Bending all types of material (steel, stainless steel, aluminum, etc.) on a press brake is not new.

Press brakes have evolved a lot since their beginnings, however, regardless of their degree of sophistication, sources of errors in folding remain present on a daily basis.

The aim of this article is not to detail each source of error in depth but rather to make a summary list, in order to give you some ideas or simply to inform you. We have subdivided these sources of error into 4 groups: the material to be folded, the construction of the press brake, the positioning of the axes and the tools used.

Material

Among these four sources, the most difficult to control is certainly the consistency of the material to be folded. All you have to do is measure the thicknesses of the same material ordered for each of your orders and request a mill test (“Mill Test”). You will be able to see that the characteristics and dimensions can vary considerably. The lower the quality of the material, the more likely you are that the properties and dimensions of the same plate or sheet will vary. The higher quality your material is, the more homogeneous it will be and the stable properties, the better the results will be in terms of folding and the less scrap you will generate. Sometimes it is better to pay a little more for the equipment and avoid some unpleasant and costly problems and expenses.

Construction

The second group concerns the construction of the press brake you are using. New generation presses are generally equipped with a compensating system for the bending of the lower apron and frames. The design and quality of these systems is essential when the time comes to bend in multi-station mode or when you bend relatively long parts and at tonnages greater than 30% (sometimes less) of the nominal capacity of your press brake. For example, if you fold to the left, center, or right of the press, the gooseneck-shaped press posts open differently depending on where you fold. This bending will influence the bending angle on either side of the part if it is not compensated correctly. In addition, this bending of the press uprights tends to shift the punch forward and consequently, if no mechanism is in place to compensate, the errors add up.

In terms of table flexion, since the objective is to obtain a constant angle from one end of the part to the other and since the hydraulic cylinders on most presses push at each end, a compensation system Bending of the lower apron (“crooning”) is necessary if you are looking for a minimum of precision. These compensation systems are mainly found in three versions on the market: mechanical, hydraulic-CNC, hydraulic-CNC & Dynamic. The first two types are based on a position estimate by the press controller. The third type, hydraulic-CNC & Dynamic, allows the deflection of the lower deck to be adjusted in real time according to the measured deflection of the upper deck. This type of system is very efficient and greatly increases the quality and consistency of folding, whether for long parts or in multi-station mode.

An equally important and rarely considered aspect is the quality and dimensions of the steel used to manufacture the press. The use of thicker steel plates with a higher elastic limit will reduce the bending of the various components, which will help to improve the straightness and precision of the bent parts.

Also at the construction level, the precision in the positioning of the different axes (top apron, rear stop, etc.) is undoubtedly a very important factor as well. Obviously, proper maintenance is required since cumulative clearances can cause significant errors. The new press brakes, Synchro type (Y1-Y2) equipped with rear stops controlled with ball screws or rack generally offer very interesting precision.

Axis positioning

A third source of potential error lies in the calculation of the final positioning of the axes. From one numerical control manufacturer to another, positioning calculations may differ since they do not all use the same calculation formulas. Consequently the operator must constantly make corrections to the position of the axes when a certain precision is sought. In addition, there are many designers of CAD-CAM conversion software that can generate a folding program from a 2D or 3D drawing and once again they do not all use the same calculations. Good integration between this type of software and your press controller is essential. In addition, the bending data (elastic limit, thickness, length, punch, die) must be entered as precisely as possible in order to minimize sources of error. There are a few real-time angle measuring systems on the market, which take a few measurements during the bending process to determine the material's Spring Back and exact positioning based on the desired angle. Although the concept is very interesting at first glance, the solutions offered to date are quite expensive and often present significant drawbacks (interference with parts, frequent calibration, slowness of the process, etc.) and are often put aside after a few uses.

Tools

The last source of error identified at this time is associated with the tooling used to bend your parts. Very frequently we installed new press brakes and tried to recover the used tools that were in the workshop. The result is more often than not very disappointing. Worn, damaged or distorted tools have a direct impact on precision and quality. Additionally, if your press brake is of a certain age or has been subjected to improper use, the condition of the punch attachment surface and the table can also cause inaccuracies. The basic rules of folding are already known and still prevail. However, you must ensure that you use precise and appropriate tools.

In conclusion, if you use equipment with uniform and well-identified properties, a quality press brake with systems compensating for the bending induced during folding, you have already eliminated a certain number of sources of error. The precision of the estimate calculated by the digital control of your press is difficult to modify, all you have to do is ensure that you use quality tools in excellent condition. Happy folding!

Yves Garant, eng.
Manufacturing Advisor

This article aims to compare two types of data in terms of precision of folded parts, namely the generic data from the Machinery Handbook and the specific data from the DIN Standard.

The widespread use in North America of the basic data from the Machinery Handbook for calculating the dimensions of folded parts generates errors which represent per fold +13% of the thickness of the material in the case of 301L stainless steel - MT and +15% for A-710 steel.

