Pivot hole size

Discussion in 'Wood Movement Clocks' started by MikeA, May 13, 2019.

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  1. MikeA

    MikeA Registered User
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    Dec 21, 2006
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    I am about to attempt rebushing a typical Terry type wooden movement by the tapered plug movement. I measure several pivots at about 0.065 inches and some holes in various plates at about 0.070. So, I am planning to use a 0.070 drill. Does this sound about right?

    Thanks,
     
  2. R. Croswell

    R. Croswell Registered User

    Apr 4, 2006
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    Mike, I would consider each pivot and hole individually and not assume that 0.070" drill is right for all. The first step is to check the pivots. These are often worn unevenly so by the time you true up the pivots and polish them that 0.065" pivot may be somewhat smaller. Just in case you aren't aware, these pivots are just mild steel and were only pressed into a hole in the end of the arbor. You may have read recommendations to simply pull out the pivot and turn it around and reinsert it to use the unworn end. While this sounds good, in most cases the holes in the arbors were not drilled straight and the pivot was bent after insertion until it ran true. This can be a problem if you reverse the pivots. It is likewise a problem if you decide to replace a badly worn pivot. During the process of turning and/or filing a worn pivot you may find that the pivot is no longer tight in its hole. Just make sure your pivots are smooth and straight and not tapered before starting your bushing work.

    There are several schools of thought about sizing wooden pivot holes, including the effect of moisture on the wood and the degree of precision in drilling and aligning the hole, and the overall stability of the movement plates. The wooden pillars are frequently loose in the back plate. You don't want the plates squirming around and introducing any misalignment of pivot holes, so take care of this first. The pillars are frequently nailed through the end grain of the wood and these nails are hard to remove so don't try. Assemble the plates with no parts between and pin the top plate in place. With the bottom side up, use thin CA glue (super glue - the water-thin stuff - not gel) and flow the glue into the joint around the pillar a little at a time until it will take no more, then set it aside until it dries.

    Now do your bushings. The important thing is that you get the hole in your bushing precisely centered on what was the center of the original pivot hole before it was worn. This can be challenging but is most important. I hope that you plan to drill the holes with a drill press; it is essential that the hole be perpendicular to the plate and directly in line with the hole in the opposite plate. As for the size of the drill, I would suggest using a 0.068" or 0.066" drill (or a bit smaller if the pivot is underside) and broach the opening to final size using the hole in the opposite plate to guide the broach. This will ensure that your pivot holes are in true alignment. You have the size right when with the pivot in the hole "tilts" equally in all directions such the opposite pivot is free to move about 2 pivot diameters either side of the opposite pivot hole AND with that arbor in place and the plates pinned together and the movement face up, when that wheel is lifted and released it will drop freely under it's own weight - repeat with the movement face down. Finally with the movement in the normal position spin the wheel and make sure it spins easily and coasts slowly to a stop. Some may recommend burnishing the new pivot hole and some may recommend fitting pivots in wooden bushing a bit looser. I no longer use wood bushing and prefer making my bushings from Delrin-AF and drilling/boring the pivot holes on the lathe when the bushing is made. Wood is preferred by some purists and served the original clock well for nearly 200 years.

    RC
     
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  3. MikeA

    MikeA Registered User
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    Dec 21, 2006
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    Thanks for the detailed response. Yes, I will be using a drill press and wood replacements. I have some empty plates with no parts and somewhat hacked up. I also have a pretty good selection of small drills, both metric and inch, so I can get pretty close to the required size. Hadn't thought about the plates alignment, so will try that procedure.

    Been waiting on a plug cutter. It was shipped UPS, then the PO took it in New Orleans (45 minutes away) and sent it 100 miles north of me to Mississippi, then to another town in MS 30 miles east. Supposed to be delivered today, we'll see.
     
  4. Jim DuBois

    Jim DuBois Registered User
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    Jun 14, 2008
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    I make my own plug cutters. I make bushings out of bone as well as oak, maple, cherry, mahogany, and lignum vitae. The thin edge plug cutters allow some very precise bushings.

    2018-06-09 09.48.42.jpg 2018-03-12 14.20.28.jpg 2018-03-11 18.06.18.jpg 20170109_134736.jpg 20170109_131908.jpg 20160925_132638.jpg IMG_1608.JPG IMG_1575.JPG 2016-09-21 14.28.21.jpg
     

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