270 Offy

Discussion in 'A Work In Progress' started by mayhugh1, Aug 12, 2019.

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  1. Oct 19, 2019 #61

    petertha

    petertha

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    Terry, once the craft vinyl (Oracal 651) is cut, do you make any attempt to dissolve the adhesive prior to installing on the metal face? I can see on one hand where some tackiness might be good in that it stays put, but then maybe the opposite when you remove. Are they reusable?

    The cumulative gear backlash during throttle variation is a really good point. I'm still not clear on your mitigating procedure though. Was the test fixture somehow variable that you could tweak the center to center gear distances right? And then once satisfied, you somehow measured resultant shaft centers & replicated these using DRO to the real part? In the test fixture do you measure play gear by gear, like hold one gear, wiggle the the other, adjust... then proceed down the line?

    A fixture for testing the 40 tooth gears was also machined to verify the meshes and spacing between them. Its design was small subset of the gear tower and was also used to tweak the end mill parameters for the bearing fits. These three gears also turned freely with minimum backlash at their theoretical spacings.
     
  2. Oct 19, 2019 #62

    mayhugh1

    mayhugh1

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    Petertha,
    I don't try to reuse the gaskets. It's easier to replace a damaged one with new material. Once it's in place, I treat it as a new permanent surface on the part.mSo far, I've used this stuff only in unpressurized areas to prevent oil leaks where sealer might have typically be used.

    With respect to the gears, the particular test set I used was only to verify the gears would turn smoothly with a hint of backlash at their theoretical spacings. If they hadn't passed that test they would have been re-made.

    I have made test sets in the past where I could vary the distance between the gears to see what my margins were around the theoretical spacing. There were too many cascaded gears in the Offy tower to start playing with spacings to conform to poorly made gears though. - Terry

    p.s. Another effect of gear slop can be large valve timing changes during sudden speed changes. Their effects are probably overwhelmed by the carburetor vacuum changes though.
     
  3. Oct 19, 2019 #63

    minh-thanh

    minh-thanh

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    mayhugh1 !
    Can you show me how to make a surface like that?
    That surface is great !
    22.jpg
     
  4. Oct 19, 2019 #64

    mayhugh1

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    Minh-thanh,
    Thanks for the complement. After machining, the surface was glass bead blasted. There's more information about my particular setup, which is nothing special, several posts above this one where someone else asked a similar question. - Terry
     
  5. Oct 21, 2019 #65

    minh-thanh

    minh-thanh

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    I missed that.
    Thanks mayhugh1 !
     
  6. Oct 26, 2019 #66

    mayhugh1

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    The backside of the front half gear tower contains a mirror image of the rear half's six bearing and gear pockets. An additional complication is the magneto drive block which is a forward extension of the tower and contains the right angle drive components needed for the magneto driveshaft. In the full-size engine, this block was an integral part of the tower's front-half casting. To simulate the original engine's appearance, Ron suggests bolting the magneto block to the tower and blending the two together with aluminum-filled epoxy fillets.

    I came up with two more approaches that were considerably more complex but avoided the need to color match epoxy to the tower. The first approach was to split the drive block housing into two pieces. The rear piece, including its blending fillets, would be machined integrally with the gear tower. The front half, a separately machined part would be bonded to its rear half with screws and a nearly invisible Loctite seam.

    The second approach, and the one I ultimately used, was to machine the entire block as part of the tower. This required considerably more machining on a hefty workpiece that was turned mostly into chips. Since the block stands over a portion of the tower, the workpiece had to be repositioned three times to get the needed spindle accesses. The third setup was required to machine some cosmetic features on the side of the block. Inevitable errors accompanied the re-referencing and tool changes that were needed, and a couple seams that couldn't be hidden with bead blasting required some manual cleanup. The long reach finishing cutters that were needed inside the deep workpiece created their own issues with surface finish and machining time.

    The front-half tower's first machining operations were the bearing and gear pockets on its backside. After verifying their fit with the already completed rear half with all the gears, shafts, and bearings installed, the workpiece was flipped over for its lengthy front side machining. More than half of the nine hour machining time was spent roughing in the shape of the tower. After the finishing operations, a quick-dry epoxy was poured over the periphery of the part to keep it connected to the workpiece during its final machining.

