Cylinder frames are bored larger because the cylinder eccentric has caused the cylinder frame hole to enlarge, usually on the bottom due to the effects of gravity and the weight of the plate cylinder.

Boring the hole larger is done so that an oversized eccentric can be fitted in to the newly bored cylinder side frame. Usually, the newly bored hole is anywhere from .025-inch to .050-inch larger than the manufacturer’s original hole size.

The difference in hole sizes is determined by the amount of wear in the frame and whether it has been previously bored. Renewing the cylinder frame hole with an oversized hole will dramatically improve registration in most cases and improve the units’ ability to reproduce quality solid and halftone images.

Bad boring?

What if the boring job goes bad? The results of having a plate cylinder frame hole bored off-center can be devastating. What I am referring to is when the boring tool slips as the hole is being bored and causes the center of the hole to be off of the manufacturer’s original hole-center.

This causes a permanent skew of the image being printed. Correcting the problem is very time-consuming and expensive and will require the cylinder to be re-bored so that the oversized hole’s center is aligned with the original manufacturer’s center.

The hole’s center can be found by stripping the unit of all of its components - allowing both the drive side cylinder frame and the operator side frame to match up and thus bolted firmly together.

Once the side frames are bolted together, using the three good cylinder frame holes as a guide, the hole can be re-centered by using a large milling machine.

The incorrectly bored hole’s center can be properly relocated using the companion hole on the drive side of the unit.

Another way to correct this problem is to have an accurate template available that exactly represents the plate and blanket holes’ centers.

The template would look just like the portion of the unit side frame needed to find the hole’s center. In fact, the template could be made from a good unit side frame, although it would be very heavy and difficult to work with.

Alternative direction

Another option to relocate and rebore the incorrectly bored hole would be to use a double eccentric method.

The double eccentric has been used on Harris presses to set bearer pressure but could be use to relocate a hole’s center as well (see Figure 1, the inside eccentric).

The theory of the eccentric is that the inner hole is offset in relationship to the outside circumference of the eccentric. This allows the eccentric’s hole center to be shifted as the outside of the eccentric is rotated inside the hole in which it’s placed, which in this case is another eccentric (see Figure 2).

If the outer eccentric is rotated while the inner eccentric is held stationary, the center hole of the inner eccentric will be shifted (see Figure 3). The green circle illustrates the changing hole’s center location as the outside eccentric is rotated, which will allow the plate cylinder to be repositioned to its correct hole center.

Keep in mind, however, that this method still requires a larger bored hole to accommodate the eccentric sleeve. But this approach will help you determine the hole’s center much more easily.

Frank Bourlon has more than 30 years’ experience in the newspaper industry. He is the executive and training director for the Newspaper Production & Research Center. He can be reached at 405.524.7774 or via e-mail at nprc@flash.net.