March 23, 2001
Prospective buyers of press brakes are advised to consider their purchase in terms of its end use, the amount of deflection likely to occur in a give machine, the inside radius of their parts, and several other factors before speding their money.
You have realized for a long time that your company should consider buying a new press brake. Now you have the responsibility of developing the specifications and recommending a new machine. This is an awesome responsibility, because if you select the wrong machine, your manufacturing costs will escalate, and the press brake will not pay for itself as soon as it should. Several factors should be weighed in your decision.
The first major item to consider is the parts that you need to produce. The idea is to purchase the shortest and the lowest-tonnage machine that will do the job.
Look at the type of material and also at the maximun thickness and length you will be working with. If most of your work is in 16-gauge mild steel with a maximum length of 10 feet, the capacity does not have to be more than 50 tons for air bending. However, if you do a lot of bottoming work, you may have to consider a 150-ton machine.
Now, suppose that your heaviest material is 1/4 inch thick. Air bending 10 feet requires 165 tons and at least 600 tons for bottoming. If most of your work is 5 feet or less, you can do very well with a new machine about half of this capacity and enjoy a substantial reduction in acquisition cost. The length of the parts is quite important in determining the size of a new machine.
You also should consider the amount of deflection likely to occur in a particular machine. Under the same load, a 10-foot machine has four times the deflection in the bed and ram than a 5-foot machine. For the shorter machine, that is less shimming is required to get good parts. Less shimming reduces the setup time as well.
The type of material is a critical factor too. As a rule, stainless steel increases the load requirement about 50 percent over that of mild steel, while most grades of soft aluminum require about 50 percent less. Tonnage charts that give you tonnage estimates per foot of length in different thicknesses and different materials are readily available from press brake manufacturers.
Next, you need to look at the inside radius of your parts.
In air bending, the inside radius equals 0.156 times the die opening. During air bending, the die opening should be eight times the metal thickness. For example, when forming 16-gauge mild steel, a die opening of inch is used, producing an inside radius of about 0.078 inch. To produce a tighter inside radius, you need to bottom-bend to produce an inside radius about equal to the metal thickness. However, bottom bending requires about four times more tonnage than air bending does.
If you need an inside radius less than the metal thickness, use an upper punch with your required radius at the tip and go to a coining method of bending. This generally requires 10 times the tonnage of air bending.
For air bending, the punch and die are machined at an angle of 85 degrees or less, with less being better. With this type of tool set, you see air spaces between the punch and die at the bottom of the stroke and have enough overbend to set the material at approximately 90 degrees, factoring in springback.
Usually, an air bend set produces an angle of 2 degrees or less on a new press brake. Air bend sets produce an inside radius that is equal to 0.156 times the die opening.
For bottoming, the tool set should be from 86 to 90 degrees. At the bottom of the stroke, there should be slightly more than metal thickness between the punch and die. The angularity is improved because the greater tonnage - - about four times as much as in air bending - - relieves the stress on the inside radius that normally causes springback.
Coining is similar to bottoming, except that the required inside radius is machined into the punch nose, and the punch and die are set at less than metal thickness at the bottom of the stroke. By applying enough tonnage (about 10 times as much as air bending), the nose of the punch is forced into the material, and springback is essentially eliminated.
For the lowest required tonnage, it is best to plan for an inside radius greater than metal thickness and air bend as much as possible. Many times, the larger radius does not affect the finished part and its subsequent usage.
Required angular accuracy is a serious consideration. It is the accuracy of your bends that determines whether you need to consider a CNC brake or a manually controlled brake. If your angular accuracy is +/- 1 degree or less, and cannot be changed, you need to look at a CNC machine.
The ram repeatability for a CNC brake is +/- 0.0004 inch, which, with good tooling, is required to form accurate angles. A manually controlled brake has a ram repeatability of +/- 0.002 inch and should form--with good tooling--an angle of +/- 2 to 3 degrees. In addition, a CNC brake provides for faster setup, which is a definite consideration when you have many small part lots to make.
Even though you have a rack full of tooling, don't assume that it is suitable for a new machine. Every piece of tooling should be checked for wear by measuring from nose to shoulder on the punches and from shoulder to shoulder on the dies.
For conventional tooling, the accuracy should be about +/- 0.001 inch per foot and should not exceed +/-0.005 inch for the overall length. For precision-ground tooling, the accuracy should be +/- 0.0004 inch per foot and not more than +/- 0.002 inch overall. Precision-ground tooling is recommended for CNC brakes and conventional tooling for manual brakes.
So often, we make the mistake of buying a new press brake but not buying good tooling to complement the brake.
A factor that is overlooked often is the width of the flange in front of the machine.
Suppose you are forming a 90-degree angle along the edge of a 5-by-10-foot sheet of 10-gauge mild steel. The press brake would have to generate another 7.5 tons of pressure to lift the sheet, and the operator would have to be prepared to lower a flange weighing 280 pounds. Several strong workers, or a crane, would be necessary to make this part. Press brake operators often have wide-flange parts to make and don't realize how much weight they are working with.
For shops doing this kind of work there is now a sheet-supporting device that can be retrofitted for new and old machinery. With this device, only one operator is needed to form wide-flange parts.