# Bit radius or guide radius



## bill0199 (Jun 9, 2009)

This may sound like a stupid question but I am routing the mortise for my insert plate and it appears the radius of the corners is 3/16. So if I'm routing the inside corners using a 3/8 template guide with a 1/4 diameter bit my corners will have the radius of the template guide (3/16) not the bit (1/8) correct?


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## bill0199 (Jun 9, 2009)

just a note. Here is how I intended to route the mortise. I know there a billion ways to do it but I thought this would be the easiet. The outside corners of the insert plate are 3/16". I made a fence of mdf 3" by 5' and cut a rabbit on the fence just a little lower than the depth of the 3/8 template guide. Then put the template guide on with a 1/4" bit and cut the fence so it is offset by the bushing (in this case leaving me with just shy of a 1/4" ledge. Then cut the fence into 4 pieces 15" long (the plate is 13" long). These will act as the four fences to surround the plate. So I have a perfectly sized offset fence on all four sides. Just want to make sure the inside corners are picking up the radius of the template guide or bit. Test run aside. Ha ha


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## RJM (Apr 11, 2010)

bill0199 said:


> just a note. Here is how I intended to route the mortise. I know there a billion ways to do it but I thought this would be the easiet. The outside corners of the insert plate are 3/16". I made a fence of mdf 3" by 5' and cut a rabbit on the fence just a little lower than the depth of the 3/8 template guide. Then put the template guide on with a 1/4" bit and cut the fence so it is offset by the bushing (in this case leaving me with just shy of a 1/4" ledge. Then cut the fence into 4 pieces 15" long (the plate is 13" long). These will act as the four fences to surround the plate. So I have a perfectly sized offset fence on all four sides. Just want to make sure the inside corners are picking up the radius of the template guide or bit. Test run aside. Ha ha


I don't really understand your explanations but if what you're going to do is route a rabbet (lip) in your table top to support a router plate then I think the pictures below will explain a lot.

I believe you're using a 3/8" diameter bushing with a 1/4" diameter bit. With a female template, the offset for this combination is 1/16" (3/8-1/4)/2.

For the template with the rounded inside corners and your setup, the radius of the corners would need to be 1/4" to give you a 3/16" radius in the inside corners of the rabbet. Note that the template corner radius must be larger than the bushing radius, or this just becomes the same as if you had square corners.

If you are using a female template with square corners, then the inside corners of the rabbet will be 1/8" (same as the bit). Your plate will fit but there will be a gap in the 4 corners.

Alternately, if you use a 1/2" bushing with a 3/8" bit, you will still have a 1/16" offset but the corners will have a 3/16" radius (same as the larger bit). This would be easier to do than making a template with 1/4" radius corners.


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## bill0199 (Jun 9, 2009)

Thank you very much that was what i was looking for.


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## bobj3 (Jan 17, 2006)

Just some more help and tips 

VIDEO
HOW TO Inset a Router Base Plate
Popular Woodworking - VIDEO — No–nonsense Router Table— Inset a Router Base Plate


Ron Fox's Routing Tips
Ron Fox's Routing Tips
Ron Fox's Routing Tips
Ron Fox's Quick Tips
Ron's Hints and Tips

======


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## bill0199 (Jun 9, 2009)

Thanks Bob. I've seen that video before. In my situation I've got a 3/8 thick aluminum plate that already has 3/16 radius cornders, and that is what i'm trying to match (I have no interest in finding a 3/8 diameter rabitting bit just for this purpose if such a thing even existed). Maybe it'll just be easier to file the corners to 1/4 radius and use a 1/2 pattern bit instead of bushings the end. Mistakes are part of learing I guess and inevitable. But that's what practice cuts in mdf are for.


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## RJM (Apr 11, 2010)

The sketches I posted were for a bushing and straight bit. Doesn't need to be a rabbetting bit.


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## bobj3 (Jan 17, 2006)

Hi 

I'm not a real big fan of the wabbit bit way I use a 3/4" dado clean out bit and a 1/2" bit with a 1" guide to drop the hole out..

The dado bit gives me a nice clean cut on the top,it's made to plunge in and the 1/2" bit makes it easy to plunge down and to cut out the hole in 1 1/2" thick stock..

MLCS Dado Clean Out Router Bits

MLCS solid carbide router bits

==


RJM60 said:


> The sketches I posted were for a bushing and straight bit. Doesn't need to be a rabbetting bit.


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## bill0199 (Jun 9, 2009)

RJM60 said:


> The sketches I posted were for a bushing and straight bit. Doesn't need to be a rabbetting bit.


Thanks. I understood from the sketches it was a straight bit. The reference to using a rabbit bit was because that was part of the method used in the video link.


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## bill0199 (Jun 9, 2009)

RJM60 said:


> I don't really understand your explanations but if what you're going to do is route a rabbet (lip) in your table top to support a router plate then I think the pictures below will explain a lot.
> 
> I believe you're using a 3/8" diameter bushing with a 1/4" diameter bit. With a female template, the offset for this combination is 1/16" (3/8-1/4)/2.
> 
> ...


