Build Your Own Soap Cutter

by Catherine Failor of Milky Way Molds

When it comes to cutting soap into bars, most small-scale soapmakers are faced with 2 options: laborious hand-carving or the purchase of expensive pre-fabbed cutters. But in very little time, you can create your own customized soap cutter for $50 or less. This simple but effective cutter can slice up to 1,000 perfectly square and uniform bars per hour!

NEW: Read tips on determining your soap cutter size, by Michael McAuley

Materials needed:

• 4ft x 4ft. sheet of plywood, at least 5/8″ thick
• 4ft x 4ft. sheet of formica
• 24 ft. of 2×2 in. fir
• 8ft. of 2x4in. fir
• 3-4 dozen wood or drywall screws, 1-1/2″ length
• 4-5 dozen wood or drywall screws, 2-1/2″ length
• 2 dozen small metal washers, 3/8″
• hex-head carriage bolts (number of bolts depends upon number of cutting wires – read below to determine your specific needs)
• medium gauge guitar wire or 20 – 22 gauge music wire
• 1 pint contact cement

teach soap tutorials

soap cutter

Your finished soap cutter

Preliminary Considerations

Look at the finished cutter, at right. Basically, your soap cutter will resemble the capital letter L. The soap slab is first pushed down one length of the L where wires dissect the soap lengthwise. The slab is then fed into the other leg of the L, where the bars are cut into their finished widths. The size of this L depends entirely upon the size of your soap slab.

For example, if you pour your soap into a tray 12″ wide by 18″ long, the long leg of the L will be 18 x 2 =36 inches (to accomodate the lengths of the pre-cut and post-cut slab of soap). Add another 8 inches to that 36 inches to accomodate the 2 x 2 fir “rim” of the cutter as well as give extra maneuvering room for your slab. So for an 18″ long slab of soap, one leg of the L needs to be about 44 inches long. For the width of this first L leg, add 12″ (the width of the slab) plus 3 inches (the width of the two 2×2’s on the borders), then add an extra inch for “slack.” That’s 16 inches wide. The length and width of the second leg of the L will be 18 inches wide (length of soap slab) plus 3 inches( two 2×2’s along rim of cutter); add an extra inch for slack. The finished width will be 22 inches. The length of this L-segment will be a maximum of 14-15 inches- that’s the width of the slab plus a couple extra inches slack.

Figure 1


1. After taking the above factors into consideration then tailoring them to your own specific soap slab, measure and cut your plywood into the appropriate size. (Fig. 1)

2. Using those same dimensions, cut the formica into an L. You don’t need special tools for scoring formica; a utility knife will do. Hold a ruler or T-square on the line you want cut and gently but firmly score it several times with the knife. It will snap along this line with light pressure.

Figure 2

3. Following instructions given on the contact cement container, spread cement over the surfaces of the formica and plywood, let dry until tacky, then press together. Make sure all warps are smoothed out. If any formica hangs out over the edge of the plywood, saw or file off. (Fig. 2)

4. Now cut the 2 x 2’s which will form the “rim” of the cutter. You’ll need pairs of four different lengths (4 lengths for the top of the cutter, 4 identical lengths for underneath). These 2×2’s will be fastened along the sides of the L, but both ends of the L will be left open so that the soap feeds in one end then exits out the other.

Figure 3feeds in one end then exits out the other.

5. Starting from the formica-covered top side, sink 1 1/2 inch screws and fasten the 2×2’s to the bottom of the plywood. To avoid cracking the formica, pre-drill small holes before sinking the screws. (Fig. 3)

6. Using 2 1/2 inch screws, attach the other set of four 2×2’s to the top of the cutter. (Fig. 4)

7. Now cut four blocks, 2″x 4″x 8″. These will form the “sidebars” which will be attached to the sides of the cutter. The 2 crossbars will eventually be screwed onto these sidebars.

Figure 4

8. Next, cut two crossbars, using 2×4″ lumber. The length of these will depend upon the size of your soapcutter. Referring to the example given under Preliminary Considerations, the width of one L leg was 16 inches, and the width of the other was 22 inches. Add 3 inches to each of these measurements in order to obtain length of both crossbars. Your twocrossbars will consequently measure 19 inches and 25 inches.

