Paper-Cutting Machines Part 2

_The Clamping Pressure_

The clamping of the stock in the machine while the cut is being made is an important feature of a modern paper cutter. This is done by means of a horizontal bar placed behind the knife and parallel with it. This clamping bar is moved up out of the way while the stock is being placed in position and is then brought down with the desired pressure to hold the pile of sheets firmly until the cut is made, and is again lifted up out of the way.

In the simpler machines the clamping pressure is obtained by means of a stout vertical threaded rod to which the clamp bar is attached at the lower end. The rod is operated by a hand wheel at the top and turns in a threaded opening in the cross-head of the frame.

Clamping pressures vary greatly under different conditions, from a light pressure to several tons. The ratio of the power applied in hand clamping to the pressure secured on the clamp is, in ordinary commercial machines, about 1 to 150 for small machines up to 32 inches wide, using a 2-foot-diameter clamp wheel and overhead screw.

The ratio of power in cutting machines 34 inches and wider, having a 1-1/2-foot-diameter clamp wheel, and worm of 3/4-inch pitch at the side engaging a worm gear keyed to the clamp shaft, is about 1 to 180. That is, a pull by the hand of 1 pound on the overhead clamp wheel will produce about 150 pounds pressure of the clamp on the stock to be cut; and a pull by the hand of 1 pound on the side clamping wheel will produce about 180 pounds pressure of the clamp, less friction. Operators will pull ordinarily from 10 to 100 pounds; _i.e._, producing a pressure of from one to nine tons.

The first type of automatic-clamp cutting machines produced always the same arbitrary pressure on the clamp, with consequent waste of power, crus.h.i.+ng and indenting the work, offsetting the ink unless it was absolutely dry, and requiring adjusting with a wrench, which was slow, dirty, and indefinite.

All modern automatic-clamp cutting machines have independent automatic-clamp pressure, and these apply the pressure at both ends of the clamp. The earliest power clamps were called "self-clamp." In these the knife and clamp were connected together (_i.e._, dependent).

The modern automatic-clamp mechanism is not only independent of the knife, but in addition practically all the power of the belt goes first to clamp the work and then afterwards to drive the knife through the cut. This separation of the clamping effort from the cutting effort increases efficiency and economy.

By a modern device on some machines this clamping pressure is instantly variable to suit different kinds of stock and other conditions.

_Cutting Sticks_

The cutting stick, into which the knife pa.s.ses after cutting through the bottom sheet, saves the edge from cutting on the iron table.

Correctly designed cutting machines have knife-bar motions that permit no "chug" of the knife into the stick and thus conserve its sharp edge.

Many designs, however, are faulty in this respect and the unrecorded expense of their operation is a serious leak.

The cutting stick is set in a slot in the table. The most common and perhaps best form, everything considered, is a 3/4-inch square hard maple stick, set so that the knife strikes it 1/4-inch from the edge.

All four faces of such a stick may be used.

A metal frame for holding a 1/4-inch square hardwood stick, or a soft metal (so-called) cutting stick, is made. A round wood cutting stick connected to a timing mechanism which rotates it automatically for every cut, or for a certain number of cuts, is valuable for hard stock requiring microscopical perfection.

_The Back Gage_

The back gage is moved and controlled on the machine table in several ways. In the simpler machines the gage is attached to a rod under the table, a long slot in the center of the table allowing the connection of the gage with the rod below. The rod is attached to the gage with a worm gear and is operated by a small hand wheel at the front of the table. In other machines the gage is attached to a steel cable, or a metal tape, or a chain which pa.s.ses over wheels at the back and front below the table.

The one-piece back gage was improved by cutting it into two or three sections so that the first and final cuts of two or three piles may be made at every stroke of the knife. Adjusting screws are provided for tilting the back gage forward or backward to compensate for the variation in the width of the top and the bottom sheets which occurs in the same machine and with the same knife when cutting hard or soft papers. A swinging adjustment is also provided to "square" the back gage parallel with the knife edge.

Various devices for taking up the slack caused by wear in the table slot guiding the back gage are furnished, but probably the simplest and best method is the replacement and refitting of the inexpensive sliding part.

INDICATOR ATTACHMENT FOR BACK GAGE

The distance the back gage is moved is read in different ways. The movement of a gage operated by a screw and wheel is indicated by a pointer on the front edge of the table overlapping the rim of the long screw wheel. If the rim of the screw wheel is lined off in sixteenths of its circ.u.mference, the pitch of the screw being one inch, each complete turn of the screw wheel means that the back gage is advanced one inch, and each one-sixteenth turn means that it is advanced one-sixteenth of an inch. It is important always to keep turning the screw and wheel the same way when so measuring, because otherwise the "back lash" (looseness of the screw and its nut to permit easy working) will cause variation.

