to machine damage, sheet damage, and incorrect forms—all while compromising operator safety.

Measuring feed clearance determines whether or not the desired form will fit in the punch press. While feed clearance commonly is thought of as the space between the upper turret and the lower turret, this is actually incorrect. The feed clearance measurement is actually the distance between the upper turret and the piercing die. Because stripper plates are slightly recessed into the machine, the lower turret is not included in measuring feed clearance.

Space and Design Considerations

When determining if the form will fit, fabricators must first assess the current limitations of the space itself, such as safety distance, the size of the finished form, material thickness, and the size of the taller die, also called the die add-on. All of these features contribute to the level of available feed clearance. Ejectors are an additional consideration, as they may hang down into the feed below the turret clearance zone. They normally do not require accommodation because the operator usually can move the sheet sideways or around the heels of the ejector to properly and safely create the form.

Design is a major component in determining feed clearance and using that space accurately. When designers understand the assets and limitations of the turret machine, required tooling, and form type, determining minimum feed clearance becomes less challenging, and they can implement simple mathematical formulas to determine feed clearance with ease.

Understanding Machine Functionality

Naturally, different turret machines provide varying limitations and assets. Taking the time to fully investigate a system can help fabricators learn how to enhance the quality of their products and better meet the needs of their customers. When figuring feed clearance in particular, understanding the specific functionality of a machine can save operators and designers significant time in troubleshooting and trial and error.

Special forming functionality available on many types of turret machines is a feature that can be beneficial for sheet metal fabricators. Turret presses often are equipped with forming stations or retractable systems, which allow operators to keep the sheet flat during forming operations. In Prima Power machines, for example, this functionality is called a retractable die station; TRUMPF calls them active die stations; and in Amada machines they are known as forming stations or punch and forming stations. Many other brands of turret machines have some type of forming station as well.

The goal of this machine feature is to help the operator maintain a flat sheet. This practice helps reduce distortion and create higher-quality parts by preventing the sheet from tilting up during the forming process. In the operation, first the punch comes down and engages the sheet, the forming cylinder or the die station comes up to complete the form, and then the system retracts. The application allows for forming the part while keeping the sheet flat.

In all types of forming applications, regardless if the machine has a retractable system, completing the forming up step in the process first gives operators more flexibility in working within the limited feed clearance. Performing the form down process last helps ensure higher-quality parts and reduces the risk of machine or sheet damage.

Formula for minimum feed clearance for positive forms.

: Figure 1: Use this formula to determine the minimum feed clearance for positive forms such as bridges, embosses, electrical knockouts (EKOs), dimples, half shears, lance and form (up and over only), louvres, ribs, thread forms, and wheel tools. The die add-on equals the form height.

Feed Clearance and Production Planning

Accounting for feed clearance in production planning is essential for increasing efficiencies in quoting jobs and estimating. Feed clearance must accommodate safety distance, the size of the form, sheet stock, die add-on, and extra- thick stripper, so all of these variables need to fit into the equation.

This is where simple math comes into play.

The safety factor, or safety distance, is a simple measurement that can be applied to most types of forming applications on the turret press. Normally, safety distance accommodates sheet distortion and the effects a tall die can have on the sheet. While the sheet might not always lie flat during the forming operation, it can hit the turret if it takes up all the space. As a result of these obstacles, the suggested safety distance is 0.12 in. Once operators become more comfortable with a particular part run, the safety factor could be reduced to a minimum of 0.060 in. This is a figure fabricators can keep in mind when implementing one of the three formulas to figure minimum feed clearance.

While universal terms for the limitless types of forms are not standardized in the industry, most can fall under three common classifications of tool design: positive forms, pass-through or pass-by forms, and wipe-up or 90-degree forms. Each form presents an individual situation for determining feed clearance, which begins with determining the die add-on measurement.

The die add-on is the measurement of how much taller the tool needs to be to correctly and safely create the form. In other words, Die add-on = Form height + Material thickness. Each of the following examples illustrates and explains the formula for calculating feed clearance for the three common types of forms.

Positive Forms

For positive forms, the die add-on equals the form height. Examples of positive forms are bridges, embosses, electrical knockouts (EKOs), dimples, half shears, lance and form (up and over only), louvres, ribs, thread forms, and wheel tools.

This is the formula for calculating minimum feed clearance for positive forms:

Minimum feed clearance = Safety factor + (2x Form height) + Material thickness

Figure 1 shows a bridge form example that exhibits a form height of 0.130 in. and a material thickness of 0.060 in. Using the maximum recommended safety distance, the minimum feed clearance in this example is 0.44 in.

Minimum feed clearance = (0.12) + (2 x 0.130) + (0.060)

Formula for minimun feed clearance for pass-through or pass-by forms.

Figure 2: Use this formula to determine the minimum feed clearance for pass-through or pass-by forms such as card guides, coining tools with a pilot, extrusions, countersinks, lance and form (up 90 with straight sides), and progressive edge forms. One material thickness equals the radius.

Minimum feed clearance = 0.44

Pass-through or Pass-by Forms

Instances where the die add-on is greater than the form height are called pass-through or pass-by forms. In these cases, the machine must shove the tool completely through the extrusion. Commonsense measures can be valuable here, such as the radius on the tool should be the same as the material thickness. In other words, one material thickness = radius.

Examples of pass-through or pass-by forms are card guides, coining tools with a pilot, extrusions, countersinks, lance and form (up 90 with straight sides), and progressive edge forms.

This is the formula for figuring minimum feed clearance for pass-through or pass-by forms (see Figure 2):

Step 1: Die add-on = Form height + Material thickness

Step 2: Use the formula from Figure 1Minimum feed clearance = Safety factor + (2 x Form height) + Material thickness

Minimum feed clearance = (0.12) + (2 x 0.156) + 0.048

Minimum feed clearance = 0.48

Wipe-up or 90-degree Forms

In a category by itself, wipe-up or 90-degree forms have a die add-on that is less than the form height. In these applications, the operator is not cutting and forming the sheet; all that needs to take place is wiping the form up and not passing beyond 90 degrees. Accommodations for these types of forms include the bend radius on the punch and the material thickness.

This is the formula for figuring minimum feed clearance for wipe-up or 90-degree forms (see Figure 3):

Formula fpr minimum feed clearance for wipe-ups or 90-degree forms.

Use this formula to determine the minimum feed clearance for wipe-ups or 90-degree forms that have a die add-on that is less than the form height. Accommodations for these types of forms include the bend radius on the punch and the material thickness.

Step 1: Die add-on = 3 x Material thickness + Bend radius

Step 2: Minimum feed clearance = Safety factor + Overall form height + (3 x Material thickness + Bend radius)

Minimum feed clearance = 0.12 + 0.502 + (3 x 0.06) + 0.020

Minimum feed clearance = 0.822

Faster Quotes, More Jobs on the Floor

While the tooling manufacturer is undoubtedly a beneficial resource, fabricators can easily determine feed clearance in-house, saving hours on the time required to turn quotes around to potential customers.

Incorporating the safety factor, die add-on height, form height, and material thickness into the mathematical formula for the required form allows sheet metal fabricators to have greater independence and confidence in answering the question, “Will my form fit?” with in-house skill and resources.

Nick Kassermelli is business unit manager, Wilson Tool Canada, 800-268-5573, www.wilsontool.com.