Press Brake

Instructions on Choosing a Press Brake Tool

Precision of shaped pieces depends on tooling. Tools of press brake machines may sometimes be considered not so important during metal formation process. Press brake machines have developed becoming multi axes, accurate machinery that are featured with self stabilization, however tool is the accessory touching immediately the piece in bend process.

Fig. 1

Different tools possess characteristics that provide top quality bending. No matter what tool or clamp technique to select as long as it corresponds to some of the least requests.

Great accuracy: To provide a high degree of piece precision tooling must possess endurance of 0.0004 inches to avoid trigging and pinching while making settings.

Sectioned segments: This makes it possible to make lines from pre cut parts. It is safe and easy to process little parts.

Self retain setting: Tool loading while the ram is raised is possible as tool keeping technique allows holding many parts in their places under the clamp pressure. (Fig. 2)

Self-seating: While applying clamp pressure, punches automatically come to proper positions. As a result, lowering punches in dies while making settings is not required.

Head load: This means possibility of installing a tool at the front part of a press brake. Due to it setting duration is shortened as there is no need to slide tooling from the back of the machine. Besides, this makes any forklift or crane unnecessary.

Standard forms: Tooling that possesses a standard height reduces the demand to adjust the press brake while realizing different tasks. Head supporting rods, back gauge height security units appear in average positions. As tools do not differ in height, making a work piece that matches the available tools.

 Most advanced tools correspond to metrical norms. Thus, a nominative size is 0.250 inch and V-opening is 6 mm(0.236 inches). Besides, plate metal bending has a little eleptic angle radius. For correction just getting closer is enough. Herein we have summed up great sizes. This information refers to air bend process as in most cases it is preferable to apply air bend process and avoid bottoming or coining. However, this technique may not be applicable to produce some kinds of work pieces. Most often work pieces are made through improper tools because of the absence of appropriate tooling. This is most often inefficient and cannot be repeated all the time. While selecting a tool, the utmost aim should be providing top-quality products in short duration.

Fig. 2

The needed tools and selection reasoning.

More varied tools are required for maintenance than for fabrication. Specify what are your requirements and budget constrictions. Say, an extra tool may be necessary for you to reduce the time of settings. Note manufacturing rules, identify advantages of achieving tooling athenaeum, referring to every press brake machine. This will help to save time and keep you from searching the proper tool. The tooling does not have to be compatible when it is transferred to another machine, as the tool is intended for the designed press brake (Fig. 3). 

In case extra duplicating tool is needed for expansion of the machine`s tooling cot, selection is much simpler. Tools should possess the utmost wear-out and shining, glossy work face. Avoid unclean and rustle tools.

Selection of dies.  

To save your budget, select as few bottom dies as possible, which can be applied in the full series of sheet thickness that your workshop fabricates. Workshops less informed and small budget had better select bottom dies applying 8×2 standard.

Identify metal thickness being bent. For instance, it may happen so that sheet of 0.030 has to be bent through 0.250 thickness. Then estimate the least required V-die, for which the slimmest sheet must be multiplied by eight. So, 0.030-in. sheet needs the least die, 0.030×8 equals 0.24, which in its turn is approximated to 0.25.

Estimate the biggest required V-die. The thickest sheet is multiplied by eight. Here the thickest sheet is 0.250 inch: 0.250×8 equals 2. So, the least die is 0.25 and the biggest one is 2 inches. Begin with the least V-die, then multiply its dimension. Here 0.25×2 equals 0.5, then multiply 0.5-inch die and you will have 1.0 inch, by multiplying once more have 2.0 inches. This makes bending of 0.030 to 0.250 in. possible.

Selection of Punches

Sheet density has to be taken into account to identify the least number of top punches. In case of 0.187 inch or slimmer a sharp off-set blade of 0.04 inch semi diameter is applicable. The sharp corner makes 90 degrees bend possible, offset allows forming J-forms. For 0.187 and 0.5 inch thickness while bending direct punches possessing 0.120 inches semi diameter are preferable to expose greater force.

Fig.3 Tool crib at every machine shortens setting duration

There are certain operations, such as processing materials of considerable thickness and extensile strength, that cause work pieces to fold, fracture and break if applying usual industrial bend norms. This is because tight punching nozzles apply a bigger strength upon bending length, a narrower V-shaped slot is added to it, which increases the strength. In case of intricate operations or more than 0.5 inch thickness have a consultation with your suppliers about punch nozzle semi diameter recommendation.

Principle of 8

V die openings must be eight times as thick as the sheet. Plate density should be multiplied by eight. For instance, for a plate possessing 0.060 inch thickness, a 0.5 inch die is needed (0.060×8 = 0.48 approximately 0.50), in case of 0.125 a one-inch die is needed. The most perfect angle is provided by this coefficient. It is called sweet point while selecting the V die.

Radius depends on V die opening. 

In the process of soft steel air bend, the inner bending radius is formed at about sixteen percent of a V die slot. In case of bending more than 1 inch sheet, the inner bending radius is 0.16 inch. Most cases show that a plate metal designer specifies a bending semi diameter (radius) that is as thick as the sheet. So, one-inch die produces a semi diameter of 0.160 inch. Then a narrow V-die can be used. The 0.75 inch die gives inner radius of 1.125 inch. (0.75×0.16=0.12) This same is applied for specifying a bigger bending radius. For example, for bending 0.125 inch thickness soft steels to 0.320 inch inner bending radius, decide on a die of 2 inches, that will create an inner bending radius of 0.320 inches. Provided that you specify that for inner bending radius, a V die slot is needed, which is five times thinner than the sheet, you will come to angle inaccuracy, the device and its tool fault creating insecure conditions.

