Tube Bender Selection
There are many types of tube bending machines on the market today and many factors need to be considered in the selection process. Some buyers will look forward and buy more capacity or more capability for future use, so potentially this may influence the decision.
For most companies, what the end decision point usually ends up being is: “Select the best machine which will do the job at the lowest investment cost”
Of course there are factors such as operator skill level, power consumption, floor space and layout considerations, process flow of parts, quality and accuracy of part shapes to be considered, however the three main factors in general are:
1- Application Constraints: What are min/max tube size, part shapes and material types?
2- Volume Consideration: How many parts are to be produced? How many hours a day will the machine run? How many times a day or week is the tooling changed over?
3- Budgetary Constraints: How much can be spent on the project and what the payback will be.
Before we can break down these three main factors, one should consider the type of machines available on the market today.
Types of Rotary Draw Tube Benders
Single Axis Semi-Automatic:
This type of tube bending machine is the lowest cost and is effective for single bend parts or simple multi-plane parts with low volume. The only controlled axis on this type of bender is the bend angle. The straight length and rotation are positioned by the operator.
CNC – Single Stack:
This tube bending machine can control the bend angle, tube rotation and straight length. It’s called a “single stack” because it has the ability to mount only one tool set at a time. This tube bending machine is the entry level CNC tube bending machine which is able to produce moderate to high volume parts.
As the name implies, this machine has two bend heads and uses a compression bending process to wrap the tube around a die. It cannot mandrel bend tight radius parts. It’s a very fast bender whose main use is for high volume, symmetrical shapes with lower bending application factors. Products such as shopping carts, furniture, wheel barrel frame, lawn mower parts, etc.
Types of Bender Drive Systems
Uses a hydraulic power system to close dies and rotate the bend die. In today’s manufacturing environment, these tube benders are usually lower cost machines which are reliable but also have speed and accuracy limitations.
Hybrid – Hydraulic/Electric:
Uses a hydraulic system for some features, such as die closing and mandrel extraction, but the bend die drive is electric servo. This gives the machine higher performance capability and better control of bend accuracy at higher speeds. HMT brand hybrid benders use hydraulic actuators for closing the dies which are automatically positioned and therefore provide a system comparable to all electric.
Hybrid – Pneumatic/Electric:
Some manufacturers build machines which use pneumatic powered actuators to close the dies and there may even be an electric servo to position the die. These machines are often passed off as “Electric” tube bending machines, but they are not true all electric. They only reason to do this is to reduce cost and provide higher margins for the machine builder.
A true all electric bender uses electric servo drives to close and position the dies and perform the bending operation. There may be some minor functions such as collet close or tube supports which are pneumatic, but the main functions which control set-up and function of the tooling are always electric driven. This gives the machine the ability to control the process variables very consistently and very precisely without as much operator intervention.
Machine Rotation Direction
A standard tube bending machine will rotate in either the left hand or right hand direction. The determination is made by looking down on the bend die. If it rotates clockwise when bending, then that is a right hand direction machine. If it rotates counterclockwise, then it’s a left hand machine.
Some tube bending machines are designed to be bi-directional meaning that they can change bending direction on the fly from LH to RH. These machines are capable of very complex shapes but can also have rigidity limitations and are usually 40% more expensive that a single direction bender.
The reason for having different rotation machines is to avoid interference of the tube hitting the machine, tooling or floor during the part forming process. A typical rule is that 85% of the parts can be formed on either direction machine, 10% can be formed on only one specific direction and 5% of the parts require both directions.
So between the types of benders, different styles of drive systems and rotation direction there are numerous choices in the market place. Below are charts which help to summarize the various choices.
|Item||Single axis||Two axis||CNC Single-stack||CNC Multi-stack||Twin head|
|Single bend parts||Y||Y||Y|
|Multi plane parts||Y||Y||Y||Y|
|Tight radius bending||Y||Y||Y||Y|
|Push roll bending||Y|
|Multi Radius parts||Y|
? = Possible, but must be reviewed case by case
Bender Drive Systems
|Low to Moderate volume||Y||Y|
|High Volume – 24/7||Y||Y|
|Accuracy & Repeatability||Good||Better||Best|
|Push Roll Bending||N||Y||Y|
*HMT USA made Hybrids have this ability, most other brands do not
Bender Drive Systems
There are so many different bending applications that an entire book could be written on this topic and still not cover it all. For the purpose of this discussion, we will cover the basics of analyzing applications to determine which tube bending machines could be used to produce different tube shapes and meet the requirements for quality and quantity.
The process includes these steps:
Determine the correct size capacity of the machine to fit the min/max size of tubes. Some applications may require several tube bending machines to meet the entire range.
Review the material to be formed on the machine. A difficult alloy material will require much more torque and rigidity than mild carbon steel. The material type and shape will also affect the type of tooling to specify. Another issue is reducing scrap being generated on the ends of the tube or during setup.
Review part shapes to ensure that tubes will not interfere and collide with the machine or tooling. This will tell you if a LH or RH rotation machine is needed, or possibly a bidirectional bender is best suited. A proper set of drawings with bend coordinates or having a sample to reverse engineer is absolutely essential to this process.
Review any special requirements, overall tolerances, Ovality or wall thinning or other quality requirements as these may require additional options on the machine.
Review tooling requirements and specify what tools are needed to accomplish the application and meet the quality requirements. Pay attention to grip lengths between bends and at the beginning and end of the tube. Die marks are another problematic issue to consider.
Perform a cycle time study to develop capacity loading and volume.
Review the upstream and downstream processes to ensure that the bending operation is not going to have an issue with the incoming tube or the outgoing formed part.
Review the process flow and how many times a day or week the machine will have the dies changed. This could affect what type of tube bending machine is best suited.
Consider the experience level of the staff which is to run the machine. As the application gets more demanding, less experienced operators will need a higher tech machine to produce acceptable part quality and production levels.
Develop project cost proposals based on the above findings for each possible solution.