9. With a straight-edged ruler or T-square, draw a line lengthwise downthe middle of both crossbars. The guide holes for the wire will be drilled along this line. The spacing for the wire holes will depend entirely upon the size you cutyour bars. For the hypothetical soap slab measuring 12×18 inches, let’ssay the slab is cut into 2×3 inch bars. So as the soap slab is pushed down the long, narrow neck of the L-shaped cutter, five wires will cut the slab into six 2-inch wide strips. These strips will then be pushed through five wires on the other leg of the L which divide the strips into six 3-inch long bars. The 12×18 inch slab therefore yields 36 bars.

Now back to the crossbars.

The line drawn on the 19-inch crossbar will be scored with pencil in 2 inch intervals. But first mark an “X” at one end; this will be the end that rests over the outside edge of the cutter (the edge your soap slab always follows). For the first of the five wire holes, measure in five inches from the “X”-ed end of the wood and mark that spot on the line.(The first 3 of these 5 inches takes into account the 1 1/2 widths of the sidebar and rim.) Now pencil 4 more spots along the line, each spaced 2 inches apart. With a small drill bit, use these guidemarks to bore five holes through the 2×4 crossbar.

The 25-inch crossbar will likewise need to be marked and drilled. Mark one end with an X and measure in 6 inches from that end for your first mark (again, the first 3 of these 6 inches is due to the widths of the sidebar and rim). This crossbar holds 5 wires which cut the bars into 3 inch widths, so the marks will consequently need to be made in 3 inch intervals. Then drill each hole completely through the 2×4.

10. Corresponding holes for both crossbars will now have to be drilled on the surface of the soap cutter itself.

On the narrow, long leg of the L a straight line (perpendicular to the sides) needs to be ruled across the surface of the cutter. For our hypothetical 12″x18″ slab, this line will be drawn at least 18 inches from the end of the cutter so that the entire slab of soap rests squarely on the cutter’s surface before being fed through the first set of wires. Otherwise, part of the slab will hang over the cutter’s edge, resulting in less control over the movement of the slab through the wires. For an 18 inch long slab, considering drawing the line 19 inches from the end, with an extra inch added as “slack.”

Starting from the inside edge of the outer 2×2″ rim, pencil in 5 marks spaced at 2-inch intervals along
the drawn line. Drill 5 holes through the plywood.

For the wider leg of the L-shaped cutter, another line will need to be ruled. The location of this line will be determined by the 12-inch width of the hypothetical soap slab, so allow 12 inches between the line and the end of the soap cutter.

After determining where to locate this line, mark your first hole 3 inches in from the inner edge of the 2×2″ rim. Mark 5 holes, 3 inches apart, then drill.

Figure 5

11. Before screwing the sidebars and crossbars to the cutter, you might want to make sure the holes on the crossbar line up with the holes on the cutter surface. If the holes don’t match at 90 degree angles, the wires will be askew, marring the finished shape of your soap bars.

To this end, set the base blocks in place(don’t attach) then lay the crossbars on top; flip the crossbars on their sides so that the drill holesare in view. Rest one edge of a small L-square on the cutter surface and check matching top and bottom holes for alignment. Re-drill any holes if necessary. (Fig.5)

12. Now attach the four 6-inch sidebars to the sides of the cutter using 2-1/2″ screws. To insure even alignment of the sidebars with the wire holes drilled on the cutter surface, draw a line lengthwise through the middle of each 6-inch block. When screwing the blocks onto the cutter sides, make sure the lines through the middle of the blocks are relatively even with the line of holes on the cutter surface.

Figure 6

13. Attach the 2 crossbars to the sidebars with 2 1/2 inch screws. (Fig. 6)

14. It’s time to begin wiring the cutter.

The wires for your soap cutter will be attached to 3/8″ hex-head carriage bolts, which function in exactly the same way a tuning key on a guitar or piano does. The wire is tightened by turning the bolts sunk into the crossbars.

