PMC IMP Build log and WIP

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Thanks for the kind comments CC, Arnold, Diymania and Tony.

Next up is the needle parts. There are two parts for the needle. The needle and the barrel that holds the needle. Lets start with the barrel.

The barrel is just a length of 3/16 brass a little over a half inch long with 5-40 internal threads in most of it, and a 1/16 hole in the rest. About half of it is turned down to 0.14 inch for the spring to slip over. After facing the end of the rod, I turned wond the end for about 0.281. Center drilled and drilled with at tap drill for the 5-40 to a depth of about 7/16 inch. Then drilled an additional 1/8 inch with a 1/16 diameter drill. Tapped as deep as I could with a plug tap.

Needle1.jpg


Cut the barrel off to length plus a little bit to clean up. Reversed the barrel in the chuck and faced it off and rounded the end a little bit with a file.

The needle is a short length of straight 1/16 music wire with a 1/4 inch long taper ground on one end. When finished it does not need a sharp point, but I find it easier to grind it to a sharp point and then to blunt the point to about 0.015 diameter. Eyeball accuracy is close enough for all these dimensions. I use a Dremel (rotary tool) with a 3/4 inch fine grinding wheel to do the grinding.

The first step is to dress the grinding wheel if it has never been dressed. A new wheel will have runout and a rough surface. If it is not dressed, the needle will have a rough surface, and probably be egg shaped from the grinding wheel bouncing. I use a single point diamond dresser that I use for my other grinders to dress the wheel. I hand guide the wheel over the dresser, holding the Dremel in one hand and the dresser in the other. To steady my movements, I rest both arms on the workbench or outdoors on the picnic table. Eye/face protection is a must, either safety glasses or preferably a face shield. Don't do this near your machine tools as the dust created is pure abrasive.

I put the music wire in a 1/16 collet on the lathe with about 1/2 inch protruding. Cover the lathe bed with a paper towel. For safety and other reasons, don't use a cloth rag. I run the lathe at about 1200 RPM. It works best if the lathe is run in reverse so the wire and the wheel are turning into each other. It gives a little smother finish, but is not absolutely necessary. I first ground a blunt taper on the end of the wire. If the end of the wire is not uniform, then the shallow taper will not be concentric when it is ground.Then the shallow 1/4 inch long taper is ground. It only takes light pressure and by moving the tool along the axis of the wire as it is being ground a very smooth finish is produced. I grind for about 5 seconds at a time to prevent overheating the wire and destroying the temper. Then let it cool for one or two seconds. It takes less than a minute to grind the taper. I ground to a point and then blunted the point. After extending the wire about an inch from the chuck I polished the ground section with a bit of 800 grit abrasive paper and polished up section of wire above the point taper so it would solder easier during assembly.

Last step is to throw away the paper towel covering the lathe ways so I don't wipe something down with it by accident.

Gail in NM,USA

Needle2.jpg
 
With the needle parts made, they need to be assembled. I use regular electronics grade soft solder with a little bit of extra flux added to the parts. It does not take very much flux or solder.

I started by installing the spray bar in the venturi and held it in place with one of the nuts made earlier. I aligned the spray bar cross hole so it pointed at the side of the venturi. This was easy to do as I had drilled the hole so it was in line with one of the points on the hex section of the spray bar.

The barrel was screwed onto the spray bar until it bottomed out and then backed off one turn. I clamped the venturi in my small drill press vice on the ends as shown in the photo. I made a ring of solder out of some 0.020 diameter electronics solder by wrapping it around the needle and trimmed it off to make one full turn with a hobby knife.

The needle was inserted in the barrel until it bottomed out in the spray bar, then was lifted up about an 1/8 inch and a small amount of flux was applied to the needle just above the barrel. The needle was slid in and out of the barrel with a twisting motion to distribute the flux in the joint. Finally the needle was bottomed out again and the solder ring slid down to the needle-barrel joint.

A small propane torch was used to heat the assembly. The flame was directed at the top portion of the barrel at a slight upward angle so the flame would not touch the solder ring. After about 2 seconds of heating the solder ring melted and flowed into the joint.

FuelAssy1.jpg


After cooling, everything was removed from the vice. The needle was unscrewed from the spraybar and given a little clean up to remove any extra flux. The end of the needle was bent about 45 degrees with two pair of pliers to form a handle for adjusting. The spring was added to the needle and then the needle was reinstalled into the spray bar. A 1/4-40 jam nut was added to the venturi and the fuel assembly was complete and ready to be installed in the crankcase.

FuelAssy2.jpg


Gail in NM,USA


 
Very nice looking carburetor Thm: And thanks for the step-by-step description :bow:
Regards, Arnold
 
Penultimate post on the IMPs.
All three IMPs are finished and test run.

