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270 Offy

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Richard Hed

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I found this item on Ebay. I've seen them come up before, but they're usually sold before I can post a URL. Evidently, a die cast manufacturer (GMP) started producing and selling these 1/6 replicas of the Offy 270 in 2002. Production continued through 2010 and although I've never seen one, there should be a lot of them around. It seems it was one of their first products, and they made lots more. These Offys probably don't have functioning internals, but their exteriors look very realistic. - TerryView attachment 118472
How much is the bidding on it?
 

mayhugh1

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Richard Hed

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mayhugh1

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Our 100+ degree days have been lasting well into the evenings during the past several weeks, and so I've been in our house working on a design for the Offy's fuel and oil tanks.

I so much wanted to do something novel with them and ended up wasting a lot of time on a faux bell housing and gear box in which I hoped to hide them. The fuel pump provides flexibility in placing the fuel tank, but it also takes up a lot of its own space. In the end, its size and form factor created so many problems that I finally gave up the idea.

The Solidworks renderings instead show what I finally ended up with. A plain combination tank was something of a disappointment especially after putting so many hours into the gear box. I added cosmetic detail to keep it interesting, but the accompanying extra machining time will increase my chances of spoiling the part along the way.

The tank's construction began with a 3-1/2" x 3-1/2" x 10" chunk of aluminum. After squaring up a workpiece, a pair of holes were drilled through its ends and then their exits plugged. Later, when the the interior of the tank is machined, these inside holes will be tapped for hose barbs and interconnecting hoses.

The plugs immediately created problems. Since I hope to anodize the tank, the plugs' alloy need to match that of the tank. They were turned for .001" slip fits inside the holes but, even with primer, neither Loctite 620 nor 680 would cure even after 24 hours and additional heat. I knew slip-fit aluminum inside aluminum is a problematic application for Loctite, but I'd hoped the primer would kick off the curing process. My eventual solution was to make threaded plugs and secure them with red thread locker. After installation, the plugs were hard pressed into the ends of the workpiece using a hydraulic press.

All four holes would have exited inside filleted edges in the ends of the tank where the zig-zag patterns of the threads would have been visible even after the pressing operations. After the plugs were installed, the filleting was redesigned to make the holes exit through flat areas.

Since I'm not an early morning person, the tank's machining will be done during the late nights of the next week or so when the outdoor temperatures are in the low 90's. - Terry

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BaronJ

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Hi Terry,

Both the Chinese and Japanese use a lot of superglue for that kind of application ! Have you tried it instead of Loctite ?

I've had issues with getting loctite to bond aluminum, Cyanoacrylate adhesive has worked for me !
 

mayhugh1

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The first machining operations on the combo tank were on its complex underside where I was able to reuse some of the CAD/CAM developed for my Knucklehead's fuel pump enclosure. Even though the two inch long 3/8" diameter end mill used for the tanks' deep pocketing operations was pretty noisy, the surface finishes came out surprisingly nice. With daytime outside temperatures still hovering around 100F, the tank's 20+ hour machining time was spread over several late evenings.

After half an hour into the bottom side machining, I realized I'd mounted the workpiece upside down in the vise. This was a problem because of the previously cross-drilled holes. After being plugged they were no longer visible from the outside, and since my ambiguous Sharpie 'topside' label managed to confuse me, I had immediately gotten off to a bad start.

The purpose of the cross-drilled holes is to create internal passages that will eliminate the need for external hoses between the fuel tank and pump. After machining away the excess stock in the valley in between the tanks, I was able to plug three of the mis-drilled passages, but my modeling showed the fourth was going to wind up as a groove across the finished top surface of the oil tank. Since I was still in the early stages of the tank's machining, I was able to save the workpiece with some last minute design changes that reduced the tank's height.

I later ran into a problem with the limited computing resources in my ancient XP computer that I use to run my dozen year old CAM software. The tank's large and highly detailed top surfaces continually crashed the software during the tool path calculations. I eventually had to divide the model of the tank's top surfaces into several smaller pieces and keep track of the connections outside the CAM tool. This required even more design changes to the tank whose workpiece had already been roughed out.

The photos show some of the machining steps as the workpiece evolved into a finished tank. Still remaining is a bottom cover and a pair of filler caps. I've probably lost my opportunity to anodize the tank due to a poor late night decision to plug the mis-drilled holes with Loctite'd threaded steel fasteners. In any event, the tank's size would likely have been a stretch for my little home anodizing setup. Instead, I've located some gasoline resistant paint that appears to reasonably match the color of the magneto. - Terry
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dsage

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That's one heck of a nice piece of work Terry. I'm amazed at the finish. Almost looks like a stamped stainless item except for the thickness. Beautiful as usual.
 

propclock

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Always amazed! could you detail how you went from the final cnc stage to the final polished stage.
Is it just hard hand work? I know you often bead blast . Possibly blast then polish?
Thanks for all your posts.
 

mayhugh1

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Propclock,
What you see in the photos is directly off the machine. No manual clean-up. It was, in fact, the reason for the long machining times and the inadequate computing resources in the computer that did the tool path calculations. I usually set the CAM up to leave no more than a .0003" scallop and sometimes leave it at its default setting of .00016" scallop.

