Ohrndorf 5 Cylinder Radial

[petertha]

I used the thickest section boring bar with carbide insert & bored to 0.940” ID. This lands me with 0.005" left to remove for target 0.945" finish bore.

 

[petertha]

I rough turned the main OD slightly oversize, then did the upper liner features. The crown is an extended lip with an undercut so the liner registers onto the cylinder top deck. Then I used some homebrew sanding sticks made from 2" wide MDF boards with wet-o-dry paper bonded with 3M spray adhesive. I found this to be an expedient way to remove the turning grooves & work the material down to size in a controlled manner. The board width spans the entire liner length which helps correct & minimize undulations that can result from traversing a narrower abrasive belt strip back & forth because the dwell time & tension can be different across the length. For reference the OS-56 liner was within a tenth OD all along the surface.

 

[petertha]

The final 0.0010-0.0015" came off with a homebrew OD lapping tool of sorts. This was kind of an experimental venture driven by how much time it took me to achieve acceptable surfaces on my prior test liners. I hoped these generic lapping blanks could be utilized used for this job & future projects if they worked out. Commercial tools are available but they are expensive, especially in larger sizes.

The idea was to have some aluminum blanks cut with the radial pie segment slit pattern you see. Then they could be bored out to the requisite ID & either lap directly on the bored surface, or perhaps in conjunction with a sacrificial split lapping collar to get more utility out of the bore size, kind of like how a collet grips a part. I made a CAD drawing & send it to a waterjet outfit. They cut the slit profile including end holes leaving me to drill & tap for a cross screw for setting lap pressure. I tried different lapping compounds & settled on some cheapo oil based AliExpress diamond paste that comes in a syringe tube.

I guess I could say my lapping tool worked out OK in that any diameter undulations (hilltops) are worked down & the diameter can become quite consistent. But I find lapping to be messy business & best confined to removing small material thickness. It also requires a bit of hand technique like stroke & dwell time & re-charging the lapping goop & fiddling with the clamp tension. Then you have to clean everything spotless, measure at various spots down the length & go at it again. It all takes time. I left the last 0.0005" for 1000 & 1200 paper using my full width backing boards & in all honesty might be just as quick as lapping. Eventually I arrived there. It’s Interesting how CI can get a finish, eh?

 

[petertha]

The skirt ID has a shallow chamfer to provide clearance for the rod motion. One last check of finish dimensions, lightly knock down any corner edges, then part off the liner. Then apply a single layer of protection tape, hold reversed in collet chuck & face the lip surface to final length.

 

[petertha]

Onto finishing the bores. A while back I acquired a Themac tool post grinder (TPG) grinder & played around with it on my previous test liners. I now have mixed feelings about TPG’s. I suppose it suites this kind of application where relatively short length work sticks out of the headstock cantilever mode. Accurate surface dimension & finish combinations can be achieved with the right technique. If the work involves hardened materials, grinding may be the only way to go, but I don’t consider CI to fall in this class. I also had hopes of using TPG for several other applications & that’s where some shortcomings & complications become apparent, especially if the project involves tailstock support. The TPG assembly just wants to occupy the same real estate as TS assembly which really is difficult to work around. I also had a tough time finding suitable wheels, meaning the right diameter & grit & composition combination. I ended up buying a surface grinder wheel from KBC & had a water jet cutter make me a bunch of wheel blanks. I was a bit apprehensive about them blowing up but so far so good. Dumore is another popular name. I can’t speak for their wheel arbors but the Themac has an oddball taper that doesn’t match any common taper, seems to be unique to them. Ultimately, I figured it out, but suffice to say making your own arbors is more work.

Most of the time you will see a TPG lathe setup where people pre-align the lathe compound at a shallow angle & finely increment cross travel depth that way. The magic trig number for convenience is 5.739 degrees compound angle (off of spindle axis) which yields the relationship of 0.001” increment on compound dial equates to 0.0001” of cross bed travel, times 2 equals 0.0002” in bore increment. Just remember the same rules of removing backlash apply. And this is not exact because unlike a cutting tool, the wheel is slowly eroding in diameter as grinding proceeds. So you have to measure & re-calibrate more so than other lathe operations.

