Help with understanding router bits

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Forum topic by Angela posted 08-12-2011 03:03 AM 5537 views 0 times favorited 7 replies Add to Favorites Watch
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205 posts in 3070 days

08-12-2011 03:03 AM

Topic tags/keywords: question router router bits

I’ve posted this on a router forum but I usually get such great advised from this forum that I thought I’d post it here too.

I was looking for a bit to do some molding. When I went on a website I saw they had a ogee and classical bit but they offered it in different sizes. I didn’t understand how to determine what size I should purchase for what I needed. If my molding is 3” X 3/4” what would I look for? Also I thought the radius was half the dia. but that won’t work with the numbers they provided.

They provided the radius, large dia, carbide height, shank size. (listed in that order. I provided the … to separate the numbers but each line is the measurements for one bit.

3/16”... 1-1/4”... 5/8”... 1/4”
1/4”... 1-1/2”... 3/4” ... 1/4”
3/16”... 1-1/4”... 5/8”... 1/2”
1/4”... 1-1/2”... 7/8”... 1/2”

I’ve done a lot of woodworking and feel this is a stupid question but still haven’t figured this out.

-- - Helping other woodworker's

7 replies so far

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Craftsman on the lake

2823 posts in 3612 days

#1 posted 08-12-2011 03:19 AM

I’ll be watching this too. I’m relegated to going down to the store. Trying to imagine the profile it will make and seeing if the size will ‘seem’ about right for what I have in mind.

-- The smell of wood, coffee in the cup, the wife let's me do my thing, the lake is peaceful.

View B0b's profile


104 posts in 2865 days

#2 posted 08-12-2011 03:23 AM

I agree with this confusion on the sizing as it seems everyone gives different info, but when routing I like a roman ogee that is 1/2” total cut height on a 3/4” edge, so you have 1/3 remaining that is perfectly square.

-- Time to get started

View Gonecrazy's profile


41 posts in 2703 days

#3 posted 08-12-2011 03:43 AM

Here this might help explain it a bit …. click the more infomation tab

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3175 posts in 3702 days

#4 posted 08-12-2011 03:55 AM

The radius that they give is the radius of the circle containing the arc (or arcs) in the profile. The diameter that they give is the diameter of the bit itself. These are not really related.

The diameter is the number that should be used to set the speed of the router when using the bit.

-- “Big man, pig man, ha ha, charade you are.” ― R. Waters

View Angela's profile


205 posts in 3070 days

#5 posted 08-12-2011 04:30 AM

Bentlyj and gonecrazy you guys are just directing me to more websites with more measurements. I’ve seen those but I don’t understand what I’m looking at or how to determine what size I would purchase for a certain size project.

Its just like the numbers I provided, they’re from MLCS website, but I still don’t understand what to look for when I see the numbers. Actually i spent a lot of time on Freud’s website last night but didn’t learn a thing about understanding the numbers I’m looking at to determine what bit I need.

bentlyj – when you go to the page you provided, are you able to determine what bit you would purchase based on the numbers provided. If so, how? what are you looking for? I understand the shank size and the overall lenght but lets say based on the chart it has 1-1/8” Dia.; 5/8” Radius; 5/32” R1; 1/2” B. Would I look at B, which looks like the overall cut lenght, and say that wouldn’t work well for a 3/4” thick wood.

-- - Helping other woodworker's

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3261 posts in 2850 days

#6 posted 08-12-2011 04:55 AM

I didn’t see what I was looking for on Bentlyj’s site but I went to the Rockler site. The ogee shown is 1/8” by 1/2” tall. It will cut 1/2 inch on the edge of a board (verticle). The 1/8” dimension is the radius of the profile. It has an inside an outside radius and they are the same. If we were looking at a simple round over bit it would be the same except it would have 1 radius instead of 2 or more. They come in 1/16” R, 1/8” R, etc. Sometimes they give the diameter of a bit because it might need to come through the opening in the base of a plunge router. They have a bunch of dimensions on some router bits. The more involved the profile gets the more dimensions they can give.

View Tootles's profile


808 posts in 2676 days

#7 posted 08-12-2011 11:42 AM

Looking at the page for the ogee bit on the web site povided by Bentlyj, there is a picture which I interpret this way:

Radius: the resultant profile cut by the ogee bit has two curves, one convex and one concave. The number given for the radius is the radius of these curves. It has nothing to do with any dimension relative to the axis around which the bit spins.

Outside Diameter: This is the largest diameter of the bit relative to the axis aroud which it spins. As said above, this is relevant for setting the speed of the router.

Carbide Height: This is effectively the “depth” relative to the router base plate that the bit will cut to. I would think, and others are free to contradict, that the bit you choose would most likely have a lesser carbide height than the thickness of the wood that you are routing – at least if you buy a bit that has a bearing tip and you intend using the bearing instead of a guide fence.

Shank size: well that has to match the collet on your router.

I hope that helps. Since I’m no specialist on routers or router bits (I’m just interpreting the diagram), I’m open to correction on any of the points above.

-- I may have lost my marbles, but I still have my love of woodworking

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