In the face of it, converting a sewing machine shouldn't be all that difficult - hence the idea. You have a needle that goes up and down at a speed fast enough to drive the saw blade, a work area with a section underneath to keep tension on the blade... What could be simpler? All you need are some basic hand tools, some epoxy, and an old unused sewing machine.
Step 1: Assess Your Sewing Machine
There are lots of leavers, pulleys, gears and pivots. Even if some of these have fallen into disrepair (as had my machine) this typically will not matter, as we will be stripping lots of it out. Compared to a sewing machine like this, the scroll saw I have in mind is actually quite simple. It helps if the motor is working and the needle is going up and down - everything else is pretty much surplus.
So unscrew the case retaining bolts and have a good look and what you have got... It might well be the case that you will have to modify thing to suit your particular machine.
Step 2: Remove Surplus Parts
As you do this consider how you are going to arrange the saw blade in relation to the underside. You can do this by releasing the plunger (see pics) and pushing it down so you can see where the blade, if extending straight out from the plunger, would rest.
As always, see the pictures to get a better idea of what I mean.
Step 3: Accommodating the Blade
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).
Step 4: Saw Blade to Feed Dog
I went about this by making a casting of sorts, from JB Weld. There are loads of pages out there that that consider this (here is one), so I will not go into too much detail here.
First fill the void in the leaver with epoxy. Then cover a sample of the saw blade with beeswax sot it will not be stuck forever, and set it into the epoxy. You probably want to periodically rotate the part to prevent the JB from sagging while drying. Once dry we can pull out the saw blade, and hay presto, a nice little blade sized holder. Now drill a hole and tread it and we can use a grubscrew tightened against the saw blade to fix it in place.
Now we just need to spring load the leaver to keep the blade in tension - that way it should stay straight and not bend all over the place. So there would be many different ways to approach this, one could mount a 'push' style spring between the leaver and the machine underside, or as I have you can have a twist style spring mounted on the shaft that supports the leaver. Either make your own from scratch (here is a good DIY spring method) or, as I did, you could probably modify one of the many springs you took away from other bits of the machine.
Step 5: Fixing the Plunger Supports in Place
Again some JB Weld or similar epoxy putty will do the job nicely (whilst also adding a bit of strength).
Take the spring mechanism out and make sure the plunger is dead centre over where your 'feed dog blade retainer' is.
Step 6: Mod the Baseplate
Don't do what I did and hit it hard with the centre punch, as it might break (DOH!). It is not of equal thickness underneath, so even when you lay it out on a flat surface, as I did, it is not supported in some mid-sections. Luckily when I did this the break was not too serious, and it doesn't hinder performance any.
I ended up borrowing my friends Dremel and spending a time grinding at it with a small milling bit. I recommend this method over drilling small holes - the drill bit skids all over the place.
Step 7: Guarding From Those Splinters
1. Every up stroke of the blade will try and lift the work piece up off the flat work surface. One could maintain extreme vigilance and hold it down at all times, but that would quite frustrating. What we need to do is bring the existing presser foot into operation.
2. As with all sawing there is the problem of splintering edges where the cutting teeth of the blade exit the wood, sometimes tearing fibres loose, making a rough splintered edge. This can be minimised by using a sharp blade, but we can do better and eliminate this almost all together, again, by using a modified version of the presser foot.
Using some transparent polycarbonate (the kind of material they use for safety goggles), we can fabricate a functional splinter guard. Any of you that read my blog will know the ironic string of failures I had trying to get this right... I will show the way that seemed to finally work.
Basically this involves cutting into a piece of the plastic with the scroll saw. This piece of plasti will become the 'hold-down splinter guard', and the slot we cut in it will be exactly the correct size for the blade. See the photos for a further description of what to do. This splinter guard is kinda optional, you might find the presser foot on its own, with a sharp blade is enough.
Step 8: Let's Make Some Jazzy Stuff!
Ok so here we go. Scrounge up some boards of wood. I recommend starting with nice thin bits and seeing how you go. As you go to thicker and harder woods you may notice the motor straining a bit... That is expected, it is just a poor little sewing machine motor. What helps massively is to put some bee's wax on the blade. Particularly if you find it is slowing during tight curves, this is great stuff as it stops the edges of the blade binding on the wood. To apply, make some cuts to heat up the blade and just wipe a bit on - it should melt and cover the whole blade straight away.
Using this method I have found it to work well for thicknesses of up to about 10mm (of solid oak). If you want to go crazy you could look out for, say a 1/4 horse power motor, bolt it to the side and transfer the drive using a belt to the hand turn wheel. I didn't think it strictly necessary - but we will see how long this motor lasts ;) To improve its chances it is important that you take your foot off the gas the second you get a hint of a complete stall - It will burn out shortly after if you keep feeding it power.
To make wooden designs, you can simply cut away at the wood, making a pattern of your own design on the fly. Or you can pencil a design onto the wood and cut round that. Another option is to draw out your design on paper and stick that to the face of the wood with glue or double sided tape. Yet another option is to print out super elaborate designs that you make or download on the computer, and stick them on.
Like the things you can draw with regular art media, the number of things you can sew-scroll are endless. You can use it to make gifts like wooden keyrings, jewellery (you could do variations, for example, of this kind of thing), interesting shaped boxes and containers, wooden toys, bird houses/mansions, ornate photo frames, wooden lettering, and so on... Let your creativity loose.
You can also make all manner of Christmas decorations like I have.
Have fun with it and do your own thing!
Step 9: Further Considerations
OK so the dust problem partly motivated a new project: The Dust Sniper. It is a super silent extractor (approved for late night use!), with cyclonic filtration. Check out the link above for a video and the full Instructable.
Other points of consideration: the blades I used will likely dull quickly when used in this way. Such blades are made for a coping saw (a hand saw). Some bi-metal HSS job, would be preferable as it would be less prone to damage by overheating. Saying that, I have not changed the blade yet, and it doesn't seem too bad. A thinner blade would be useful for those tighter curves, and more intricate bits. Also, if you attempt this project, this forum topic has some more ideas and might also be of use.
Anyway, let me know if you give this a go and how you get on. Merry Christmas!