Another Knucklehead Build

[kuhncw]

Terry,

I just received some 7/16 C544 bar from Online metals. The bar has the spiral marking I'm used to seeing on 660. The 544 from Online Metals measures just about exactly 0.437. I've always found 660 bar to be oversize. For example, my 3/8 660 measures 0.410. Did your C544 from McMaster measure 0.75 or was it oversize?

Thanks.

Chuck

 

[petertha]

I'll have to look at my OLM 544 & 660. But what is the significance of the spiral marking?

 

[mayhugh1]

Chuck,
The rod stock that I received from both MSC and McMaster-Carr were, in fact, .002" undersize.

Peter,
I'm not sure what that spiral coloring is.

Terry

 

[mu38&Bg#]

Spiraling like that on bronze is continuous cast.

 

[e.picler]

Edi,
I'm using Solidworks2010 for CAD and Sprutcam7 for CAM. Sprutcam is distributed in the US by Tormach. - Terry

Thanks Tarry!
I will start using Sprutcam very soon. There is a Tormach here in Brazil. I will have the training a couple weeks. They will also supply the Post Processor for the milling machine and the lathe. They told me that the Sprucam is easier to use then MasterCam.

Edi

 

[mayhugh1]

I next worked on the rocker shafts since their fits in the head assemblies will provide some feedback on the construction's accuracy so far. The shafts are large custom machined bolts that support the rocker arms which themselves are fairly complex three-piece subassemblies. These multi-part assemblies were necessary to solve an otherwise impossible rocker arm installation, but their individual components require careful machining.

I turned the bolts from 303 stainless and polished their heads to match the rocker box covers. Their lengths were also polished to reduce wear on the rocker components. The ends of the bolts were lathe threaded to insure the threads were straight and concentric with the shafts. These bolts, which thread into the rocker support brackets, pass through cylindrical tunnels in the valve boxes. The backsides of the heads, which fit into recesses in the rocker box covers, are grooved for o-rings which seal them to the covers. Fortunately, all four of the bolts installed with no issues.

The real test, however, was a check of the concentricity of the shafts to the circular tunnels inside the valve boxes. The cylindrical components of the rocker assemblies rotate on these shafts inside the tunnels, and there's little clearance. In fact, the drawings call out the same diameter for the o.d.'s of the rocker arm components as for the i.d.'s of the valve box tunnels. Their actual concentricity, however, has been affected by at least a dozen machining and assembly steps in getting to this point. Even with perfect machining, the diameters of the rocker components will have to be reduced to prevent binding.

In my case the concentricity was determined by trial-and-error machining the o.d. of a bushing that was slipped onto each of the installed rocker bolts. I reduced the diameter of the bushing until it spun freely on the worst-case shaft, and then I reduced its diameter another four thousandths for margin. I used this diameter to modify the design of the rocker arm components before they were machined. In my case, including the margin, the diameter was reduced .010" less than the tunnel diameter.

Each rocker arm assembly contains a pair of arms attached to either end of a cylindrical sleeve that rotates on a rocker shaft. One arm pushes down on the valve in reaction to the other one being raised by a pushrod. The effective rocker arm ratios are not identical for the intake and exhaust valves. The ratio for the intakes is .841 and for the exhausts it is 1.08. The sleeve and its two arms are actually three separate parts coupled together by a circular array of dowel pins on either end of the sleeve. The pins were pressed into the ends of the sleeves, but the holes in the rocker arms were drilled and reamed one thousandth oversize for slip fits.

I machined the rocker arms from 7075 aluminum. This alloy has 30% more tensile strength than machinable brass (recommended in the drawings) and is about 15% harder for a little better wear resistance against the valve and pushrod ends. The arms were machined in two groups: four valve rockers and four pushrod rockers.

