Originally Posted by woodflow
I, too, have broken 2 1/8" spiral bits. I have several different MilesCraft Spirocrafter sets and I wanted to know what bushing sizes to use with what size bit to make inlays. They wrote back with this guidance:
If you use a 1/8" bit, then the size of the bushings should be two (2) times the diameter of the bit. Therefore, 1/8" bit usesbushings 1/4" difference, eg., 1/4" bushing to make the inlay-pocket, and 1/2" bushing to cut the inlay material. Or, 3/8" bushing for the pocket, and 5/8" bushing for the inlay material.
Can anyone help a math-deficient person to figure out what size bushings I should use if I use a 1/4" bit? 1/2" bit?
Let's start with the math and then add a few thoughts......
Like they did when they added the bit size 1/8" and 1/8" to get the 1/4" you have to do the same thing with the new bit size. So 1/4" + 1/4" = 1/2" or the difference (off-set) needed. In a standard guide bushing set you need to find a guide bushing that will be large enough for a 1/4" bit to work in. In this case let's say that you have a set with a 3/8" bushing... but when we go from the standard set again the second size you need would be 7/8", one that you will most likly not have. This pushes you to a 1/2 bushing and then 1".
For the 1/2" bit we do the same thing 1/2 + 1/2= 1", then the first bushing needs to be a 5/8" to fit the bit, the second one would be 1-5/8 but for a typical pc bushing you have gone over the maximum size.
Now the thoughts:
First I don't know how well changing bushing will work for doing inlays... I've never tried it. I do know that the 1/4" kit works just like the 1/8" one (exceptions for radius issue).
Second, every time you increase the guide bushing size you make the inlay smaller and it can change the shape of the inlay if the guide can not touch all through the process (if the corner radius is smaller then the radius of the guide bushing, you can read this as the inlay not working).
If you want to want to play with this let us know how it turns out.
If this is still unclear let me know I can make up some sketches......