The Least Flange length 

The factor of flange length is very important in V-die selection. The least flange formed by V dies may be about 77 % of a V-die slot. A one-inch V die produced piece needs minimum 0.77 inch flange. (Fig.4)In accordance with 8 principle sheets of 0.125 thickness need one-inch V dies, which in their turn demand a work piece possessing minimum 0.77 inch flange. Then a narrow die may be used. V dies of 0.625 inches may produce pieces possessing a flange of 0.5 inches. (0.625×0.77 equals 0.48, approximately 0.5).

Fig. 4

In case flanges require dies five-times-less-than sheet density breadth, you may face inaccuracy, device and tool faults causing unsafe situation.

Principles on Choosing Punches

Nearly all punch forms are acceptable to make an L shape. That is why while choosing a punch for a piece, L shapes can be the last to consider. While shaping L forms choose a punch applicable in different punches formation, instead of applying additional tooling. Tool selection is acceptable as long as it reduces setting duration through decreasing tool form numbers, that are required at shop floors.(Fig.5)

Other forms demand particular principles while choosing punches. While shaping J forms, instructions are given in Fig. 6.

  • Goose-neck punches are needed if the little top leg exceeds the down leg in length.
  • Almost all punches are acceptable if the down leg exceeds the little top leg in length.
  • And offset sharp punches are needed in case of their equality.

So nearly always work piece determines punch choosing process. Here bend simulating software plays a major part. Provided that the software is not accessable, make use of manual handling. (Fig. 7)

Off-set Principles

While applying a traditional tooling, two ram-cycles are needed for the formation of an off-set and Z shape. Instructions are as the following;

  • Central leg, i.e. web should exceed the V die breadth in size. Note it is the whole die breadth, but not V die slot.
  • V die highness together with riser highness should exceed the lateral leg.
  • If central leg is no more than the half of a V die, a particular tooling is needed to form two bends in a single impact. There is no necessity to upend the sheet at the upside. As to the down side, it requires a triple more than standards bend strength.

Instructions on bending Across Cut-outs and Mitres

All materials without support in V dies are inclined to get deformed, inside a hole or a cutout this deformity appears in the form of rupture.(Fig. 9) If the hole close to a bending line is little, the rupture is also little. No particular instruction exists connected with a perfect V die breadth in case cutouts appear close to bending lines. If the flange, cutout, miter appear near bending line, decide on a rocker die. This type of die rotates supporting materials while the bend procedure is going on, eliminating blowouts.

You can see pieces with a cutout near bending line. The telltale blowout in the front is shaped through a traditional V-die. The one in the back is shaped through a rocker die. Both oval shapes upon left-side possess the similar breadth and space from bending lines. But they are not the similarly long. The oval possessing more length bears more blowouts.

Punch Height For a Particular Box Depth  

For a three or four-side box height of punches is very crucial. Sometimes a short punch forms a three-side box, when one of the shaped sides hangs off press brakes in the process of conclusive bending. In case of four-side box formation, decide on high punches spanning box highness along diagonal lines. (Fig. 10)

Fig. 5 For most pieces punch shapes do not insist bend restriction

The least height of a punch to bend boxes is equal box deepness/0.7 + ram density/2. In the absence of upper flange or when the upper flange protrudes outwards, there is no need for a big gap in the midst of upper and bottom dies to move the piece away in the aftermath of bend process. If it happens so that a returning flange appears (the upper flange protrudes inwards) on each side, the gap should be enough to move boxes away in the aftermath of bend process.

Combination Bends and Hems

Hem-bending tools shape hem-edged pieces through just one setting.(See in Fig. 11) For hemming more than 0.125 inch thick material customizing of tools is needed to handle with the necessary great force. Pre-bending of 30 degree for hemming requires quite long least flange, which is nearly 115 % of chosen V die opening due to sharp corners. For the formation of materials of more than 0.375 inch, flanges should be no less than 0.431 inch. (0.375×1.15)

Pieces Without Scratches 

There is no V-die tool that does not make a mark on pieces. More often marks are allowable. Marks may be avoided if shoulder semi diameter is increased. In an application when the least mark is not allowed, for example in the bend process of pre-painted or burnished material apply a nylon insert to avoid scratches. (Fig.12) Bending without scratches is necessity in the fabrication of aircraft pieces, as in case of inspection it is nearly impossible to identify between scratches and cracks.

Simplicity provides efficiency.

The advanced accuracy tools as well as press brake machines are capable of reaching high precision. Having proper tooling and materials press brakes provide bending of flanges at particular angles possessing a particular inner bending radius. The inner bending radius is formed according to some percent of die openings. So proper tooling is crucial. In case you specify various, strictly tolerant radius, tool cost will rise. More tooling brings about more rearrangement again increasing the cost.

 Here are three main instructions to be followed during piece design process to select tools and operate the whole bending more easily. 

  1. Inner bending radii must be 1.5 times as thick as the sheet.
  2. Flanges should have length minimum 6 times more than the sheet thickness. This is about holes as well, i.e. they must not be close to bending lines. The space should be minimum 6 times the sheet thickness.
Fig.6 The rule to select punches for a J shape is as the following: if the upper and down legs are equal, use sharp off-set punches, (see  left side) if the upper and down legs are different in length, use goose-necked punches (see right side)

For bending tight radii or short flanges narrow V die openings are suitable. However, bending excessively acute radii causes bending lines to crease, exceeds tonnage rate for tools as well as for press brakes. Then bending narrow off-sets requiring certain tooling or tonnage rate is unavoidable. In case the piece does not require short flanges, confined off-sets, tight radii, just note the three instructions mentioned above, which offer improved angles, short duration of setting as well as reduced tooling costs.

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