Author Topic: Another version of how not to build a model engine  (Read 88540 times)

Offline Darren

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Re: Another version of how not to build a model engine
« Reply #25 on: November 13, 2009, 01:32:37 PM »
How about making a tap on the lathe?  :dremel:

You can have whatever pitch you need then.
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Offline sbwhart

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Re: Another version of how not to build a model engine
« Reply #26 on: November 13, 2009, 01:40:20 PM »
You can get 60 TPI  (0.423mm pitch) from her http://www.tracytools.com/tapsanddiesfinethread.htm

Hope this helps

Stew
A little bit of clearance never got in the road
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Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #27 on: November 13, 2009, 01:45:41 PM »
How about making a tap on the lathe?  :dremel:

You can have whatever pitch you need then.

It's indeed possible, but I'm completely newbie, what comes to required hardening process. On the other hand, if the tap is used to thread ptfe, it wouldn't need any hardening.... The needle body doesn't have to be made of metal?

Anyway, thanks for the tip, I'm going to test how I can manage :thumbup:.

Offline Darren

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Re: Another version of how not to build a model engine
« Reply #28 on: November 13, 2009, 02:00:40 PM »
If you want to tap a softer material you could thread a Silver Steel/Drill rod to the sizes required.

Then grind a groove down one side, or maybe two sides. Touch it up with a stone to sharpen the cutting edge.

I'm guessing, cos I have never done it, then you will now have something between a regular cutting tap and a thread forming tap.

If you want to harden it, just heat to a bright red and quench in water. Then re-heat noting the colour change required and allow to self cool in air.

You say a newbie as if that should limit you?  Not so ...ask they guys here where they started ... just go for it  :thumbup:
« Last Edit: November 13, 2009, 04:30:43 PM by Darren »
You will find it a distinct help… if you know and look as if you know what you are doing. (IRS training manual)

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #29 on: November 13, 2009, 03:32:58 PM »
Thanks, I will :dremel:.

I tested making a 0.2mm pitch thread on the lathe, and found out, that the cutting tools tip has to be very sharp, without any radius.

I wonder, how watchmakers sharpen their thread cutting tools. With the aid of microscope perhaps?

Back to sharpening I go ::).

Offline Darren

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Re: Another version of how not to build a model engine
« Reply #30 on: November 13, 2009, 04:33:15 PM »
Forget the radius, far too fine a pitch to manage that I would have thought.

In essence you don't need to worry about exact angles and stuff because you will make both the male and female parts from the same cutting tool  :thumbup:
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Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #31 on: November 14, 2009, 07:24:43 AM »
Another thing popped to my mind. There is noticeable difference between the glow engine needle system, and the one I made.

The former doesn't have its needle going across the carb throat. Thus allowing smaller throat size.
When using the latter with same throat size, needle itself restricts the air flow.

So I drilled the throat to 1.9 mm size, and the engine starts easier with that. Engine flooding is also reduced greatly.

Offline NickG

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Re: Another version of how not to build a model engine
« Reply #32 on: November 14, 2009, 09:52:25 AM »
Was just thinking, have you considered making it glow ignition? It might be easier to trouble shoot carb problems if you have reliable ignition initially? Just a thought.

Nick
Location: County Durham (North East England)

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #33 on: November 14, 2009, 10:24:35 AM »
The ignition seems to work, as the engine did just run at a horribly fast speed. I thought it was going to explode :borg:.

To tame it, I'll add an air shutter to the throat.

Glow ignition would be simpler, but it wouldn't allow low rpm.

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #34 on: November 14, 2009, 06:09:21 PM »
Is a compression ratio of 3:1 high enough?

Nick

The approximation, that I presented about the compression ratio, is based to my own calculations, and has proven to be somewhat misleading.
The math really isn't my strongest virtue :smart:.

The engine behaves like it had something like 8:1-10:1 comp ratio on it, regarding the speed it has run.

Next time I'm going to warm up the tachometer to get some readings.



Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #35 on: November 15, 2009, 08:57:23 AM »
The shutter at the carb's throat doesn't seem to solve the adjusting problem :scratch:.

Anyway, I finally got a tachometer reading, about 4100 rpm, but the engine goes even faster than that, if allowed. Mostly I have to kill the spark to stop it.

Any advices on that carburetor thing would be greatly appreciated.

