Ohrndorf 5 Cylinder Radial

[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]

(Note, I posted a variation of this information elsewhere on the forum, so if it looks familiar, that's why)

I recently found this website which is an excellent resource for model engines http://sceptreflight.com/
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.

[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 & 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 in the chuck & transferred to bandsaw vise where it was lopped off.

[petertha]

The plans had some internal pocket relief (milling) features & contouring around the lifter area which I stared at for a long time. It looked very nice & it yielded a slightly thinner case section in certain areas. But to my eye actually seemed a bit thin around the front mounting bolt head recesses. So, I opted to modify the section profile a bit so that all the internal surfaces could be done in the lathe in one setting as a series of steps. This provided a bit more meat around the bolt heads, but same annular thickness around the bushing radial holes. It cost some grams of aluminum which I didn’t really care about anyways but erred on the side of bit more strength since it was 6061 & not 2024. I also wasn’t entirely clear about how the pushrod tubes were being retained on the conical lifter bushings, but trusted the plans for now.

[petertha]

Next step was to chuck the part for the rear side internal cavity work. A spacer was positioned between the chuck face & nose as a mechanical stop. I wrapped some tape on the OD surface to protect it from jaw bite. The DTI said my 3-jaw was accurate, but in instances where required, an extra layer of tape under a jaw allows you to micro-shim. In hindsight the 4-jaw chuck would have been a better choice on 2 counts. You can dial it in exactly & also it provides one more gripping surface. This can be an issue on thin-walled parts where high initial gronk when the part is solid can result in slight deviations when the internal surface is machined out. Another thing I have subsequently learned is that not all tape adhesives play well with cutting fluids. They can dissolve, become unglued & I suppose risk the jaw grip.
I now favor an aluminum tape for protection applications like this. It also works well in shimming applications where its preferrable to pre-attach the material. I think this tape is used for furnace duct work.

So, after some material hogging, I just had to be careful as I approached the ID lip surface which becomes quite thin & must fit the gear plate OD properly c/w with its O-ring in place. It’s important to let the part cool to room temperature & take spring passes at different feed settings.

[petertha]

Some interim assembly testing pictures

[petertha]

Next step was to make a fixture to hold the nose case by the ID lip so that the part could be gripped in a rotary table. It has a threaded hole for a retention stud. After some trial fit-ups that were a bit too tight & concerning moments where the parts were firmly stuck, I put a smear of anti-seize on the surface.

The fixture/part assembly was gripped in a RT & 4-jaw chuck & 5 bolt holes drilled & counterbored. Now the RT was flipped upright. I used a parallel & DTI to register what is equivalent to horizontal reference off 2 bolt holes & then proceeded to drill & recess the holes for the lifter bushings. These are offset on either side of center nose case center & also offset fore & aft corresponding to the respective cam plates positions.

[bent]

Nice work Peter!

[Craig DeShong]

Hi Peter

I’ve been following along; your radial and my rotary share many similarities.  I especially enjoyed your discussion regarding the cam disks.

Your work is superb.

[petertha]

Thanks for the kind words.

The lifters (or maybe they are called tappets, I’m never sure) are made from nominal 3mm O1 tool steel. The internal end is a dome shape which runs along the cam plate profile. The external end has a partial depth 2mm OD spherical socket seat which mates the ball ended pushrods. The lifters slide up & down within bronze valve guides. So, I made some test guides first so that the drilled & reamed bores could be used as guides for lapping the lifter stock OD.

The male dome profile was formed by a ball turner accessory. Then the part was flipped in the collet, trimmed to length & the female radius profile made with a ball end mill. These lifters were sent at the same time to the same heat treat guy who did the cams so I wanted to get the finish as good as possible now. The lifters would have been easy enough to heat treat with a torch to ‘some’ level of hardness but I wanted them to be a few points softer than the resultant cam plate hardness so as to preferentially wear the (easier to replicate) lifters. I didn’t trust my repurposed toaster oven to deliver accurate temperature for tempering. The male end was polished by gripping in my Dremel chuck & lightly running in a shallow well drilled in MDF wood with a smear of compound.

[petertha]

The lifter guides are made from bronze as per plans. The turning profile is pretty straightforward. I used my 5C collet chuck & sharp, uncoated insert like I use on aluminum. The holes were drilled & reamed. Its interesting when you make a bunch of the same parts, you gain an appreciation for variation. Some lifters slid nicely in the guides as planned, others felt a bit scratchy on entry or exit. The bore looked good. Turns out my handheld hole chamfer gizmo was leaving a micro burr, so I used a small polishing point to dress the edge.

The lifter guides will eventually get bonded into counterbored holes in the crankcase with Loctite. The conical shape is intended to accommodate the ID of the pushrod cover tubes which meet the guides at a mild 3D angle relative to the lifter axis. At this point I’m not really crazy about the metal on metal contact & mitered end. I would prefer some kind of rubberized or silicone material between tube & cone or somehow making a union. The plans call for the tubes being retained in the rocker perch with a small lateral set screw. I have some hopefully better ideas to test but don’t yet have a clear game plan. So, I have deferred this for now & maybe some divine inspiration will occur closer to final assembly. If I have to re-make the guides m, it’s not a big deal.

[Roger B]

Looking good  Radial engine cam rings and valve gear looks an interesting topic