Difference in tongue and groove router bits

Question

Whilst looking for router bits I came across 2 different types of tongue and groove bits.

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Set A

This set I call normal

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Set B

are these strange ones

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There is only £1 difference in price, so I am wondering what the pros and cons are for each type.

Answer 1

Short Answer

I like @Graphus' comment where he/she states that Set B is mainly intended as shown in the image, for attaching solid-wood trim to the front edge of plywood or other manmade boards. However if you are using these bits for jointing solid-wood, there are applications where Set B could be better than Set A.

More surface area within the joint = stronger joint

therefore

For pulling mechanical forces

Butt joints are ever so slightly stronger with Set A (although I doubt much practical difference)

Corner Joints can be stronger with Set B

For flexing mechanical forces, Set B wins all round for strength
(in certain situations)

Detailed answer

For a detailed answer we need to look at the properties of the joints possible with each set.

Butt Joints Set A

In order to answer this question more effectively, I have found dimensions for this set of bits.

With this set, you will have a joint plane length of 1¾"

¼" + ½" + ¼" + ½" + ¼" = 1¾"

Butt Joints Set B

Now this is a bit more complicated and takes me back to my geometry classes in school. To help us with this I looked at the demensions marked up on the wood joint image provided.

The squared bit of the tongue and groove seems square, so for arguement sake, we will treat it as square. That part of the joint has three ¼" planes to make a total of ¾". For the slanted parts, this is the bit which uses geometry. I could get this to display better if I could use MathJax within the writing of this answer, but here goes...

If you create an imaginary triangle behind the top slant, the horizontal part would be equal to ¼" and the vertical part would be 0.375"

(1" - ¼") ÷ 2 = 0.375"

The slant is calculated using pythagorus therorum 'The square on the hypotenuse is equal to the sum of the squares on the other two sides'.

so, the slant equals the square root of ¼" squared + 0.375" squared.

0.25² = 0.0625
0.375² = 0.1407
0.0625 + 0.1407 = 0.2032

therefore the length of the slant equals

√0.2032 = 0.4507"

Because there are 2 slants, that gives a total joint plane length of 0.4507" + 0.4507" + ¾" = 1.6514"

This is slightly lower than one tenth of an inch less than Set A so not a lot of difference.

Corner Joints Set A

As with butt joints, you will have a joint plane length of 1¾"

¼" + ½" + ¼" + ½" + ¼" = 1¾"

Corner Joints Set B

Now this is where Set B is much better than Set A.

I am no good at drawing, but if you can picture in your mind the way the joint will be produced, if you are corner jointing a piece of wood which is at least an inch thick, that piece can be routered so that you can use the ½" depth available.

This will give you a total joint plane length of 2.1514" (0.4014" more than Set A)

0.4507" + 0.4507" + ¾" + ½" = 2.1514"

Corner Joints with Set B would be weaker when using wood less than 1" thick under the same flexing mechanical forces.