sizing a router bushing & bit for a jig

[Brian Runau]

Need to ask for confirmation if I understand this correctly. Any help appreciated.

I plan to use a 3/4" bushing with a 1/2 in cutting size bit. The finished size routed rectangle I need is 1-3/4 x 3/8. The 3/8" size comes off the edge of the wood.

As I understand it, the math is as follows, 3/4" bushing - 1/2" bit = 1/4". So for the 1-3/4 length I need to be 2" long on the jig and the 3/8" size I need to be 1/2" since it is open to the edge on one side so I only need 1/2 of the 1/4" or 1/8"?

Is this correct?

Thanks.

Brian

[Ron Citerone]

If 3/4" is the outside dimension of the bushing, it sound right. I ALWAYS do a trial in scrap piece though. ;)

[Brian Runau]

Yes, thanks Brian

[Myles Moran]

Your math works out to me as far as I understand it.

One thing that's helped me with using bushings is to make sure that I do the math on the radius not the diameter. In your case I'm seeing that the radius difference is 1/8 so I look at each edge routing as an additional 1/8". However you make sense of it though is what's important.

[Orlando Gonzalez]

Your math is correct. A couple of things:

1. Make sure that the same part of the bushing is up against the template all the way around because the bushings are not totally concentric.
2. If you re using MDF, plywood, or whatever you are going to use to guide the bushing, make sure that they are 1/4" wider & longer than your finished dimension to account for the 1/4" offset between the the outside of the bushing and the router bit's cutting edge.

Another option is to make a template the dimensions of your rectangle and use a pattern bit to rout it.

[Brian Runau]

[QUOTE=Orlando Gonzalez;3072210]Your math is correct. A couple of things:

1. Make sure that the same part of the bushing is up against the template all the way around because the bushings are not totally concentric.
2. If you re using MDF, plywood, or whatever you are going to use to guide the bushing, make sure that they are 1/4" wider & longer than your finished dimension to account for the 1/4" offset between the the outside of the bushing and the router bit's cutting edge.

Orlando, here is a sample I did this morning. Came out routed tot he correct dimensions, but for some reason the routed area is offset by 1/16" to one side VS the other?

Thanks.

Brian

445467445468

[Jamie Buxton]

... Came out routed tot he correct dimensions, but for some reason the routed area is offset by 1/16" to one side VS the other?..

That sounds like the bushing and the bit are not concentric. I find this is common on plunge routers. Folks do make adjustable bases to reduce the offset, or you can sometimes just live with it.

[Lee Schierer]

[QUOTE=Orlando Gonzalez;3072210]Your math is correct. A couple of things:

1. Make sure that the same part of the bushing is up against the template all the way around because the bushings are not totally concentric.
2. If you re using MDF, plywood, or whatever you are going to use to guide the bushing, make sure that they are 1/4" wider & longer than your finished dimension to account for the 1/4" offset between the the outside of the bushing and the router bit's cutting edge.

Orlando, here is a sample I did this morning. Came out routed tot he correct dimensions, but for some reason the routed area is offset by 1/16" to one side VS the other?

Thanks.

Brian

445467445468

If you are using a plunge type router the motor can tilt slightly in the housing which will also throw the router bit off center on your bushing.

[Brian Runau]

Thanks Lee.

[Brian Runau]

Thanks Jamie.

[Orlando Gonzalez]

From the looks of it, it seems that the bit and bushing are not concentric as Jaime said. Looking at the picture it looks like there's more distance between the cutting edge and the outside of the bushing, but that may be due to the angle the picture was taken at. Take a ruler and check that the distance from the outside of the bushing to the cutting edge are the same all the way around. If not, you probably need to center the base to the collet using a centering cone that centers the base to the collet which may help with making it more concentric. Bushings are machined aluminum and there can be small variations in the outside dimensions of them.

Lee's statement about the plunge router motor possibly tilting is also correct.

Sometimes it's better to get as close as you can then finish with a chisel.