Those who have followed any of my previous builds might recognize the construction technique that I like to use when machining these kinds of parts. Each group of rockers was completely machined from the topside of its workpiece in cookie sheet fashion, and then the machined troughs around each individual part were filled with Devcon 5 minute epoxy. This glue held the parts in place while the workpiece was flipped over and faced to bring them to their final thickness. Heating the workpiece to 350F cleanly released the finished parts. In the past I've use a heat gun to warm the epoxy, but lately I've been putting the whole workpiece in an oven for half an hour or so. It's important to wear gloves and push the parts free from the workpiece while they're still hot. If allowed to cool, the Devcon will re-bond to the parts.

The sleeves between the rocker arms were turned from 6061 since strength and wear are less of a concern with these parts. A pair of radial oil holes drilled at either end of the sleeve and connected by a longitudinal distribution slot provides an oil film between the sleeve and shaft. This slot was also used to align the drilling of the circular hole arrays on the ends of each sleeve in order to align the rocker pairs. Spacers between the pushrod-end rocker arms and the rocker covers fill the remaining space on the rocker shafts.

The locations of the pushrod-side rockers with respect to the holes drilled for the pushrod tubes in the rocker covers were also verified. A snug-fitting dummy rod with a pointed tip was used to mark the ends of the rocker arms to transfer the centers of the pushrod tubes to the tips of the rockers. An oil hole was through-drilled and a hemispherical cavity was plunged into the tip of each rocker at this location.

When assembled, there was no discernible lash measured at the locations of the valves due to the clearances in the parts making up the rocker assemblies. - Terry

 

[mayhugh1]

Scratch-made model engine oil fittings are among my least favorite parts to work on. For what they do, they require an inordinate amount of work and use tiny little workpieces that continually go missing in my shop. Whenever possible, I cheat and adapt commercially available parts - usually 1/8" tube compression fittings that I find in auto parts stores. These can be a bit large for the scales involved, but they seem to look at home where I've used them. For this engine, though, I'll have to 'knuckle' under and fabricate them from scratch because there's just no room for commercial parts.

Inside the rocker boxes there's very little space for the soldered spray bars used to lubricate the rocker arm assemblies. They're tucked behind the rear-most rockers, and their soldering has to be very carefully done so that what little clearance is available isn't taken up by over-soldered joints. The edge of the circular mounting recess inside each rocker box also has to be chamfered to clear even the tiniest solder fillets on the rear of the assembly's copper nozzles.

The drawings specify 1/8" copper tubing for plumbing the entire oil system. The bottom end, including the cams and crankshaft, are splash lubricated by a wet sump. There is an oil pump, but it's only used to feed the spray bars in the engine's top end. If the gravity-fed oil returns in the valve boxes can't keep up with the pump, the boxes can fill with oil and flood the valve guides. If this happens, it may be necessary to reduce the oil flow to the top end. After the engine is completely assembled and running, a relatively painless way to do this will be to add a restrictor in the feed tube to the rocker boxes. Limiting oil to the top end might even reduce some of the leaks for which the full-scale version of this engine is famous.

An arbitrarily short restrictor in the feed line to the spray bars won't necessarily achieve the desired effect, though. In fact, a restrictor that's too short can increase the velocity of the flow without significantly reducing its volume - sort of like holding your thumb over the end of a garden hose. The flow Q of a liquid through a tube is given by:

Q = (pi)(dia^4)(P)/(u*L)

where P is the applied pressure, u is a viscosity constant, L is the length of the tube, and dia is its i.d.. This equation shows that the flow is proportional to the square of the area of the tube rather than just its area. This may explain why trying to control flow with a quarter-turn ball valve can sometimes be so frustrating. An important point, however, is that flow limited by a restriction will be dependent upon the length of the restriction.

Since I might later need to fractionally reduce the flow, and since absolute numbers are dependent upon a pesky viscosity constant, I can ratio the equations written for a restricted and an unrestricted case and eliminate the need to know the viscosity. For example, if I want to cut the flow in half by using 3/32" tubing (.063" i.d.) in the feed tube but continue to use 1/8" (.094" i.d.) for the rest of the tubing, then after a bit of math,

Qrestricted/Qunrestricted = 1/2 = (.063^4/.094^4)*(Lunrestricted/Lrestricted), or

Lunrestricted/Lrestricted = .5/.2 = 2.5

This means that if the unrestricted case happens to use a total of 14 inches of 1/8" tubing between the pump and the spray bar, I can cut the flow in half by replacing 4 inches of the 1/8" tubing with 3/32" tubing.

