Examining tube mill roll tooling, setup, and maintenance
In today's competitive market, two of the most important considerations for high-quality production are proper roll tooling setup and mill alignment.
Poor tooling setup and mill alignment can be costly, especially when they result in decreased production quality and scrap. Both areas must be maintained continuously.
A secret to achieving maximum roll integrity is simple, but often forgotten: Roll tooling design is based on known parameters and goals. Using this approach as a road map to quality can save money, downtime, and frustration.
A good approach is to start with a part drawing which should contain the following information:
1. Tube size, dimensions, and tolerances
2. Material type, gauge, hardness, and finish
3. Length minimum and maximum
4. Notching or secondary operations
5. Reference to mating parts
6. Attention to critical areas
If a worn-out sample roll is submitted instead of a roll drawing, the manufacturer should take the time to reverse-engineer the roll, make a new drawing, and get it approved by the customer to avoid any misunderstandings when the new roll tooling is delivered.
If manufacturing managers get involved during the initial stages of a new product, they should point out trouble spots and help designers to develop a roll that does the job and is easy to produce--a move that benefits both parties.
Trouble Spots. When a company receives a roll drawing with trouble spots, it ought to review it with design engineers and manufacturing managers, ask for design changes, and explain why it wants the changes.
The roll drawing is an engineer's main reference. Engineering's purpose is to establish a common ground from which different parties can work.
When laying out tool drawings over a flower progression, the material flow should be considered. Manufacturers should try to provide lead-in angles with generous radii to let the previous form flow in. Companies also should consider leading out into the next station—sometimes center fins can scrape a section's edges if stations are close.
Splitting. Splitting rolls sometimes makes it possible to produce economically an otherwise difficult roll. Splitting rolls, or piecing them together, allows for sectional replacement rather than repetitive new tooling purchases. It also makes setup easier in some cases. It is easier to put one piece on at a time than it is to struggle with heavy mating top and bottom rolls.
Splitting does add to tooling costs. However, machining a part and wasting tool steel is less expensive than boring and facing an extra small piece.
Setting Up, Aligning the Mill
Roller die setup should be a simple procedure. A setup person should only need to put the rolls on and adjust the vertical center distance equally.
The first step in mill alignment is cleaning everything, especially bases and shafts. Then, operators should make sure horizontal centers are parallel by using micrometers to measure inboard and outboard distances.
Most machines have a double bearing mount on inboard bottom shafts or an outboard that is keyed in place. If the bottom shafts have single inboard bearings and no outboard key, an operator may need to install taper pins in the outboards after setting them parallel to each other.
The next step is to adjust the vertical center screws. This can be done by measuring with micrometers just like horizontal centering or by placing gauge blocks between the shafts.
Once shafts are parallel, the datum plane must be checked.
Tool design starts from an imaginary line on one side of the paper, which represents any station's top and bottom shaft shoulder. By going from station to station, a plane is established. The top-to-bottom relationship is most important because it affects design clearance. The horizontal line carries less weight but should be as straight as possible.
Horizontal alignment should be checked first along the bottom (fixed) shaft shoulders. A straight edge works best on small mills. On both large and small mills, an optical scope with cross- hair sight can be used for better accuracy.
Vertical alignment can be checked with short, straight edges or with similar top and bottom slugs that can be depth-milked from one face to another. Once a deviation is recorded, bearing assemblies can be taken apart and corrected, or a set of compensating collars can bring shoulders into a flat datum plane square to all parallel shafts.
Installing the Tooling
Once a mill is aligned, rolls need to be installed.
Two basic methods for putting rolls on a mill predominate. One is to put all the rolls on at once, and the other is to put one pass on at a time while adjusting and checking profiles along the way. Most rolls are designed with rims and hubs using a stock thickness for clearance. This way, vertical centers can be set and an operator can ensure shafts are parallel at the same time.
After rolls are installed, strip is run through one station at a time. At this point, fine adjustments can be made for any play in the vertical adjusting screws. This ensures that the rolls are set up the way the designer intended.
The first goal in line troubleshooting is to define the problem. If dimensional problems arise, exactly which one is wrong? Is the material within specifications? Has a different wall thickness just been tried? Is the temper and chemical composition within specifications, or has it changed recently? Are galling problems present? Is the material too heavy? Is the surface softer than the specifications call for? Have frequency marks appeared, possibly from a chip on a roll?
One way to fix problems on a particular dimension is to look through the tooling progression and find a pass that looks like it controls the dimension without having a major effect on the rest of the section. It is usually best to make only one adjustment at a time.
Another tip is to look for damaged or worn tooling that may no longer define critical bend points and to make sure the entry table guides the material evenly and squarely. If possible, the operator should back up the machine and look at how the progressive forming flows.
Operators should look for proper tracking and possible climbing or pinching. Sometimes, severe straightness problems are created by overworking one pass. If backing up does not show excessive pressure, an operator can try putting a coat of heavyweight lube oil on a roll.Running the roll around once can show high and low pressure points. If this fails to work, an operator can run the strip out, check tool clearance with a mirror, and go through the setup procedure as previously described before looking for possible tool design errors.
Maintaining the Tooling
The heart of roll tooling maintenance lies in good record keeping. Production records can help in finding trouble spots and predicting future problems. They can help justify regrinding an entire set or replacing key high-wear points.
Maintenance starts with periodic visual inspection during production runs. This includes checking product dimensions (especially tolerance deviations), finish deterioration, noticeable tool wear, and especially damaged tooling.
In between production runs, when tools are off the mill, it may be good to check bores and faces for wear. Worn bores can loosen dies, which can cause wandering part dimensions. Worn faces can create uneven clearances and pinching, which might lead to straightness problems.
Regrinding versus Replacing
Should rolls be reground or replaced? Sometimes companies have no choice in this matter--if a machine has little or no vertical adjustment, one cannot just sink the rolls to clean them up.
The best thing to do is push back female tools and replace key male tools. After doing this a few times, a fabricator may be forced to replace the entire set anyway.
On machines with vertical adjustments, a variety of options is available. Rolls can be shimmed after regrinding to maintain the passline height, or the drive shafts can be idled on some applications.
However, after replacing high-wear key forming tools a few times, it usually becomes necessary to regrind the entire set.
Roll tooling manufacturers typically measure an entire set of tooling and record the information on a regrind chart, noting maximum wear on tooling. Certain highly worn tools may be replaced to avoid sinking an entire set more than necessary. Careful attention should be paid to situations such as an unequally geared machine. Worn faces can be welded up and reground. Worn bores can be plated and the inside diameter ground.
It is usually less expensive to regrind a set of tools than to replace it, but production volume and records may alter this. For example, if many parts require replacement, the cost approaches that of a new set.
Once a new set is in service, the old set should remain available as a standby. Also, if volume does not permit lengthy downtime while tools are being reground, a new set is the best choice because it only requires changeover.
Again, good record keeping makes good maintenance. It also helps setups.
Some minor adjustments are going to be made on a machine. One pass may be brought down a little hard, and one may be brought up a little bit. There may be a reverse reaction in the straightness adjustment. Little things like these can waste hours of setup time. All an operator has to do is write those things down, and eventually the set should go in little time.
With highly maintained roll tooling and good record keeping, a properly aligned mill can produce high-quality parts. Roll tooling can be replaced to ensure proper dimensions are consistent on all products. If the mill is out of alignment, its roll tooling will never be used to full capacity.
In today's workplace, lost production time resulting from emergency maintenance is no longer acceptable. Properly maintained roll tooling and mills that are aligned regularly can result in continuous quality. A good preventive maintenance program helps keep all tooling and mills in top working condition.
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