Singer 48K is a transverse shuttle machine like no other: it can make a flexible stitch with up to 60% stretch on the finest jersey. I haven’t found another type of machine that can do that. But how does it do it? What’s the secret?
First I thought it was the feed because it’s different from the later machines.
I called this type of feed “spring-driven feed” because I first thought that the spring to the left of the feed dog height adjuster pushed the feed dogs upwards. That was a mistake! At a closer inspection, it pulls the feed dogs down so that they remain submerged on the way back. It doesn’t actually drive the feed, so my term for this type of feed is not right.
Here is how this feed works. The long bar that is connected to the “bell” joint is what drives the feed. The joint moves the right end of the bar back and forth and left and right on an elliptical path. The bar is pivoted on a point within the bracket in the middle. This bracket is rigidly attached to the bar above it which is the slider that sets the stitch length. Move the slider – and the bracket – to the left for a shorter stitch and to the right for a longer stitch.
You can see that when the pivot point is moved left, the left side of the long bar is getting shorter, so for the same amount of movement on the “bell” joint on the right, the left end moves less.
This is a close-up of the feed dog drive by the height adjusting screw. There is a wedge* mounted to the bar that pushes up the feed dogs as the bar moves left and right. You can see how shifting the wedge left or right can change the maximum height of the feed dogs. But the bar end also moves back and forth – it’s on an elliptical path – and that is what moves the feed dogs back and forth. Moving the pivot point to the left reduces this back and forth movement and makes for shorter stitches.
*Different makes have a different design of this bit, but the effect is the same. Here is an example from an 1885 Wertheim TS:
Here the wedge is made to be a part of the driving bar and you regulate feed dog height by moving the plate in the centre of the photo left and right. The hole in it is elongated and the screw has a grip ring.
In both design variations the spring pulls the feed dogs and the driving bar down against the bar holding plate making sure that the feed dogs ride the wedge rather than float above the needle plate. Of course, with the foot down there would be no floating anyway, so the net effect of the spring is to keep the feed dogs submerged on the way back.
This feed design does not allow for reverse. The elliptical movement of the driving bar is locked to the “bell” joint and cannot be changed. The later feed design where the feed dogs are moved with a system of joins and axes, does allow to reverse the movement, which must have been the reason it replaced the old feed design completely.
The feed dogs in these machines sit on a long bar that rides under the bed or under a cover plate.
This is Wertheim TS with the cover plate removed. The feed dog bar actually pivots on the driving bar under the bed, so if you push on the feed dogs, the front bit of the bar lifts up.
Of course normally the cover is on, so the front end doesn’t actually rise above the surface. 😉 But on Singer 48K there is a substantial play here – at maximum foot pressure the feed dogs are suppressed to half of their regular height.
On the face of it the old feed appears to deliver the same action as the later feed design. Yet in practice it feels differently – it feels a lot softer. It does not mangle, rip or chew even the finest of fabrics, and it does not stretch out jersey.
I’ve examined and tested several classic TS machines, and they have all successfully sewn the finest materials without any damage. Yep, the old style feed is indeed a lot softer than the new one. But none of those machines managed to make a flexible stitch – the bobbin thread tension was always much too high. (Although I have now figured out how to add stretch to a TS stitch with a 1920s Vesta. This might work also for some other machines!)
The problem is in the shuttle. TS machines normally use a “boat” shuttle, but Singer 48K uses a “bullet” shuttle like a VS machine. A “bullet” shuttle can deliver much lower thread tension than a “boat” shuttle. There are a few other makes that introduced a “bullet” shuttle to a TS machine, namely Pfaff around 1900 and Frister & Rossmann (Gritzner) in 1938, possibly others.
So, a “bullet” shuttle is required to make that extra flexible stitch, but is the old style feed also required or merely optional?
I compared the action to vibrating shuttle machines – they’ve got the same shuttle with the new style feed. I have several vibrating shuttle machines of different designs, and they can all deliver a flexible stitch with 40% stretch on fine jersey, but none can do it with 60% stretch.
To get 60% stretch on a thin material you have to use very short stitches about 0.5mm, aiming to sew pretty much into every knitted stitch in the jersey.
Those are incredibly short stitches and require a very gentle feed to pack them in without stretching out the material. My vibrating shuttle machines cannot sew fine jersey with these ultra-short stitches without stretching it out – the feed is too strong, even when I reduce foot pressure to zero, and even if I remove the pressure adjuster altogether. The feed dogs in my VS machines are set to the same height as in Singer 48K, but they are all larger than those of 48K which gives a better grip and a stronger feed.
Compared to the similar one-sided feed dogs of Singer 27K, the feed dogs of 48K are much smaller: 15mm x 5mm with 8 rows of teeth on 48K versus 20mm x 7.5mm with 10 rows of teeth on 27K. Besides, the teeth on 48K are not all the same: rows of tall teeth alternate with rows of short teeth as you can see in the photo above. This appears to be quite important. Compared to Wertheim TS which has a similar feed mechanism but larger feed dogs with 10 rows of sharper teeth, the feed of Singer 48K is noticeably softer, although Wertheim is in turn softer than the new style feed on VS machines.
And then there’s the matter of foot pressure. Old style feed mechanism normally goes together with an old style presser bar of a classic TS design. The presser bar spring is quite small and the range of foot pressures is quite limited. But Singer 48K has a new style presser bar, the same as the VS machines. It has a wide range of foot pressures, from none at all to barely being able to lift the foot off the ground. I believe that this increased foot pressure that is the cherry on the cake. Singer 48K drops bobbin tension with increased foot pressure.
This last bit is an extremely peculiar property. I could reproduce something similar with Vesta Saxonia which also has a modern foot presser system. But the 48K works differently. On the 48K if you are not getting enough stretch in your stitching, you should lower the feed dogs a bit – the same thing that we see in many later machines with their “silk” setting (lower feed dogs to half height). On Vesta TS I had to raise the feed dogs slightly to add stretch (which is extremely weird, I agree).
Taking it all together, the magic is not in any one thing. Singer 48K is a classic example where the whole is greater than the sum of its parts. The magic happens because everything fits together exactly right.