Checklists can prevent costly tube bender damage

Oct 14, 2024

Damage to tooling is clear after overclamping and running dry. Images: Jay Robinson

I received a text last week from a number I did not recognize: “We need help.”

A few minutes later, a picture of a machine came through, and I definitely recognized that. It was a tubing bender that one of our customers had recently bought. We had helped the company get started bending parts. The picture included one of the machine’s motors, normally mounted to the carriage, lying beside it on the floor.

My heart sank.

I called to ask what had happened and could feel the shoulder shrugs coming through the phone. “We’re not sure.”

Taking a straight tube and forming it into a complex shape is not an easy or entirely predictable process. If you operate a CNC tube bender, sooner or later it’s going to crash.

I drove to the customer’s facility on a Saturday and spent the rest of the weekend, plus Monday and Tuesday, trying to figure out what happened, how much damage was done, and if we could get the bender back together enough to get the company back into production.

A few years ago, I was lucky enough to learn how to fly an airplane. Today, if the weather cooperates, I fly myself to almost every customer visit that I do. I am much less a slave to the airlines’ schedules, it helps me provide more efficient service, and, most important, I am able to spend a few more nights a year at home with my family.

Learning to fly can be pretty overwhelming, though. It is easy to forget a crucial step or overlook something that might make it unsafe. This is why pilots use checklists—a lot of checklists. I use them for many steps and scenarios: preflight, before startup, cold engine start, hot engine start, after engine start, warmup, before taxi, ground runup, before takeoff, after takeoff, climb to cruise altitude, at cruise altitude, before descent, before landing, after landing, shutdown, and parking.

I also have checklists that I double-check from time to time for various emergency situations.

Those flight checklists came to mind because, as we went through the events that caused the customer’s bender to wreck itself, I realized that its operators had missed several crucial steps when setting up for production.

A series of miscues (that might have been prevented with a checklist) bent the shaft in one tube bender’s carriage gearbox, breaking a couple of teeth in the drive pinion gear.

It occurred to me that, had the company’s operators followed a checklist before they hit the go button, I might have been able to spend a relaxing weekend at home instead of digging out broken bolts and straightening bent sheet metal.

After years of working on and around tube benders, I have a mental checklist of things to verify before I let the machine take off. But often as not, I have to stop the machine because something just isn’t right.

As I was working with the team to get the bender back into production, we created a before-production checklist on a whiteboard:

Make sure you mount the correct tooling. Just like there are many variations in tubing, there are countless variations in tool design. Tool mounting seems straightforward, but it can be easy to miss things. I have seen bend dies mounted upside down with the clamping section opposite the clamp die—or even upside down and backward with the clamping section facing the carriage. The tools need to fully seat against their mounting platens, and heights must be adjusted so that centerlines match. The clamping position needs to be set to create enough pressure against the tube being bent but not so much that it damages the tools or the machine.

In the crashed machine, the clamp die platen was removeable, and the part that the customer was running had a platen designed specifically to create clearance for the part. The platen was mounted upside down, which moved the clamp about 3 in. forward, causing the clamp to press on the previous bend. This did not cause any damage to the machine (thankfully) but certainly could have damaged the machine and possibly the tools.

Operators also had improperly set the close positions on the clamp and pressure die, setting them to stroke almost 1 in. past fully closed. The machine has an independent bend die, and overclamping this much pushed the bend die over enough that a limit switch and its bracket were damaged. An alignment plate that holds the independent die in place during bending also was damaged to the point that its bolt broke and shot off the machine.

Set and check centerlines. Depending on the design of your machine, either the bend head or the carriage and mandrel assemblies will be horizontally adjustable. On an all-electric machine, this will be set by the control system, but it should still be verified. This can act as a cross check that you have the correct bend die radius on the machine.

I typically do this by moving the machine horizontally and getting it close to position using an eyeball measurement, usually making sure it is a little bit wide of the bend die. Then I clamp a tube in the collet. (If your part uses one, it may be easier to do this before installing a mandrel.) The correct position is set by adjusting the machine horizontally until the tube nests in the bend die.

This may mean that I am manually adjusting an acme screw that moves the bend head or carriage, or it may mean adjusting it by jogging a motor. On a machine with a servo horizontal adjustment, the control should allow you to jog it to the programmed position.

On the machine that crashed, operators had just done a changeover from a rectangular tube with a large centerline to a smaller square tube with a much smaller centerline, and they forgot to reset the centerline for the new part.

This is a large machine that has a small motor to manually adjust the carriage and mandrel assemblies to the correct centerline. Even though it is using a motor, it still requires a manual adjustment with no position feedback to the control system.

When operators set the machine into automatic mode, two things happened: When the clamp and pressure die came in, the carriage was forced into position by the act of the tube being bent. Also, the returning action of the pressure die assist shoved the carriage toward the back of the machine.

Check PDA speed. On machines that require it, I typically set the PDA speed manually with no tube and often will remove the pressure die. This helps prevent the tools from colliding and then dragging, which can cause significant damage if the PDA speed does not match the bend arm speed.

If the PDA is too fast, the face of the pressure die may be forced into the back of the clamp die. If it is too slow, it may cause too much wall thinning or even a broken tube during production. After this speed is set with no tube, it may need to be adjusted during bending.

This step is generally not necessary on machines with a servo PDA, as the control calculates and sets the follower speed based on the theoretical speed of the tube moving forward. However, for these calculations to be correct, the centerline radius entered must match the CLR of the bend die being used. If your bend dies are undercut (talk to your tooling supplier) to allow for radial growth, the actual dimension (not the expected result) must be entered into the control for it to calculate speeds correctly during bending.

On the machine that crashed, this was not checked. After the initial crash, the shop ran the machine dry. Because the tools were still set to overclamp and the PDA was running much too fast, significant galling of material on both the bend die and pressure die occurred. The pressure die also cut a groove into the back of the clamp die.

Most of these steps could each have their own checklist to make sure those steps are completed. Does your production floor have a before-bending checklist to follow? What would you add to the one we created above?

Here is what happened to the crashed machine and a list of damage we found:

It took working late into the night, through the weekend, and the first part of the following week to get the bender back into production. More work will be needed once some parts that had to be ordered or fabricated arrive—all because of damage that might have been avoided by following a checklist!