This is a very good question, and one I hear often from 401, 500, and 600 owners.
This simple answer?
It depends on how creative you want or need to be. As shown on the flip-top chart, the 401 has an impressive array of built-in stitch patterns with almost infinite range of length and width variations.
Singer 401 built-in stitch chart
Singer 401A built-in cam-stack.
If you look at the 401's built-in cam stack each cam represents a different stitch pattern. And when you factor in that the 401 can "combine" two cams to create additional stitch patterns, it's pretty awesome what the 401 can do without Special Discs.
But if you want to get the maximum creative use out of your 401, you will definitely want a set of Special Discs.
Special Disc patterns from 500A manual.
The original accessories kit for the 401 included 5 Special Discs (numbered 1-5) for stitches that are not built-in. These 5 discs can be combined with the built in Primary stitches to produce additional patterns, such as scalloped zigzag.
However, Singer produced a total of 24 Special Discs for the Slant-o-Matic and 600-series Touch & Sew family, which means there are 19 additional discs available.
Some of these additional Special Discs are patterns which are not built-in, while others are duplications of stitches built into the 401. However, even duplicates can add to the range of stitches the 401 can produce.
"If it's already built-in, why would I want the Special Disc too?
The answer lies in the distinction between "Primary" patterns and "Combination" patterns.
"Primary" patterns use a single cam in the cam-stack to produce the stitch. This means that the stitch pattern is unchanged regardless of width and length settings. In other words, a scalloped stitch simply gets wider or narrower but still looks the same. In the sample shown here, the width of Special Disc #12 has been set progressively wider.
Special Disc #12 at widths 2, 3, 4, 5 (Primary)
On the other hand, "Combination" patterns use two cams in the stack to produce the stitch pattern. Which means that a change in stitch width affects the movement of both cams, which can greatly affect the overall appearance of the stitch. Note in the sample shown here how the appearance of stitch setting DP changes when the width is set progressively wider. The zigzag blocks get narrower as the offset gets wider.
Built in stitch DP at widths 2, 3, 4 (Combination)
While this does add some variety to the built-in stitches, it also means you are somewhat limited when using built-in stitch patterns if you want the pattern wider or narrower. The 401 and 500 have seven built-in "Primary patterns. They're shown on the top row of the chart (see photo above) inside the flip-up lid.
Which is where Special Discs come in. Special Discs are considered "Primary" patterns, so they look the same regardless of width. Clearly an advantage in some situations.
Another advantage of using a Special Disc is that two-needle stitching only works with "Primary" patterns, because the left-hand stitch selector has to be set on "A." So if you like the look of a "combination" stitch but want that stitch in double-needle, you'll need to use a Special Disc.
Built-in Primary patterns can also be combined with certain Special Discs to produce additional stitch patterns.
The final advantage of Special Discs is that they are just plain easy to use. Pick a pattern, pop it in, and away you go. No messing with dials or charts.
So, DO you need a complete set of Special Discs for your 401, 500, or 600 sewing machine?
Bottom line, it's up to you. The choice is yours.
Happy Sewing! Barbara OldSewinGear...dedicated to helping you get the most our of your old sewing gear.
OldSewinGear is the collaborative effort of retired repairman Gary and daughter Barbara. We love old sewing gear and enjoy sharing what we've learned in our vintage sewing machine adventures. We are located in Roseburg, Oregon.