Sheet Metal Bending Angles
Sheet Metal Bending Angles for All kinds of Pressbrakes
Sheet metal Springback is the bane of air bending, and it gets worse as workpiece materials become stronger. Here’s an update on how major press brake builders are coping with it.
Variations in material thickness, hardness, and grain direction result in angle variations when air bending, slowing production and producing scrap.
Fortunately, technology has come to the rescue. Many of today’s machines offer adaptive, in-process control over the bend angle to make adjustments in suit to ensure that precise angles are met—from the first part to the last.
One of the biggest issues is setup times, especially for short runs, You’ve got these short runs and you spend so much time on setup, which kills you on short run applications. It’s important to be able to get in there and bend that thing up quickly and accurately without a lot of fussing around. Sometimes you can’t even discern where the inaccuracy started and ended, because it can occur anywhere in the process.
According to Trumpf pressbrakes, the manufacturing tolerances during sheet production allow sheet thickness fluctuations of up to ±10%. Also, the sheet thickness can vary between the sheet margin and middle due to the rolling process. With air bending, the angle is determined by the plunging depth of the upper tool. Thus, a sheet thickness deviation of 10% already leads to an angle deviation of 3 to 4 degrees, without considering other variables.
If there was no springback,it would be easy for control software to calculate how deep the ram needs to go to get the angle, and that would be the end of it. Right now, operators program the angles they want and then they’ll bend, measure, make an adjustment, then maybe re-hit that part. Or what some guys do is they’ll have blanks of the same material so they can test bending and sneak up on the bends that way. Or, what happens quite often: if they need ten parts they’re going to cut fifteen for all the test bending. That’s the first benefit of these angle correction devices. They’ll eliminate that completely. If you need ten blanks, you can just cut ten blanks and you’re going to get ten good blanks.
Typically, in gauged steel, 12 – 16 gauge and lighter, we’ll see a degree and a half of springback. That’s a mild steel. You get into stainless steel and you get up to three degrees of springback -- sometimes more, depending on the alloy of stainless you’re bending. If you get into some of these exotic materials you’re talking double-digit springback. It can be huge.
Trumpf has two different angle-measuring systems. Which to choose depends on what the customer’s application requirements are. There are a lot of specialized applications in sheet metal fabrication; a heavier plate in agricultural industries, stainless steel, and aluminum in aerospace. There are two approaches that are mostly used on the automated systems, what Trumpf refers to as their ACB and TCB systems, which represent different styles.
ACB, or the Automatically Controlled Bending angle sensor, is integrated into the tooling. It makes one of the press brake die segments basically an electronic protractor for the machine. During bending, two sensor plates integrated into the top tool measure the actual angle and the resilience of the bend part, and control the beam at the desired angle. ACB is extremely flexible because it works virtually independently of the shaft length, measuring angles ranging from 35 deg. to 145 deg. The system is rated to an accuracy of plus or minus 0.3 degree.
The TCB, or thickness controlled bending, is an automatic compensation function for material thickness variation. It measures the material thickness from each incoming part. Through sensors in the machine, the actual sheet thickness can be detected and the plunging depth of the upper tool automatically corrected. In this way, the machine achieves an angle quality independent of sheet thickness and with no loss of productivity or need for calibration or extra programming effort.
Then it will adjust the bending position based on material variations. The TBC system, because it’s a thickness compensation system, is very sensitive. It responds to differences on the order of 0.01 millimeter, or 0.001 in.
Amada has two bend-angle measuring and correcting systems. There is a touch-probe system, called the Bend Indicator Slide. A mechanical probe comes up on either side of the part. It’s mounted on a unit in front of the machine, and it’s programmable. An operator can choose if he wants to measure all the bends on a part, or if he’s got one bend that’s really critical for him, he can program it to automatically come over just for that bend.
It will come up and take its measurement position during the bend, adjust the bend, and then move down out of the way. It will work on thick material and up to a 1.5 in. V. The probe is harder to use on larger Vs; that’s where Amada’s laser device, which scans the part for angular tolerance, comes in. The Bend Indicator Slide is highly accurate: 10 - 15 minutes of accuracy, or a quarter degree, plus or minus.
The automatic laser Bending Indicator (BI) is Amada’s latest innovation in advanced bending technology. Mounted on a linear guide rail running across the front of the lower ram of the press brake, the BI can be positioned at any bending point on any work piece along the ram. In operation, the Bending Indicator uses laser sensing to zero its position relative to the die. It then laser-senses the position of the workpiece as it is being bent, feeding that information back to the machine’s CNC, which stops the bending ram only when the programmed angle is
accurately achieved. Then the laser helps to complete the bend with an accuracy of plus or minus a third of a degree.
Bystronic’s Optical Bend Guiding System is based on optical LED. A bar of LEDs is located above the tool clamping in the ram, allowing easier tool positioning, more effective station guiding, clear back-gauge positioning, clamping open, error, and end-of-job indications. It guides the operator through all of the machine setup and bending sequences, reducing setup times and ensuring part quality, says the company.
For the highest part accuracy from the very first bend, Bystronic offers its Laser Angle Measuring System (LAMS). The LAMS system automatically positions itself and measures each angle that is produced, to accuracy approaching a quarter of a degree of the programmed angle. The Optical Bend Guiding System and the LAMS can be combined.
A lot of intelligence and experience is built into the control. The idea is that an operator, if he knows how to operate a brake, can work on different materials even though he has never bent that material before -- and be successful at it.
The optical bend guide system informs the operator where to put tooling during the cycle. It will actually tell the operator where to put the tools. It also tells him where the back gauges are, so it will illuminate only those areas. It illuminates only where the active bending station is.