Ohrndorf 5 Cylinder Radial

[petertha]

The gear idler shaft is made from 5mm O1 tool steel. The end has a M2.5 threaded hole for flathead screw that retains a brass end washer in position. Including some pictures of one of my (many) lapping trials. My experience with drill rod is that is always within the stated tolerance, but is often eccentric in cross section. So, the purpose of lapping here is to bring it to size with appropriate finish, but also make it circular section. Here I have a steel clamp with a thumb screw tightening nut. It holds a sacrificial aluminum lap, slit through & also some internal relief slots made with a jeweler blade in scroll saw. I have a selection of inexpensive (AliExpress/Ebay) diamond lapping compound of graduated grits. The method worked reasonably well. But I have subsequently come up with an easier, less messy tool which is now my go-to method. I’ll show that tool a bit later.

After lapping & parting off, I torch heated & oil quenched. Then into the toaster oven to tan brown. I discovered it is sufficiently hard because I discovered the countersink was a teeny bit shallow & had to grind it bit to deepen, because my HSS tool just rubbed it. All good, everything seems to run smooth.

 

[petertha]

Cam Plates. I only have limited hardening experience with O1 tool steel & that was confined to relatively simple parts using a torch. I don’t have heat treating equipment, but I discovered a local fellow who does heat treating for knifemaker community. He has all the appropriate equipment & experience with the many flavors of air quench blade steels. Considering the work that would go into producing the cams & the form factor, I had visions of it distorting into a Pringle chip, or cracking across the internal holes. I figured successfully heat-treating knife blades would present a tougher challenge than my cam plates. So, I sourced the (Starrett brand) A2 from my local KBC dealer, choosing a bit thicker Imperial stock which had to be thinned to prescribed metric dimensions.

I made a simple aluminum fixture puck for the lathe with 2 threaded holes. After a facing the puck face true, the A2 stock was mounted with matching screw holes & brought to thickness. The ID was rough bored with annular cutter then finished with boring bar.

 

[petertha]

The cam outline was band sawed roughly to outline but leaving 2 sacrificial ears with the original screw holes, which now served second duty to secure the part to the mill fixture. The ears correspond to where the cam lobes will occur. The lobe profiles are identical shape between intake & exhaust cams, but the four M3 mounting holes are angularly phased different to each other to achieve proper timing. One plate has M3 clearance holes, the other plate holes are threaded. Therefore, the cams don’t lend themselves to be stacked together to make both intake/exhaust plates simultaneously.

The rotary table was first zeroed to the quill center. Then the fixture assembly was positioned concentrically on the ID hole with DTI & also along an edge of rectangular jig plate. Now the M3 holes could be drilled as well as the array of larger holes. These holes were a bit of foresight on my part relating to the same possibility that mist lubrication might be in my future, because drilling these holes after the cams were hardened would be very difficult. So, I came up with a CAD pattern which I could also replicate on the ring gear cup which the plates mount to & this would allow mist lubrication to flow through from rear to front. I was a bit concerned these holes would be great places for the cam to crack during heat treating but it turned out OK. Actually, the plans called for larger holes & non-symmetric spacing so it was a bit of faith.

I used an endmill & rotary table to cut the main profile, which is the valve closed, non-action surface. Overall, it went according to plan. Just have to be careful about entering & exiting the cut accounting for RT direction & backlash. The lobe ramp profile shape was created by the radius defined by the EM diameter as per the plans. You can see I have a small Sherline 4” RT clamped in my mill vise bolted to an intermediary plate held in my main 6” vise. I feel the RT was accurate enough but found myself doing light cuts because I could feel the cutting action on the handwheel. Next time I would use my larger RT which is a bit more solid.

