Raab Style Heibluftmotor

[JasonBModerator @jasonb]

Or a Raab Style Hot Air Engine to you.

I have always liked the look of the Raab hot air engines with their typical buttress fins around the top of the displacer cylinder and while browsing the web as you do came across this listing in a German auction for toys and collectables.

I don’t think it was actually made by Raab but certainly had the look and if built at 1:1 would make a managable sized model so the image was filed away and a few others of similar engines added over time. Earlier this year I decided to start drawing it up, luckily the power cylinder scaled out around the 25mm mark so I decided to do it the same 24mm bore that I have used for a lot of other engines.

Starting with the most obvious part which is what would have been the main casting with it’s webs a piece of bar for the upper plate was cut and squared up, faced to thinkness in the lathe and then clamped to the rotary table to mill a circular groove and some radial slots to locate other parts of the assembly in.

A larger square for the lower plate was bored in the lathe

Then after sawing off the corners held by the bored hole to turn it round.

Out with the rotary table again to do the radial slots. One was done deeper and wider and will act as the linking passage between power and displacer cylinders which was an easier option than trying to drill in from the side at a late date.

I toyed with a few different ways of shaping the webs – cutting the profile while held flat and then using a 1.5mm roundover cutter along each side of the curved edge, saw and file, etc. In the end I used the manual mill to do the two straight edges and a couple of steps then milled a piece of corian to allow the webs to be held curved edge up in the CNC mills vice.

First an adaptive to rough out the shape

Then a scallop to smooth it all out, Th ecutter moved back and forth along the work in a shallow U shaped path stepping over 0.2mm per pass

The central cylinder was turned from a bit of old galvanised water pipe

Which then had eight locating slots milled along its length

There was also a round part that the power cylinder will sit on to complete the total of 12 to make up the “casting”

[DiogenesParticipant @diogenes]

Looks an interesting one – waiting with interest for the ‘sheet metal’ episode..

[JasonBModerator @jasonb]

I’m working on that at the moment. The hardest part is out of the way which was having to eat two tins of biscuits!!

Cutting the 66 slots is going to be easy compared to that.

[Robert Atkinson 2Participant @robertatkinson2]

I have a small stash of deep drawn stainless steel pots They come in pairs that nest together with a small gap. The larger is about 130mm dia by 160mm high. They have a rolled flange and a turned aluminium ring to clamp it down onto a thick alloy base. The inner one is flangeless. Sounds like they might suit this type of model…

 

[JasonBModerator @jasonb]

I think any thin walled tube could be put into service, I’ll mention a fe wof the options I considered when I get to describing the base construction.

[JasonBModerator @jasonb]

The various slots and tabs made the “casting” almost able to hold itself together, I just added a M3 threaded rod through the middle to make sure nothing moved as the water in the solder boiled off. A short length of 3mm bar locates the cylinder base. After a good application of HT-5 flux I set the part up outside on a 25lts drum so that I could get access all round. Starting at the top I worked my way round feeding the stick of solder in under the top plate and used the heat to chase the solder down each vertical joint. Then back round the other way applying solder to the bottoms of the webs and lastly to the cylinder base.

Half an hour in the pickle had it looking a bit more presentable. That discolouration is not rust but a coating of copper that has dissolved into the pickle over time, must get a new can as the mat grey it used to leave looked a lot nicer. I’ll grit blast it before painting anyway.

Into the 4-jaw to give the base a skim just in case anything had distorted during soldering and the machined finish should give a better seal for the displacer cylinder than the hot rolled surface of the raw material. I also used a boring bar to true the underside of the top plate and drilled & tapped for a bronze bush for the displacer rod.

With the freshly skimmed bottom clamped to the mill table with some packing to lift it up a bit the top plate was milled to final height as was the power cylinder pad. M3 holes in the top for the two bearing supports and some 3mm holes around the bottom to attach the “casting” to the base.

The outside of the power cylinder was shaped from some 40mm dia cast iron bar, A gauge plate form tool was used to shape the bead around the top.

After sawing off from the length of bar it was held the other way round, the bottom faced and then drilled and bored to the required 24mm diameter

I did not take any photos of the piston being machined but it is basically a cast iron cup, 24mm OD x 22mm ID with a 2mm thick bottom. You can just see it in the background where I am doing the final lapping of the piston into the cylinder bore using 1000g silicon carbide powder and oil. There will be no piston rings on this engine just a good friction free fit to seal without adding any drag that a ring would.

[JasonBModerator @jasonb]

The crankshaft is supported by two “A” frame standards. After sizing some aluminium plate I reamed for a bearing and drilled & tapped the two foot holes so that I could use them with a block to give me three holding points while the frames were cut on the CNC. You can see the first one partly done and the prepared stock for the second one below

After doing the first side of both they were then held the other way round to complete the cutouts on the opposite side

This shows the cutters used

1. 3-flute 6mm dia aluminium specific carbide for the roughing adaptive cut and outside contour
2. 3-flute 4mm dia aluminium specific carbide to get further into the internal corners and the upper triangle of the central web
3. 4-flute 2mm dia carbide to do the final inner profiles of the webs
4. 3-flute 4mm with 1mm corner radius to complete the inner edges of the main A frame and leave a fillet where they join the web, also to do a scallop to round over the clamp bolt lug.

With the CNC work done it was back to the manual mill to drill out the tapped foot holes to 3mm, drill and tap the clamp lug M2.5 and saw that in half. The original would most likely have had the crankshaft run straight in the iron castings but I made a couple of plain bronze bushes.

The Crankshaft webs were from flat mild steel bar, drilled & reamed then the holes used to mount them on an arbor to turn the taper. The same arbor was used to round over the ends. Some 8mm PGMS was used for the shafts which were silver soldered together.

A quick trial assembly confirmed the crankshaft turns round OK.

[JasonBModerator @jasonb]

Back to some more predictable materials.

I had some thick wall brass tube that was suitable to make the displacer from. First I bored it out to size coming in from each end as it was 112mm long and I left a 1mm thick central web to add a bit of stiffness but not too much weight. I also opened up the first 6mm of one end to 0.2mm larger diameter to give me a shoulder to locate the top plate against. After making a couple of hardwood plugs I turned down the OD to leave a 0.4mm wall thickness.

Top and bottoms were made and silver soldered into place with the top one being relieved to clear the guide bush in the top of the main “casting”

To keep the weight of the displacer rod as low as possible I used a length of thin wall KS type brass tube and soldered in the partly machined ends. Once soldered a thread was cut on the end of the Hex to screw into the top of the displacer and flats machined on the top spigot before cross drilling and reaming for the wrist pin.

No photos of making the displacer cylinder but it is just a length of 1mm wall stainles ssteel tube with a disc of 1mm stainless Sif brazed to the bottom and a brass flange silver soldered to the top. You can also see that I cut a gasket from car exhaust gasket material with the metal facings removed to both seal the cylinder to the base as well as offer some heat insulation.

The two conrods were partly machined on the manual mill drilling and reaming for bronze bushes then completed on the CNC to give the shape of the cast originals.

The two flywheel blanks had all the turning and boring done in the lathe before moving over to the CNC to cut the spokes. The two flywheels are slightly different, the power piston side one has a boss in one of the spokes for the crank pin. The other has an enlarged hub on the outside face which is tapped to allow a small stepped pulley to be attached.

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