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Micro-Adjustment Mechanism #2: Improved alignment of the threaded rods

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Blog entry by DaveFFMedic posted 625 days ago 4265 reads 2 times favorited 6 comments Add to Favorites Watch
« Part 1: How it Works Part 2 of Micro-Adjustment Mechanism series no next part

After sharing my micro-adjustment mechanism with my dad and discussing its shortcomings, he suggested epoxying the two threaded rods together to achieve better alignment. So I gave it a try.

I began by creating an alignment jig using two V-blocks and some rubber bands.

I notched out the V shape in the end of a piece of scrap MDF on the table saw. Next I drew a few lines on a scrap piece of pine and predrilled some holes in the bottom. To ensure the two V-blocks are lined up, I clamped a solid piece of threaded rod using rubber bands and screwed the blocks to the pine.

Because the threaded rods are different diameters and the V-blocks are the same height, I needed to hold the threaded rods at the same height. Because the threaded inserts are the same diameter, I put two on each threaded rod and clamped them in the jig.

I used JB Weld because it was the best value at my hardware store. It gave a strong bond and seemed to have the most glue for the money. The only down side is the long set time. I don’t have much patience, so waiting overnight to see if it worked was torturous.

I put a dab of epoxy on the ends of the threaded rods and then on the nuts. I added some locktite to the threaded rods for good measure.

The end result: almost perfect alignment.
Other issues:

1. backlash is apparent, a preloading setup is needed for the fences to maintain the accuracy

2. the thumbwheel is not an integral part of the mechanism any more.

More to come!



6 comments so far

View ptweedy's profile

ptweedy

75 posts in 1991 days


#1 posted 625 days ago

one way of eliminating backlash is to use grade 8 nuts instead of insert nuts. They have smaller tolerances and so less backlash. grade 8 allthread is also available.

View Bogeyguy's profile

Bogeyguy

457 posts in 665 days


#2 posted 625 days ago

Having missed your first posting I wonder if you could explain what you are trying to accomplish. What will this device be used for??

TY, Art

-- Art, Pittsburgh.

View DaveFFMedic's profile

DaveFFMedic

67 posts in 764 days


#3 posted 625 days ago

Art,

The micro-adjustment mechanism can be used to dial-in a jig for accuracy. I created this mechanism to adjust the squareness of my crosscut sled fence. The next project is a box joint jig. I want to create the precision which can be had by the Incra iBox jig.

The micro-adjustment mechanism takes advantage of the difference between the two thread pitches. One side is connected to a fixed fence and the other side is connected to a moveable fence. As the mechanism turns, one side lets out and the other side takes in. Since the two threads have different pitches, they let out and take in at different rates. So, the mechanism creates a sort of expanded scale. For every revolution of the mechanism, the moveable fence moves 5 thousandths of an inch.

I hope this explains a little better what I’m trying to achieve here.

Dave

View Earlextech's profile

Earlextech

903 posts in 1288 days


#4 posted 624 days ago

Or – http://www.microfence.com/ – where Rich Wedler manufactures fantastically precise tools that will do exactly what you trying to accomplish.

-- Sam Hamory - The project is never finished until its "finished"!

View MitchMan's profile

MitchMan

75 posts in 946 days


#5 posted 624 days ago

Great solution! To get even more dead-center you could even make your left and right holders be the same piece of wood, and just cut a divot in the middle to allow space for the two nuts. Then you wouldn’t even have to align the left and right holders.

View DaveFFMedic's profile

DaveFFMedic

67 posts in 764 days


#6 posted 617 days ago

I wanted to post an update on this project, but it doesn’t quite deserve its own blog entry. Since my last posting, I reworked the fixed and movable fences using MDF. I figured it would be a softer material and easier to install the threaded inserts. I was correct in that it made the installation easier. It had plenty of hold and adding two additional threaded inserts (one to each fence) allowed me to preload the threads and eliminate any backlash. Unfortunately that is all of the good news. I was using a dial indicator to show the accuracy in the mechanism. It didn’t move with the accuracy I thought it should. The mechanism moved with 1 thousandth of an inch some of the time, other times it moved with 5 thousandths of an inch accuracy. I was not happy with this variation in the accuracy.

As I was filming a video for my next blog post, the micro adjustment mechanism broke. The two halves separated at the epoxy joint. This was especially disappointing for me because I used a very strong epoxy. My guess is the joint was under too much stress from a slight misalignment of the mechanism and fences.

So, its back to the drawing board. I’m exploring multiple solutions: one is to use bolts instead of threaded rod. The thought is that the larger surface area of the bolt head will give a stronger bond. I also am taking more care in preparing the surfaces before gluing them together. I also plan on using two threaded inserts on two thumbwheels and gluing them together to give a strong bond. The second solution is one I am revisiting from years ago: build the mechanism from one piece of metal. Not having a lathe or any measurable metal-working skills, I attempted to use my drill press and files and saws to create the threaded rod. My first attempt was , in my opinion, pathetic. The threads were very rough and broken. It also took way too long to make. I really need to find a friend with a metal lathe to turn these pieces. If I’m still struggling with this after Christmas, I may make a trip down to Harbour Freight and buy the Mini Lathe and teach myself. Although, spending $500 on a machine so I don’t have to buy a $150 jig doesn’t make financial sense.

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