 

[petertha]

The cam part was released from the mill fixture & ears band sawed off. Now I could transfer to another lathe fixture, this time with a boss which fit the cam hole ID & retained with screws through the holes. Now I could turn the cam lobe OD which is the valve open segment. That just left a small radius to blend the ramp segment to the open segment which was done by hand. I didn’t take a picture but basically, I blued the part, scribed a line using a radius gage tangent to both surfaces & filed it to shape. That left finishing the outer profile with rubber abrasive in a Dremel.

 

[petertha]

Once the cams were finished, I shipped them to the heat treat person along with some sacrificial coupons. A few weeks later they arrived back by mail. He verified the hardness & came as you see here. There was negligible distortion. They fit the ring cup the same way. The bolt threads engaged nicely so I was happy.

 

[petertha]

Some partial assembly pics.

 

[minh-thanh]

Thanks for sharing !
Does your engine pump oil out of the crankcase ?

 

[petertha]

Minh Thanh, no there is no physical oil pump in this engine. Oil is pre-mixed with the (methanol) fuel like a typical RC engine. Intake charge (mist) enters from the carb/intake manifold at rear of engine. The mist wets the rotating master rods & link rods within the crankcase before getting sucked into each induction pipe to the head intake port. Also some mist continues forward into the nose case through the various passage holes in the ring cup & cam plates, which is intended to lubricate those parts. There will be a drain nipple at the bottom of the crankcase to allow any pooled residual oil to be vented. At least that is what I have seen on commercial radials like the OS. The concern there is accumulation of oil or fuel residue risking hydraulic lock of lower cylinders.

You might be thinking of the Edwards radial engine which is also methanol fueled glow ignition but has a dedicated pump that circulates oil to various internal components & also drains from a lower collection sump. It is actuated off a crankshaft lobe. Personally I think that is a better system, but it is incorporated into the original design. It would not be an easy add-on to this engine. Having said that, there appear to be many successful multi-cylinder glow engines without pump. I think in the methanol pre-mix engines, wherever mist can circulate internally is likely to be oily by circulation/blowby alone. Even up to the valves. At least that's the hope! LOL

 

[petertha]

The 40-tooth module-1 steel ring gear blank I purchased needed to be reduced in diameter & also in thickness. I purchased 2 gears just in case, but managed to get it completed on the first try. I first made a lathe fixture with a shoulder boss sized to tightly fit the tooth crowns. With this boss feature turned, the gear was positioned to preserve concentricity. It was held with CA glue on the fixture back face a clamp plate sandwiching the gear to the fixture with a cap screw. It held sufficiently tight & the material machined quite nicely. I was able to turn the OD to size by eventually cutting both the gear & fixture cap until there was some remnant gears to trim off. Maybe there was a better machining sequence to accomplish this, but the end result doesn’t leave much of a gear one way or another. It worked out in the end.

 

[petertha]

The ring gear cup is made from 2024 aluminum. It is supported on the crankshaft with the 2 smaller intermediary bearings. Once the timing is set, the ring gear is permanently bonded to the inside cup lip with Loctite. The cup has 4 countersunk holes for the M3 flathead screws to secure the cam plates. The cup was subsequently drilled with lubrication mist passage holes that match the holes in the cam plates so that lubrication mist can frow from crankcase into nose case & hopefully wetting everything in it’s path with oil film. Aside from careful turning to match all the fit tolerances, it was pretty straightforward machining.

 

[petertha]

Ring gear cup machining.

 

[petertha]

Subsequent modifications to add mist passage holes.

 

[petertha]

Before leaving the construction aspect of gears & cams for now, this might be a good opportunity to discuss the engine timing. The O5 plans provide all the necessary dimensional information to construct the cam drive train, but the specification sheet did not express inlet/exhaust timing in terms relative to piston TDC/BDC. I’m not sure why some engine designers omit this information, but it is what it is. The instructions are also somewhat ‘abbreviated’ in terms of how to set the timing. Translation from another language probably doesn’t help matters. So, I wanted a firmer grasp of this stuff & also understand how the O5 timing compares to other 4-stroke (methanol/glow) model engines. Not that I felt qualified to modify it, but more for the sake of interest & future projects as well.