The main thing still to be done is to build a display stand to hold all three engines. When that is done I will take a decent photo and post it. It may be a little while as I am still sketching what I want and have some other projects going right now.

At some point in the future I may anodize the V3 version with a red head and black crankcase. Probably not, but I have been thinking about it.

From left to right in the photo below.
V1 which is a fairly close replica of the original IMP with a few minor internal changes to get rid of the insane exhaust timing.
V2 is the same as V1 internally, but with external cosmetic changes.
V3 Same external as V2 except intake a exhaust interchanges and internals changed to match.

Check the first post of this thread for a link to V2 running.
EDIT: run link video added.
[ame]http://www.youtube.com/watch?v=8x3PalJlz0w[/ame]

It's been fun trip. Thanks for joining me and thanks for all the comments along the way.
Gail in NM,USA

IMPsX3.jpg
 
Gail,

Wonderful work. :bow: :bow: :bow:

Smooth runner.

Best Regards
Bob
 
Great build, and a fine build log too, Gail. They look just pretty as can be, and the one in the vid runs as well as any I've seen. Sounded like it was running where it should when you got the comp and mix adjusted. Impressive work!
 
great work Gail, many compliments, very nice running engine :bow: :bow: :bow:

but... did you make 3 engines? in the same time or in different periods?
 
Gail, Three stunning little engines in a row! :bow:
Thanks for the write-up and video! - surprising how reactive the engine is to the compression screw changes.
Kind Regards, Arnold
 
Very nice work Gail :bow: cracking runner too

CC
 
Beautiful runner Gail! :bow:

Rick
 

Great work! :bow:

Sounds awesome.
 
Thanks to everyone for all the kind comments.

Ariz: All three engines were built at the same time. It took about three weeks to make the parts and assemble the engines. I would guess that it would have taken about 2 weeks to build only one. Most of the parts are either common to all engines, or have a minor variation from one engine to the other. Once a set up is made it only takes a little bit of time to make a second or similar part. In contrast it took me about three months to write everything up for the thread. Of course a lot of that is bacause I would rather machine parts than write about it.

Arnold: The sensitivity of the compression adjustment is because the thread pitch is rather coarse on the screw. While the thread is not the same as the original production engine, it is quite close. More modern engines often had a finer pitch screw, but I was trying to keep the engines in at least the spirit of the original design. Most small commercial engines of that era had similar sensitivity. Two other factors are that the propeller is quite large for an engine of that displacement and that I live at about 5300 feet (1600 meters) elevation and the thin air affects performance quite a bit.

Gail in NM,USA
 
GailInNM said:
Arnold: The sensitivity of the compression adjustment is because the thread pitch is rather coarse on the screw. While the thread is not the same as the original production engine, it is quite close. More modern engines often had a finer pitch screw, but I was trying to keep the engines in at least the spirit of the original design. Most small commercial engines of that era had similar sensitivity. Two other factors are that the propeller is quite large for an engine of that displacement and that I live at about 5300 feet (1600 meters) elevation and the thin air affects performance quite a bit.

Thanks for explanation Gail :) - And it is good to know about the elevation issue for when I build an IC engine one day; Windhoek is also 1600m elevation.
Kind Regards, Arnold
 
Final photos of the IMP's.
They are now in the display case.
For reference, the base of the display is 6 inches (15cm) wide.
The mottled appearance of the bottom photo is a reflection of the wood surface it is sitting on.
Thank you to everyone who joined me for the ride.
Now it is time to move on to the next project. Drawings in progress.

Gail in NM

Final1.jpg


Final2.jpg
 
Once again Gail - WELL DONE!
I wish I can see this display in real-life one day.
Kind Regards, Arnold
 
Top class Gail, from start to finish, including this excellent thread, and the final pictures do you proud :bow:

and very enjoyable 8)

CC
 
Thank you to all who commented on this build.

For those who might be interested, the IMP build has been included on Ron Chernich's website in the Gallery on pages 12 and 15. He also gave a nice plug for this HMEM thread in the second link.

http://modelenginenews.org/gallery/p12.html
http://modelenginenews.org/gallery/p15.html

Ron did the reverse engineering of the original IMP and produced the plans that started this project off. Thank you Ron.

If any of you have not visited his website, you really should do so if you are interested in building model aircraft type engines. His site has a most comprehensive list of "how to's" on most problems faced when building this type of engine, and a fantastic photo gallery of both homebuilt and commercial model aircraft engines. When I first discovered it I got lost for several days while reading it, and it has a much larger content now. It is updated at first of each month.
Gail in NM
 
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