ML Midge Build

[jack620]

Progress has been slooow. I finally finished the fuel tanks and lids.

 

[jack620]

OK, third time I've typed this because I got 'token expired' messages on my first two attempts.

I resumed work on my spraybars today. I drilled 14mm deep with a 1mm drill bit then reamed with a D-bit made from the same 1.5mm music wire that the needle valves will be made from. I slowly peck drilled, withdrawing the D-bit each time to clear the swarf. It seemed to cut very nicely, but as the depth increased the spraybar developed a quite pronounced bend. It was very strange. Maybe the step from 1 to 1.5mm was too big for the D-bit? I would have liked to drill to just under 1.5mm, but I didn't have a drill bit between 1 and 1.5mm. I'd better get some.

The music wire is a nice snug fit in the spraybar, but that may be because of the bend in the spraybar. The first pic is prior to drilling so the spraybar is straight. The second blurry phone pic was taken before the bend had developed very much. The bend is only in the threaded section.

Does anyone have any idea why the spraybar developed the bend as it was being reamed?
Any tips for straightening the spraybar?

Chris

 

[jack620]

I finally got a 1.3mm drill bit and had a crack at the second spraybar. This time the bending was WORSE. There's something about reaming the spraybar with a music wire D-bit that is causing the spraybar to develop a bend.

I finished the spraybar and decided to try and straighten it using the thimble. Here's the result:

 

[Ramon]

Hi Chris - just caught up with your difficulties - sorry to hear you're having problems.

Firstly though I have no idea as to why the threaded portion should bend as you describe I can only hazard a guess that the stresses induced when cutting the thread may be releasing as you 'ream' the hole out using your D bit. Bending is usually stress (relief of) related.

I use the wire as a 'reamer' but only to take a couple of thou off. You may have already seen these images but this is how it's ground (just a long slow flat taper) and used delicately.

The hole diameter is the nearest drill size to the piano wire - you may have music wire which is usually 1/16" so a 1.5 drill would be fine. It's important to withdraw the tool and clear the swarf very frequently as swarf build up will quickly jam on the wedge shape however if this reaming is done slowly and carefully then the cutting of a size hole is very good indeed.

That first pic is reaming a 7BA threaded rod cut about 7/8" long - no room for wander as you can imagine, the second shows about the maximum swarf build up per peck for trouble free operation.

Personally, given the size of the spray bar, I would go for 18swg wire - about .048" - and still going about it the same way. That will leave more wall thickness on the spray bar thread which will help too

Hope this helps a little

Regards - Ramon

 

[jack620]

Thanks Ramon. On my second attempt I drilled to 1.3mm before reaming. When I make a replacement for the broken spraybar I will drill to 1.4mm then ream with a new music wire reamer with a tapered end like yours. I might even made a threaded sleeve to screw onto the spraybar thread and hopefully prevent the bending.

If that doesn't work I'll track down some 18swg wire and try that.

Regards,
Chris

 

[jack620]

I tried to straighten the other spraybar today without success. After many attempts at gently bending the spraybar I could only get the thimble to screw on about 6mm. I then tried re-cutting the thread with a button die. That just ruined the thread. That's two ruined spraybars. :wall:

Tomorrow I'll have to make two new spraybars. Hopefully Ramon's tips will stop the bending I experienced on the first two.

Here's the other piece of scrap brass. Excuse the dodgy solder job. I was going to clean that up, but there's no point now.

 

[jack620]

I made two more spraybars today. I used the music wire reamer with a tapered end like Ramon's. I also drilled with 1.5mm drill which was slightly undersize before reaming. The needles aren't quite as tight as they were in the first spraybars, but at least the threads are straight and the thimbles screw on easily.

Thm:

Chris

 

[Ramon]

Hi Chris good to see you got there in the end.

I'm not wanting to push info before you need it but this is a precaution

Be careful not to use too much solder if you are soldering the needle into the thimble - it can run through and into the threads .

I use to do it fully assembled ie screw the thimble on, back off 1-2 threads, push the needle home and solder the thimble. Now I do the same but mark the needle position where it enters the thimble and remove both parts setting them back up to the mark before soldering. This only works if the needle is held straight concentrically enough by the thimble.

Good to see your steady progress - not long now eh?

Regards - Ramon

 

[jack620]

Thanks Ramon. I soldered the needles last night. I used Ron Chernich's method of marking the needle with a china-graph pencil either side of the solder joint. It prevents solder sticking where you don't want it. I was able to solder the needles while they were inserted into the spraybar without solder flowing into the spraybar. I did apply too much solder to the top of the thimble though which looks awful. I'll make up a 1/8-40 threaded mount and turn the solder blobs off the thimbles.

My knurls never arrived from China, so I got a refund and now I have some on the way from England. Once the prop drivers are knurled I just have to make two compression screws and I'm done!

Regards,
Chris

 

[jack620]

I finished the carby/tank assemblies today. Just the prop drivers to knurl and compression screws to make and their done.

 

[jack620]

Here's how I mounted the thimbles to turn off the excess solder. Just a piece of 6mm silver steel turned down to 1/8", threaded 1/8-40 and drilled 1/16" to accept the needle.

 

[jack620]

Today I finally knurled the prop drivers. I also assembled engine #2. I would have attempted to start it, but I couldn't mount it. I bought some 2.5mm cap screws a while ago, but stupidly didn't buy any nuts! I think I must have been thinking I had a 2.5mm tap. Oh well, I'll get some nuts later in the week.