The use of the same data (generic formulas) for all ranges of carbon and stainless steels mainly explains the excessive errors that we encounter in the majority of companies when parts of high thickness and with multiple plies are manufactured. Data from drawing software and digital folder programs often comes from these generic formulas.

The general tolerance for a bent part is ±0.062″. The probability that the first part manufactured will be out of tolerance is therefore very high. Additionally, cumulative errors are generally found on the last section of the part where the +0.061″ is already equal to the maximum tolerance.

The error being proportional to the thickness and cumulative to the number of plies, the consequences of using a generic formula are enormous. Thus, for the same part of 0.250″ thickness, the cumulative errors would have been +0.077″/−0.013″ with an excess of 0.015″ over the maximum tolerance of +0.062″. It becomes easy to imagine the difficulties encountered when the thickness of the parts is greater than 1/4″ and when they have more than 2 plies.

The use of a specific table (Figure 1) for each type of steel according to the European DIN standard allows precision such that the thickness and number of plies do not risk placing the part out of tolerance.

This table was produced from tests carried out on 301L-MT stainless steel, all parameters of which have been factory-controlled to ensure reliability. Here, for a folding factor R/T = 2, the position of the neutral fiber is located at 37% of the thickness from the inner radius.

Here is a real example of a case encountered where the calculations show the error per fold that the generic formula generates in comparison to the data for a specific bend curve.

Basic data: T (thicknesses) = 0.187”, R (radius) = 0.375”, A = 3”, B = 3”
L = (R + DT) 1.571: formula for a bend angle of 90°
Or
L = length of the ply at the position of the neutral fiber D
D = position of the neutral fiber determined in % of the thickness from the interior radius

Note: for reference D is equal to K/2

  • Generic formula: D = 0.45 (according to data from the Machinery Handbook)
    L = (0.375″ + 0.45 x 0.187″) 1.571= 0.721″
    DIFFERENCE: +0.024″ or +13%  thickness
  • Specific formula: D = 0.37 (according to DIN standard with specific formula)
    L = (0.375″ + 0.37 x 0.187″) 1.571 = 0.697″
    DIFFERENCE: ±0.003″ or ±2% thickness

Following this example, we see that if we manufacture a part larger than 0.250″ with more than 2 plies, it should inevitably be out of tolerance. Unfortunately this phenomenon occurs frequently with several manufacturers who have not considered questioning the reliability of the basic data of their calculations. If you believe that we can be of any assistance to you in calculating your unfolds, do not hesitate to contact us.

Mr. RICHARD VAILLANCOURT,
STEEL BENDING TRAINER

This information might be useful...

Steel plate rolling is not new but even today, despite the degree of sophistication of new rolling machines (sometimes called benders), the fact remains that the perfect roller does not exist and that A competent operator is often the key to success. However, the new 4-roller rolling machines equipped with digital controls (NC or CNC) can greatly help the cause.
We will take a few minutes in this communication to tell you a minimum of things to know when you are looking for a roller. Particularly in terms of capacities and design.

Abilities:

The plate rolling machines available on the current market are mainly manufactured in Europe. Given that the steel available in Canada is generally stiffer (“yield strength” or greater elastic limit), many buyers often find themselves biased by the specifications provided by the European manufacturer. It is therefore important to specify to your advisor the elastic limit or at least the material that you roll most of the time as well as the harder materials that you might be prone to rolling.

In the world of rolling, subcontractors who roll several part sizes and different thicknesses generally have to use two or three different rollers to be able to cover all needs. For example, a high capacity roller (say 10' x ½''), must have a camber (ovality) at the level of the upper roller to compensate for its bending. It's a bit like a crowning system on a press brake. Therefore, if you are rolling thin material using this same roller, this camber will look to roll more from the center otherwise rolling your piece to the middle of the roller. In addition, it is impossible to roll parts with a diameter smaller than the upper roller, just as it is impossible to roll thick plates with a low capacity roller. Hence the need to have rollers of different capacities.

Most rollers are defined by their ability to “roll” AND “pre-roll” a certain thickness. For example, a roll with a capacity of 10' x ½ generally means it can "roll ½" and "pre-roll 3/8". The pre-rolling capacity is always lower than that of rolling because pre-rolling requires offset use of one side of the roller and therefore all the force available at the hydraulic unit. From one manufacturer to another, this capacity may vary but it is important to know that if you want to minimize flat sections at the entry and exit of parts, a good pre-rolling capacity is important. In addition, some manufacturers set the rolling capacity at 3X (3 times) the diameter of the upper roller while others do it at 5X. Which means that the 5X manufacturer manufactures a roller that is less rigid (diameter and roller material, motorization, etc.) than the one that can roll the material at 3X. For example, a roller with a capacity of 10' x ½'' having a top roller of 11'' could roll, in one case a plate of 10' x ½'' in a diameter of 33'' and in the otherwise, he could not roll it in a diameter smaller than 55''. This is a significant gap and it is important to know this nuance.