My compound leadscrew is in pretty good shape. But when I secure the compound dovetail between each pass, the clamping action itself can drift the actual position a bit. I’ve made some improvements to the lock, but it’s still there. So, I don’t completely trust this setup on my machine although I do like the fine incremental feed aspect. Alternatively, I made a fixture to hold a tenth’s reading dial gauge to bear against the end of the cross slide itself which directly measures Y infeed displacement that way. This removes both backlash & table lock issues simultaneously. A sensitive dial gage also acts as a sober indicator of machine vibration which is flowing through to the grinding wheel. The needle fluctuates on either side of the true reading. Ideally, deflection is low & somewhat dependent on where the indicator is mounted & how things are tightened down. I think the TPG is probably approaching the limits of my lathe rigidity & spindle bearing condition. If you don’t have dovetail locks, my opinion is that TPG might be the wrong weapon of choice because once the motor winds up, the sliding surfaces can become ‘buzzy’ & prone to free floating, even with a good quality TPG. A suitable DRO can measure displacement independent of dials so long as you have the right resolution. But consider even typical 0.0005” step increment on DRO display represents 0.001” of bore gone. That’s what made me a believer in a mounted analog indicator. So, after some trials I kind of considered the TPG as a ‘truing’ tool, not a finished bore tool. At least on my lathe & limited experience. That leaves the last thou or more for lapping but TPG is still a time saver. Whether this warrants the expense & setup of TPG is probably another discussion. So, who knows, it may get traded for a TIG welder one day which costs about as much & would see a lot more use in my shop, but that’s another story. What I REALLY need is a Sunnen hone LOL.

Back to liner grinding. I plugged off the back end of my collet chuck so grinding debris would not migrate in behind. I used a single wrap of tape on the liner OD for protection & gripped it in 5C collet chuck.

The wheel was dressed in-situ with a homebrew diamond tool holder. Cover the machine & use a vacuum for this operation please. Then it was a matter of selecting a low spindle RPM, power feed the TPG on low traverse setting & start opening the bore in small ~0.0005” bore increments. It is a somewhat satisfying experience to see partial patches being removed under grinding, meaning non circular sections features you thought were quite true by a boring bar alone. The liner never got warm because of low DOC & more time measuring & futzing. I tried a fluid but t didn seem to be adding any value, the wheel looked clean at these removal rates. The bore finish was ‘ok’ not stunning. The geometry & roughness seemed acceptable but I think it exhibited skip or maybe secondary vibration. I’ve been told my grit selection was too fine, that actually coarser would be better. Grinding is a science in itself. But they popped off pretty quickly. I have been making 6 cylinders, 1 guinea pig & hopefully 5 keepers.

 

[petertha]

TPG grinding examples

 

[petertha]

At this point I was ready to lap the liners using a brass Acro brand lapping tool and a tenths reading bore gauge. I used my OS-56 liner as a glorified bore gage setting tool.

 

[petertha]

Many hours later of mind-numbing work I had 6 liners to a nice matt finish & within a tenth. And all this was basically a warm up run.

 

[petertha]

Now we arrive at the nasty bit. I put a cylinder in the oven at 450F for a set time & dropped in an ambient liner with no drama. But as mentioned previously, once the assembly cooled, the bore had reduced differentially. About +0.0015” near the top & ~ 0.0005” near the skirt. So not only a significant bore diameter change, but the barrel walls were no longer parallel either. I returned the assembly to the oven & repeated what I’d done a few times already on the tester; heat up & separate to evaluate what was going on. Well, this time even a light love tap after heating was not removing them. This always worked with my test cylinder.

When I carefully remeasured all the cylinders, I had more questions than answers. Maybe because the new cylinder design had more mass. Maybe the reaming was not quite as perfect as I imagined. Maybe the cylinders relaxed a little post-machining because I could measure as much as 0.0005” oval in spots. Or maybe there are micro undulations in the surfaces kind of acting like a screw thread where the mean distance between hill tops is correct, but the surface itself can act like a secondary grip once the shrink has occurred? I decided I wanted these parts separated to be utilized so had to resort to light torch heat. They finally let go. The liner came out a light tan color but amazingly neither seemed worse for wear. They had the same dimensions as when they started out.

 

[petertha]

At this point it was time to weigh my options. I already sunk some effort making the liner OD’s nice & consistent between one another. But the interference just seemed a bit excessive now. It was obviously shrinking the bores quite a bit. Had the liner bores remained unfinished at this stage, it would be of no real consequence, I could have just carried on. I no longer saw much merit pursuing the ability to swap in a replacement liner at some later time because I had already decided to make a complete 6th cylinder assembly spare to just bolt on the crankcase if something bad happened.

I tried mounting my test liner to an offshore expanding arbor held between centers in the lathe to see how easy it would be to somehow lap off a thou in a controlled manner. I’m not sure if I received a Monday arbor but it did not turn concentrically. I put a dial on the OD & it had about 0.002” TIR. I didn’t need more bad geometry problems so abandoned that idea. Glad I didn’t buy a complete arbor set!