Note I'm using an M4 cap screw as a temporary compression screw. The plans call for 4mm all-thread to be used to make a simple bent compression screw. That's a joke. The only all-thread I can find is a very sloppy fit in the cylinder head. I've made 3 attempts at cutting my own screws on the lathe, but all have ended in failure (broken screw). Clearly I need a sharper thread cutting tool and lots more practice. I am attemting to make screws like the one by Ramon in the picture below.

I've also included a photo of all the fixtures, laps, jigs, templates, etc that I have made along the way.

Chris

 

[jack620]

Success of sorts. I was able to get engine #2 started fairly easily, but it runs slowly and very rough, so I need some advice.

A couple of points I observed:
The engine runs with the needle valve screwed fully in, so I obviously didn't solder it into the thimble properly.
The engine runs with the needle valve fully removed! I may try running the engine with the other carby fitted.
There is a very black pool of oil building up on the top of the cylinder head. There are bubbles in this oil from around the comp screw. I guess I have a loose CP? I'm pretty sure I can hear the CP moving up and down when I flick the prop over.
Fuel is 25% ether, 42% kero & 33% castor oil plus IPN.
It vibrates so badly it loosened the screws that secure the back plate.

Whats with the black sludge? Is that a normal colour for diesel fuel exhaust? I hope it isn't cast iron powder from the piston or CP. I cleaned everyting in an ultrasonic cleaner and/or nitro thinner before assembly.

Here's a video of the engine running (spluttering actually).

[ame="http://youtu.be/WR1X5HOWRPU"]http://youtu.be/WR1X5HOWRPU[/ame]

Chris

 

[Swifty]

Must be very satisfying to at least have it running. Not really having any ideas about fuel ratio, but maybe there is too much castor oil in the mix.

Paul.

 

[Ramon]

Hi Chris - first off well done for producing a running engine despite its 'apparent' problems. It sounds and appears however that it may be oscillating which is a sign of over compression and too rich a fuel setting. It will vibrate considerably if it is doing this. If you can't close the needle then you have no control over the fuel setting.

Your fuel make up sounds ok but how much IPN did you use - Did you actually get IPN (lucky fellow). Also what is the prop size.

Other than that taking it one thing at a time -

Bubbly oil coming from the comp screw means only one thing and that is as you rightly say a contra piston too loose. You probably can hear it going up and down with the piston. I wouldn't continue to run it like this until you have sorted that out. If you made the contra piston out of cast iron you may be able to grow it by heat treating it - check out the Eta thread again. I had to do this on two CPs on this last lot. It's a good way of recovering a loose CP but you can only do it the once.

Black gunge coming from the exhaust is usually one of two things especially early on like this. As said you may have it overcompressed a bit so it's running 'hard' which will sound rough also getting hot very quickly which will create a carbon rich exhaust. Once it's running right, it's probably just the parts bedding in and the black content will soon diminish. Again you can see this on the Tigres and the Etas

Setting the needle to re-solder - screw the thimble right in then back off two turns. Heat the solder then at the same time push the needle inwards to seat and close the orrifice - you can check this is closed by putting some fuel tube on the inlet and blowing down it. Being able to run with it right out probably means no more than that the orifice is very small - sufficiently small enough to let the engine run rich.

Don't despair - the fact that it's firing speaks volumes. You'll certainly know when you have the settings right - if it will run like this at the moment it should be more than capable of running much better once you sort these small issues

Any further advice on running just ask - I'll be glad to help
Good Luck - Ramon

 

[jack620]

Hi Swifty,
After a bit of research, 33% castor seems to be what is needed for a plain bearing motor to prevent excessive wear.

Ramon,
Thanks, you've put my mind at ease a little.

I bought this fuel from my local model club. It was originally 20% castor, so I added more. That dragged the ether content from 30% down to 25%. I believe it has 1.5% IPN.

Prop is 8x4. I also have 7x4 wooden props. Should i change to those now?

I'll re-solder the needle first and see if I can get it to run smoother.
Then I'll try to grow the cast iron CP.

Hopefully I'll have some good news later this afternoon.

Chris

 

[jack620]

Today came and went without much workshop time. I swapped the carbies over and the other carby does allow me to shut the fuel off. However the motor doesn't run any better. I have removed the CP and I'll try to 'grow' it during the week using Ramon's technique (heat & quench in oil).

With the head & liner removed I was shocked at how much the piston wobbles on the conrod. I don't mean in the direction you would expect it to wobble, but front to back. It wasn't like that when I installed it. I must have used a very soft grade of aluminium. The rods were cut out of a piece of 3mm thick aluminium angle. I don't know what grade is typically used to make angle. I think I'm going to have to make two more rods from a tougher aluminium. The plans call for 2024. Wikipedia says this about 2024: "It is used in applications requiring high strength to weight ratio, as well as good fatigue resistance." I think I'd better get some.

Chris

 

[jack620]

After a bit of research it seems likely my conrods are made from 6060 aluminium. While not a strong as 2024, it still has good strength, so I'm surprised my top end has worn so quickly.

 

[Swifty]

Why not make them out of phosphor bronze? The little bit of extra weight surely won't make much difference.

Paul.

 

[jack620]

Sounds good, but where do I get it Paul? I only need a piece 30x15x3.