Mods 15

 

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A Replacement Crank

After many years of use the plastic crank handle bushes of my WoodRat became very worn. I also had occasional trouble with the internal star retaining washer slipping on the crank resulting in too much up and down play. After ordering new bushes I took the opportunity to make some modifications. To make it easier to "sneak" up to precision positions when tracking the carriage I made a crank handle with a greater throw to in effect gear down the travel movement.

After a good rummage in my metal scrap box I found some bits of alloy. After a bit of cutting and filing I made this. The shaft is a 10mm silver steel bar which is available from any engineering supplier. The handle now has a 4" throw. There is still plenty of clearance between the wall and router. The vertical handle bar is a piece of chromed bar that I threaded and screwed into the alloy.

The advantage of silver tool steel bar is that is is accurately ground so is a good fit in the bushes.

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The chrome handle is nice and slippy in the hand and saves all the bother of making one with bearings.

(Donated from a B&Q kitchen pot stand!)

The silver steel bar sits in a 10mm hole in the alloy. I drilled into the alloy and completely through the bar and threaded it with a 4mm tap. A screw holds the two parts together. Since there are two alloy components to the handle, I could make a replacement top part with an even greater throw with ease.

I made two retaining fixings for the shaft. A big brass nut was modified to fit onto the shaft and a 5mm  thread cut into one of the flats. This thread accepts an allen cap head screw which in turn engages with a small flat filed into the shaft to keep the assembly from slipping. There is a nylon washer between the brass nut and the under side of the channel. Fortune was with me and the head of the allen screw just clears the back of the channel when the handle is turned although I did have to file the flat down a bit. You may have noticed the heavier gauge wire. I purchased this from a cycle shop. Its a gear change cable and only cost 1.50. ( Even the nipple is a good fit into the hole in the channel). Although teflon coated, the cable has not slipped once. I think the silver steel gives more grip than the  nickel/chrome original.

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I turned a longer spigot on the end of the shaft so that it protrudes out of the bottom of the channel by about 6mm. The shaft end was drilled and tapped to 5mm. I cut a piece of molybdenum filled nylon bar to fit over this spigot and secured it in place  with a bolt and washer as shown. I did find that I had to use some 'Locktight' thread adhesive to prevent the screw from gradually loosening.

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Well maybe the whole thing is a bit of overkill but I am very pleased with the end result. The design was dictated purely by the scrap material that I had to hand but I am very pleased with the smooth travel action. The greater throw of the crank makes a very noticeable improvement in precision tracking to a line or point on the digital sliding scale. Since there are no modifications to the channel the original parts can be replaced.

In my time I have replaced a few drive cables and so I hope this one may prove a little stronger.


 

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