By: Chris Rindels, Chief Mechanical Engineer, AMSAW
Virtually all sawing systems require a method for accurately measuring the length of the bar being cut. Usually the exact length of each piece is known before making a cut. Sometimes it is also necessary to measure the entire length of the rough stock prior to processing. This is often done to establish a cut plan for maximizing material usage and minimizing scrap.
Getting a precision length measurement is also a crucial factor in avoiding additional operations after the material is cut. Selecting the right measurement system for your application will help control your process flow, decrease waste, and increase productivity.
Sensor Measuring Methods in Billet Sawing Operations
Sensor measuring systems are a non-contact method that quickly and semi-accurately establish the length of a part. There are several sensor methods available. The type of sensors used can depend on variables such as accuracy, range, environment, material type, and material surface.
Laser Surface Velocimeter (LSV) for Length Measurement
Laser Surface Velocimeter (LSV) is a non-contact optical sensor that measures velocity and length of moving surfaces. These sensors can be used on all commonly sawn materials including steel or aluminum bars, billets, tubing, rail, hot or cold stock. The sensor uses the Doppler principle to evaluate the laser light scattered back from a moving object to understand its speed and and time to measure the length.
LSVs can accomplish various tasks, including:
Cut-to-length control
Part length and spool length measurement
Speed measurement and speed control
Differential speed measurement for mass flow control
Encoder calibration
And, many others
Ultrasonic Proximity sensors
Ultrasonic Proximity is used in situations where the length of measurement is shorter and accuracy is less. These sensors normally have a range of 20mm (.79 inch) to mm (157.5 inches) with an accuracy around 3mm (.12 inch). Proximity sensors are often used in conjunction with a mechanical hard stop.
Photoelectric/
Retro reflective Length Measurement System
Photoelectric sensors consist of an emitter and a receiver. The emitter sends out a beam of light and the receiver picks it up or senses it. With a diffused and retro reflective type sensor, both the emitter and receiver are placed in the same housing and are configured for light to be reflected back to the sensor. These types of sensors normally have longer sensing range than the retro reflective type.
Retroreflective sensors use a target to reflect the light back to the sensor and will sense an object that breaks the beam&#;s path. The diffuse reflective type senses an object when the light beam is reflected back to the sensor. One advantage of this type of sensor is that the emitter and receiver are contained in the same housing. A disadvantage is that the object might be reflective which can cause false readings.
Linear Measuring Methods in Billet Sawing Operations
Index Style
Measuring
Measuring your length using &#;indexing&#; or measuring slide involves clamping the part with a clamping mechanism attached to a slide on linear guides. The slide is controlled by a drive and uses rotary or linear feedback for accurate positioning.
This method is much more reliable with regard to accuracy and repeatability. As the material is conveyed into the measuring system a sensor or laser detects the front end of the incoming material. Once the end of the material is sensed, the conveyor stops and the measuring system clamps and grabs hold of the material.
The material is driven forward by the measuring system drive until the leading end of the material is in the saw. The saw clamps the front of the material and the measuring system unclamps and retracts a measured amount and reclamps. This is the index distance.
The saw would normally cut a short length off the front of the material called a crop cut. Once this crop cut is made the measuring system can accurately index the material the desired distance for the cut length. A servo drive is often used for driving the index slide.
Friction
Roller w/ Encoder
A friction roller can be coupled to a rotary encoder. This device is used to provide length measuring.
The roller is brought into contact with the moving material. As the material moves, friction causes the roller to turn, which then causes the encoder to generate pulses in response to the linear displacement of the material.
It is a matter of simple math for the receiving device to convert those pulses into a distance. It is crucial that the friction wheel never loses contact with the moving material or is allowed to slip as this will produce measurement errors.
Pinch Roller w/ Encoder
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A pinch roller system is similar to the friction roller system with the exception that it not only provides the feedback, but it also drives the material to and through the saw system.
The material is forced down onto a driving roller usually by hydraulic actuators. The force must be sufficient to assure the necessary contact friction to prevent slippage or lost motion. The driving roller&#;s circumference provides the distance of the material per revolution.
The driving system of the driven roller provides the velocity and position feedback needed for accurate length measurement.
Positive Stop
The positive stop measuring approach can be a simple manual operation or it can be fully automated. A measuring gauge stop is positioned at a specific distance. The material is conveyed or driven to the stop which sets the measured length.
Stop gauges can be very simple using a hand crank with a rack and pinion and a measuring tape, or they can be designed for fully automated operation with a servo system and feedback devices as described above.
The positive stop is an accurate method for measuring. There are, however, a few things to consider when implementing the positive stop:
The mass of the material being cut--the incoming material that gets driven into the stop causes a significant shock load to the stop device. This is often addressed by designing a shock absorbing feature into the stop mechanism.
The design of the gauge stop--often the gauge stop must be rotated or lifted out of the way after the stop to allow the outgoing material to pass by it. If accuracy is important then a sensor should be included in the design of the stop. This assures that the material is in contact with the stop face before clamping and cutting.
Linear scales versus encoder feedback systems--most sawing machines are priced to include an encoder feedback system. Some machines are upscaled and designed with a linear scale, which is more accurate than an encoder because there is no backlash. Linear scales, however, can be susceptible to contamination and sawing systems are often used in harsh environments.
How to Select the Best Length Measurement Method for Your Application
Selecting a length measurement method depends on a variety of factors, including:
Repeatability and Resolution
Before you select a length measurement system for your application, you should ask yourself the following questions:
How consistent do I need my measurements (repeatability) and cuts to be?
How accurate do my measurements (resolution) and cuts need to be?
There are several factors that affect the repeatability and resolution of your cuts, including what type of material you are cutting, the squareness of each cut, and the condition of the tool you are using to make the cuts.
The Material Being Cut
What type of material are you cutting? Has that material been left outside or placed in an environment where it could have rusted or been damaged? Are there any hotspots in the material you need to consider with thermal expansion?
The type of material and the condition that it is in determines how well it cuts once it enters the sawing machining. It also determines how it will affect the cutting tool that is used to cut it.
The Squareness of the Cut and Condition of the Cutting Tool
If you&#;re buying a saw you have to keep in mind that the saw doesn&#;t cut real square. To maintain consistent and accurate repeatability and resolution, a certain amount of tolerance has to be given to the cutting process (standard squareness tolerance is .002 inch per 4 inches).
The kerf or the condition of the cutting tool can affect how the material enters the cutting machine, which then affects the squareness of the channel as it cuts. Once the cut starts, if the blade pulls to the side due to a dull insert, then the cut will be angled and slightly off. That angled cut will generally follow all the way through the material as it continues being cut. Adding a blade guide can reduce this problem but it can never be completely eliminated.
Different blades have different characteristics such as blade body, thickness, tooth grind, tooth coating, tooth kerf, gullet size, etc. Manufacturers should also consider the blade geometry and thickness of the blade:
The thinner the blade the less vibration frequency, which creates instability in the blade. Circular saw tensioning induces stresses in the sawblade such that the periphery is pulled in to tension. These stresses alter the saw vibration frequencies, and when favorably distributed, the can significantly improve sawblade stability. The purpose of tensioning is to increase the lowest (most unstable) backward travelling wave frequencies.
Chipped or dull teeth on the blade can also affect the squareness of a cut, so monitoring the condition of your tool is important.
Summary
There is a lot to consider when selecting the right length measurement system for your application. It is a matter of discrimination on how accurate your measuring needs to be and the costs benefit of each method that is available.
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