Can anyone tell me the radius relationship between bushing, router bit and inside corner of a female template. By this I mean, I took my insert plate, which has 3/16 radius corners and routed around it with a 3/8 template guide and 1/4" bit resulting in a 5/16th (5/8 all around) from the newly cut male template to the resulting female template. Just curious how the female template corners are formed. I checked and they have a 1/2" radius. What is the formula for this?


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## bill0199 (Jun 9, 2009)

Is the calculation 5/16 (template offset from router baseplate plus 1/4" bit diameter) plus 3/16th radius of the base plate corners? I need coffee.


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## jschaben (Jun 21, 2009)

bill0199 said:


> Is the calculation 5/16 (template offset from router baseplate plus 1/4" bit diameter) plus 3/16th radius of the base plate corners? I need coffee.


Hi Bill - Sounds reasonable: Female template radius = offset + radius of male template (for lack of better term). 
Plotted your original situation out in DeltaCad and it works out to 1/2" as you got. Will need to plot out several different combinations of bits-bushings-radii to check it for sure. Hopefully someone else will be able to come up with a mathmatical proof/explanation before then.:blink:


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## xvimbi (Sep 29, 2009)

The way I see it:

A 3/8 bushing with a 1/4" bit will get you a 1/4" groove centered at a distance of 3/16 from the template (assuming the bit is centered in the bushing).

The outside of the groove is 5/16 from the template, and the inside is 1/16 from the template (adding or subtracting, respectively, 1/8 = half the width of the bit).

The radii of the groove can be found by adding the respective distances to the radius of the template:

- radius of near edge of the groove: 4/16 = 1/4
- radius of far edge of the groove: 8/16 = 1/2
- radius of center of the groove: 6/16 = 3/8

Now the formula:
distance of center line from template = (diameter of bushing) divided by 2
distC = diamBu/2

distance of near edge from template = (distance of center line from template) minus (half the diameter of the bit)
distN = distC - diamBi/2

distance of far edge from template = (distance of center line from template) plus (half the diameter of the bit)
distF = distC + diamBi/2

radii: add respective distances above to radius of template
rC = rT + distC = rT + (diamBu/2)
rN = rT + distN = rT + (distC - diamBi/2)
rF = rT + distF = rT + (distC + diamBi/2)

There are certainly different ways of looking at it, but that's how I like to look at it. 

Of course, I could be totally wrong...

Cheers! MM


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## jschaben (Jun 21, 2009)

xvimbi said:


> The way I see it:
> 
> A 3/8 bushing with a 1/4" bit will get you a 1/4" groove centered at a distance of 3/16 from the template (assuming the bit is centered in the bushing).
> 
> ...


Sounds like it will work. Took 4 cups of coffee but I think I understand it.:wacko:


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## bill0199 (Jun 9, 2009)

Thank you very much for that detailed explanation Mischa. Very helfpful. Now I assume if the male template has no corner radius (90 degree corner) that does not factor in and your outside radius would be the sum of the offset plus bit diameter. In this case a 3/8 bushing with 1/4" bit would give you a far radius (female template) of 5/16. Correct? What I want is to use the template i have with 3/16" corners to route a female template(does not matter what size overall as it's just the corners I want) that has 3/4" inside corner radii. So, with that calculation I could use a 5/8 bushing with a 1/2" bit and should get 3/4". male template corner= 3/16 plus offset (1/16) plus bit diameter(8/16)= 12/16 = 3/4.
That should do it. I was confused about this originally as the JessEm plate has 3/4" radius and calls for a 1 1/2 pattern bit to acheive this (which i have no intention of buying) or i could use a forstner or hole saw bit the same size. Again, I have neither of those but i do have the bushings and straight bits. Their manual says you can buy their template but again more money. The strange thing is the template says to use a 3/4" bushing with a 1/2" bit. Assuming a) the corner radius is larger than 3/4" wouldn't that result in a corner radius of 5/8 (3/4 less the offset of 1/8)? If the corners were smaller than 3/4 then you'd have a 1/4 inch radius.
Take a look at their instruction manual and tell me if i'm wrong:
http://www.jessem.com/CURRENT_PRODUCTS/routrplate.pdf


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## xvimbi (Sep 29, 2009)

bill0199 said:


> Thank you very much for that detailed explanation Mischa. Very helfpful. Now I assume if the male template has no corner radius (90 degree corner) that does not factor in and your outside radius would be the sum of the offset plus bit diameter. In this case a 3/8 bushing with 1/4" bit would give you a far radius (female template) of 5/16. Correct? What I want is to use the template i have with 3/16" corners to route a female template(does not matter what size overall as it's just the corners I want) that has 3/4" inside corner radii. So, with that calculation I could use a 5/8 bushing with a 1/2" bit and should get 3/4". male template corner= 3/16 plus offset (1/16) plus bit diameter(8/16)= 12/16 = 3/4.
> That should do it.