For a back gage moved by a chain or a wire cable or a metal tape, a graduated dial is attached to the top of the cable hand wheel which reads from a pointer attached to the front edge of the table.

Both these ways of reading require the operator to look down.

For a back gage moved by power, a steel indicator ribbon pa.s.sing around a wheel overhead in the frame cap and attached to a standard on top of the back gage (see Fig. 7) enables the operator to read the position of the back gage without looking down, and a second wheel indicator and pointer permits reading to less than one-thousandth of an inch. Similar indicator ribbons are attachable to the back gage for screw, cable, chain, and metal tape movements.

These indicator ribbons are usually graduated and marked for inches, halves, quarters, eighths, and sixteenths. They are also furnished with metric system measurements, graduated and marked for centimeters and millimeters. They are also furnished extra wide so that both the English inch measurements as well as the metric system measurements may be put upon the same ribbon. When graduated to thirty-seconds of an inch and millimeters a good magnifying gla.s.s of about three inches diameter, adjusted in front of the pointer, enables easier and more accurate reading. An easily operated lock for holding the back gage fast to the table at any exact mark prevents variation in the width of cutting.

Fixed distance gage rods and suitable engagements with the back gage are provided for cutting at any time duplicates of exactly the same width, especially valuable for loose-leaf ledger work. Pins and holes drilled in the back gage and table also secure uniform locations impossible to get solely from reading the overhead ribbon or an indicator dial, which latter may be read incorrectly because of poor light or variation in the operator's position or eye.

Locking devices for the back gage ordinarily consist of (a) a friction grip around the moving screw, or (b) about the cable hand wheel, or (c) a clamping device which holds the back gage, or (d), best of all, a fixed grip rod-holding device operated from the front of the table, thus eliminating any possible lost motion through connecting parts from the jogging or chucking of work.

A back gage is split so that different width piles may be cut at the same time, such as tr.i.m.m.i.n.g the heads, tails, and fronts of books. The fingers at the splits are placed smooth, so that they may be used as side gages to enable the wear of the knife to be taken its full width, instead of just at the usual left-hand end.

_Power Back Gage Movement_

The larger sizes of cutting machines are equipped with a labor-saving power connection to the main driving shaft which may be thrown in by the operator at will to move the back gage forward or backward by power, a micrometer reading the position to thousandths of an inch.

_Special Devices_

Special s.p.a.cing devices for the back gage are revolutionizing many branches of work. The usual screw or cable is relatively slow and undependable and requires care and time to move the work the exact distance required.

The new way is to equip the cutting machine with a back gage operating mechanism having stops which can be set for any width to be cut. The operator simply pulls a lever between the stops, which instantly moves the pile and measures it exactly. Production has been increased six hundred per cent. with such s.p.a.cing devices.

Gages for measuring the width of the cut may be simply a sample cut the desired width laid upon the pile, or an exact size wood or metal pattern, or the width may be determined by reading on a steel indicator-ribbon attached to the back gage the distance its face is back from the knife edge.

These methods are only approximately accurate, however; for exact cutting, steel distance-pieces of correct lengths to give the different widths of cut required are set against a fixed stop in the back table and the back gage run back until it grips the steel distance-piece.

This brings the face of the back gage the desired distance from the knife.

Modern patented s.p.a.cing devices for the rapid duplication of exact widths in succession, while the machine runs continuously without stopping between cuts, have been perfected so as to enable, on some cla.s.ses of work, one cutting machine to do the work of six and still obtain accuracy. These s.p.a.cing devices operate the back gage through a chain or a screw or a gear by means of a lever driven by hand or by power between accurately set stops fixed for any desired width, and thus eliminate the time ordinarily lost measuring the width for each cut.

Among other conveniences on the large modern cutters two starting levers, one at each side of the machine, or a starting bar extending across the front, save several motions at each cut.

A flat piece of metal, called a clamp face, which may be quickly attached to the under side of the fingered clamp, is used to prevent these fingers marking soft or delicate finished stock.

A snake gage is a folding lattice used in front of the back gage of a solid wide face clamp machine to enable the back gage to push the pile up nearer the knife.

For large pamphlets or magazines an extra clamp attached behind the regular clamp to hold the back of the pile down gently by spring pressure will prevent the sheets springing up and away from the back gage, caused by the folds and air between the sheets.