The canvasworker's best friend or worst enemy is the sewing machine. Fortunately, each one of us can make sure that the relationship is "friendly." The more we understand our machines, the better they work for us.
It is surprising that sewing machine companies do not make information concerning their machines' mechanical needs available. Their handbooks are written as though companies expect mechanical skills that are completely beyond the user. The fact is that sewing machines are relatively simple. Their adjustment is easy for the average user willing to acquire a little knowledge. The following will help in this regard.

MACHINE ADJUSTMENT
The Class 15 Machine

Most home sewing machines have class 15 shuttle hook systems. The gib hook in the bottom of the machine moves around the bobbin case back and forth (Figure 1). It oscillates half way round and then back the other way a half turn. The purpose of the hook is to pick up the upper thread at the needle and carry it down around the bottom of the bobbin case where the upper thread loop is pulled up and tight by the take up arm above the needle bar. Once this task is completed, the oscillating hook reverses its direction and returns to its original location. This movement is created by a simple yoke and cam arrangement on the top shaft of the machine. It is a relatively inexpensive design but quite reliable and wholly satisfactory for sail & canvas work.

The Two Most Common Sewing Machine Problems

But no matter how good the machine is there will be times when you want to throw it out the window. When this happens just pick up these instructions and let us help. There are two problems which occur frequently with most machines: skipped stitches and improper stitch tension. There are a number of mechanical adjustments that can overcome these problems. In order to utilize them properly, let us again review the fundamental operation of a lock stitch sewing machine.

To make a stitch a sharp "hook" (the gib hook) rotates around the cage that holds the bobbin thread under the machine. See Figure 1. This hook passes right next to the rising needle and catches a loop formed in the upper thread by the needle as it comes up. After catching this loop, the hook pulls it down until it circles around the entire bobbin of lower thread. The two threads are thus interlocked and a stitch is formed. As the needle continues to rise, a "take-up arm" also rises to pull the excess thread up from the bottom of the fabric. The thread comes up out of the cloth because of the tension disks that tightly clamp the thread on the spool side of the take-up arm.

Skipped Stitches

If your machine is skipping stitches your zigzags may appear like a straight stitch on either the right or left side with a proper zig stitch being formed only once in a while. We know from the discussion above that the hook is not catching the thread consistently.

It is usually because either the thread is not being held down by the fabric as the needle is withdrawn and, thus, a loop of thread is not formed for the hook as it passes the needle. Or the hook may not be passing the needle at the proper time, i.e., it may be passing the needle before a loop is formed or, at the opposite extreme, after the thread has been pulled upward out of the path of the hook.

What to Do

1. The first thing to do is simply change the needle. A bent needle will cause skipped stitches because the loop is not where the hook "expects" it to be. The old needle could also have become fouled with adhesive if you are using basting tape or sewing insignia cloth. In either case, the new needle will resolve these problems.

2. Next check for adequate pressure foot adjustment. Heavy, closely-woven materials like sailcloth and canvas can make the withdrawal of the needle from the fabric difficult. If the presser foot is lifting as the needle comes out of the cloth the effect is the same as if the needle were not going far enough into the cloth — the loop that it forms will be too small. To test if this is the problem, hold your finger (with care to avoid the needle) on the presser foot so that it cannot "bounce" as the needle is withdrawn. If there are no longer any skipped stitches, you know that more pressure down on the foot is required.

The presser foot is spring loaded and it is possible to adjust the pressure in two ways. The first means of adjustment is limited but it is generally sufficient. There is a roughly 1/2 inch plunger that protrudes out of the top of the case just above the presser foot (Figure 2). Push down on the ring at the base of the plunger and the plunger will snap up to its full height. At this point pressure down on the presser foot is minimal. Pushing the plunger down will increase presser foot pressure by compressing the spring underneath. Most sail and canvas work will benefit from a good deal of pressure.

The second means of adjustment is to change the presser foot spring itself. Generally the machine comes with a light presser foot spring. It should be used with more delicate fabrics. Purchasing and using a heavier presser foot spring will allow you to put more pressure down on the presser foot. To change springs, loosen the lock nut and then the set screw to the left side of the upper presser foot bearing (just under the plunger release ring described above). See Figure 3. That will make it possible to remove (lift up) the release ring. The presser foot spring can then be pulled out and the smaller one inserted in its place. Tighten the set screw and the lock nut after replacing the plunger release ring. Do not over tighten the lock nut. Its purpose is simply to keep the set screw from working its way out. Tightening the lock nut to a snug position is adequate.