# Size of tongue in T&G bit



## Val123 (Jul 17, 2013)

Just starting to learn routing.
Building a garden table using hardwood boards 20mm for the top. Want to join them together using T&G. 
Question is - looking at T&G set (e.g. from Trend) the thickness of slotting cutter is 6.35 mm, which is a bit too thin in comparison to the thickness of the board. Would it make sense to increase the gap between cutters and use separate slotting cutter for groove (I have one ~8mm). Looking around - majority, if not all, T&G sets come with ~6mm/1/4" slot size. What's the rational here?


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## Semipro (Mar 22, 2013)

Val
I use tongue or groove 1/3 thickness of board in imperial measurement 3/4" board 1/4 groove


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## jw2170 (Jan 24, 2008)

Hi Val.

Welcome to the forum.

Do you have a router table? 

IMO, 6.35mm is close enough to 1/3 the timber thickness to do. (6.35 x 3 =19.05mm)

I believe the T&G is more to stop the timber twisting than for other reasons.


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## cagenuts (May 8, 2010)

jw2170 said:


> I believe the T&G is more to stop the timber twisting than for other reasons.


I agree with James. It's just a method to help with aligning the panels for glue-up. You could use biscuits, nails, dowels, a routed glue line or simply nothing.

If however you're after the V-groove look then again as James said, 1/4" is fine for 20mm stock.


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## Val123 (Jul 17, 2013)

THanks a lot James, et al

yes, just built my own router table - proud of it 

I see your point. I was more considering it from the point of view of equalising strength of both pieces. I am joining them at 90degrees - crosswise boards fitting into 2 edges lengthwise boards.


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## cagenuts (May 8, 2010)

In that case you need to be careful of wood movement. Research breadboard ends.


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## Cherryville Chuck (Sep 28, 2010)

You can go a little thicker if you want. The 1/3 rule is probably mostly for flooring. If you had a gap under the groove side of a board and applied pressure to that edge then the weight would be held by the wood above the groove. Too thin and it would want to split. I would say that the 1/3 rule has less to do with the tongue than it does with the groove side.


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