Perpendicular Bat Rolling
Perpendicular bat rolling is a process that rolls a softball or baseball bat through two nylon or hard rubber rollers. This rolling process will roll the entire length of the bat as it passes through the rollers on approximately a half inch line on both the top and bottom of the bat. The bat is rolled on 8-12 points around the bat which breaks in almost the entire bat as it passes through the rolling machine. The bat will make a slight crunching sound to an all out crackle and pop when going through the rollers. The sound depends on the type of bat and the bat's thickness. If the bat has a thinner wall the bat will tend to pop or crackle sooner than a thicker walled bat. Certain bats can take more pressure than others during rolling. The key is to know how much pressure to place on the bat or the thinner walled bats will break during the bat rolling process. A Miken Freak, slowpitch softball bat, is able to with stand a lot of pressure and also needs that extra pressure to be fully broken in during rolling. TPS bats have a thinner wall and need less pressure when rolling, too much pressure will definitely break a bat in one pass going through perpendicularly.
Now that a bat has been rolled on 8 to 12 points perpendicularly there should be peaks and valleys along the length of the bat. These highs and lows are created by rolling bats along certain points and not all points. Here lies a catch 22; the bat needs to be rolled completely but if it is rolled on too many points around the bat the potential for cracks increases substantially. When the bat goes through the rollers perpendicularly the rollers flatten the bat causing the resin (or glue) to break up, this is what gives the extra flex and makes rolling favorable to players. When the bat is rolled perpendicularly along too many points the area of the bat gets rolled twice, thrice, or maybe 4 times too much causing weak spots in your bat. Perpendicular rolling will break in your bat but not to its fullest potential; that is where the parallel rolling comes in.
Parallel Bat Rolling
Parallel bat rolling is a process that rolls the traditional sweet spot of the bat. As you could guess the bat is placed parallel with the bat rollers. The theory behind this is that the entire sweet spot of the bat would be rolled with out missing any areas within the sweet spot. Some bat rolling machine builders have taken the idea a little too far by expanding the length of their rollers to encompass almost the entire barrel. This type of bat roller is destined to cause problems. The area that is rolled is too large and will cause a decrease and an increase in pressure along the barrel. What do I mean, you ask? The mechanics of a bat roller with longer rollers will not allow for consistent pressure points along the barrel. Depending where the mechanism is placed to tighten the rollers down is where the most pressure will be on the bat and then diminish as it goes down the length of the bat. This means the composite will be broken in more in certain spots and not in others. Trying to break the bat in fully with a longer bat rolling machine could compromise the integrity of the bat in certain areas. The rollers that surround the sweet spot only are shorter and have a small disproportioned distribution of pressure which breaks in a bat more consistent.
The parallel rolling machine should be used after it is run through a perpendicular rolling machine. This will allow the parallel rolling machine to break in any spots the perpendicular rolling machine has missed. The peaks and valleys that were discussed earlier will be flattened out by breaking up the resin and glue in those areas. This is the most consistent and even bat rolling you can accomplish. As a stand alone the parallel bat roller will work, as long as it has the shorter rollers (around 6 inches, which is the length of a sweet spot). The longer rollers will not roll a bat consistently and there is the danger of breaking a bat with an over zealous operator trying desperately to break in the entire barrel the same.
With all that said, I would suggest getting your bat rolled with a parallel rolling machine with shorter rollers which also does a perpendicular roll first. Go ahead and ask who ever does the rolling if this is the way they roll to get a better sense of security. And if you are buying a rolling machine make sure to get the parallel one with the shorter rollers.
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