Motore Glow da 5cc

[Admiral_dk]

A very nice crank 

[Jasonb]

The prop driver was next on the list, this uses a "D" shaped hole which located on the flat previously milled on the crankshaft to stop it rotating. On previous engines that use this method I have cheated and bought spare parts for OS engines but thought I should have a go this time.

A test broach was roughly made from a bit of silver steel (drillrod) to see how it would work and I did not bother to harden & temper it for the initial test. It actually worked very well so I did not bother to make a proper hardened version but just drilled and parted off a couple of blanks and used my high tech press and spacer to drive the broach through with a squirt of tapping fluid to help it on its way.



And the resulting hole it produced



In this photo you can see the "c" shaped pieces of waste cut out by each sucsessive step of the broach.



To make sure everything was nice and concentric I then used the crank as an arbor to hold the prop driver so that the external machining could be done.



With the part now in a 5C collet a small boring bar was used to recess the central area followed by knurling the teeth that grip the prop.



From the side you can see that by holding a two wheel pivoting type knurl as high as possible in the toolpost it can be used like a single wheel one



The finished teeth





The Conrod is from HE15 (2014) and I found it cheaper to get a round bar and reduce to the required rectangular section than buy square stock.

Once the basic shape was squared up the first job was to drill and ream for the wrist and crank pins.



Next the central section was taken down to thickness, I probably should have left a slight fillet on the internal corners but as my engines only get run a few times to establish that they work I hopefully won't get any stress fractures starting from those sharp corners.



With a cutter in one hole and smaller drill in the other I could easily set the angle to machine the tapered sides, this time I did use a cutter that I had knocked the corners off of which left a nice little fillet.



It was then back to the rotary table again to round over the ends carefully blending the cut into the tapered edge



A spotting drill conviniently located the hole and counter sunk it in one which was followed by a small 1.2mm drill to allow some lubrication to the moving parts. One cole for the wrist pin and two for the crank.



Job done

[Allen Smithee]

That's really nice stuff, noted down for future reference!

So the broach is just a bit of silver steel bar with turned steps and a milled flat? What step sizes did you use? And are you going to bush the big/little ends of the rod?

AS

[Jasonb]

I was not really certain how it would work so just free handed it.

The lead diameter is the OD less twice the depth of the flat, in this case 7.8mm less (2x 0.9) so 5mm and the 7.8mm at the bigend. I used a fairly pointed HSS tool which gave each tooth a small amount of top rake then just made a series of cuts which look to be 0.100" long (one turn of the handwheel) increasing the depth by about 0.010" depth of cut on each sucsessively shorter cut. 5mm pilot hole.

The flat does tend to push the broach slightly to one side so make your blank a bit bigger than needed. Not too sure how it would work in steel but was pleased with the cut in ali.

The drawings don't show any bushes in the conrod, I've done a couple of other engine designs  ( 2 and 4-stroke) without any and have not had problems with them picking up or ware, maybe the copper content of the HE15?

[Roger B]

I like the broach and the flat knurling     I would say not long now, but I guess that it is actually already finished 

[Allen Smithee]

This is all being filed away for future reference - much appreciated. Where I've made rods for my pylon racing engines I always tended to bush them, but these were 25-30,000rpm beasts which were expected to do up to 2 hours of race-speed running between piston/loner/rod replacements so it's not really comparable. It was one of the standard tricks to "re-life" a worn but undamaged piston/liner set to make a new rod that was up to 50 thou longer to move the pinch point up the bore, and then re-cut the ports and clean up the liner with a gentle lap. The bores were hard-chromed and tapered, so this would move the operating point to a fresh bit of chrome. Obviously the head buttons had to be shimmed up by a similar amount, but the head-shimming was one of the standard "tuning adjustments" done on the day to match the prop, pipe and weather anyway.

One other question on the crankshaft - would you tend to harden it or list leave it as-is?

AS

[Jasonb]

I made it out of EN8 and will just leave as is, again done it on several engines. EN24T is a bit stronger but not as nice to machine or you could do as Ramon does on his 5cc diesel engines and use a HT bolt as raw material. I think without a proper oven and quenching you could quite easily end up with a bananna crank trying to do it in the home workshop

[Jasonb]

The pistion was turned a little oversize from some cast iron bar then counterbored to form the skirt before being held in an ER32 collet block to first mill the 10mm wide slot for the conrod.



Then laying the collet block on it's side to keep things at right angles while to wrist pin hole was drilled and reamed 6mm.



It was then parted off the bar, turned to length and the domed head formed with a combination of round nosed lathe tool, handheld scraper and files.