So, I bought another Acro brass lap barrel to slightly enlarge the cylinder ID, thus reducing the amount of liner interference & simultaneously tuning up the surface geometry & finish. Hopefully I could use the liners as they were & reduce the amount of subsequent bore correction once shrunk again. Kind of bass-akwards to the original plan but seemed like the best option. This worked quite well. I haven’t lapped aluminum like this before with brass but it yielded a nice light matt finish & the bore gage said it was consistent down the length.

 

[petertha]

Some trial & error futzing, eventually I arrived at combination where the liners could be re-inserted & the bores were undersized in the 0.0005 – 0.0010” range.

 

[petertha]

So once again, the liner bores were re-lapped to target ID, this time as a mated assembly. I will now call these good. I had the bright idea to submerge the assemblies in my ultrasonic cleaner again to remove trace lapping compound. But very shortly thereafter I noticed some corrosion stains starting to form on the liner surface, so I immediately pulled them out. Swabbing & rinsing with mineral spirits is a better way to go. Then a light coat of oil for storage.

Next engine I would leave the liner bores unfinished (undersized) at the boring bar stage. Some of the interference surface conditioning could probably be minimized, particularly if there was no need to pull & replace the liner. The cylinder assembly would now have to be jigged so that the grinding / lapping / honing operation could proceeded on the mated surfaces.

 

[cwelkie]

You are a very patient fellow with results to prove it!
A lot of hours but you are achieving your expectations and standards.

 

[josodl1953]

Hello Peter,
I wonder what the point is of a heavy interference fit between cylinder barrel and liner. I think the main objective is a good heat transfer between the two parts. I gave my Edwards barrels and liners only a light interference fit and had very little deformation issues with these parts.

Jos

 

[petertha]

Hi Jos. The short answer is that I agree with you. I don't think there is an advantage to excessive interference, in fact several potential disadvantages. For example, in my case where the cylinder shape is tapered & thicker wall near the top can contribute to different shrink force on upper vs. lower liner. Pretty much every RC engine I have disassembled requires oven heat to install & remove the liner, even when brand new.

My initial heat testing was based on a separate spare test cylinder which was the original design, not my subsequent modified design. But I think either reamed ID surfaces were not as consistent, or possibly they stress relieved a bit. In this kind of application, a half thou one way or another seems to make a big difference.

My longwinded story was just to say in hindsight I would not bother grinding & lapping the liner bore until mated to cylinder & completely stabilized. Unless you have good shop methods to very tightly control both OD & ID dimensions & finishes. Jung provides these instructions in his 5-cylinder engine which is probably not far off how mine ended up after lapping the cylinder ID (0.02mm = 0.0008”).

To ensure optimum heat transfer, the cylinder liners must be shrunk into the cylinders. The inner diameter of the cylinder is about 0.02 mm smaller than the respective outer diameter of the liners to unscrew. After uniform heating of the aluminum cylinder by means of gas burner or hot plate (to about 200 ° C), the cold liners are inserted into the cylinder.

 

[petertha]

Carrying on with what surely must be the most drawn out & convoluted engine build post in the history of model engineering, now onto the head assembly. I hope I can accurately recall what I did at the time. Here goes.

I suspect heads are one of the more challenging build aspects of most 4-stroke engines. Maybe because they are the intersection point where so many important & inter-related components must come together. Internally are the valves, valve cages, port passages. Externally the valve spring assembly, rocker assembly, ignition plug, cooling fins, INT/EXH piping connection, mounting holes & fasteners. There is a lot to get right. This is also where many important tolerances, clearances & motions occur.

The reason I mention this is because I tried to stick with the original design intent for the most part, but there were several mods along the way. I pretty much had to make prototypes of most parts beforehand. With heads, a change in one component inevitably has a domino effect to other things. To name a few, the O9 had wider valve seats than what I preferred. So that changed the valve cage, its placement in the head & valve itself. The threaded portion of INT/EXH ports were quite short. Very few threads for tubing couplers which was another motivation to increase the head diameter slightly. Plans are metric & which included tubing sizes I could not get. I found a few (minor) dimensional errors only by transcribing the drawings into 3D CAD. So, what you will see here is more or less the end results for each the various components. Some will also appear out of order construction wise.

 

[petertha]

Here are some CAD assembly pics of where this is going.

The heads are made from 2024 aluminum (or was it 6061, sheesh!). First operation was in the lathe. Turning the blank OD, turning a lip boss which fits snugly within the upper liner ID, the hemispherical combustion chamber profile and the radial cooling fins all in one setup. I acquired a used Radii internal/external spherical turning tool which has come in handy, but it’s a bit fiddly to set up. The combustion chamber bowl profile has a specific radius & depth combination which needs to be dimensionally correct to achieve target CR. After turning, the head blank was parted off & finished to length. After several prototypes leading up this point, I made 7 heads ‘production’ heads. 5 for the engine, one as new spare and one sacrificial spare to commence each operation.