    A portion of the workpiece had to be cut away to gain access to the port side of the drive block. This side contains some contouring added to the design to smooth the transition of the block into the tower. Once this area was machined, the part was returned to its original position with its backside facing up so the periphery could finally be machined. The front half of the gear tower has a convenient bead around its periphery that allowed the part's front and back surfaces to be machined and the cut free of its workpiece without leaving an annoying seam. This bead, a part of the original engine's casting, was probably used to help remove the front tower's half during maintenance/disassembly. This final operation separated the part from its workpiece with a slot milled around the outside of the bead. The epoxy kept it safely attached to the workpiece until it could be removed with heat. After a half hour in a 300F oven, and while still hot, the (Devcon 5 minute) epoxy could literally be rubbed off the part. - Terry


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  7. Oct 26, 2019 #67

    RonC9876

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    Looking good!
     
  8. Oct 26, 2019 #68

    propclock

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    You just made my morning!
    I always enjoy following your builds. Very inspirational.
    I guess I should make something this morning before they turn my power off.
    California.
     
  9. Oct 27, 2019 #69

    bobden72

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    I have the drawings and the book for building the "OFFY" and is going to be my next build. After seeing your build though I don't know if I can do it justice but will give it a damn goo try.
     
  10. Nov 2, 2019 #70

    mayhugh1

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    The cam gear covers bolt onto the upper ends of the gear tower and will eventually enclose it around the camshafts' driven gears. These covers are small in size and rich in detail and will be troublesome parts to machine. They'll wind up in prominent locations at the top of the engine, and so the effort they'll soak up won't go unnoticed. I scrapped four parts and the better part of a week coming up with a pair of covers that I was satisfied with. Again, my efforts were focused on CNC machining them from single blocks of metal. Ron describes a somewhat simpler method for creating them from several manually machined pieces.

    I started with a pair of 2" x 2" x 3/4" aluminum blocks and was able to mill most of the parts' intricate features while working on their topsides. Once I completed this step on my first pair of parts, I noticed the covers' thicknesses were .005" less than that of the assembled gear tower. Both the covers and the tower seemed to match my understanding of the drawings, but I felt the thicknesses should probably match. So, I added another .005" to the cover design and started over with a second pair of parts.

    Work holding issues showed up during the second step which involved opening up the sides of the parts to accept the cam box and end plate covers. This step included cutting the parts free of their workpieces on their exact centerlines. I built up temporary epoxy bridges to hold the parts in place during the milling operation that would free them from their workpieces. But, with little excess stock to adhere to, the epoxy failed on both parts and the cutter scarred them as they were freed. Yet another pair of parts had to be started.

    The final machining steps opened up the interiors of the covers for the clearances needed around the cam gears. This was done in two operations using 1/8" diameter end mills and working down through the bottoms of the parts. The first roughing operation was performed with a cylindrical cutter, and the finishing operation done with a ball cutter. The finished diameter of the opening had to be increased slightly beyond the dimension given in the cover's drawing in order to accommodate the fillet left behind by the finishing cutter.

    After machining, the covers' mounting screw locations were transferred to the temporarily assembled gear tower, and its eight 2-56 holes were drilled and tapped. The bores for the end plates were also finished using a boring head, and their 0-80 mounting screw holes were drilled and tapped. The pair of openings on the backside of the assembled tower shown in the last photo may look mis-machined, but they're designed to fit around the ends of the sheet metal covers that will later enclose the cam boxes. - Terry

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  11. Nov 6, 2019 at 10:44 PM #71

    Gabe J DiMarino

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    My one piece cover.
     

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  12. Nov 7, 2019 at 12:49 PM #72

    nj111

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    I do enjoy reading your build threads, all very informative, excellent photography, lots of great ideas, thank you for sharing your knowledge and techniques!
     
