Manson Hot Air Engine Build

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pete
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Re: Manson Hot Air Engine Build

Post by pete »

Is that a set you have Rmac? If so then today there not all that common at least in the tool dealer catalogs I have. My best guess and because of there length, there meant for use more on a lathe than in a boring head. Three grooving bars of various widths and eyeball measurement only, what looks to be a 60 degree internal threading bar. A very nice set for sure. But grooving bars like that really can't be used in a standard boring head. There meant for doing a specific job and not simple boring, that takes a different tool point geometry. You'd need a boring and facing head with the geared automatic feed. And cnc for the threading tool even with a boring head on a mill. Your also probably correct about most bars like that are now solid carbide. Even more so for bars of that length. Directly compared, solid carbide will allow at least twice the unsupported length or tool stick out for deeper holes over HSS. If your trying to figure out if any tool is solid carbide or HSS, it's pretty easy just by picking it up. Carbide is approximately twice as dense as HSS. So it's noticeably a lot heavier for the same cross section and length.
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

pete wrote:Is that a set you have Rmac?
No, it's just a picture I grabbed off the internet. I do have a similar set that I inherited along with my lathe and mill and everything else. I realize that they're for the lathe and not for use in a boring head. My set happens to have an internal threading tool and three or four generic boring tools of different sizes. They're reaching the end of their useful life after having been sharpened a million times. I thought that places Little Machine Shop and Grizzly all sold them, but I guess not any more. Looks like most everything is either indexable or else solid carbide these days. I guess that's not all bad.

-- Russell Mac
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

I finished the connecting rod and the power piston end cap today.

connecting_rod_and_piston_end_cap.jpg

The connecting rod was straightforward, but I thought for a long time about how to hold the end cap for milling the flat part and drilling the hole through it. "Collet block!" says everyone, which is fine except I don't have one. So I bit the bullet and made a substitute by drilling a carefully centered hole in a carefully milled bit of aluminum. It worked okay, and I guess it might even find some use in future if I happen to need the same size hole (and if I can remember where I put it).

Does anybody have other ways I might have done this?

piston_end_cap_milling_setup.jpg

Sometimes I'm tempted to make such fixtures out of wood instead of metal. It would be a LOT faster and probably good enough for some things.

-- Russell Mac
pete
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Re: Manson Hot Air Engine Build

Post by pete »

A few thoughts Rmac and this is certainly not meant as being overly critical. I'm trying to be practical and leaning towards what good engineering practice might be. While I've yet to build a hot air engine, I've studied enough drawings to run across more than a few mentions they don't produce that much power for there size. Inertia loads due to component weight is just one part of the over all problem. More prototypical to something used to do work might be putting a full radius on the outside ends of your connecting rod ends. No it won't remove that much weight, but every little bit will help, and that would always be done at full size since the squared off ends add nothing to the parts available working strength. Leaving aside what someone might want for overall aesthetics since something like this is after all a model engine and it's what pleases you personally. Then engineering wise that connecting rod is massively over built for the job it's doing. Weight equals that higher inertia load and more frictional drag. More drag requires a higher rpm to stay above those loads and maintain the engines operating cycle or it will slowly bring itself to a stop. More weight in an unbalanced assembly also creates more vibration and rougher operating.More vibration almost always comes with an increase in friction.

I've seen quite a few pre assembled hot air engine kits and all the moving parts are always built as light weight as possible. With a scale engine where your trying to duplicate even the metal type used in a full size engine that vastly restricts your options. With a non scale engine the sky's the limit for the general design and materials. So it helps to think somewhat like what a formula 1 design engineer does as an example. Were only seeing parts of the whole engine so my points might be completely unworkable for what your actually building. What about first turning the center section of that connecting rod between centers on the lathe from aluminum, then forming the ends back on the mill as a one piece light weight connecting rod? Yes aluminum itself is a really poor bearing material, so it's long term durability when used as a bearing are a joke. Press fit sleeve bearings or bolted on connecting rod end caps holding bronze sleeve bearings in place would be the usual choice. It's less known by a lot of hobbyist's so I think it's worth mentioning, but using those bronze bearings to get the durability that bronze is a lot harder than most think and generally harder than mild steel. Ideally you'd then use hardened and ground steel pins so the cheaper sleeve bearings would have most of the wear after long use. In the sizes your using it's still dirt cheap to buy those off the shelf since that's what guaranteed straight, round, hardened and ground gauge pins are. Plus they can be had in almost any + - size you want. Even a cut off disk on a Dremel will cut them to rough length. Some type of rudimentary tool post grinding might be needed to properly finish those rough cut ends though. Again these are only a few thoughts and suggestions and you might have very good reasons to ignore all of it.