Figure 7

Figure 7

The number of bolts you’ll need depends upon how many bars your soap slab is cut into. If we again refer to the 12×18 inch soap slab (cut 6 bars wide by 6 bars long), 5 vertically-strung wires will be needed for each leg of the L-shaped cutter. Two wires will share one carriage bolt; both crossbars will therefore require 3 bolts apiece, for a total of 6 carriage bolts.

A hole needs to be drilled through the upper shaft of each bolt. You can do this yourself by clamping the bolt in a vise and drilling; each bolt will take a few minutes of drilling time. Or you can have a machine shop do it for you. (Fig. 7)

Holes for the carriage bolts now need to be drilled into the top of each crossbar. Since two vertical wires share one bolt, drill the carriage bolt holes somewhere in between the wire holes. Use a 5/16″ drill bit for the 3/8″ bolts. A slightly undersized hole is important, otherwise the tension in the tightened wire loosens the bolt in its channel, resulting in slack wire. Drill completely through the 2×4 inch crossbar and sink the bolt 1/4″ to 1/2″ into the wood.

Figure 8

15. To wire your cutter, thread a length of wire (approximately 1 1/2 to 2 feet long) through the hole in the bolt, loop the wire once around the bolt then tie the wire in a single knot. You’ll probably need pliers to do this. Cut off any excess wire.

16. Now tighten the wires by turning the carriage bolts clockwise. Before the wires are completely tight, flip the cutter over and adjust the washers. Each hole should have a washer resting squarely underneath it; this keeps the wire from cutting into the soft plywood when the wires are further tightened. (Fig.8) Give your wires a final tightening.

figure 9

Figure 9, Your cutter is complete

Your cutter is complete! (Fig. 9)

Tips on Using Your Soap Cutter

1. When pushing your soap through the cutter, push in a diagonal direction: into the outside rim of the cutter as well as forward through the wires. If you only push forward, the soap can wobble, resulting in wavy” soap bars.

2. Use a small block or board to push the soap. It makes for more even, consistent pressure on the slab.

3. After the soap has been pushed through the first set of wires, you might want to flip all the strips up onto their sides for the next pass through. If there is any wobbling in this cut, it doesn’t show on barswhich are standing on their ends. Bars which lie flat through the second pass will show any and all inconsistencies.

4. For ease of pushing, lightly mist the formica surface using a spray bottle filled with water. This easesthe friction between soap and cutter surface.

5. If your 2×2 inch rims on the outside of the cutter are somewhat rough, you may want to sand them smooth and/or shellac them. This will help the soap pass more easily and cleanly through the cutter.

6. If your raw soap slab comes out of its mold with imperfect sides (due to wrinkle marks from a plastic liner, for example) which need “cleaning” before being cut into bars, you might want to consider a slightly different wire configuration than the one given above. Instead of setting your first wire a bar’s width from inner edge of the cutter’s rim, set this wire a mere eighth of an inch out from the rim. As the rough soap slab is run through, a eighth-inch thin sliver of soap will be shaved from the rough edge. Likewise, set the outermost wire an eighth of an inch closer in, so that a corresponding sliver will be shaved from the other side of the block. If you align both sets of wires in this fashion, all four sides of the slab will be cleanly and evenly cut. A little extra scrap is generated this way, but lots of time is saved which would otherwise be spent on cosmetic makeovers for your soap.

7. If you pour your soap slab 2 bars thick, you can divide the block horizontally by sinking a carriage bolt into a sidebar and weaving a horizontal wire through the first set of vertical wires. Two holes will need to be drilled through the sidebars and rims of the cutter, so it’s best to plan ahead and do this drilling before screwing these pieces to the cutter surface. If your soap is one inch thick, the two holes will have to be drilled through the rims (and sidebars) one inch above the surface of the cutter. The wire can be screwed down to the other side of the cutter in the same fashion that any “odd-numbered” wire is screwed to the underside of the cutter in Step 15 above. If your soap slab is 3 bars high, you may have trouble pushing the soap through, since that much wire will offer a lot of resistance.