Offline John Hill

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Re: Another version of how not to build a model engine
« Reply #36 on: November 15, 2009, 01:33:49 PM »
How about a restrictor on the exhaust?  That should tame the beast! :coffee:
From the den of The Artful Bodger

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #37 on: November 15, 2009, 04:02:39 PM »
How about a restrictor on the exhaust?  That should tame the beast! :coffee:

Thanks for the tip :thumbup:. I was just getting to testing it, when I heard 'crack' and the engine stopped suddenly, resulting this:


Reason was the M2 nut, that keeps the connecting rod in place. It was loosened, and finally it fell off.
So far, I haven't found any other damages.

Lesson learned: The nut only isn't good way to secure the connecting rod end.

As an alternative, I'm going to attach a bearing disk to crankcase backplate.

What comes to crankshaft, only the shaft is bent, so I'll drill it off, and replace it with a hardened 3 mm rod.

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #38 on: November 16, 2009, 07:01:31 AM »
Here is the crank disk and new rod, that are to be silver soldered together:


Bits and pieces:


Another view of cylinder head and liner:                                    From the top:
                         

Crankcase is fastened to base with two M3 screws:



Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #39 on: November 16, 2009, 11:22:20 AM »
Repaired crankshaft with ball bearings:                                 

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #40 on: November 16, 2009, 03:52:31 PM »
About that bearing disk, that I mentioned earlier; it really isn't 'bearing' disk, as it's a static part.
Since I don't have suitable size steel stock, I'll make that disk by stacking an M8 washers, and silver solder them together:

before...                                                                       ...and after.
               

The disk in place:                                                           
               

It needs to be shortened, so that it can be attached to crankcase backplate.


If my memory serves, this kind of system has been used on glow engines.

Offline NickG

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Re: Another version of how not to build a model engine
« Reply #41 on: November 17, 2009, 04:02:01 AM »
sorveltaja,

I think the nut would have been a fine way of stopping it coming off. It shouldn't have come undone, the only reason I can think it did was if the conrod was slightly thicker than the length of the crank pin, it should have been the other way around.

Still, your new method should work as long as there is no binding between rod and that disc.

Nick
Location: County Durham (North East England)

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #42 on: November 17, 2009, 04:23:50 PM »
Nick, the recent nut-system should have worked, as my purpose was to make a slow runner. But the engine runs at too fast speed, causing more vibrations, than I ever expected.

But it's just one of those bugs, that needs to be stomped.

Here is the bearing plate attached to backplate. Fastening is done by two M2 screws:


And here are the plate + crankcase backplate in place:


So it's about time to re-assemble rest of the engine :ddb:.


Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #43 on: November 18, 2009, 01:06:18 PM »
Before assembly, slots for primary gear and flywheel's grub screws needs to be done:


Testing that they fit ok:

bogstandard

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Re: Another version of how not to build a model engine
« Reply #44 on: November 18, 2009, 01:19:36 PM »
In all honesty S, your over revving just might be because you have a too large carburettor throat. I would try about half the diameter you have now and see if it makes a difference, you can always open up in stages of 0.1mm until you reach a good compromise. On small engines like yours, 3mm or 4mm would be classed as a racing bore, designed for getting the engine into a higher rev range by over fuelling it.

The reason I suggested sewing needles is because they have a very shallow taper, so allowing you to use a coarser thread for adjustment, plus they will be much smoother than a needle you make yourself.


Bogs

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #45 on: November 18, 2009, 02:02:45 PM »
Bogs, you are right, but as I have tested throat sizes from 0.6 mm to 1.9mm, there doesn't seem to be any sweet spot.

Too small throat size causes flooding, no matter what; with ~1.2-1.9mm throat, the engine starts easier, but runs too fast.

Maybe the compression ratio is too high; at the beginning of this project, I was going to make a cylinder head, that allows the comp ratio to be adjusted.

Back then, it did seem too complex thing to achieve. Not impossible anyway.

Lowering the comp ratio is just one of the options to test.

But in the mean time, I'll assemble the engine.:
   



Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #46 on: November 18, 2009, 02:25:03 PM »
Cylinder head/liner combo in place:

Offline sorveltaja

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Re: Another version of how not to build a model engine
« Reply #47 on: November 18, 2009, 03:11:28 PM »
And finally, the assembled engine:




Offline Darren

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Re: Another version of how not to build a model engine
« Reply #48 on: November 18, 2009, 05:22:37 PM »
Sure is an interesting looking engine ... nice to see you got it fixed up after the little disaster :thumbup:

Now don't you be going thrashing it again ....  :wack:
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Russel

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Re: Another version of how not to build a model engine
« Reply #49 on: November 18, 2009, 10:35:28 PM »
Wow, until you added the matchbox in the photo I thought your engine was about 6" tall. I'm impressed, a very nice little engine!

Russ