Yes. In other words: you are looking for a far radius of 3/4".

with distC = diamBush/2
and rF=rT+distF = rT + (distC + DiamBit/2)

=> rF=rT+distF = rT + (diamBush/2 + DiamBit/2)
with rF = 3/4 and rT = 3/16

3/4" = 3/16"+diamBushing/2 + DiamBit/2
9/16" = diamBushing/2 + DiamBit/2
9/8" = diamBushing + DiamBit

As long as your bushing and your bit add up to 9/8" you are fine (of course, diamBushing > diamBit)




> I was confused about this originally as the JessEm plate has 3/4" radius and calls for a 1 1/2 pattern bit to acheive this (which i have no intention of buying) or i could use a forstner or hole saw bit the same size. Again, I have neither of those but i do have the bushings and straight bits. Their manual says you can buy their template but again more money. The strange thing is the template says to use a 3/4" bushing with a 1/2" bit. Assuming a) the corner radius is larger than 3/4" wouldn't that result in a corner radius of 5/8 (3/4 less the offset of 1/8)? If the corners were smaller than 3/4 then you'd have a 1/4 inch radius.
> Take a look at their instruction manual and tell me if i'm wrong:
> http://www.jessem.com/CURRENT_PRODUCTS/routrplate.pdf


For an inside template you need to subtract the radius of the far edge of the groove from the radius of the template (as opposed to adding the radii for an outside template). The 3/4" bushing with a 1/2" bit gives you a far-edge radius of 5/8" = 10/16". So, I assume the JessEm template has a corner radius of 13/16", which would give you the desired 3/16".

My head is spinning...

Cheers! MM


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## bill0199 (Jun 9, 2009)

My head is spinning as well. Here is what i know. The JessEm base plate has outside corners radii of 3/4". In Roberts post earlier his thumbnail showing rounded inside corners (like the template you can buy from JessEm) stated that as long as the inside radius of the corner was larger than the radius of the bushing, then the resulting "far edge" radius would be the same as that of the bushing less the offset. Is this incorrect? You said 
"you need to subtract the radius of the far edge of the groove from the radius of the template (as opposed to adding the radii for an outside template). The 3/4" bushing with a 1/2" bit gives you a far-edge radius of 5/8" = 10/16". 

So, knowing that the outide corner radii of the base plate are 3/4" or 12/16, and the resulting far edge radius of bushing plus bit is 10/16, We are missing 2/16. Therefore The JessEm template corners would need to have inside radii of 22/16 or 1" 3/8. (22/16less far radius 10/16 = 12/16 = 3/4). Or am I lost?


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## bobj3 (Jan 17, 2006)

Hi Bill

I don't get it why all the fuss over the radius on the corners  they are not a big deal as long as the sides are true with the edges of the plate and the plate can drop in easy, you can a 1/4" gap on the corners and the plate will still be just fine and do the job you want it to do..  you can clip the corners off the plate with the band saw /table saw because it's not a big deal....

It's nice to get just right but it's not a must have thing, you can drive yourself nuts playing with that...

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## xvimbi (Sep 29, 2009)

bill0199 said:


> My head is spinning as well. Here is what i know. The JessEm base plate has outside corners radii of 3/4". In Roberts post earlier his thumbnail showing rounded inside corners (like the template you can buy from JessEm) stated that as long as the inside radius of the corner was larger than the radius of the bushing, then the resulting "far edge" radius would be the same as that of the bushing less the offset. Is this incorrect?


No, that is correct. 

But I think we are getting 'far edge' and 'near edge' mixed up. The 'far edge' in my made-up nomenclature refers to the edge of a groove farthest away from the template. Likewise, the 'near edge' refers to the edge of a groove closest to the template. 

For this discussion, when making a template from your base plate (I think you are referring to this template as a female template), the far edge becomes important. Now, when making a table cutout/rabbet from that female template, the near edge becomes important.



> You said
> "you need to subtract the radius of the far edge of the groove from the radius of the template (as opposed to adding the radii for an outside template). The 3/4" bushing with a 1/2" bit gives you a far-edge radius of 5/8" = 10/16".
> 
> So, knowing that the outide corner radii of the base plate are 3/4" or 12/16, and the resulting far edge radius of bushing plus bit is 10/16, We are missing 2/16. Therefore The JessEm template corners would need to have inside radii of 22/16 or 1" 3/8. (22/16less far radius 10/16 = 12/16 = 3/4). Or am I lost?


I was wrong. You need to subtract the radius of the near edge (offset), not the radius of the far edge, just as it is shown in Robert's picture. The 3/4" bushing with a 1/2" bit gives you an offset of 1/8" = 2/16". To achieve the 3/4" = 12/16" radius, the radius of the JessEm template would have to be 14/16" = 7/8".

I hope that is correct now.


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## bill0199 (Jun 9, 2009)

Bob you are absolutely right! But at this point it is a math problem I just want to solve because... well I like to understand causal relationships and I'm ALREADY nuts so it's not a long drive. Ha ha. And Mischa I understood your definitions of far and near edge I think with Robert's picture i confused template with template 'guide'. I think i've got it now. Thanks to all of you for helping me through this. I didn't mean to make this a 10 page string. Sometimes it's better to just dig into a 2$ piece of hardboard to test, than to try and map everything out on paper.


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