A mandrel was knocked up that was a good fit up inside the skirt and a screw and tommy bar used to hold the piston in place.



I  could then finish turn the piston and lap it until it just started to enter the cylinder at which point a little 1000g lapping paste and some oil was added and the piston lapped to the cylinder. This is where the taper on the cylinder that I mentioned before reared its head again and the lapping took a lot longer than it should have and the piston was a bit looser than I would have liked when low in the bore, more on that later.




The cylinder head was a fairly straight forward turning job starting with a spigot to fit inside the cylinder and a shaped end to mirror the domed top of the piston. The plug thread was also drilled and tapped at this setting.



With the head parted off and held by the spigot a hole was counter bored for the plug. This had to be deeper than scaled as the length of the plug stayed the same unlike the thickness of the head.



Finally over to the mill to add the 4 holes and counterbores, like the other fixings I stayed with M3 which allowed the smaller heads to be set flush with the top of the cylinder head.



Not much more to do now so should be able to cover that in teh next installment.

J

[Tennessee Whiskey]

Nice work J. Aren't collet blocks the best thing since sliced bread and indoor plumbing  ? I'll have to admit,  you and the Queen of Model Machine,  sure are making these little aeros intriguing  .

Cletus.

[Jasonb]

The Prop nut is a bit unusual as rather than just being a hex nut it has a long spigot on it that fits inside the propellor. This was turned from some round steel, drilled and tapped M6.



Then over to the spin indexer on the mill to cut the hex.



After which it was parted off and the edge of the hex chamfered. The long internal thread also allows a grub screw to be inserted which locks the nut on and prevents the prop unwinding itself in the event of a miss fire (Jo you should have something like this now that you have got a collet rather than a pin to retain your propdriver)



The wrist pins were just short lengths of Silver steel (drill rod) opened out to 3mm and bronze pads inserted



The spray bar was turned, threaded and drilled from a piece of round brass before going into the spin indexer to cut a 5.5mm hex, this is the standard size hex for M3 fixings.



After parting off and forming a spigot the end of a small parting tool was ground to an angle and used to cut some barbs which should help the fuel tube stay in place.



The rod for the stainless steel needle was supported in a groove in a bit of MDF while the point was shaped with a needle file and then fine emery.



The needle "nut " was turned and the straight knurl cut then after slitting the end was squashed slightly so the friction woul dstop the needle comming loose. The nut was screwed home and then backed off two turns, the needle inserted with a dab of locktite and a quick blow to check the needle was closing the fuel supply before it set. That way you know the needle will be long enough and the fuel supply can be shut off if needed.



And assembled on the engine



Well I think thats all the parts covered so here is a group shot



And a few of the completed engine









As I have mentioned earlier the bore had a bit too much taper and was not making compression until right at the top of its stroke so the first runs were not ideal and the only way I could get the engine to keep running was to keep my finger over the venturi inlet and almost close it off.

<a href="https://www.youtube.com/watch?v=q7QP1tJtKys" target="_blank">http://www.youtube.com/watch?v=q7QP1tJtKys</a>

I stripped the engine back down, trued up the bore with a HSS boring bar and lapped it again then made a second piston which was lapped to the new slightly larger bore, also sleeved the venturi down slightly. I still would like the engine to have a bit more life but at least it will now run unaided and 5500rpm is not dad with fuel that is at least 25years old.

<a href="https://www.youtube.com/watch?v=H3sy_L0yQcE" target="_blank">http://www.youtube.com/watch?v=H3sy_L0yQcE</a>

J

[vcutajar]

Nice job Jason.

Vince

[Roger B]

Very nicely done    Keep well away from the finger chopper 

[Jasonb]

Very nicely done    Keep well away from the finger chopper 

Don't worry I won't be going near Jo for a while

[Dave Otto]

Nice work Jason!

Thanks for sharing your work with us.

Dave

[Jo]

A safety observation:  Running model engines inside is dangerous.

While Jason is running his engine inside I am sure that he has all the doors and windows open to avoid a build up of Carbon Monoxide which is an odourless gas that will kill you.

The other disadvantage is that your workshop (and you) will stink of fuel for days afterwards which I am sure your Wives will really appreciate  And if you are using a nitro based fuel remember that the same rule about cleaning out the engine so that it does not rot the engine components goes for whatever it may have sprayed the exhaust all over

The long internal thread also allows a grub screw to be inserted which locks the nut on and prevents the prop unwinding itself in the event of a miss fire (Jo you should have something like this now that you have got a collet rather than a pin to retain your propdriver)

The prop is not friction driven it is pinned onto the rear carrier.

I am sure that Jason won't be avoiding me for long 

Jo