"POPPIN" Vacuum (Atmospheric) Engine - Part 4

[00:00:01]Welcome back. This is part four of the Poppin vacuum engine build.

[00:00:07]This video describes the machining of the cylinder shown at the top right of the drawing and the piston and circled in the middle of the drawing. Both are cast iron.

[00:00:18]The piston is a simple design with no rings um and a very thin wall.

[00:00:27]A quick calculation shows the limits we have to work to with these items.

[00:00:32]Atmospheric pressure at 14.7 PSI and at a piston diameter of 16 mil means that even a absolute vacuum will give you a maximum of 4.58 lbs of piston force.

[00:00:46]So, you need a close fit but not binding in any way. A smooth, free-running fit.

[00:01:25]A quick check to make sure the spigot on the cylinder blank is a good fit in the recess on the standard.

[00:01:33]I'll cut the cooling fins on the cylinder blank before moving on to finishing the ball to a lapping size.

[00:01:42]There are 12 grooves to be cut, 3 mil deep, to form 11 cooling fins 1 mil wide.

[00:01:51]I'm using a dial indicator to accurately set the pitch of the grooves.

[00:02:01]>> Doesn't take long to get sick of hand feeding in the parting tool.

[00:02:06]Here I'm using a power feed of about 2,000 per rev and the machine running in back gear.

[00:02:31]Oops, bit of a miscalculation here. I have to turn the finished diameter a bit further to get the 12th groove plus the um end face of the cylinder in.

[00:02:44]The best way to get a parallel cylinder ball to minimize your lapping time is to line ball the cylinder. In other words, to mount the cylinder in V blocks or on a face plate on the saddle of the machine and hold the boring bar in the chuck so the cylinder is stationary but the boring bar rotates.

[00:03:06]However, it's only a very small short cylinder so I'll try standard boring first.

[00:03:14]The 12th groove is cut. We're ready to part off.

[00:03:18]As far as I can measure, I've got about 1/100 of a millimeter taper in the ball.

[00:03:23]I'll try turning a cylinder around in the chuck setting it up closely and taking a dust cut to reduce that amount of taper before lapping.

[00:03:34]Well, I didn't film the lapping process cuz it's messy and you get emulsified carbon dust on everything you touch.

[00:03:43]This is the cylinder where the outside turning and the ball has been completed to within about half a thou of what you want the finished lap size to be.

[00:03:57]The fastening holes on each end plus the hole for the oil cup will be drilled after I've lapped it.

[00:04:04]I use a carborundum grit uh mixed with a few drops of water as a lapping paste.

[00:04:11]The manufacturer of the lapping grit uh specifies water as the medium to mix it, not any uh oil-based substance.

[00:04:20]The crumbled grit and water wash out of the finished cylinder very easily.

[00:04:26]You start with a coarse grit uh working towards the finest grit depending on what finish you need.

[00:04:35]And the main aim is to get a parallel bore with a smooth finish.

[00:04:41]The lap I use is homemade. It just consists of a tapered mandrel and a matching tapered shell that to get a larger size, you just tap the shell further along the taper on the mandrel.

[00:04:55]The material is soft aluminum so that it retains the grit.

[00:05:00]The shell could be regarded as a disposable item. I've used this one to lap two previous engine cylinders and after this engine, it'll be thrown away.

[00:05:11]The flexible three-leg hones used to hone a cylinder in automotive work are not suitable for doing this because because of the flexible legs will allow the hone to follow whatever taper is in the cylinder rather than remove it.

[00:05:30]Because the piston is made to suit the bore of the cylinder, whatever size you end up with is not critical.

[00:05:38]The essential part is to get a parallel bore with a smooth finish.

[00:05:43]Regardless of how parallel you think you've finished lapping the ball, any irregularity will show up as soon as you start finishing the piston to the bore of the cylinder, and you may have to revisit the lapping.

[00:06:00]A final thought about lapping.

[00:06:03]Carborundum based grit crumbles away to nothing and washes out of the cylinder when you're finished the job.

[00:06:10]Diamond based grit will cut more effectively, but don't break down, and some of the compound will be retained in the metal of the cylinder, and continue grinding away when you're running the engine, because diamonds are forever.

[00:06:34]The fastening screws at each end of the cylinder are UNC 2 by 56.

[00:06:42]Checking that the valve plate fits.

[00:06:46]The surface of the valve plate still has to be lapped so that you get a good smooth finish for the valve to operate against.

[00:06:54]The accuracy provided by a DRO on the machine means that you can make a part like this with zero clearance allowed on the bolt holes, and yet it still fits well.

[00:07:06]It's good aesthetics to have the fastening holes on each end of the cylinder aligned with each other.

[00:07:14]Easiest way I could do this was to use a straight edge bearing against the heads of screws threaded into the fastening holes on the other end, and measure the gap between the straight edge and the top surface of the V-block until you get an even on both sides.

[00:07:32]The straight edge is held up against the cylinder by a magnet underneath.

[00:07:39]Here I'm using the two screw heads set against the top surface of the vise to ensure that the oil cup is drilled along the center line between the um screws on either end of the cylinder.

[00:07:57]The left valve plate is fitted.

[00:08:00]You notice the screws are different because those supplied were too short.

[00:08:06]The warehouse people seem to have measured the overall length of the screw rather than the under the head dimension.

[00:08:13]And these leftovers from the up-sure engine project.

[00:08:21]The screws now have enough protrusion to allow for a gasket if required.

[00:08:26]However, the lap surfaces should seal with liquid form-a-gasket.

[00:08:34]These are the components of the piston.

[00:08:37]The piston on the left, strangely enough, has a hole in the middle.

[00:08:41]That's because the yoke is screwed to the inside of the piston with that button head screw on the right.

[00:08:48]The gudgeon pin is of a length that fits neatly within the internal dimension of the piston.

[00:09:16]The piston is fitted very closely to the bore of the cylinder.

[00:09:20]Making fine adjustments where required with 1,500 grit wet and dry paper until you get a piston that runs freely over the full length of the bore with a close fit over the full length.

[00:09:38]That wraps up the construction of the cylinder and the piston.

[00:09:42]Hopefully the time taken to get a a free running but close-fitting piston will pay dividends when it comes to running the engine.