I really won't know if there's an issue until the engine is running, but if there is, I'll have a backup plan that won't require disassembling the entire engine. At this point my goal is to wrap up work on the heads, and this will now require some half dozen oil fittings. I started with the spray bars that lubricate the rocker arm assemblies. Each of these is simply a pair of copper nozzles soldered into a brass puck that fits into a recess in the rear wall of each rocker box. Reducing the diameter of the nozzles to 3/32" from the drawing's recommended 1/8" helped a bit with the clearance issues, but things are still pretty tight in that area.

A custom compression fitting that receives oil from the feed tube is next threaded into the spray bar through the rear of each rocker box. I basically followed the drawing for these fittings, but added a groove in their bases for a 1mm x 4.5 mm o-ring to seal them to the rocker boxes. Their machining was many hours of mind numbing work mixed with a lot of procrastination. I didn't bother taking photos since the steps themselves were trivial, and there were lots of them. All totaled, there were some two dozen individual manual lathe operations and half as many setups involved with making each three-piece fitting. The starting workpiece was a twelve inch 3/8" diameter brass rod, and all the operations were done on a manual lathe. The ferrules were machined with flat rear surfaces so I'll have the option of soldering them to the tubes before assembly. This will eliminate one of the two crush seals typically found in larger commercial fittings. I ran across this trick for improving the reliability of miniature tube fittings in a small scale live steam text.

The Knucklehead drawings actually show soldered-on threaded fittings for the tubes making up the returns from the valve boxes. I couldn't visualize an assembly/disassembly strategy associated with using them, and so I took what I thought was a safer approach and used compression fittings here also. I may find out later that this was a mistake because of space limitations. After working the kinks out of the processes for making each of the three parts, I made what I hope are enough to finish the entire engine plus a few spares. - Terry

 

[johnmcc69]

Great post. I like your thinking with the oiling system & especially the O-Rings.

John

 

[Buchanan]

Terry.
I am enjoying this build as much as all your others.

I was looking at the beautiful work in the heads and a thought passed through my mind. Just think how good a Knucklehead radial would look! You might need to rearrange the direction of the head fins though, and a few other details that I haven't thought of.
I really like your work.
Buchanan.

 

[mayhugh1]

The Knucklehead's port hardware consists of two pairs of stainless steel flanged tubes that will eventually mate with the intake manifold and exhaust pipes. The tubes themselves are close fits inside the holes that were bored into the areas behind the valves during the head's initial machining. They will be held in the heads with setscrews threaded through the bottoms of the heads. These screws won't be accessible after the heads are bolted to the cylinders, and so each flange is machined as a separate component that's captured to a tube in a two-piece assembly. Being separate will allow the flange to rotate on its tube so its bolt holes can be aligned with those on the flange(s) of the intake manifold or the exhaust pipes during final assembly. Adding this complication to the port-side flanges will allow simpler flanges to be used on the much higher risk intake and exhaust components later.

The parts were all machined from 303 stainless, and the flanges were each tapped for six 2-56 SHCS's. When tapping stainless or ferrous metals I use a dark brown moly pipe threading oil that I found in a hardware store some twenty years ago. This oil makes tapping operations in steels seem effortless, and I don't think I've broken even an 0-80 tap while using it. You have to clean the workpieces right after using it, though, or it will stain them and probably one's flesh as well. Portions of nearly all my tools have been patina'd by this stuff over time. Unfortunately, I've not found anything that works as well with aluminum. (I currently use lowly WD-40.)

The joints between the mating flanges will need to be sealed to prevent leakage. The original design recommends a .010" thick copper washer to do this, but I happened to have some -012 o-rings left over from previous projects that I used instead. During assembly, an o-ring will be compressed between the mating flanges and will seal the interface between them. The o-ring fills a machined groove that's equally divided between the two components of each port-side assembly, and so it will seal the gap between them as well.

