The Return of No. 83, a Hot Air Engine

[vtsteam]

I finished drilling and boring (not shown) the new plug, giving it an I.D. equal to the I.D. of the stainless cylinders. Formerly that hole was equal to their O.D. I also drilled and tapped 4 mounting holes for a new flanged cylinder

I then started a third cylinder and press fitted it into an aluminum flange (which will provide additional cooling sink). The flange was drilled for four fixing screws. It's shown here fastened to the newly sleeved and bored bulkhead. The previous two cylinders are sitting next to it for comparison.

The new one will be shortened so that the overall length including the bulkhead is the same as the other two. I haven't capped it yet-- I've left it open to be able to see how the displacer is riding in the bore.

I'm happy with today's work and the new cylinder mount. 


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[vtsteam]

I shortened the third displacer cylinder, brazed a cap on it an then thinned the walls to .014". It's mounted here on the aluminum cooler/bulkhead. This time I used copper instead of stainless steel as the cap.

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[vtsteam]

The brass displacer rod guide is somewhat worn/oversized and the displacer can sometimes contact the bottom of the displacer cylinder if things aren't adjusted to compensate for the droop. That's not ideal, since the displacer is then no longer parallel with the cylinder, and there's not a lot of leeway for this -- the cylinder is ~ 3" long, and the clearance is only .073".

The wear is in the brass guide, not the rod. Guides on a horizontal displacer hot air engine receive a lot more wear than a vertical.

Another detriment due to wear here is pressure leak. There's no seal other than a good fit over its 3/4" length. As built, the guide was permanently brazed in place. I've decided to make it replaceable by drilling and threading the existing guide 1/4 -28 to take a screw-in bushing.

I've got some sintered bronze material in the scrap box, and I think I'll make a guide bushing out of that. That material should reduce wear and friction compared to brass, and obviate the need for oiling.

Here's the guide, now opened up and  threaded:

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[tom osselton]

 

[vtsteam]

Thanks, Tom 

I spent time in the shop today adding a motorized spindle to a milling attachment I had once built for my old Gingery lathe. I found that the milling attachment does mount onto my newer homemade lathe with T-nuts in the boring table.

That was a fun, overdue project -- I've had the spindle for a year. The milling attachment was built in 2002. Neither had seen service on my present lathe until now.

I had planned to use this new rig to mill down the top of No 83's valve guide socket, since the new threaded bushing will have a head that would otherwise increase it's height. Seemed like a good reason to make up the tooling.

Got the rig working and set up, but  I found I didn't have a 1/4" end mill to fit the spindle. All my 1/4" end mills have a 3/8" shank. The new spindle has an ER11 collet system which only handles up to a little more than 1/4". I thought about mounting a 1/8" end mill, but the one I had turned out to be too short to clear the power piston cylinder.

So in the end I went with the old fashioned way: I chucked an oversized drill bit in the drill press, and drilled out most of the way down on the valve guide, then finished up with a file to square the top off.

Oh well, glad to have the new powered milling attachment made up. I've ordered some 1/4" end mills with 1/4" shanks.

[vtsteam]

Here's a pic of the old milling attachment with the new spindle on the homemade lathe. The milling attachment can be used this way with work held in the chuck or faceplate, which can be locked or used as a rotary table for drilling a radial hole arrangement, or radial slotting, or grinding.

The attachment can be mounted crosswise as well. It's held down to the lathe's boring table with tee-nuts.

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In today's work on No.83, I found I'd tapped the displacer rod guide off-vertical  . So I mounted the whole power cylinder/guide mount in the four jaw and very carefully bored the entire displacer guide out to start over again.

I didn't have a reference location for the guide, so it was a matter of taking small cuts on the off center hole with a thin homemade boring bar, checking for concentricity with the walls as the they got thinner, and adjusting the 4-jaw. Finally when I broke through it was just a very thin foil all around and the bore was concentric. One step forward one step back!

The silver lining, or lemonade, or whatever for the messed up tapping is that it will now be a lot easier to make up a new guide and threaded bushing in the lathe before mounting them in the plate.

[vtsteam]

Today I turned, threaded, and pressed into place the displacer bearing boss. I used brass, as before. Then I started on the sintered bronze bushing. It was turned from a rough square cutoff piece from the scrap box. By supper time I had it turned to size, but not yet threaded or drilled.

