The bench jig saw, also known as a scroll saw, is a woodworking machine used for making tightly curved cuts in wood and other materials up to around 2 inches in thickness. It uses a specially designed blade called a jeweler’s blade, fitting to upper and lower chucks. The blade is powered by a motor to produce an up and down motion, like that of a reciprocating saw, rather than a continuous one way motion such as in a band saw.
Most bench jig saws feature a metal C-shaped frame base, an electric motor driving the blade assembly via a drive belt, and a metal work table. Some tables are capable of tilting to allow for beveled cuts.
Jig Saw Blades
Jig saw jeweler’s blades for cutting composites, metal, plastics and wood are available. They come in widths of between 1/32 inch to ¼ inch, and with pitches of from 7 to 32 teeth per inch. Blades can be fitted sideways, with the teeth to the left or right of the operator, or straight, with teeth facing toward the operator.
Behind the blade is a hold down assembly with a sprung foot that is vertically adjustable to ride on the top surface of the work piece. The hold down assembly will also usually incorporate a blade guide, designed to prevent bending and twisting of the blade. The guide is a slot in a rod or disk that is adjustable to the size of the blade, with a slightly sprung backside to support the blade’s back edge.
To fit a new blade, remove the table insert and raise the hold down assembly. Manually move the drive belt so that the bottom assembly is at the top position of it’s travel. Next, locate the bottom end of the blade in the bottom chuck, with the teeth facing downward and forward, then tighten the lock screw.
Check with a metal square that the blade is normal to the table, both faces and edges. Finally, pull down the upper chuck assembly, fit the top of the blade, and tighten the lock screw.
If you are changing from a thick blade to a thinner one, you may need to add tension to the blade as well. The tensioning adjustment is located in the upper chuck assembly. Check for proper tension by cutting a piece of scrap. If the cut tends to wander, add more tension.
Insufficient tension can also cause a blade to break prematurely. Both the above faults can also be caused by the blade guide alignment needing adjustment.
Since the material being cut on a scroll saw is usually relatively thin, it needs to be held down on the work table to prevent chatter and break-outs. The hold down guide should just the touch the work surface so that it exerts a light pressure; too much pressure will make it hard to feed the work in to the blade and can ruin the surface with the marks left.
Choosing the right speed for the job is next. Your machine’s manufacturer will have supplied speed recommendations for different materials and thicknesses, so consult the handbook.
A rule of thumb is that the heavier the blade, the slower the speed, but the faster the speed, the better the quality of the finished cut. Prior to switching on the machine, it is a good practice to move the blade by hand at least one revolution to make sure everything is adjusted properly.
Making Straight Cuts
When making a straight cut, it is a good idea to clamp a temporary fence to one side of the table to use as a guide. The thickness of the fence material should be more than that of the work material. With the fence as a guide, feed the work along it into the blade, preferably with a push stick.
Tight curves are where the jig saw excels. For tight radius complex curves, use a narrow blade; for simpler shallow curves, use a broader blade. Feed the work slowly, keeping a steady pressure on it. It is important to not allow the blade to twist, but if it dos and leaves the cut line, make sure it is on the waste side of the cut or the work will be ruined.
One trick for cutting curves in thin materials is to sandwich the work between two plywood sheets; this prevents distortion and reduces burring.
Interior hole cuts can be made in materials by a process of first drilling a hole in the waster area for the blade to move through. You will need to disconnect the blade from the top chuck, thread the work over the blade, then reconnect the top chuck before cutting.
If you need to make a cut with a bevel on it, this can be done by tilting the work table. Raise up the hold down assembly, loosen the tilt clamp and tilt the table manually to the angle indicated by the angle on the built-in protractor. After you have the correct angle, make to sure to fully tighten the angle adjustment clamp before switching on the saw. You will also need to adjust the hold down assembly to align with the angle.
You can also fit specially made files to the jig saw. These are available in a range of cross sectional shapes, from oval to triangular to square, with shanks that fit into the lower chuck’s V-block. They are used with a special table insert which has a ½ inch diameter through hole in the center.
It is important to check the orientation of the blade after fitting to make sure it is normal to table insert; use a 90 degree try square. Run the jig saw at slow speeds when filing, running the work edge against the file until you get the proper finish, avoiding excessive pressure.
Another attachment you can get to fir in place of the blade is a sanding rod. Fit into the lower chuck similar to a file, these are abrasive cylinders available in various grits that can be used at slow speeds for finishing edges.