Silicone has a temperature rating of nearly 500F which should be more than adequate to handle the temperature rise of the components heated by the exhaust gasses. I've recorded the header temperature measurements on the five multi-cylinder model engines I've built so far and have never seen temperatures greater than 350F. Silicone will dissolve in fuel, however, and so Viton o-rings will be used with the intake flanges. - Terry

 

[ddmckee54]

Terry:

You've got an O-ring sealing between the tube and its' flange and the fit between the tube and the head is a close fit. What's going to be the seal between the head and the tube? I know that you're good and it WILL be a close fit, but you're still going to get a vacuum/exhaust leak at that point won't you?

Don

 

[mayhugh1]

Don,
Those will be Loctited in place. Thanks for asking.
Terry

 

[ddmckee54]

Thanks Terry, that explains the need for the flange to rotate on the tube.

Don

 

[mayhugh1]

A parts list found in the download describes the Knucklehead's valve springs as having an o.d. of .375" and being wound with .035" stainless spring wire to a free length of .500". An included commercial part number adds to the specs a minimum compressed length of .230" and a 13.6 lb/inch spring rate.

In his own build, Steve mentioned that he ran into issues with spring bind. Based on his comment I deepened the valve boxes almost .10" over what was shown in the drawings when I machined them, and I shortened their mounting screw heads. This effectively increased the springs' installed height. I also shortened the guide portion of my valve cages which brought their tops down an additional .18" below the rocker arms in order to eliminate any possibility of interference with the upper spring cup. Since I had some .031" diameter stainless spring wire on hand, I decided to also wind my own springs.

After some experimenting, I came up with a .36" diameter spring having a .15" minimum compressed length using four turns of my wire wrapped around a .240" mandrel. A check of the spring rate showed a value similar to that in the Draw-Tech spec. In addition to using smaller wire, I was able to reduce the springs' compressed length a bit more by carefully forming and grinding their ends flat without adding any inactive turns. Flat ends can be beneficial for valve springs since they help keep the valves consistently centered on their seats especially if there is excessive clearance in the guides.

The finished springs were heat treated for 45 minutes at 400F. This partial annealing relieves internal stresses created by the winding process, and it improves the spring's ability to hold its free length and spring rate over time. After completing six springs I carefully measured their spring rates using a height gage and a small force gage that I snagged from eBay. The average was approximately 13 lb/inch which was very close to the original spec.

As mentioned earlier, the designs for the spring cups and valve guides were in the pdf that was missing from my download. Steve sent me a copy of the one he used since Draw-Tech may have pulled it from the download in order to look into the binding issue. I machined the cups from phosphor bronze while making minor modifications to them in order to reduce their contributions to the spring stack-up. This included changing the valve retainer on the top cup from a pin to a 'U' clip which increased the spring's installed height slightly.

In a low-revving model engine the valve springs need to be strong enough to keep the closed valves tight on their seats especially during starting. While leak-checking valves during my previous engine builds, I concluded that 1 to 1-1/2 pounds of force on the face of the valve under test was sufficient to obtain consistent leak-down times on usable valve/seat combinations. With a spring rate of 13 lb/inch, my scratch-wound springs will create just over 2 pounds of force at their installed height which results from .15" of compression. With a simple test fixture using one of the scrap valve cages machined earlier from some questionable bronze, I measured the maximum available valve travel to be .165".

The cam drawing shows the exhaust valves' lobe heights to be .065", and since their rocker arm ratios are 1.08, the expected exhaust valve lift will be .070". Similarly, the expected intake valve lift will be .060" x .841 = .050". The exhaust valve lift will create a worst-case pushrod load of just under 3 pounds. With this 3 pounds I'd be a bit more concerned about wear if the engine wasn't using a roller cam. Even so, I'm certain I have some .026" wire stashed away somewhere in my shop, and I may re-visit the springs if I can find it.

