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Target for This Month: A 3D Printed Engine

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SillyOldDuffer14/06/2022 14:42:26
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Slow progress with the engine, mainly because I keep stopping to solve new CAD problems. The latest is I needed to split the existing solid crankshaft in two so the crank can be fitted.

cranksolid.jpg

 

crankrearpart.jpgcrankfrontpart.jpg

Then the two parts changed to fit securely back together:

crankfrontspigot.jpg

crankrearspigot.jpg

Easy when you know how which I didn't! Quicker to have remodelled the crank in two parts from scratch, but breaking a part into bits and then editing them separately seems a handy skill. After a few hours cursing and crashing SolidEdge, I got there. The second attempt took a few minutes, no swearing, and nothing broke.

The latest section shows the engine is nearly ready to print, valve position roughly right, and the parts should fit together. The main thing missing is the fasteners needed to hold it all together. I'm also inclined to put a lot of holes around the periphery of the flywheel that can be loaded with modelling clay or tungsten putty to store more energy. Another useful change might be to taper the rotary valve to improve the seal. Finally, I can save a lot of fettling by opening up parts that plug together. Expecting a 4mm printed plastic shaft to slide neatly into a 4mm printed plastic hole is too much! I also want to see if a 4mm nut and bolt to hold the crank together can be printed: the normal metric pitch is too fine, and I'll probably have to go coarser to suit the material - Whitworth!!!

e3dengwithtwopartcrank.jpg Dave

Edited By SillyOldDuffer on 14/06/2022 14:43:28

SillyOldDuffer15/06/2022 22:27:10
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Despite being out this afternoon, I got closer to finishing the engine design, and was able to take a peek at the next stage - printing it!

Not quite finished but the engine is now held together by clips. Not entirely happy with them and I might look for alternatives. The nut and bolt needed to hold the crankshaft together aren't started at all, and I have grave doubts about them.

3dengassywithclips.jpg

Solid Edge's 3D print functions seem straightforward except the first test export resulted in a tiny plastic model - 2.8 x 3.7 x 4.7mm. Naughty, bad Solid Edge. Despite the model being metric throughout, Solid Edge defaults 3D prints to inches and has to be ordered to use millimetres. That fixed, Cura loaded the model correctly.

3dengassycura.jpg

However, Solid Edge can check the design, and now I have to look at the complaints! For example, next image shows all the areas of my model where the plastic skin is less than the recommended 2mm thick, which would cause structural problems. The deliberately thin for lightness piston is the worst, but the flywheel, head and parts of the crankcase need attention too.

3dengtoothin.jpg

Another worry, although Solid Edge and Cura both accepted me printing the whole engine as a complete Assembly, I'm far from convinced it would work. Cura says the model will take 22 hours and 23 minutes to print, and I fear ending up with a clump of parts stuck solidly together. Love to believe a complex assembly of parts can be printed in one go, but I think it much safer to print the parts one at a time so they can be fettled as necessary to fit and move smoothly. Also be good if two or more parts can be printed side by side to save time. More research needed - I'm out of my depth again.

Dave

lee webster15/06/2022 23:24:23
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With printers such as my Ender 3, putting more than one part on the bed for printing saves very little time. The only time saved is how long it takes you to get one part off the bed and telling the printer to print the next file. If you had a resin printer the story would be different. Printing say six parts, all on the bed at once, would take the same time as printing one. That and the improved quality of the finish is why I am thinking of buying one.

I wonder if a disc valve would have less friction than a rotary, tapered or not, valve? They seem to work well on my bathroom taps!

SillyOldDuffer16/06/2022 10:13:04
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Posted by lee webster on 15/06/2022 23:24:23:

With printers such as my Ender 3, putting more than one part on the bed for printing saves very little time. The only time saved is how long it takes you to get one part off the bed and telling the printer to print the next file. If you had a resin printer the story would be different. Printing say six parts, all on the bed at once, would take the same time as printing one. That and the improved quality of the finish is why I am thinking of buying one.

I wonder if a disc valve would have less friction than a rotary, tapered or not, valve? They seem to work well on my bathroom taps!

I was hoping to save the bed heating and cooling time which is significant when printing lots of small parts, but this engine is all medium sized parts, so you're probably right.

