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properly sizing router bushings to router bit size

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Forum topic by worldgeezer posted 09-27-2020 09:16 PM 436 views 0 times favorited 7 replies Add to Favorites Watch
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worldgeezer

43 posts in 700 days


09-27-2020 09:16 PM

Looking for a data table (or something like that)
(using Porter Cable dovetail jig, to cut box joints)
which tells the the result when I use a specific router bit with a specific router bushing.

For example, if I use a 5/8 OD bushing and a 1/4 router bit, then what is the width of the cut?

I’m trying to set up the P/C dovetail jig to cut 1 inch box joints in 3/4 inch plywood.
This is for fairly large storage drawers in the garage / workshop.

I scrounged around and found a box joint template for 1” box “tabs”, but can’t figure out what size bushing and what size bit to use.

First pass yesterday afternoon, I used a 1/2 router bit in a 5/8 OD bushing … but the resulting box joint was definitely not a one inch box joint.

Hopes all that makes sense.

thanks


7 replies so far

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therealSteveN

6462 posts in 1458 days


#1 posted 09-27-2020 09:25 PM

I don’t know of an iron clad list of all bushings to bits, it’s more of a spatial distance between the OD of the bushing, to where the bit starts cutting, and depending on bits, and or bushings used it’s fluid.

Maybe this page will help?

http://www.newwoodworker.com/guidebush.html

-- Think safe, be safe

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Rich

5987 posts in 1473 days


#2 posted 09-27-2020 10:50 PM


For example, if I use a 5/8 OD bushing and a 1/4 router bit, then what is the width of the cut?

I m trying to set up the P/C dovetail jig to cut 1 inch box joints in 3/4 inch plywood.
This is for fairly large storage drawers in the garage / workshop.

First pass yesterday afternoon, I used a 1/2 router bit in a 5/8 OD bushing … but the resulting box joint was definitely not a one inch box joint.

- worldgeezer

There’s no such thing as a chart. It’s simple arithmetic. Take the bushing diameter minus the bit diameter and that will give you the difference between the width of the opening the bushing is riding along and the width of the channel the bit cuts. Half of that difference will be the difference between the edge of the bit and the edge of the bushing.

To start with, if you use a 1/4” router bit, the width of the cut will be 1/4” regardless of the bushing. Regarding your 5/8” bushing and 1/2” bit, that’s a classic combination. Say you’re doing a hinge mortise, just cut the template 1/16” outside the line you want to cut, in this case the hinge.

Any combination of bushing and bit can be used as long as the bit is smaller than the inner diameter of the bushing. For example, if you use a 1/4” bit with a 1” bushing, then the difference is 3/4”. In other words, if you used a template with a 1 1/4” wide opening, the resulting groove would be 1/2” once you make passes along both sides of the template. If you were to use it to cut a pattern, the template would have to be 3/8” smaller than the piece you’re cutting.

For your box joint example, the cutting guide would have to have openings 1 1/8” wide to get a 1” wide cut. To space them correctly, the guide would have those 1 1/8” slots 7/8” apart.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

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worldgeezer

43 posts in 700 days


#3 posted 09-27-2020 10:58 PM

Makes sense …
given the “pattern or template” (the box-joint guide on my P/C dovetail jig), is 1” wide tabs (which guides the bushing so the router cuts between them) ... then what combination of bit and bushing do I need to get a 1” box joint?
Seems that I need a bit that is wider than the bushing?
The bushing rides against the template, which then creates the offset, thus material that is cut out is less than 1”.
(I’m learning to use this P/C jig … next job will be drawers for the kitchen … so I need the practice.

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worldgeezer

43 posts in 700 days


#4 posted 09-27-2020 11:48 PM

Rich,
quote Any combination of bushing and bit can be used as long as the bit is smaller than the inner diameter of the bushing
endquote

I understand the mathmatics … what’s the rationale behind it? Suppose I have a 3/8 OD bushing and a long-shank 1/2 bit. The offset is then towards the guide rather than away from it.

But maybe there’s something I’m missing here.

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northwoodsman

389 posts in 4630 days


#5 posted 09-28-2020 12:08 AM

You can’t use a router bit larger than the bushing because it won’t through the bushing. I use a Leigh D4 Jig with the finger joint template. Their manual lists the most common size combinations but I went a little further and created an Excel table. Once you determine the basic size, for every joint you have to add or subtract something like 1/64” or something like that to accommodate error. I don’t want to rain on your parade but dovetail joints are much stronger. In my opinion box joints are more for looks. If these are shop drawers there are certainly easier and better joints to use. Your tear-out on the joints in plywood is going to be awful!

-- NorthWoodsMan

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Rich

5987 posts in 1473 days


#6 posted 09-28-2020 12:25 AM


Seems that I need a bit that is wider than the bushing?

- worldgeezer

Impossible. Read the next post.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

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Rich

5987 posts in 1473 days


#7 posted 09-28-2020 12:27 AM


I understand the mathmatics … what s the rationale behind it? Suppose I have a 3/8 OD bushing and a long-shank 1/2 bit. The offset is then towards the guide rather than away from it.

But maybe there s something I m missing here.

- worldgeezer

You can’t fit a 1/2” bit in a 3/8” OD bushing. If you install a 1/2” bit with that bushing and try to plunge the router, you’ll be heading out to buy a new bushing and a new bit.

The bit must be smaller than the inner diameter, which as a simple matter of physics will always be smaller than the OD.

The bottom line is that you can’t cut 1” finger joints with a box joint template where the guides are 1” apart. Go back and read the instructions for the jig.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

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