To prepare for machining and leak-checking the valves, I permanently installed the port hardware. Since my leak test will involve pulling a vacuum behind each valve while installed on its seat, the o.d.'s of the port tubes need to be sealed to the heads. For this I used 620 (high temperature) Loctite. Draw-Tech's design includes 6-32 setscrews threaded through the bottoms of the heads to secure the tubes. I ground points on these setscrews before installing them so they would bite into the tubes for a little more security. Both the intake and exhaust tubes will eventually be expected to carry some cantilevered weight while the engine is shaking on its mounts. - Terry

 

[doc1955]

Been following along I'm actually been planning on building this one. You are doing some might fine work looks nice!!
I did start modeling things up I want to get a model completed before diving into it. I hope to start building this fall here is what I have so far. I've only been putting things together model wise when I have spare time. The drawing of the heads was somewhat confusing but I see you did a nice job on them with the info you had NICE!

My model so far.
its a 3d pdf file.

 

[stragenmitsuko]

Lovely , another build by Terry . :thumbup:

Pat

 

[mayhugh1]

Been following along I'm actually been planning on building this one. You are doing some might fine work looks nice!!
I did start modeling things up I want to get a model completed before diving into it. I hope to start building this fall here is what I have so far. I've only been putting things together model wise when I have spare time. The drawing of the heads was somewhat confusing but I see you did a nice job on them with the info you had NICE!

My model so far.
its a 3d pdf file.

Doc,
I can't seem to bring up your pdf. My Windows PC's may be too old, but I thought my new Mac laptop wouldn't have an issue. Terry

 

[doc1955]

You have t down load then open with adobe.

 

[mayhugh1]

The Knucklehead's valves were machined from 303 stainless. Compared with those in other multi-cylinder engines I've built, these valves are huge at .625" diameter. Earlier when I machined the valve cages, though, I reduced them to .570" so the edges of the combustion chambers would have a bit more safety stock.

When I was making valves by the dozens for my radials, I learned to machine them in one large lot that I shepherded through six machining operations. Before jumping into production, however, I completed at least one valve that I trial fitted into all the cylinders so I could check for machining inconsistencies. There's often a number of machining operations that can affect the requirement for a valve's length, and some of these can be hard to control. Although it may sound risky to put off lapping and testing until after all the valves are machined, I probably saved a lot of time and avoided errors by not changing and recalibrating the set-ups for each and every valve.

The first operation is to create the starting workpieces. For the Knucklehead this involved sawing a 5/8" diameter 303 rod into short lengths so a complete valve could be machined on either end of a central one inch work-holding spigot.

The second operation involves roughing out the valves on both ends of all the workpieces. This is my favorite step because I can turn my little Wabeco CNC lathe loose with very little handholding. I typically use a CCMT 21.51 roughing insert or sometimes even a dull finishing insert that's been retired from service. Surface finish isn't yet important because this step leaves a diametrical excess of .020" stock. A tailstock isn't bothered with since stem taper will be dealt with later. However, before starting the operation the ends of the workpieces are center drilled to prepare for the next step.

The next step is the finishing operation, and it uses a very sharp DCMT 21.51 insert typically intended for aluminum. Three passes are used to remove the remaining stock except for the last 1 to 1-1/2 thousandths. Either the tailstock is used for this operation, or I will back up the free end of the workpiece with a piece of leather held in my right hand. (I realize this sounds wonky, but after a little practice it actually works pretty well.) The speed and feed are adjusted for the best possible surface finish. During this step my lathe requires some hand-holding. Upon completion of each valve, its stem diameter is measured, and the lathe's work offset is corrected as needed before taking on the next valve.

The valve stems are brought to their finished length in a fourth (lathe) operation. The workpiece is inserted through the rear end of a 5C collet so the valve to be trimmed can be gripped close to the end of its stem. The groove for the retainer clip is also cut in this step.

In the fifth step the workpiece is re-chucked on its spigot so the last thousandth or so can be polished from the valve stems. I start with 400 grit paper if I have more than a thousandth to remove. The last thousandth is removed with 600 grit paper, and the stem is mic'd along its length during the process. When finished, a dab of white buffing compound on a rolled-up paper towel moved along the entire spinning valve brilliantly colors it.

