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Step 2 –Accommodating the blade

Once you work out where the blade will be moving, we just need to work out how it will remain in tension when the the main ‘plunger’ pushes down (so it doesn’t bend up or break).  We can minimise this problem by setting the blade to cut on the up stroke – although this has problems of its own (more on that later).  There are a few ways one could approach this: 

1.  Modify the drive that goes to the bottom and moves the swing arm (see photos) up and down and side to side, so that it just goes up and down (and not side to side) the same distance as the main needle plunger.  This would be a nice but complicated solution because you could have the blade being driven, or pulled, from both sides.  I discounted it as horrendously complicated.

2.  Keep the blade in tension by some direct connection to a bungee or spring on the underside.  Hope that this will work well enough to pull the blade down as it will not be cutting on the down stroke anyway.  The problem here is that the main plunger moves a fair old distance and any directly opposing spring is going to have a hard job being effective over such a range of motion.

3. Similar to 2. but use some extra mechanical advantage to reduce the space requirements and demand for high performance bungee or spring.  In my case, this involved spring loading the long arm that controlled the ‘feed dog’ (the bit that moves the fabric).

Notice that the swing arm (below) has been liberated of its crankshaft overlord, and now flaps freely.
The swing arm moves side to side on a central pivot – we will want to stop this movement. Drilling a hole in the side and taping a thread to accept a grub screw should be all right for now.  It moves up and down on the pivots above and below – we want to keep this motion.
Removing the swing arm. As luck would have it, it already has a hole, prime for tapping to a M4 thread.
Take the ‘feed dog lever arm’ out to reveal the bare underbelly!
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