Our readers are often our best source of information, especially for tips. For example, in ShopNotes Issue No. 88, Clark Robbins of White Lake, Michigan sent in an idea he came up with for an easy way to make micro-adjustments to his router table fence with just the turn of a screw.
Now, for most operations, a tap to the fence is all that’s needed to make adjustments. Occasionally though, a more precise adjustment is required. That’s where Clark’s micro-adjustment tip comes in.
But according to David Eaton of Anaheim, California, there was one important piece of information left out of the article. “I like to know how much adjustment I’m making when I turn the screw?” To help him determine how much his fence is moving, David uses his Screw Accuracy Chart as a handy reference.
Here are David’s comments from an email he sent to ShopNotes readers’ mail:
I have been reading Shop Notes from your 1st issue. Keep up the good work.
In Issue No. 88 on page 6, you have an article about how to Micro-Adjust Your Router Table Fence. Good idea. But in none of your 88 issues has there been anything published about the accuracy of a screw to control movement for positioning.
Engineers (and metal workers) use micrometers for making precise measurements. The spindle of an inch-system micrometer has 40 threads per inch, so that one turn moves the spindle axially 0.025 inch. As you can see in my chart, a screw with a 6-40 thread has the same movement as a micrometer: One full turn equals 0.025″ or approximately 3/128″ (Note: all decimal to inch conversions are rounded up to the nearest 1/128″.)
I found the best 3 screw sizes to use are:
- 6-40 thread, one full turn = .0250″ (3/128″)
- 10-32 thread, one full turn = .0312″ (1/32″)
- 3/8″-16 thread, one full turn = .0625″ (1/16″)
Editor’s Note: I’ll use the chart for reference, but I still like to make test cuts in a piece of scrap, just in case.