I've started to worry about the material properties of the various plastic filaments and FDM generally for printing anything that needs to be strong. For example, because the print is developed layer by layer, parts have a grain like wood, and are probably weaker up down than across. So for strength something like a conrod should be printed flat, not upright. Unlike model engines fabricated or made from metal castings, where the strength of the metal makes careful design unnecessary, I'm pretty sure plastic needs careful geometry - thick plastic at weak points and thin plastic everywhere else.

All your fault Lee. As a result of your comments, I've been looking into Resin printers! Can I send you the bill please?

Only reason for me going for a rotary valve is they're simple to design.  Unfortunately, there's a lot of friction, and it may not work!  

Dave

Edited By SillyOldDuffer on 16/06/2022 10:15:16

JasonB16/06/2022 13:05:25
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A lot will depend on the size/shape of the parts much as it does with CNC. If they fit closely then there is little travel time between parts when the cutter is not cutting or the printer not printing. On the other hand if there is a lot of air between the parts then they may be best done individually.

lee webster16/06/2022 15:24:36
116 forum posts
10 photos

Dave,

I watched a video on youtube where someone printed two epicyclic gearboxes. One at high res on an FDM printer, not sure what filament he used, the other gearbox was printed on a resin printer. He then tested them using a very professional setup. The resin gearbox failed a few times whereas the FDM gearbox soldiered on. The teeth on the gears were tiny, I think the gears were about 12 maybe 15mm dia. If I can find the link I will post it here.

The youtuber Myfordboy has both types of printer but favours the FDM's when he wants to print something tough enough to stand up to sand casting.

If I had the spare cash to buy you a resin printer I would spend it on myself instead!

Lee

lee webster16/06/2022 19:22:05
116 forum posts
10 photos

Dave,

This is the video I mentioned about 3D printed gears.

https://www.youtube.com/watch?v=d9P5LBQqgFo

I hope the link works

SillyOldDuffer19/06/2022 11:29:46
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Still slogging away at this! Spent a good deal of time investigating Solid Edge misbehaviour that turned out to be due to me telling it to use advanced GPU settings that don't fully match my older model graphics card. Everything seemed OK and it worked a little faster, but CAD models gradually started to misbehave in odd ways. Fingers crossed, reverting to 'Accelerator Basic' has fixed it, and I don't have a faulty card!

The misconfiguration may have confused my rework of the rotary valve. Jason suggested altering the crank so the valve rocks rather than rotates and, without thinking about the details, I made the change successfully except the ports were in the wrong place. Correcting them and other small details caused the engine simulation to jamb, and I guessed the error was due to me not understanding the crank relationship geometry. Now I'm struggling with Grashof's Law. And when that roadblock is removed, I've noticed the ports and timing are both misconceived, and the whole head assembly needs attention.

Dave

SillyOldDuffer24/06/2022 14:48:47
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Thanks to the forum I had an urgent rethink this morning and designed a second, simpler engine. Despite many lessons learned, the Mark 1 engine has a lot of moving parts, hence friction, and will take about 24 hours to print, with no guarantee of success.

The second engine doesn't solve everything, but the rotary valve is incorporated in the main shaft and there's only one crank. Fewer joints to leak, only one fixing (holding the flywheel on), timing easier to set up, less plastic filament, and it will 'only' take 9 hours to print. Most important, I think it has a reasonable chance of running.

simpleenginefront.jpg

Air enters at the side and is fed to the cylinder via a slot cut in the rotary valve. It pushes the piston up to TDC at which point a groove cut part way round the rotary valve allows air to escape through a hole in the centre of the shaft, exiting at the flywheel. Section shows the slot connecting air to the cylinder while the outlet slot is closed.

simpleenginefrontsectionjustafterbdc.jpg

Thinking about valve timings revealed another huge gap in in my engineering understanding, so I guessed! The exhaust gap is set to nearly 180 degrees, which is probably fair enough, but the input is open for 38 degrees. That seems like a lot, probably allowing more energy in than is needed to spin the engine. Might blow the piston clean out of the cylinder in which case I shall claim to invented an air-gun...

Anyone know what valve timings are suitable for an air engine of this type? I guess they depend on the required power output, which is unquantified, the friction to be overcome, which is unknown, and the amount of energy in compressed air at operating pressure.