I just received a no-name 1/8" reamer, but I mic'ed it at .1260". I think it will cut larger. I'll have to try it on a test drilling to see if it's worth using. If not I'll make a toolmakers reamer from the pushrod stock. It's .1250"

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[vtsteam]

After a test in some scrap material, the reamer worked out as a good fit on the existing displacer rod. That rod was made from 1/8 drill rod stock (silver steel). Recently I received a few pieces of polished stainless rod, also supposedly 1/8" and trying one of them, they were actually too large. But the displacer rod was a nice close sliding fit in the scrap, so the reamer was proven out.

I drilled the bushing in the lathe with a #31 drill (.120"), and then used the new reamer to bring the bore to size. Then it was time to thread the outside of the bushing 1/4-28. I don't have a tailstock die holder for my lathe (yet), and it doesn't have screw cutting capability (yet), so I used the expedient of squaring up one of my tool holders to the lathe ways, and using that to press against the back of a hand held die handle -- which kept it square to the workpiece. I rotated the chuck by hand, and wound in the leadscrew at the same time, which kept up square pressure on the die, and the threads came out nicely.

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[vtsteam]

I parted off, then screwed the bushing into in the brass displacer boss for length. A little filing of the of height and depth brought it flush at both ends. Finally I slotted the top with a hacksaw.

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[vtsteam]

And here's the finished displacer guide with the displacer in place. Easy sliding, and yet a close fit, with self-lubricating bronze. Now also replaceable if ever needed. I like the change!

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[RussellT]

Hi Steve

I'm following with interest.  What's the slot in the guide for?  The original version didn't appear to have one.

Russell

[vtsteam]

Hi Russell! 

It's just a slot for a screwdriver.

(The original was just one piece brass, silver brazed in place. Now it's threaded and two-piece.)

[RussellT]

It's just a slot for a screwdriver.

[vtsteam]

  Russell. Most of what I do isn't too sophisticated!

[vtsteam]

Speaking of not sophisticated, when I tried reassembling the engine the power piston was ultra tight in the bore. One might even say stuck. I got it out, and then tried to figure out why. Measuring the bore revealed the fact that it was ovaled by about 2 thousandths now. Thinking about that, I kind of remember when I was boring the displacer boss I "ticked" the power cylinder against the toolholder. 

Total disappointment! I was all set to try out the new changes. But no go is no go.

So of course I tried un-ovaling it in the vise, and went about a thousandth in the opposite direction. Still, it was an improvement. The only course open at that point was to hope that lapping the cylinder would bring it back into round enough to try the engine out again.

It might mean making a new piston too though, since it will result in an overbore. Well, lapping first, and we'll see what we've got.

[vtsteam]

After a half hour of searching I couldn't find the original lap I'd made to build this engine. So on to making one.

First I found an old 3/8" bolt. Cut the head off, and screwed it into a chunk of aluminum scrap.

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[vtsteam]

Then I turned it around in the chuck, and turned the end to 2 thou under the piston diameter, and reduced the inner end.

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[vtsteam]

Checked fit in cylinder for binding.

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[vtsteam]

Then slit it with a hacksaw in the vise.

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[vtsteam]

Then drilled through the base of the slot.

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[vtsteam]

I tapped one side to add a 3/16" grub screw for adjustment.

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[vtsteam]

And lapping with some valve grinding compound to start. I ran the lathe extremely low speed, with a cloth over the ways to protect them. You can also do it in the drill press with less worry, but mine doesn't go slow enough not to worry about a catch.

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The bore was eventually cleaned up back to round, with about a thousandth taken off. I reassembled the engine and there was still good compression, so I gave it a run this evening. It struggled to break 900 rpm, and at that speed I could hear some pretty loud clacking sounds.

Upon disassembly and checking parts, these noises turned out to be piston slap. The piston is definitely undersized now despite the good compression, and I could feel a fair amount of slap-stick when I exercised it by hand. Not surprising the engine couldn't run well.

As I figured, time for a new piston.

Unfortunately a couple hour search for the stick of graphite I'd had eighteen years ago when first building the engine turned up nothing. I know I still have it somewhere. Extremely irritating! But finally I gave up. I sent for another which will take about a week to get here. I know it will turn up again, some time after the new one arrives.

Never mind, I'll think about making other improvements in the mean time.

[shipto]

I like that milling atachment, my lathe does not have the T slots unfortunatly so cant use that type on mine.

[vtsteam]

Dwayne, what kind of lathe do you have?

[shipto]

Dwayne, what kind of lathe do you have?

Old Harrison 14" like https://www.lathes.co.uk/harrison/page18.html although mine is in a bit better condition.