Maybe in commercial shops today it's used far less, but wood has a pretty long history of use in machine shops. I've seen it mentioned being used for temporary plugs in large tube ends to turn the O.D. between centers or trim the tube ends square. And I've seen it mentioned as temporary low part count fixtures when the machining loads aren't above the materials own strength and rigidity. Plus as you mentioned it machines faster, easier on cutting tools and can easily be built up into any cross section you need with a few cents of wood glue. The 3D CAD guru's here will laugh at me, but I've used it as a prototype material to check out rough design ideas for anything I've missed on paper or what I should have caught with better visualization and the eraser. Yeah having a full wood working range of machines would help, but even that's easy and fairly cheap to get around since your machining the material to the accuracy you want anyway. A hand saw and that glue if needed will get you to a half decent starting point.

At our mills reduced spindle speeds I've found standard metal cutting end mills will give a cleaner cut than even proper router bits can that require the much higher speeds to make those clean cuts. An end mill isn't quite as good as a router can do, but better than the router bits can at our lower rpms and I've compared both. A medium priced set of brad point and forstner bits are also worth while. Solid wood and even the best Birch plywood still has that grain structure, so backing up the cutting tools exit side by clamping any construction grade pine or fir scrap wood isn't really optional or you will get break out chipping on the materials perimeter. MDF isn't great for almost anything in our shops because it isn't that strong or durable. But it's pretty damn flat, on a mill it's at least useful as a back up when your clamping parts to the table, drilling through where you might touch the vise bed without parallels, rotary table tops, or anywhere the tool tip might hit your expensive mill tooling and leave those ugly apprentice mark reminders. I've bolted MDF to my lathe face plate then screwed or through bolted parts to it so a full cut on the parts O.D. can be made just by cutting a few thou into the MDF facing. But the other thing to keep in mind when using wood is it's pretty easily compressed. You have to keep that deflection in mind when fixturing metal parts to it or you can end up with finished part accuracy issues.
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

Wow! Thanks again, pete, for your helpful hints. I see what you mean about the massive connecting rod. That kind of evolved from 1) a desire to use something other than aluminum for the bearings, 2) a desire to keep things simple (so no sleeves or bolted-on end caps), and 3) a baseless tendency to copy parts of the Moriya engine. So ... when/if it doesn't run, I'll definitely try to address some of the things you've mentioned. At this point I have to confess that I made the round part of the connecting rod out of steel instead of aluminum for absolutely no good reason. At the very least I need to fix that.

Glad to hear that wooden fixtures aren't totally out of the question for some things in the metal shop. As it turns out, I've worked a lot more wood than metal, and am pretty well set up with woodworking equipment. I get really frustrated sometimes when it seems to take me and my despicable round column mill/drill forever just to square up some little chunk of aluminum when I know that I could do the same thing in wood in just a few minutes.

Only a few parts left to make now on the Manson engine--the flywheel, the two support posts, and the base. Maybe I'll make the base out of walnut!

-- Russell Mac
pete
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Re: Manson Hot Air Engine Build

Post by pete »

Ok, that's great and saves mentioning a bunch of items you'd already know. But since you've done a fair amount of wood working then you already know it's capable of moving around across the grain just due to humidity changes. For a close fitting mechanical parts assembly just remember that movement could easily change enough to create binding in the working parts. So typical wood working methods of slightly over size holes or short slots for the mounting bolts might be worth considering if you do make that walnut base. And think of full size parts for general design. Automotive connecting rods are a good example. There sort of I beam shaped with relieved areas either cast or forged in to save as much weight as possible but still be strong enough. Yeah more work to machine, but there's good solid engineering reasons there shaped in the way they are. A whole lot heavier built of course, but afaik all top fuel drag engines producing well over 1200 HP per cylinder still use aluminum connecting rods. Titanium rods would be better, but actual rod failure in those engines is pretty rare. Hydraulic lock with unburned fuel usually takes out the piston tops and blows the heads off, or a no longer attached rod end because the piston disintegrated punches that loose rod end through the side of the block. Good enough for top fuel then good enough for me. Formula 1 engines I think mostly use titanium rods today, but probably more for the now roughly 14k-in the past up to 20k + rpm levels. If you can stay below 10,000 hp and 10k rpm, aluminum should work ok for a rod in your hot air engine. :-)
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