  13. Nov 7, 2019 at 11:30 PM #73

    mayhugh1

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    I was hoping to tie up the ribbons on the gear tower and have at least one fully machined, gasket'd, and working assembly with moving parts that I could show off. However, the plans call for a trough to be machined into its backside for the purpose of carrying pressurized oil to the engine's top end. Since I'm not comfortable with beginning work on the engine's oiling system by starting in its middle and working outward, the tower will have to be torn down later for some additional machining.

    The tower/drive block combo has a number of cover plates that are easy to machine and, after polishing, add nice contrasting touches to the tower's simulated cast aluminum surface finish. The more challenging parts, though, are the internals of the magneto drive block. Ron's parts list calls out what are now obsolete Small Parts numbers for the miter gears that he originally used. However, I wasn't able to cross-reference them to currently available parts. The BOM shows them as 48 DP, but the G479-Y and G461-Y eighteen tooth 48 DP miters currently available from Boston gear appear to be too small to handle the modifications implied in the drive block's assembly drawing.

    I happened to have a pair of Boston Gear G462Y 32 DP 16 tooth gears left over from an earlier project that I decided to use. I increased their original .188" bores to .2495" and turned down their o.d.'s from .536" to .480" in order to fit them inside the drive block's half inch bore. My modifications were very similar to the ones shown in the drive block's assembly drawing, but I was initially puzzled about why the drawing also seems to show most of the output shaft's gear hub machined away.

    It was then that I realized that my drive block's housing is wider than it's supposed to be. There are several dimensions associated with the drive block that are left for the user to fill in, and evidently the value that I'd estimated for its width from a Xeroxed photo in the manual was incorrect. The reason why this width is important is that there must be room between the drive block and the magneto for a faux coupler between the two. Embedded in this coupler will be a pair of magnets needed to trigger a Hall device located on the side of the magneto. (Although the manual recommends making the magneto's input shaft from stainless steel in order to minimize field distortions around the magnets, I made the drive block's output shaft from stainless as well.)

    Rather than re-machine the drive block, I designed a new inboard cover that would allow me to move the output shaft's inboard bearing entirely within the housing. I'm fairly sure I picked up enough space for the coupler, although it too may require a minor redesign. These changes turned out to require shortening the hub of my miter gear after all. It's probably important that these gears be bored for tight press-fits onto their shafts since, after shortening, there may not be enough surface area for Loctite to adequately hold them, if loosely fitted, in place. The inertial loads of the coupler and magneto won't be insignificant during abrupt speed changes at the rpm's this engine will be capable of.

    Fitting the gears inside the drive block would have had to be done blindly, so I cross-bored a scrap piece of metal to try to duplicate it. After opening an inspection window on its top, I was able to iteratively modify the gears and begin the shimming process for an optimum engagement. The fixture got me into the ballpark, but I had to complete the process inside the actual housing. In previous miter gear (distributor) applications, I've had gravity to help out with setting the engagement depth for silky smooth low-backlash operation. Gravity seemed to work against the right angle drive's engagement, however, and setting up its gears was a lengthy and tedious process.

    The drive block's components complicate the assembly and disassembly of the gear tower. Although the miter gear and its 20 tooth drive gear must be hard pressed onto the block's input shaft, the bronze shop-made bearing which is captured between the two must turn freely on the shaft and slip into the drive block during assembly. Simultaneously, the rear end of the input shaft must slip into the ball bearing located inside the rear half of the gear tower. I've included photos of my two shaft assemblies showing the shims I used. I didn't include any dimensions since they would apply only to my particular (mis-machined) drive block. The output shaft's outboard bearing is a slip fit inside my drive block, and so I added a spacer between it and the nose of the miter gear to prevent the bearing from walking inward during normal operation.

    The next step will likely be to machine the cylinder liners so I can finish up the block. - Terry

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  14. Nov 8, 2019 at 12:15 AM #74

    Scott_M

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    Hi Terry
    As always, beautiful work !
    But I really wanted to thank you for sharing your mistakes as well :) and the really awesome and clever engineering used to right the wrong. You always come up with an elegant solution.

    Many thanks for sharing !

    Scott
     
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