In the last operation, the two valves are sawed free of the spigot which is gripped in a vise attached to my bandsaw. With some care, the larger diameter spigot prevents the polished valves from being marred. The stem of each freed valve is then gripped in a 5C collet up close to the valve head so its face can be finished in the lathe. Again, light passes with a sharp finishing insert produce a nice surface finish without deforming the completed valve.

Before starting my little production run, I used a test valve to search for an optimum stem length for both the intake and exhaust valves in both heads. The length I was looking for was one that would rotate the pushrod rockers perpendicular to their pushrods with the rocker arms resting on their closed valves. Somewhere between the valve and pushrod drilling operations, though, one of my compound angled setups must have been off. As a result, two valve lengths will be needed - one for the valves in the left side of the rocker boxes and another for the valves in the right side of the boxes.

The need for two valve lengths created another issue. Measurements using my previously finished springs showed the pushrod forces now approaching four pounds in the valve trains with the short valves. I had already been uneasy about these relatively high forces when all the valves were the same length and longer. As a result, I decided to make a new set of valve springs.

I machined the two different length valves in a mini production run. I also machined a lap which was nothing more than a completed and polished valve that wasn't parted off from its spigot.

Unlike what I've done on full-size engines, I like to finish model engine valve seats with a separate lap rather than lapping them to their mating valves. After all, the valves come off the lathe with correct and concentric geometry, and they're brilliantly polished as well. The seats also have the correct geometry, and they're concentric with their guides thanks to the piloted seat cutter used to cut them. Their sealing surfaces, though, have marks left on them by the seat cutter that, at the scale of a model engine, can create leaks.

Over time I've 'honed' a technique that helps to minimize the marks left by the piloted seat cutters I use. I coat their teeth with cutting oil and their pilot with motor oil. The oil reduces the chatter that can sometimes be felt while turning the cutter. I'm also careful to apply only a minimum force to the cutter. Pushing too hard can gouge the seat requiring it to be made wider than necessary. I usually orient the head so the cutter is vertical and allow its own weight to do the cutting. I try to keep the final seat width on the order of .005" in order to minimize the amount of material that will have to be removed to clean up the marks.

Lapping can damage a perfectly good valve especially if the marks on the seat are prominent. I feel these marks are best removed using a sacrificial lap and a very fine lapping compound such as Timesaver. The whole process takes less than ten minutes including the time used for leak testing along the way. I perform my leak-checks by pulling a vacuum in the port behind the closed valve under test using a Mity-Vac. My own subjective goal is to achieve a 25 to 15 inHg leak-down time of ten seconds or more. This is probably overkill but it's not difficult to achieve when the seats have the proper starting geometry. This check can also take leaky valves off the table as a potential problem when trying to start an engine for the first time. If valve cages are used, their geometry can be verified with a very light pass of the seat cutter before the cage is installed in the head.

After leak-testing the valves, I turned my attention back to the valve springs. I wound a new set of springs on the mandrel used for the first set, but this time I used .026" diameter spring wire. I ended up with approximately the same i.d., o.d., and length but with one less turn. The finished springs had a measured spring rate of 5 lb/inch which reduced the worst-case pushrod loads to a bit more reasonable 2.3 lbs.

This finally completes the work on the heads. I'll likely move on to the cylinders next as I work my way down from the top of the engine. - Terry

 

[mayhugh1]

The cylinder design has good looking proportions and cosmetic detail that requires some effort and care to machine. The finished cylinders will look similar to those on the full-size engine except for the radial notches for the head fasteners.

I had some 12L14 stock left over from my radial builds that was ideal for the workpieces. After band sawing them to length, each was chucked in my Enco lathe so their ends could be faced and center drilled. Their centers were drilled out in three separate steps and then bored to within a thousandth or so of their final value in order to reduce the amount of honing that will be required later. A heavy bar with a sharp insert was used to obtain the best possible surface finish. I also skimmed a short length of each workpiece's o.d. so I'd have an indicating surface for later use.