pete wrote: you already know it's capable of moving around across the grain just due to humidity changes.
Sure enough. Wood will move if you just look at it sideways.
pete wrote: For a close fitting mechanical parts assembly just remember that movement could easily change enough to create binding in the working parts.
Again, sure enough. I was sort of kidding about the walnut. If those two support posts move relative to each other, things won't end well. As an aside, in a post last summer I mentioned a simple engine that I made almost entirely from recycled HDPE. It ran great on compressed air when it was first built, but even that plastic warped enough over time to cause binding that kept it from running.
pete wrote: If you can stay below 10,000 hp and 10k rpm ...
I'll do my best. :D

-- Russell Mac
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

rmac wrote: Only a few parts left to make now on the Manson engine--the flywheel, the two support posts, and the base.
I cranked out those parts yesterday, but had to back up one step today. I originally had the flywheel spinning on a single ball bearing set into the top of the support post. That sort of worked, except the flywheel was way too wobbly. So I made a longer shaft for the flywheel and a new support post with room for two bearings separated by a spacer bushing. That got rid of most of the wobble.

Tomorrow I get to see if I can cut some test tubes to length for the displacer and its cylinder without shattering them.

-- Russell Mac

first assembly.png
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

rmac wrote: Tomorrow I get to see if I can cut some test tubes to length for the displacer and its cylinder without shattering them.
One of the neighbors does glass art and she has a nifty little bandsaw-looking machine with a diamond blade--Just the thing for cutting test tubes. After practicing with it a bit to make sure I could get a square cut, I had no trouble making the engine's displacer and its cylinder.

The shiny aluminum and glass looked pretty spiffy when the engine was all put together, but (alas) it wouldn't run. Nothing seemed to be binding, and there was evidence of appropriate compression, so I decided to take pete's advice and try to reduce the weight of the moving parts. The biggest offender was the steel power piston, with an OD of 7/16" and 1/16" thick walls. I drilled out the inside enough to leave about 0.025 thick walls, and while I was at it I opened up the two vent holes in the piston end cap just a little bit. Then I put it all back together and it ran!

But (there's always a "but"):

- Sometimes it will fail to run on several start attempts, and then suddenly spring to life for no apparent reason.
- Sometimes it will be humming along fine and then just suddenly stop, again for no apparent reason.
- It should run equally well in either direction, but it definitely favors clockwise rotation.
- The flywheel still wobbles a little bit.
- There is some definite looseness in the crude bearings on both ends of the connecting rod.
- There is a slight bit of play between the power piston and its cylinder.

I don't think the intermittent starting and stopping are due to thermal effects because they happen too quickly. Instead, I think something is shifting around and causing some bit of internal friction to come and go. Likewise, the engine's preference for clockwise rotation indicates some machining asymmetry somewhere.

So I'm thinking it might be a useful exercise to start over with the same fundamental design, but 1) do a less sloppy machining job, 2) put less weight in the moving parts (thanks, pete), and 3) use tiny ball bearings on both ends of the connecting rod.

-- Russell Mac
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Harold_V
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Re: Manson Hot Air Engine Build

Post by Harold_V »

rmac wrote: Sat Oct 30, 2021 7:57 pm and 3) use tiny ball bearings on both ends of the connecting rod.
Do you have any?

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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rmac
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Re: Manson Hot Air Engine Build

Post by rmac »

Harold_V wrote: Do you have any?
No, but McMaster-Carr does. There are also lots and lots of the little guys listed on eBay. No guarantees of quality with eBay, of course, but probably any real bearing will be better than the sloppy pins in drilled holes that I have now.

Update on the engine: A friend suggested that any hint of oil on the power piston can gum up the works, so I took the engine apart and carefully cleaned it up. And it now works much much MUCH better in either direction. So there's hope.

-- Russ
Last edited by rmac on Sun Oct 31, 2021 10:27 pm, edited 1 time in total.
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Harold_V
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Re: Manson Hot Air Engine Build

Post by Harold_V »

I have a respectable number of small bearings, still in individual plastic packaging, new, acquired long ago from an auction. I think they're 1/8" bore. If you're interested in a few, please let me know. No charge. Happy to mail them to you, and I can provide more information if you desire. I'd have to go out to the shop to retrieve them to do so. No big deal. Let me know.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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