The following can result from incorrectly adjusted front lays
a. The sheets being hard to feed.
b. Bad register
c. The edge of the sheet being torn at one or both of the front lays. If the tearing is at both lays it could be incorrect setting or twisting, but if the tearing is only at one end then it is possibly the setting only.
d. Slurring and creasing caused by buckling at the front edge.
Control of the sheet on the cylinder
The sheet after having been taken by the grippers is smoothed before impression by the cylinder forwarding wheels, the cylinder bands and the cylinder brush. The purpose of each and the method of setting and positioning is as follows:
THE CYLINDER FORWARDING WHEELS must be positioned clear of the grippers, shoo-flies and wet printing. They are adjusted to lightly contact the correctly packed cylinder. Besides smoothing the sheet to the cylinder, these wheels perform the additional operation of helping to forward the sheet from the cylinder over the strippers to the delivery tapes.
THE CYLINDER BANDS perform two duties. They smooth the sheet and prevent it from contacting the inking rollers. To set the bands, the cylinder is placed on impression until the grippers are just past the tips of the cylinder bands. The bands are then placed evenly along the bar to almost contact the cylinder making certain they are not in line with the grippers. It is an advantage to set centre cylinder bands slightly closer to the cylinder than the outer bands.
THE CYLINDER BRUSH is used to hold the sheet flat against the cylinder and to prevent paper dust and spray from falling onto the forme. Correct setting of the brush is essential or the following troubles may occur:
(b) bad register.
(c) Filling in of the forme.
(e) Folding back of the grip edge of the sheet.
Scratching of the printed image will also occur if the brush is not cleaned regularly.
PRECAUTIONS WHEN SETTING THE BRUSH
The brush is set correctly when it is parallel to the cylinder and touching it lightly. This may be varied to an advantage by packing the centre of the brush, thereby causing the brush to touch with slightly more pressure at the centre. Some cylinder brushes are caused to lift to clear the front edge of the sheet and grippers. If this is so, make certain that the brush is not set in the lifted position.
The cylinder must be correctly packed and down on impression when making any adjustments to the brush.
The test for correct setting of the brush is by pulling on strips of paper placed between the brush and the cylinder at each end and in the centre. A slight drag only should be felt on each strip. Always pull the strips in the direction of the bristles.
ADJUSTMENT OF THE DROP GUIDES
The drop guides work in conjunction with the feed guides (lays), and their object is to hold the sheet in position and keep it steady during the period when the feed guides (lays) are lifting and the grippers are about to take hold of the sheet (called "snatching").
SETTING THE DROP GUIDES
1. Turn the cylinder until the grippers, about to grip the sheet, are open one inch (25mm).
2. Place the drop guides in position with the stop against the pin and the drop guide link in its correct position.
3. Evenly balance the drop guides on the sheet and set each one down firmly to the thickness of the stock to be printed.
TIMING THE DROP GUIDES
1. Turn the cylinder until the grippers close and then move them forward a pica (1/6'').
2. The drop guides are now timed by turning the set screw down until it just touches the feed guide lever.
Note: The drop guides are essential for register work. It is an advantage to have the drop guides timed to lift immediately the grippers close and grip the sheet. This overcomes any marking or scratching of the sheet, particularly so if multi-colour work.
Buffers (or Air Cushions)
The purpose of the buffers is to save wear and tear on the reciprocating parts of the machine (type bed, ink table, rack, frame, etc.) The buffers are securely fixed on each end of two air cushion rods, these rods being held in position underneath the type bed by brackets which are firmly bolted to the underside of the type bed . By choosing the point at which the air valves close by means of a governor and cam, the requisite amount of cushioning can be obtained which will just counteract the momentum pressure. At lower speeds much less cushioning is required than is necessary at higher speeds; further, when starting the press or just prior to stopping, it's preferable they remain inoperative.
To ensure the buffers are working correctly, they should be soaked in neatsford oil once every six months. To increase the compression in the buffers, the buffer head is removed by unscrewing it off its shaft. The cone inside the buffer is then tightened which spreads the leather casing outwards, thereby making a bigger circumference and increasing the compression by allowing less air to escape.
The stroke of the buffer can be increased or decreased also by screwing it in or out to suit on the shaft. A butterfly nut at the back serves as a lock-nut.