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Back in the metal shop after a number of woodworking projects (RC tug and submarine, carvings), and decided to make something a little different but fairly straightforward. I've always liked the #23 'Tiny' wobbler engine from Elmer Verburg, built one a number of years ago, and figured a fun variation on that theme would be a radial engine version of it. I did a little sketching, and it looks like a 6 cylinder version of it will just work at the original parts size - as long as I bevel the bottom of the cylinders a bit so there is clearance between them.
It seems like someone must have done this at some point, but so far I have not found one, other than the slightly larger 3-cylinder plan that Elmer did. I am going to stick with the original plans for the cylinder, crank, and piston designs, but rather than the original pedestal base it will mount to a disk base with the 6 engines all off of one crank pin.
After looking at some larger radials I did in the past, it became obvious that having the piston rods attach to a disk on the crank pin as in most radials would not work at this size, for two reasons: it would need to have the cylinders slightly farther out, plus as the master pin rotates the disk, it would change the angles on the other pistons too much which would effect the valve timing (on a fixed-cylinder engine the piston shaft is pivoting in the piston end, but on a wobbler it moves the whole cylinder). So, back to using a single crank pin and will have the piston rods end in flat plates that are offset from each other along the length of the crank pin. Will show that later, as the parts come together...
So, off to the shop...
First up is to make a set of six cylinders. These are to the original Tiny plans, made out of a length of 5/16" brass square bar. Each piece was long enough to get two parts out of - that left enough length to hold in the chuck till final parting off to length. The pieces were chucked up in the 4-jaw, centered with a dial indicator, then offset to one side since the cylinder bore is offset to leave room for the pivot rod on the back of the cylinder.
The holes were center drilled,
drilled slightly undersized,
then reamed to final dimension,
and cut off to length.
Next step was to mount the cylinder sideways in the 4-jaw, with the back side facing out, and offset so that the centerline was where the pivot rod will go. The piece was given a light surfacing pass to ensure it was nice and flat (bar had some dings in it), then a shallow recess was cut around the pivot rod location,
then the hole for the pivot rod was drilled.
At this point, the cylinders are all shaped for the functional surfaces, just need to bevel off the front/side corners.
For the bevelling step, the 4-jaw was moved over to the tilt table on the mill and the table set to about 55 degrees to do the ends of the cylinders. With the parts set in the 4-jaw using a 1/4" square bar as a guide to how far out the end projected (that way all the parts could be milled with one setup without having to readjust the cutter every time), the front and side corners were milled off the top, and just the side corners milled off the bottom (since the cylinder bore is so close to the front side). Between cuts the rotary table was cranked around to bring the next face up for cutting. With this setup it all went pretty quick.
Next, the table was lowered to a 45 degree angle, and the front sides were milled in the same manner.
At this point, the cylinders are done, ready to have the pivot posts installed. Those will be made later, once I have the back plate done and know the length they need to be (fun part about building an engine as you go!).
With that done, I turned (pun, yes) to the pistons. They were made from some 303 stainless rod. Since the shape of the end where they will attach to the crank pin is different on each one, I am going to make up the pistons and rods as one piece, and add the connecting part later - this is the first place where I am deviating from Elmer's plans. With the rod chucked up in the lathe, the piston rod end was turned down to size, and the sides of the pistons themselves got a light cut to get a good fit in the cylinders, plus some oil grooves.
Last two photos show the parts so far....
Next up will be to cut the disk for the backplate - will be using a big chunk of 303 stainless left over from some train wheels made for an previous model (got a 3' length cheap from Yarde Metals Drop Zone as a drop cut - handy site to pick up stock).
It seems like someone must have done this at some point, but so far I have not found one, other than the slightly larger 3-cylinder plan that Elmer did. I am going to stick with the original plans for the cylinder, crank, and piston designs, but rather than the original pedestal base it will mount to a disk base with the 6 engines all off of one crank pin.
After looking at some larger radials I did in the past, it became obvious that having the piston rods attach to a disk on the crank pin as in most radials would not work at this size, for two reasons: it would need to have the cylinders slightly farther out, plus as the master pin rotates the disk, it would change the angles on the other pistons too much which would effect the valve timing (on a fixed-cylinder engine the piston shaft is pivoting in the piston end, but on a wobbler it moves the whole cylinder). So, back to using a single crank pin and will have the piston rods end in flat plates that are offset from each other along the length of the crank pin. Will show that later, as the parts come together...
So, off to the shop...
First up is to make a set of six cylinders. These are to the original Tiny plans, made out of a length of 5/16" brass square bar. Each piece was long enough to get two parts out of - that left enough length to hold in the chuck till final parting off to length. The pieces were chucked up in the 4-jaw, centered with a dial indicator, then offset to one side since the cylinder bore is offset to leave room for the pivot rod on the back of the cylinder.
The holes were center drilled,
drilled slightly undersized,
then reamed to final dimension,
and cut off to length.
Next step was to mount the cylinder sideways in the 4-jaw, with the back side facing out, and offset so that the centerline was where the pivot rod will go. The piece was given a light surfacing pass to ensure it was nice and flat (bar had some dings in it), then a shallow recess was cut around the pivot rod location,
then the hole for the pivot rod was drilled.
At this point, the cylinders are all shaped for the functional surfaces, just need to bevel off the front/side corners.
For the bevelling step, the 4-jaw was moved over to the tilt table on the mill and the table set to about 55 degrees to do the ends of the cylinders. With the parts set in the 4-jaw using a 1/4" square bar as a guide to how far out the end projected (that way all the parts could be milled with one setup without having to readjust the cutter every time), the front and side corners were milled off the top, and just the side corners milled off the bottom (since the cylinder bore is so close to the front side). Between cuts the rotary table was cranked around to bring the next face up for cutting. With this setup it all went pretty quick.
Next, the table was lowered to a 45 degree angle, and the front sides were milled in the same manner.
At this point, the cylinders are done, ready to have the pivot posts installed. Those will be made later, once I have the back plate done and know the length they need to be (fun part about building an engine as you go!).
With that done, I turned (pun, yes) to the pistons. They were made from some 303 stainless rod. Since the shape of the end where they will attach to the crank pin is different on each one, I am going to make up the pistons and rods as one piece, and add the connecting part later - this is the first place where I am deviating from Elmer's plans. With the rod chucked up in the lathe, the piston rod end was turned down to size, and the sides of the pistons themselves got a light cut to get a good fit in the cylinders, plus some oil grooves.
Last two photos show the parts so far....
Next up will be to cut the disk for the backplate - will be using a big chunk of 303 stainless left over from some train wheels made for an previous model (got a 3' length cheap from Yarde Metals Drop Zone as a drop cut - handy site to pick up stock).
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