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How tight is "too tight" when setting up the breakdown section?
Robert A. Sladky
Vice President Tube Mill Engineering
When you ask a mill operator the question: "How tight do you adjust your driven breakdown passes?" 95% of the time your answer will be: "by feel". Whereas a seasoned operator develops a "feel for the mill" from years of experience, it is hard to teach this "feel" to other operators, especially the new inexperienced individuals.
Just as any other parameter, or setting on a tube or pipe mill, the breakdown passes must be set properly per the design of the tooling, in relationship to the size and gauge the mill will be running at any one time.
Whereas most set up charts from any roll supplier will give you dimensions to set up to in the fin, welding and sizing, there is usually no specific measurement provided for in the driven breakdown section. Common sense will tell us the gap between the top and bottom driven rolls will be the gauge of the material being run on the mill. But how tight is too tight in each of these driven breakdown stations?
It is important that the operator "validate" their feel for the settings of each driven breakdown stand by taking some sort of physical measurement. This in turn can be the basis for any setup and the maintaining of the same of all sizes and gauges run on the mills, be it a seasoned or new operator. This builds a standard for all to follow.
Taking a physical measurement will insure the top shaft is parallel to the bottom shaft, and the pressures in all driven breakdown passes are equal so you do not under form in some passes, or over form in others because of too much pressure.
As illustrated in the picture below, the drive point on the driven breakdown tooling with virtually every roll manufacturer in the industry is usually in the center of the driven rolls with clearances on the outside edges. The only exception would be breakdowns that incorporate "W" style design.
When starting a new set up, cut a "coupon" of the gauge of the material you will be starting up on. Next, position this "coupon" between the top and bottom driven rolls, on center, in the first driven breakdown pass and adjust the top shaft parallel, until you feel the pressure of the top roll mating with the bottom roll with the coupon between. (See picture below) Then jog the coupon out of the first driven pass and continue the same process for each of the driven breakdown stations. To insure you were not too tight, you can mic the material to make sure you were not thinning the coupon.
Once all the driven breakdown passes are all set with the coupon, the operator simply has to taper the end of the strip for easy threading of the material. The tapered end not only allows for easy entry into each driven breakdown pass, but it will insure an easy thread through all side pass stands, and especially in the driven fin section where the taper ends end up as an angled entry on the top of the tube as it enters into the fin blades, thus preventing fold over's and possible breaking of the fin blades during threading.
Next as you thread the material through the breakdown section, adjust the pressure of the first driven pass to the "feel" of proper tension. Since we know the drive point is in the center, look for excessive pressure marks, etc. which would indicate too much pressure. An experienced operator will know, just by looking, if the pass is too tight, however, you can have appreciable thinning of the strip on some gauges without obvious visual impressions. So in order to insure we are not thinning the strip, and since we know the drive point is in the center, we can measure this area with a ball, or pin micrometer, in the center of the strip, to insure the pass is not too tight. (See picture below)
One of the most popular, and dependable ways to validate these pressure settings of the operators in the breakdown section is to use common solder. The pressure from the adjustment will thin the solder, which one can then measure this thinning out of each driven breakdown pass to insure all the stations are parallel and set to the same pressure settings. It is sort of like plasti-gauging the bearings and connecting rods when rebuilding an engine.
Lay a piece of solder on top of the strip in front of each of the driven breakdown passes, and then jog the solder through the driven pass. (See illustrations below)
After the operator has verified that the first driven breakdown pass is filled out, and does not appear to be too tight, the next step is to mic the thinnest part of the solder in this pass. Whatever measurement this is, this will become the standard measurement the operators will set the rest of the driven breakdowns to. All too often, if the operators set all the breakdowns to the same "feel", the later breakdown passes, the ones with the narrower top rolls, (which have less mechanical coverage to the strip), will have a tendency to thin the material in the area of contact of the top rolls, resulting in marking and "de-gauging" of the tube.
Do a progressive set up. This means jog the material through the first driven breakdown, through the first 1-2 side roll pass, (if your mill configuration has side passes) and into the second driven breakdown, then stop. Many firms jog the material all the way through the entire mill before starting any set up checks. This can lead to marking of the strip/tooling and wastes material.
Now check your pressure setting in pass number one with solder as outlined previously. Next, measure the tube size coming out of side pass 1-2 with a pair of calipers and compare it to your set up chart.
Continue to jog the material through to the remaining driven breakdown passes, one pass at time, checking the pressure of each driven stand with solder, and the dimensions of each side pass until you reach the first fin pass.
Look for continued articles to follow on the proper set up of the fin section, weld box, and sizing section here on Roll-Kraft's web site, which offers the most comprehensive, informative and interactive technical information in the industry.
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