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Indicators of Roll Form Tooling Failure
By: Bret Molnar
Roll inspection and maintenance are important to extend roll life and improve performance. Regular inspection with micrometers, gauges, and a trained eye can spot areas that require attention before wear can get out of control.
Complete failure should never occur on properly maintained rolls, unless outside forces, such as impact or temperature variations, affect it. High-carbon tool steel can become very brittle, if subjected to long periods of abrasion and heat. This surface cracking can travel into the roll body and propagate, leading to complete failure.
Complete inspection must include all roll surfaces. Even the most basic roll program should include a visual inspection and the use of a blade micrometer to measure root diameter wear. Since the root diameter, or throat, is always in contact with the strip, it is a great indicator of when a roll should be sent for rework. Guidelines for how much wear is allowable should be set up, based on past performance.
The next step for inspection is to check the roll contour, using templates, gauges, or plugs. Round washers are the most common and least expensive, and with a trained eye, can be very accurate. Proper training to read a gauge correctly is important.
Rolls that are overloaded or misaligned can be spotted by the wear pattern in the contour. The cause should be found, to determine if the rolls were set up improperly or there was a possible mill problem. Uneven wear can be caused by a design flaw that did not take into account the type of material to be formed or maximum gauge thickness. Roll bores and faces that are worn beyond normal limits can indicate shaft problems or, again, set up.
The outboard stand must be tight, or rocking or slipping will occur, wearing the roll faces and bores. A dial bore indicator is the most accurate way of checking the roll bores. A depth micrometer can measure the face wear. Roll bores and faces should be repaired by chroming, if they wear more than .001 per inch of shaft size. Example: A maximum of .003 over the high tolerance of a 3.000 diameter shaft. Roll faces can also be corrected by grinding the entire face of the roll and making a new spacer. This is a more cost-effective way to eliminate wear, since if the roll face is worn, so is the spacer. All roll spacers should be regularly checked and replaced, if worn more than .001 per inch of shaft size.
Weld integrity is a common problem that many times can be avoided completely by monitoring the wear on the fin blades. A grooved fin blade cannot properly prepare the strip edges before the welder. Spare blades should be on hand to change out, as needed. The blades should not be allowed to wear more than 1/4% to 1% of the gauge thickness. Example: For .030 thick material, allow no more than .003 wear. Fin blade maintenance is one of the most overlooked areas in mill setup.
Many weld problems can point back to improper strip preparation. Strip edge condition is especially critical in welding coated materials, to keep the weld zone free from contamination.
To complete the roll inspection, roll diameters and widths should be checked. To have a complete record of all dimensions, all rolls should be checked for run-out from bore to contour. This can be done by placing smaller rolls on a tapered arbor or measuring from bore to 0.0. with a micrometer. If any run-out is found, it can indicate a bad shaft or bearing problem (assuming the roll is made correctly in the first place).
A good roll program can stop many problems before they develop. Once a limit is set and held for your own internal quality, scrap, setup time, and rejects can be kept to a minimum. Tooling should never be used until it reaches the point that you just cannot run product.
Roll design can affect how tooling wears. A set may not be properly designed to handle the material to be run. Many tube producers try to run everything their customer demands on one set of tooling. Sometimes it can be done, sometimes not.
When a roll set is sent in for rework or ordered to be made new, all parameters should be explained to the roll supplier. This will enable them to determine if a set requires a design analysis. All sets of tooling should be examined for the following areas:
1) Proper strip formation. Are all the rolls on the mill doing their job? A badly designed set can cause wear and make setup difficult.
2) Are the side rolls forming properly? The side rolls are there to support and present the strip between driven passes. If they are not designed or used correctly, problems can occur.
3) Are the breakdowns cleared for the maximum gauge? If the top breakdowns are not cleared for the correct gauge, it can cause many problems. If the strip to be formed is thicker than the rolls are designed for, the result will be thinning, walking of the strip, and excess wear to the first pass. If the top breakdowns are not ceared for the lighter end, flat spots may occur that can cause welding problems and buckling.
4) Are the fin passes designed for the type of welder and material? The fin passes are a critical area of the roll set. If they are not designed for the correct type of welder or material, it may be impossible to weld.
5) Determine the range of materials to be run. The roll set must be made to handle the maximum gauge and highest tensile strip to be run. All other materials will be a compromise that mayor may not be compatible.
6) Will this set of breakdowns be used to run more than one size? Most tube producers will try to cut down setup time and save tooling costs by running more than one size on a set of breakdowns. Mild steel is very forgiving, and this is perfectly acceptable, with certain limitations. A suggested maximum is 80% of tube size. Example: 1.0000.0. breakdowns to run down to .800 0.0. Note: Stainless steel and other high-tensile materials cannot be formed correctly this way and usually require a dedicated set of breakdowns.
7) Mill configuration. Has there been a pass added or removed? Mill configuration plays a crucial part in how and what materials can be formed.
To run light-gauge and high-strength materials, proper breakdown design is a must. There are four types of forming.
- Conventional or single-radius design is good for mild steel with a TID ratio between 3% and 8%.
- Standard edgeform is the most widely used, because it forms mild steel, higher tensile, and a greater gauge range very well.
- Versatile edgeform works very well on light-gauge and higher tensile material. It forms more of the strip with the smaller edgeform radius in several passes. This gives greater stability, less buckling, and provides a rounder tube when entering the fin passes.
- "W-style;' or reverse bend, works very well on high-tensile, light- or heavy-gauge material. It can be combined with a versatile design for maximum results. Whatever design is used, it is important to have the correctly cleared top rolls for the gauge to be run. The strip memory must be worked out with the proper radius and pressure. The final tube use must be examined also, to ensure the correct design is used.
A reverse bend works very well in forming the strip, but with the increased amount of cold work, can raise the hardness of the tube. However, if bending, expanding, drilling, or other operations are to be done to the tube, reverse bend may not be the answer.
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