General rule, if you want to roll less than 3X the upper diameter, you can do so but for thinner plates. But if the plates are too thin, the camber of the upper roller could harm the result. Some models of plate rolling machines offer the possibility of changing the upper roller. Although the variation in roller diameter is not major, it is possible to play with the camber and give your rolling machine a wider range of possibilities. Your advisor, assisted by the manufacturer, will be able to make certain recommendations on this subject with a list of your rolling parts.

If you are looking to roll cones or minimize the length of flat sections, it is also a good idea to specify this to your advisor. Typically flat sections can vary from 1.5 times to 3.5 times the thickness of the material, depending on the thickness and grade. Cone rolling requires considerable effort on the mechanics of the rolling machine. Therefore, the maximum cone forming capacity is greatly reduced. Certain types of rollers (particularly those with 4 rollers, double hook) will make this application easier, although it is not as easy as it may seem in the videos found on the web. A well-trained and competent operator will quickly become skilled after a while. It is important to clearly specify all the parameters required for cone rolling.

Rolling hard plates (stainless steel, Hardox, etc.) requires increased effort. Choosing a rolling machine of sufficient capacity is important. This should also be specified upfront since if you frequently roll these harder materials, there are two types of steel for making rollers, one being more suitable for these types of parts. Likewise, the hardness of the rollers and the level of polishing could have a short and long-term impact on the lifespan of your rollers but also on the finish of your parts.

Design:

In terms of design, there are several configurations on the market. 2, 3 and 4 rollers, single hook, double hook, variable geometry. All its configurations have their advantages, whether in terms of price, handling or results. Here you will find the main ones with a brief description:

Three asymmetrical rolls (single fang)

This roller is the most common configuration due to its low purchase cost and the rolling principle, as you can see, is relatively simple. It is generally used for low capacity rolling machines.

Three reels in pyramid layout (double fang)

This type of roller offers an interesting compromise by allowing even thick plates to be rolled without too much handling. However, it requires a good level operator since it is easy to lose hold of the plate and squaring is not so easy to carry out.

Three rollers with variable geometry (double hook)

Unlike standard rollers, the top roller moves up and down. This type of roller is gaining popularity thanks to its ability to roll thicker plates than standard rollers. It is generally considered for rolling plates of 3″ and above. In addition, it offers an interesting overcapacity since with the side rollers opened to the maximum, it is possible to roll up to thicker 50%. Another interesting feature: the length of the plate at the start of rolling is shorter than on other types of rollers.

Two rollers with the lower roller coated in urethane

Most often recommended in a context of large production of relatively thin plates (3/16'' or less). Some can even roll up to 250 pieces per hour! In this system, roller A functions as a die, it determines the radius of the parts. So if you want to change the radius you need to change roller A with another roller or a suitable sleeve.

Four rolls (double fang)

This type of roller offers several advantages when compared to the three pyramid type rollers:

  1. Possibility of having a digital controller (NC or CNC): Step-by-step programming, complex parts, etc.
  2. Ease of execution and precision thanks to the pinching of the part with rollers D and C
  3. Squaring is easily done using one of the side rollers as a stop.

The 4-reel is by far the most popular configuration on the market today.

Other aspects of design

Other aspects are important in terms of design to guarantee you better results. For example, the positioning of the crushing roller (the one which determines the rolling diameter) must be ensured precisely. There are significant differences in design on the market. The principle which ensures the greatest precision is the electronic balancing of the roller(s). When electronically balanced, it is a bit like a Synchro type press brake, each side of the roller is controlled independently and the position measured using a sensor or encoder which guarantees very precise positioning. Some roller models on the market with a simpler and less expensive design use a “torsion bar” type mechanism which is more prone to causing inaccuracies in positioning and parallelism. If you are looking for high precision, this design should be avoided. The electronic swing is therefore more precise and also offers more possibilities if you are rolling cones.

Make sure the rolling machine you are considering is equipped with adequate roller drive. For example, if you need to roll small diameter coins on a 3-roller, it is better that all three rollers are driven otherwise there is a good chance that your coin will slip between the rollers. On 4-rollers, typically the side rollers are not driven since the two in the center pinch the material. This pinch is another important aspect if you are considering purchasing a 4-reel. If the pinch is ensured only by pressure maintained by hydraulic cylinders, you may encounter deviations in your rolling because although the oil is only very slightly compressible, it is slightly compressible, which means that to adjust the rolling diameters according to the pressure exerted by the side rollers. A hydraulic-mechanical system will ensure uniform pinching and greater precision.

As for rolling machines equipped with digital control, there are several versions (NC, CNC) with somewhat different possibilities from one model to another. The elaboration of all these possibilities would require a complete article alone, so if you have any questions at this level, we invite you to contact us for more information. I hope this information is of interest to you, we are at your disposal for any questions relating to rollers but also for any other manufacturing equipment that may interest you.

SYLVAIN DAGUERRE
YAN DESCHENES
YVES GARANT

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