The bead blast step is usually just bead blasting. Sometimes there will be an issue with referencing the workpiece in multiple setups and I'll end up with a 'seam' that I'll need to manually fix. This is almost always caused by an error on my part. I just bead blasted this part a few hours ago, and it won't need any manual clean up. I was going to paint the tank but have since decided to leave its bead blasted surface as is. - Terry
 
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ddmckee54

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Terry:

Just out of idle curiosity, what was the machining time on the tank?


Don
 

mayhugh1

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After machining a bottom cover plate, the individual tanks were leak-checked. I don't yet have the proper size o-rings for the bottoms of the tanks, and so the o-ring grooves were temporarily filled with Viton cord. A machined slot across the cover's top surface between the two tanks will provide a drain for the motor and pump compartments in the event of a leak. After installing the components for the pump, the fuel tank was filled and the loop exercised for several minutes feeding the Offy's carburetor bowl. The engine control module constructed earlier was used to supply the pump's control voltage to hopefully avoid any surprises later.

After bead-blasting the tank I had second thoughts about all that red paint, but I didn't want to leave it's surface in bare metal either. Instead, I sprayed it with matte gray Gun-Kote. I've used this oven-cured paint on several other projects, and its gasoline resistance puts it on the short list of robust gas tank coatings. The filler caps were done similarly in matte black.

The engine's display stand was cut from a sheet of 1/4" hot rolled plate and painted with Rustoleum textured paint which nicely covers its beat-up surface. After a several day cure, it will become sufficiently resistant to gasoline. A functional radiator is still needed, but it will require a hole through its center for the starter shaft, and so I'm considering drill starting the engine from its rear. In the meantime, though, I plan to start work on the crankshaft. I thought I'd already ordered a chunk of Stressproof for it, but it's nowhere to be found. - Terry

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Scott_M

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Hi Terry
Awesome as always !!!
Not sure if you knew that panel mount XT60 connectors are available. Or if your design would tolerate them, they do need a bit more room and require some possibly ugly screws. But wanted to make sure you knew about them. Here is an Amazon link, but they are available from any good RC dealer.
amazon.com/XT60E-M

Scott
 

mayhugh1

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No, I wasn't aware of those Scott. Thanks! - Terry
 
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mayhugh1

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I came up with 9.7 for the quarter scale Offy's stock compression ratio which agrees favorably with the 9.5 specified in the manual. I couldn't find a head gasket in the drawings and so one wasn't included in my calculation, but Ron has mentioned solving leaks using a gasket and sealant. The .020" Teflon head gasket that I plan to use will drop my c.r. to 8.7, but I've also decided to reduce the stroke from 1.094" to 1.000" to further reduce the compression ratio to 7.8.

Crankshaft construction began by band sawing a 9-1/2" length of 1-3/4" diameter Stressproof (from Speedy Metals) which was long enough for a pair of end spigots. The spigots were faced and center-drilled, and the entire workpiece o.d. was skimmed. After moving the workpiece to the mill, a pair of vertical reference flats were machined on its ends. The workpiece was then clamped vertically on the flats and, after carefully indicating the center hole, a pair of holes were center-drilled for the crank journal turning operations. The other end was similarly prepared.

It's important that the offset center-drills be identically placed on both ends of the workpiece. The axes of the crank journals must wind up parallel to the axis of the main journals to avoid binding the pistons in their bores. These were later verified to be parallel within a thousandth or so using a dial indicator with the workpiece mounted between centers in the lathe. If needed, the spigots were long enough to give another chance or two to get it right.

Since most of the crankshaft will be machined between centers, a drive dog similar to the one constructed during my Merlin's build was made to fit the faceplates on both my lathe and the fourth axis rotary on my Tormach. A pair of setscrews snugged against protective pads on the crankshaft's flats secure it to the drive, and the drive is secured to its faceplate while monitoring the workpiece TIR with a dial indicator.

The main journals were roughed out using the mill with the workpiece supported in a vise and manually indexed in 60 degree increments leaving a minimum of .038" stock for later finishing. After returning the workpiece to the lathe, the main journals were semi-finished to .525" diameter (finished diameter will be .512"), and the workpiece was skimmed once more. The resulting TIR of the main journals as well as the rest of the workpiece was essentially zero at this point. After being left undisturbed in the lathe for a day, however, the TIR of the journals (and much of the workpiece) had increased to some .002".

The warpage was disappointing. Not all 1144 alloys behave the same, and 'Stressproof' is manufactured to minimize warpage but evidently doesn't totally eliminate it. A single-piece multi-cylinder model engine crankshaft is one of the most difficult parts to accurately machine. For my own education, I decided to carefully track all the important TIR's through each of the remaining machining steps. Out of curiosity, I left the workpiece in the lathe undisturbed for 24 hours at two different orientations to see if gravity made a difference, but nothing changed.

The workpiece was then returned to the Tormach where the rod journals were hexagonally roughed similarly to the mains but this time using a four axis indexed setup. In addition to the tailstock, the center of the workpiece was stabilized with a fabricated support.

The workpiece was returned to the lathe where the TIR's of the main journals were re-measured. They had actually improved to .001" indicating that the workpiece had moved around some more. The rod journals were semi-finished in several steps to .550" (finished diameter will be .530"). The spaces left behind by the roughed-in rod journals were temporarily filled with close-fitting metal spacers during these turning operations to prevent bending moments during the cutting operations increasing the TIR of the journal being cut. Since a spacer can't be used with the journal being cut, some runout from a real-world cutter will be expected. I'm still working to see how small I can make this. - Terry

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