Saddle stitcher calipers are a key component in any stitching system. When set correctly they’ll detect missing or double signatures in the book, then stop the stitching mechanism and reject the book so signatures can be re-used.
They can also be a source of paper jams and mis-registered books, especially as the machine ages. Construction of the calipers varies substantially with the make and model of stitcher but the concept is usually the same—the book is guided between two wheels that are set to the exact thickness of the correctly assembled book. Missing or extra sheets trigger the sensitive mechanism which in turns activates the stop-stitch and reject tray.
If the book isn’t guided in to the caliper properly it can jam and over time these jams can knock the caliper out of alignment and even bend and damage parts. Once out of alignment the caliper can then literally squeeze the books out of register since the force isn’t being applied equally and accurately in the proper direction.
You can imagine that this scenario has a cascading effect; the mis-aligned caliper is now harder (or even impossible) to set correctly so the operator chases the correct setting, often causing even more problems. We know that a mis-registered book can have numerous possible causes (see previous posts here...5 Tips and Using the Reject Tray) Since the caliper is usually low on the list of suspects and may not even enter his mind, this gremlin can sneak up on the operator gradually.
However, once the caliper is flagged as the culprit for the mis-register problem, it’s a must-fix item. Mis-register at the caliper is nearly impossible to completely fix with brushes or other remedies. The fix could be as simple as re-aligning the caliper components or it may require the services of a stitcher technician.
Details will vary. The calipers on old McCain saddle-stitchers are built like tank parts but I’ve personally seen components with no visible damage, which when replaced, solved all the caliper problems. The damage, though nearly invisible was in fact a big enough problem to warrant replacement of some pretty hefty parts. You wouldn’t think some little pieces of paper could bend a shaft a couple inches thick!
According to Bill Klansko of Ace Grinding and former saddle stitcher product manager for Muller-Martini, Muller calipers must be adjusted perfectly horizontally and vertically, with centerlines of upper and lower wheels in perfect alignment in both axes. He’s encountered this simple but not so obvious defect numerous times. Re-alignment easily fixed the mis-register.
Calipers are very precise mechanisms and as with all things mechanical will wear with age and abuse. Unless you turn the caliper off (a risky and wasteful proposition) you’ll have to find a way to set the caliper to function correctly. When parts wear, this becomes increasingly difficult and thus can be a contributing factor in a signature mis-register problem.
The bottom line: don’t forget this important component when mysterious register problems appear. A careful examination at the caliper as you jog a book through and as it’s running will usually be enough to determine if it’s the culprit. When the usual adjustments don’t fix the problem, it’s worth a phone call to your technician.