Monarch Model 61 Apron Worm Assembley Repairs
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  1. #1
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    Default Monarch Model 61 Apron Worm Assembley Repairs

    The worm assembly on my Monarch Model 61 was worn out, and after looking at its design, I decided to make repairs and to redesign the assembly.

    Before I go any further, I want to thank Cliff Holm, a member here, who sent me a worm, driver, worm wheel, and half nuts. The condition of my worm and worm wheel were comparable to Cliff’s, but his half nuts were much much better than mine. Thank you Cliff.

    Using Monarch’s nomenclature, the worm assembly consists of a collar, three bronze bushings, a driver, a driver plate, and a worm. The picture below shows the parts less the collar and bronze bushings.

    worm-1.jpg

    My idea was to redesign the assembly pictured by repairing/redesigning the worm, and making a new driver that combined the driver with the driver plate (the two lower parts in the picture). I also planned on making the center bronze bushing longer to better support the joint between the driver and the worm.

    The drawing below reflects the design changes I made and the repairs/redesign of the worm.

    worm-repair.jpg

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    The first thing I did was to regrind the bearing surfaces on the worm. I was careful not to grind the thrust bearing diameter. The finished diameter was used as the OD of the new driver so that all the bronze bushings had the same ID.

    The first picture shows how I dressed the die grinder wheel. The second picture shows the grinding of the bearing surface.

    worm-2.jpgworm-3.jpg

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    The new driver was machined from a bar of 4140, C25 steel. I am only a shade tree machinist and I thought C25 would be about as hard as I could machine. I might have used harder material, but I do not know how hard a material I can turn.

    The first two pictures show the machining of the driver blank. The third picture shows the new driver blank and two old worn out drivers.

    drive-1.jpgdrive-2.jpgdrive-3.jpg

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    I used a Type C broach to broach two 3/16” keyways 180deg apart in the driver blank. To do that, I made a new broach bushing to fit the driver blank. The bushing had one slot for the broach, and a short pocket for a 3/16 key 180deg from the broach slot. The short key allowed me to index the bushing after making the first keyway in the driver blank. I ended up broaching the driver blank from both ends to get a full keyway depth.

    The first picture shows the bushing being machined using a dividing head and a Bridgeport mill. The second picture shows the broach, the bushing, the indexing key and the driver blank.

    key-1.jpgkey-2.jpg

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    The next picture shows one of the keys being machined from 4140, C25 steel. The key has a cylindrical secondary key machined perpendicular to the key. The secondary key fits into a 3/16” diameter hole drilled into the center of the integral collar that is part of the driver blank. The broach bushing was used to index the driver blank into the dividing head so that the hole was drilled into the center of the integral collar and the center of the keyway.

    key-3.jpg

    The fourth picture shows the finished keys and the fifth picture shows the new completed driver alongside an old worn out driver.

    key-4.jpgkey-5.jpg

    Not shown is the machining required to form dimples in the driver blank for the three set screws that hold on the collar shown in the final picture, or the bonding of the keys into the driver blank using Loctite 638.

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    The next task was to clean up and deepen the dogs on the worm.

    The first picture shows the dogs being ground using the dividing head in the milling machine (the dust hood used to suck up most of the grinding dust is not shown). I felt that the worm was too hard to use HSS or carbide on, so I ground it. The second picture shows the finished dogs on the worm and the worn dogs on an old worn out worm.

    dogs-1.jpgdogs-3.jpg

    The grinding wheel was centered on the wall thickness of the worm, and the top of the wheel was aligned with the centerline of the worm. Care was taken to make both dogs the same and to make the dogs 180deg apart.

    The next picture shows the machining of the dogs on the driver blank. I used a carbide milling cutter on the driver and took care to make sure the dogs on the driver would engage the dogs on the worm.

    dogs-2.jpg

    It should be noted that the depth of the dogs was increased to .300” to account for the relative softness of the new driver and to improve life. It should also be noted that a 3” diameter wheel or cutter is about the limit that can be used without the wheel or cutter striking the opposite side of the driver or worm. If the dog depth is increased beyond .300”, it is likely that a smaller wheel and cutter will be required.

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  13. #7
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    The picture below shows the final assembly (less the bronze bushings) along with a worn out original assembly. Note the hole for the secondary key in the integral collar of the driver.

    finished-assembly.jpg

    The original bronze bushings were identical, and all were held in place via tapered pins. The tapered pins were offset to the left side of all three bushings. I decided to make the center bushing longer so that it supported the joint between the worm and the driver. This meant that the center bushing was a little over a half inch longer than the other bushings, but the recess for the taper pin was still the same distance from the left side.

    The next pictures show the machining of a 932 bronze bushing, and using a ¼” ball end mill to rough in the recess for the taper pin. The recesses were reamed with a taper pin reamer after installation.

    bushing-1.jpgbushing-2.jpg

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  15. #8
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    Nice work!

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    Great detail!

    One question, my eyeball says the worms aren't the same pitch....but calibration of my glasses could be off and/or perspective or camera angle.

  17. #10
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    Matt
    They are the same.

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    Very cool! I might try and copy your instructions!

  20. #12
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    If you have any questions, I would be happy to help if I can.

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    If this thread isn't made a "sticky", I at least need to make a hard copy of it (should anyway). These are excellent ideas and this day of reckoning DOES come when you refurbish the apron & carriage of a series 60/61. I'm glad the parts I installed were usable, but I wish I had dared to do something like this.

  22. #14
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    I don't want to get ahead of myself too much. I have not put it into operation yet. But if the keys stay in place and the new driver is hard enough, then I think the fix will outlast me.


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