The cylinders' machining operations require full access to the outsides of their workpieces, and so I prepared an expanding mandrel to support them by their i.d.'s. The 5C machinable mandrel used for the Merlin's liners needed only a bit of material removed from it in order to fit these new workpieces. After turning its o.d., the mandrel was left captive in my Wabeco's collet chuck until all the cylinder lathe work was completed. I loaded and indicated the workpieces on the mandrel a couple times while checking their TIR's. They were consistent to within a half thousandth even though the cylinders' machining wouldn't require such accuracy. Mainly, I was curious about the mandrel's consistency.

The cylinder drawings specify through-hole clearances for 10-24 mounting studs for securing the cylinders to the engine's (split) crankcase. In the original design, even small pattern nuts would have overhung the cylinders' mounting flanges and bugged me to no end. Enlarging the flanges might have opened up a can of worms that infected the crankcase, and so I changed the clearances to use 8-32's instead. Even then, I had to move their locations inward a bit in order to keep the even smaller nuts within the edges of the cylinders' mounting flanges.

The remaining lathe work began at the bottoms of the workpieces which will become the cylinders' simple ends. The material to be removed above their mounting flanges was essentially a deep wide groove for which I used the bifurcated grooving insert created a couple years ago to turn the rod bearings on my Merlin's crankshaft. This grooving operation had to be performed on my Enco lathe since the Wabeco doesn't have the rigidity nor the low speed torque to handle an eighth inch wide insert in steel. After machining the workpieces above and below their mounting flanges, they were moved to the mill where the perimeters of the flanges were milled square and their mounting holes drilled.

The hole patterns for the head mounting fasteners on the tops of the cylinders need to be accurately oriented with the flange mounting holes on the bottoms of the cylinders. If not, among other consequences, the intake manifold will become even more difficult to fabricate than it's already going to be. The cylinders' mounting flanges were clamped in the mill vise for alignment of the workpieces to the mill so the clearance holes for the head studs could be properly located and drilled.

The cooling fins have cosmetic detail that requires extra effort to machine, but the final results are well worth the effort. The edges of the fins and the valleys between them are fully radius'd. Since I had machined similar fins on the cylinders of my last radial, I resurrected the CAM software and tooling that I developed for them. I ran into some issues back then, though, that I now had to revisit.

The first problem is that the mandrel'd steel workpiece has significant unsupported stick-out and at 1300 rpm can only marginally handle a .040" wide radius'd insert without raising chatter resonances between it and the lathe's lightweight cross feed. As a result the machining needs to be done in a series of brief pecks. This also means that the thin fragile insert also has to handle a side-to-side clean up pass to remove scallops left by the pecking. The insert I'm using isn't really designed to do this.

Another issue is that the CAM (Sprutcam 7) software that I'm using doesn't seem to have the flexibility to peck-turn arbitrarily shaped workpieces. It has a grooving operation, but it doesn't seem to know how to distribute the pecks within the workpiece when the workpiece is something other an ordinary cylinder. Used per its manual, the software tends to generate g-code that causes the tool to spend most of its time pecking air outside the complex contours of my roughed-in cylinders. In order to get it to behave, I found that I had to describe its job assignment as I would if I were milling the part rather than turning it. The turning operations usually know how to deal with turning assignments, but they have to be coaxed (or tricked) into working with milling assignments. After spending countless hours playing with things that shouldn't matter (every part seems to be a special case), I finally came up with usable code. Those hours spent getting usable code were well worth the effort back when the goal was to machine 20 identical cylinders. For these two cylinders, however, a form tool would have been more efficient although less satisfying.

To make things a little more interesting, the Knucklehead's radius'd grooves were also a bit deeper than those for which the insert and its holder were previously modified and used. Another .030" had to be removed from them both in order to obtain the clearances needed for the deepest grooves. Most likely this didn't help with the chatter, and I was very lucky the weakened insert didn't fail during the finishing operations.

Two more operations remain before the cylinders can be finally honed and blued. -Terry