Watch this space, I'm about to start printing. We could run a lottery on the results:

  • After mild fettling, engine runs first time
  • After lots of fettling, engine breaks at the first puff
  • Engine can't run because of basic design errors
  • Engine can't turn because the piston stanchion and con-rod bend rather spin the crank
  • Engine quickly seizes due to plastic on plastic galling

Only two cheats intended: although all parts will be 3D printed in PLA, the outer ring of flywheel holes will be loaded with plasticine and PTFE lube will be sprayed inside.

Dave

JasonB24/06/2022 15:50:50
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If you look at a simples single acting wobbler the inlet is open for 170-180deg and similar for the exhaust . Remember when running on air you will not get expansion like you do with steam so even more need to keep things open for longer.

A double acting steam engine with no lap will also be close to 180/180.

I think poking some ball bearings into the flywheel holes would be more effective and look better too,

duncan webster24/06/2022 15:57:16
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Without a crosshead guide the con rod will impose a side load on the extended piston and might stop it running at all. Even with a crosshead, the con rod is well of to one side. I liked the previous one better, and you could incorporate the crankshaft valve easily.

SillyOldDuffer24/06/2022 16:31:45
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8903 forum posts
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Posted by JasonB on 24/06/2022 15:50:50:

If you look at a simples single acting wobbler the inlet is open for 170-180deg and similar for the exhaust . Remember when running on air you will not get expansion like you do with steam so even more need to keep things open for longer.

A double acting steam engine with no lap will also be close to 180/180.

I think poking some ball bearings into the flywheel holes would be more effective and look better too,

By popular demand, ball-bearings it is! (Luckily I have some the right size.)

The valve news is distressing because the printer is running and changing the model would have been easy! However, I expect my milling machine will be able to open it up. (Worry beads out: PLA might splinter -it's brittle, the core is a sponge, and I've never tried to machine it.)

Duncan's comment are valid too, though I did think of it. In theory (which means I guessed), the length of the piston flanks will compensate for not having a cross-head. The flanks can be increased if need be, but it's more friction. The idea of the T piston top is to hang a balancing weight on it if need be.

Like simple wobbler's I think the design is a dead-end in terms of scaling up. Blasting exhaust out of the centre of a spinning flywheel would be anti-social, while the towering heavy piston stanchion and bendy con-rod can't be good.

Print progress: cause for concern. I'm printing 4 parts together and it looks as if one of them has no overhang support. Don't think I told Cura to support overhangs...

sad

SillyOldDuffer25/06/2022 10:35:13
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Yesterday's progress was 4 out of 5 parts printed, which I hope to clean up before attending a compulsory family do. As can be seen lots of support plastic to be removed. Usually comes off without fuss, but there are always small imperfections to be smoothed out.

Unfortunately won't be able to print the last part today because I prefer not to leave the printer running completely unattended.

dsc06609.jpg

Dave

SillyOldDuffer26/06/2022 13:21:34
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Last part to be printed, the cylinder block:

dsc06610.jpg

As can be seen, lots of support plastic to be removed before cleaning up and fitting.

dsc06611.jpg

Went well until I broke the crank off the rotary valve whilst sanding it smooth in the lathe.

 

dsc06612.jpg

As the photo above shows the inside of the rotary valve is largely hollow, consisting of thin triangulated walls forming cells. Here, one of the triangular cells is oversized causing an area of weakness where the crank joins the shaft. Not sure whether to repair with epoxy or superglue, or to print another one, perhaps modified to force more solid internals.

Always been concerned about the strength of printed engine components. Partly because PLA is about 10% the strength of mild-steel, and partly because this type of 3D print isn't solid and I don't know how to control the internal web. The slicer software can be ordered to increase the density of the entire internal structure but I only want more plastic at defined weak points like the crank end.

Dave

Edited By SillyOldDuffer on 26/06/2022 13:21:52

SillyOldDuffer26/06/2022 18:12:10
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All fits together:

dsc06613.jpg

The cylinder and piston are too stiff to risk putting air into it, more sanding needed and the piston tends to wedge just as Duncan said.

Two other problems I should have anticipated - there's nothing to stop the piston turning in the cylinder as it lifts the con-rod, and the con-rod is inclined to fall off even when the engine stopped!

Made a mistake cleaning off support plastic as well: I officiously sanded off a ring on the flywheel intended to space it away from the cylinder block. Now I'll have to print a special washer.

sad

Dave

Edited By SillyOldDuffer on 26/06/2022 18:12:19

Neil Wyatt26/06/2022 22:35:35
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I would not be so enthusiastic about support...

