Ohrndorf 5 Cylinder Radial

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The 5mm external radius fillet profile around the top perimeter was milled with a bull nose HSS cutter. This is best done while the block is solid for vise retention. I blued the surface & made some sanity witness lines beforehand. You can in-feed horizontally or vertically but I found setting the datum vertically was a bit easier on this particular cutter. Then progress in from the fixed jaw side until the radius just kisses both witness lines. About 0.050" DOC/pass. Rinse & repeat.

 

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The pocket milling was done in a few setups. I’m not sure if this is the right way or not. I find these types of profiles, especially deeper ones, a bit tricky to hit the dimensions & finish simultaneously. End mills can misbehave in confined space surrounded by their own chips. So, first I made a roughing hole on one end to drop the endmill into making an undersized roughing slot. Clearing chips is important or it can make a mess by regrinding its own swarf or sticking to the cutter.

 

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With most of the material out of the way, next was finishing the pocket profile to final dimension. I plunge milled what become the internal corner fillets to depth in one go with table locked. Then the perimeter was milled in a few passes to final depth. The same climb mill direction was repeated & I stopped a thou or so before the corner coordinate, otherwise it can get drawn into the stock.

 

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The covers were blued again to help with the corner blending from the 3 corner fillet profiles. They were hand profiled using files & finished with rotary tool rubber abrasive wheels. I thought I would be using my carbide burrs for profiling but the technique was evading me. I was always feeling like they wanted to dig in when I wasn’t paying attention. Maybe mine are too coarse or wrong tooth profile.

 

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Group shot how they looked at this stage.

 

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The link rods were made from 7075-T6, mostly for strength. These do not have bronze bushings on the ends, just the aluminum itself and 2 oil holes. Four link rods are required plus spares.

The stock was milled to correct thickness. Two hold down holes were drilled on either end which aligns to a matching fixture plate. The wristpin & bottom end pin holes were drilled & reamed to size 5mm & 6mm respectively

 

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Preserving this same setup, the sides were then milled to shape the dog bone profile. I set my table stops to the start & end points so this could be replicated. This was still my prior RF-45 mill, but I find these stops to be very useful so made the same for my newer mill

 

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A center flute was milled using a ball end mill, again using the table stop. Then part flipped to do the other side.

 

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The sacrificial ends with the temporary hold down holes were sawn off. The remaining part was relocated to a different fixture, mated to dowel pins corresponding to the wrist pin & bottom pin holes. It was secured with a small strap bar across the middle. This fixture was aligned to my rotary table, actually a tooling plate on top of the rotary table. The masking tape is to keep swarf out. Each end was milled to profile the round over ends.

 

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One more fixture to align the part for drilling oil holes on both pin ends. The holes are at an angle to one another, but also slightly offset either side of center to help promote lubrication path.

 

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Group shot after some chamfering & finishing.

 

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I made a few modifications to the master rod design. Nothing relating to important geometry or dimensions, more cosmetic fillets, profile transitions, tooling availability or where I thought machining steps might be easier. Some dimensions on the plans were a bit confusing to me. So rather than invite a machining error on what is a time-consuming part, I tried to match features to fixtures or jigs at the same time.

I already had developed some prior section views checking for an issue to beware of on radials. That none of rod elements intersect with anything, in particular the link rods with the skirts of cylinder liners. I’ve read about some builds requiring unwelcome notches having to be cut late in the game. Because it’s kind of a 3D problem related to rod length, stroke length & part dimensions rotating & displacing, it may not be easy to visualize. Handy hint if you don’t have a means to detect interference. Do a lateral cross section at the face of the rod section, not through the center (which may falsely show sufficient clearance). It’s usually the corner of the link rod that will contact the bore at first at some angle. In my case, I had to pay attention to the liner bottom chamfer as well as the corner chamfer/radius on the link rods.

 

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The master rod was made from 7075 aluminum for strength because it also supports the loads of 4 link rods and some of the cross sections there become pretty skinny. The stock was face milled to thickness as a block. All the holes were drilled and reamed where required in one setting for dimensional accuracy. You can see some holes become the internal fillets profiles between the link rod bottom end pin features. There are 2 small M2 screw holes to secure the crank pin bushing part. So, there is a front & back face to the master rod, otherwise the part is symmetrical.

 

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The stock was then reduced to form the main rod section profile. The big end used a larger diameter ball end mill vs the wrist pin end, then connected by a flat bottom end mill at their tangents. No fixture was required, just a vise stop so the part could be flipped & operation repeated on the other side.

 

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The excess material was cut away on the bandsaw. The part was aligned to a fixture using the crank & wrist pin holes & corresponding dowel pins which then set the taper angle of the main rod profile. Then it was just a matter of milling down the stock until it was same depth as the edge face of the fixture (by design). I left the full thickness of stock near on the wristpin hole end for this step so the part could securely gripped vise. The part was flipped in the fixture & operation repeated for the opposite side.

 

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The part was blued & link rod profile scribed around the holes using a simple turned profile template of the outer knuckle radius. The part supported in vise jaws by a dowel pin through the link rod hole & excess stock was milled away by eye. At this point I thought I could finish the profile by hand filing using steel guides on either side. This was a bit fussy, at least given my filing skills, so I switched to a milling method using a simple turned fixture held in the rotary table. I didn't want the part to snatch given the way it was clamped & variable excess material, so I stayed about 0.010" outside the radius & crept down on 0.050" increments. The tangent marks were scribed beforehand so e blue so on final pass did the full cleanup. The knuckle OD profile was then milled away full width in one go. I left just a bit of material to be hand blended where the 2 curves meet one another.

 

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The master rod has a center slot to accommodates clearance for the link rod bottom ends. I turned a fixture to be used in the rotary table. It mates the crankpin hole with a snug fit & the part is secured by a single screw. I was a bit concerned the part might slip on the fixture rotationally because of the interrupted cutting, so it was re-checked a few times during milling. Fortunately, it stayed put. I have learned not to trust my 3-jaw chuck for things requiring close tolerances & this part qualifies, so the 4-jaw independent chuck was used to dial in the fixture.

The part was first datum referenced angularly using 2 link rod pin holes square to the table as shown. From that the recess was rough milled in the center by progressive cutting depths. Then the inside face of the recess was climb milled to final dimension & also meeting the terminal radius fillets on either end. One thing I failed to consider is that EM’s have a slight corner radius and bottom relief, so slight machining variations show up in the clearance slot depending on its angle & offset position. My upper link rods were binding a bit until I realized what was going on. The bluing makes it look worse but it took some careful hand filing & scraping to get the inside corner sharp without mucking up the faces. So, I recommend having the link rods done first or at least a representative tester.

 

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The wrist pin round over profile was similarly done by bluing & milling & finishing using steel guides.

 

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The oil passage holes were drilled into the wrist pin hole. During fitting, if any final lapping was required on holes, usually more related to de-burring, these Acro-Lap tools worked well.

 

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The link rod pins were made from O1 drill rod. Other than being center drilled through, similar method of lapping them to dimension using the wet dry abrasive paper method. They were hardened as well although I’m not sure it’s required running in aluminum. The trick is they have to be a nice sliding fit within the link rod, but also a reasonably snug fit within the mater rod. So, you have to kind of pick specific your specific reamer.