 

[petertha]

Next operation was in the mill. I first drilled the 5x clearance hole pattern for M3 hold down screws. One hole occurs at different counterbore depth as a function of head orientation. I made an angle fixture/jig which was used for several subsequent operations as well. Alignment of head to the fixture depends on the combination of the cylinder lip mating the fixture’s center hole and constrained radially by the 5 hold down bolts. Because the heads are in & out of this fixture, this normally may not be an optimal method to align things because of slight bolt/hole clearance. But with all 5 bolts in place & the collective machining deviations, I found they the head didn’t have much if any free movement at all, so called it good.

The glow plug hole, counterbore & thread was done in one operation. Once I figured out how much to offset the hole from the head edge, all the heads were done in one setting. Then remove the head, replace with another, preserve the setting & repeat.

Next was milling the facet faces which become the area to which the rockers boxes are mounted. The valves will exit perpendicular to this face. Once the depth was established, it was just a matter of lopping off the material with a face mill. The fixture allowed me to rotate the head blank 180-deg & repeat left & right side.

 

[petertha]

At this point, I had already made the bronze valve cages and valves in advance so that when it came to drilling the valve cage hole, the target diameter & bore depth plan was fully defined.

I was aware of a few different paths to take when installing valve cages/seats. Some press or shrink the cage into the head with interference fit & valve seat is subsequently profiled. Some strive to have valve/cage sets fully meeting seal test outside of the head (usually by vacuum test), then slip fit the cage into the head Loctite to try & maintain the seal condition as much as possible avoiding any distortion. That was actually my initial plan too, but for unanticipated reasons explained later I ended up doing the seat cutting once in the head. There is only about 0.010” of seat contact chamfer to be removed from the cage lip so it’s not like a lot of material. The risk of course is that if you screw up the seat & cannot get seal or lap your way out of the problem, because the cage would have to be torched out in order to be replaced, hopefully leaving the head in decent shape.

I bored the valve cage hole from the combustion chamber side, out through the top side facet using the same angle fixture, but now inverted. I worked out the required offset enter distance in CAD using the jig edge itself as reference. First a smaller hole was drilled through the head, this mates to the valve guide segment of cage. Then the larger counterbore for the valve chamber body. It was a bit tricky to establish this bore depth because of the curved hemi shape and tool changes. It has to be a flat bottom hole so that when the cage is inserted & bottomed out, the seated valve ends up just beneath a flush position, conforming to the hemi shape as much as possible. For that I undersize predrilled most of the material away and then used a flat bottom end mil to make the hole ID and to target depth, pre-referencing the EM off the fixture as a datum for repeatability

Things went mostly to plan but somewhere between the hole ID, cage OD & surface finish/dimensions I had a few with slightly tighter feeling fit on the cage. Still within the Loctite gap allowance, but mostly I didn’t want to risk having them hang up during glue assembly because bronze triggers accelerate the Loctite cure significantly faster than steel alloys. End mills are not reamers so maybe that should have seen anticipated. Possibly using a boring head might have assisted here, but it seemed like more work & still leaves the issue of a flat bottom hole requirement unless one has an automatic (Wohlhaupter style) head. The end mill was a special order (metric flat bottom) style. In the end I made a simple lapping tool to condition the hole & that worked well.

The cages were glued with Loctite 680 high temp retaining compound. Lastly, I should mention that aside from the Loctite, there is no other mechanical retention of the cages in the head. I did consider pinning them or threading them but I just couldn’t see a good method I could accommodate or successfully pull off with the dimensional constraints. I did a torch test on a dummy assembly & the cage stayed mated, so I guess time will tell.

 

[petertha]

The heads & fixture was again flipped to do operations on the facet side of head. The valve hole is first located & centered, then counterbored to accommodate the valve spring body. Then holes drilled & threaded for mounting rocker cages.

A flat fixture was used to hold the head to mill the center groove using a ball EM. Then flipped on its side for sawing the vertical cooling fins which are at various progressive depths. I used a 0.045" slitting saw ~ 0.200" deep about 0.025" DOC per pass. About 200 rpm felt right on 3" OD cutter. I’m still not super confident with this operation. I kept a steady feed rate, used lots of WD-40, cleaned the teeth & trench of swarf between passes because there is very little clearance & aluminum is a gummy material. I've read horror stories where guys folded up the cutter & destroyed the part on the proverbial 'last pass'. At this stage a lot of work has gone into the part.

Lots of edge deburring & cleanup. I use these cheapo rubber abrasives in the Dremel, they work quite well. This pic shows the same tool held in a needle file handle for hand work.