Neil

Dave S27/06/2022 08:27:42
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Posted by SillyOldDuffer on 26/06/2022 13:21:34:

Always been concerned about the strength of printed engine components. Partly because PLA is about 10% the strength of mild-steel, and partly because this type of 3D print isn't solid and I don't know how to control the internal web. The slicer software can be ordered to increase the density of the entire internal structure but I only want more plastic at defined weak points like the crank end.

Hobby Slicers are pretty 'general purpose''. The hollow things out because in general a machined design would be solid and that takes ages to print. Often a hollow thing is 'sufficiently not shit' to be useable - especially for the general 'landfill ready' prints a lot of people make.

If you reframe a little: if you were designing this as a set of castings you would model all the cores and wall thicknesses exactly as you want them.

Think of 3D printing as extremely slow casting process with a digital mould and maybe you need to do the design of the parts a little different.

When I did some design for a 3D printed prototype I did all the internal work in the CAD and sent the print out (this was 15 odd years ago) to get made as per design. These were SLS Nylon, but the principle is the same - if it needs to be a specific shape or wall thickness then the generic slicer hollow fill is unlikely to be the best option.

Dave

SillyOldDuffer04/07/2022 16:47:21
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Posted by Dave S on 27/06/2022 08:27:42:
Posted by SillyOldDuffer on 26/06/2022 13:21:34:

Always been concerned about the strength of printed engine components. Partly because PLA is about 10% the strength of mild-steel, and partly because this type of 3D print isn't solid and I don't know how to control the internal web. ...

 

Hobby Slicers are pretty 'general purpose''. The hollow things out because in general a machined design would be solid and that takes ages to print. Often a hollow thing is 'sufficiently not shit' to be useable - especially for the general 'landfill ready' prints a lot of people make.

... if it needs to be a specific shape or wall thickness then the generic slicer hollow fill is unlikely to be the best option.

Dave

 

 

Talking of 'landfill ready' prints, I'm back!

dsc06619.jpg

Spent several hours finding ways of increasing strength. Cura isn't best slicer for this but it has useful features:

  • The whole model can be printed solid - takes about twice as long. I made a solid replacement clip to hold a wet and dry cleaner together and positioned it so the plastic layers went sideways across the main bending forces.
  • There are several fill patterns, all designed to save time and plastic, but some are stronger in 3D than others, and the fill density can be increased.
  • As Dave says, Cura ignores physical strength when slicing, even leaving large hollows under what should be strong. However, Cura's 'Support Blocker' and 'Per Model Settings' can be used to change several selected volumes within the model to different fill settings. Not fine control but much better than just letting the whole model print on the default fast, low strength settings. CAD could identify weak spots with a Finite Element Analysis, but basic engineering principles would probably do: thickening stressed areas, rounding sharp corners, adding webs & fillets etc.

Quite a lot to this game: anyone who thinks CAD and 3D printing with plastic filament is cheating is in for a shock! At least if anything about the model has to be strong.

Back to the engine, which I tested on compressed air this afternoon.

The prototype wasn't printed for strength and nothing broke, even though the crank end of the rotary valve is hot glued on. (Broken removing support plastic with a big knife.)

After polishing the bore, piston and rotary valve the engine turns moderately well by hand, though markedly stiffer than a metal engine. Applying air, it did 3/4 of a revolution and I had high hopes adding weights to the flywheel would provide enough energy to power the exhaust stroke. The flywheel weight holes were too small for Jason's pretty ball-bearings so I used 12 M5 nuts, a nice push fit.

Didn't work. Plenty of power in the up stroke, but the piston sticks coming down to exhaust. Duncan Webster predicted my design would wedge, it does, but I don't think it's the main problem. Seems PLA bearing on PLA gets stickier as it warms up due to friction - even a few degrees cause bother. Engine gets stiffer the more it's rotated and loosens again up when allowed to cool. Lubed with dry PTFE, not tried anything else. Rethinking...

sad

Dave

 

Edited By SillyOldDuffer on 04/07/2022 16:49:36

lee webster04/07/2022 17:35:57
116 forum posts
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Dave, A very simple form of lube is pencil lead. Could be worth trying. Rub the business end of a large dia pencil on the areas that need lube. It works on door latches!

Lee

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