What follows is not intended as a detailed How-To. More of an overview of how I stumbled my way through this timing aspect, which is kind of a reverse engineering process starting with the parts drawings & references. Hopefully this will be of value to others.

First, one needs to know the gear ratio between the crankshaft (CS) & cam plate. As mentioned, the O5 planetary gear ratio is 4:1 which comes as a result of each gear-to-gear tooth count in the train from CS to idler cluster to ring gear. The rotational direction is also important. The O5 cams rotate in the opposite direction of the CS as shown by the sketch looking at the engine from the front.

 

[petertha]

One needs to understand the intake & exhaust cams relative to each like a sub-assembly. The exhaust cam is to the front, intake to the rear, specifically orientated to one another with index bolt holes. Each cam pushes on its own dedicated lifter & respective pushrod / valve rocker. It is also important to be aware of the lifter geometry relative to the overall assembly. The sketch shows the O5 lifter action (red lines) extending radially from the CS center. Therefore, the cam contacts are occurring at different clock positions relative to the cylinder engine datum. This is important because other radials may orient their lifters differently. For example, if the lifter axis were coincident to the cylinder center (purple line) then the inlet/exhaust timing relative to TDC/BDC would be different on the exact same cam plate, all other things equal.

 

[petertha]

This sketch is a bit busy, but shows how I then superimposed the lifter reference lines on the cam assembly. Then I determined the corresponding rotation angles marking the beginning & end of each lobe event which correspond to intake open, intake close, exhaust open, exhaust close.

 

[petertha]

These numerical values were input into a homebrew spreadsheet from which I could calculate timing metrics in more familiar terms relative to TDC & BDC. I also determined valve overlap, lobe separation angle & made a plot to better visualize things.

 

[petertha]

I recently found this website which is an excellent resource for model engines Sceptre Flight

What is particularly useful is the library of past engine review articles from magazines & other sources back in the day. Some reviews were very detailed. They disassembled, measured & photographed parts & assemblies & bench tested engines to provide useful power & rpm information. So just for the sake of a gut check comparison to my radial, I limited the data extraction to O.S. 4-stroke engines, although other brands are also represented. O.S. are generally considered to be reliable, powerful sport engines & encompass a wide range of displacements & layout’s including multi-cylinders. Of course, many design factors influence resultant engine timing which is outside the scope of this post. I have also been adding a few engines of interest here & there so don’t read too much into the individual data points. It’s kind of a work in progress. You can see the O5 timing as it relates to other engines. The overlap is relatively narrow & (I think) the values suggest conservative timing, but I’ll leave that for you to decide.

 

[petertha]

Also, because there have been a few builds of the Edwards 5-cylinder engine posted on the forum & the two engines are similar in many respects, I did a timing plot overlay for comparison. If anyone spots any errors along the way, please let me know.

 

[awake]

You are doing marvelous work - thanks for the detailed discussion of how you are solving the questions and problems that emerge!

 

[petertha]

The nose case was machined from a round of 6061-T6 aluminum. I can’t recall if I chose incorrectly from my intention to use 2024 or it was my subconscious saying ‘odds favor a mess up somewhere along the way’. I decided to machine the outside profile first, then flip the part around to do the inside surfaces. This seemed like a better way to grip the part for ID hogging & hopefully concentricity on internal features.

The front was first drilled for the crankshaft clearance. Then a recess feature counterbored for the front bearing, which is a light press fit. The section profile contour was defined in the plans by various blended radii. I generated a series of corresponding X,Y intercept dimensions in a shop drawing. I cut these stepover terraces with a parting tool & then blued the surface. Then finished the surface with file & sandpaper until the blue was gone & finished off with a 3M pad. I left the part this way still held in the chuck & transferred to bandsaw vise where it was lopped off.