How to cut tongue and groove for 1 3/4" boards on a bevel?

[Sparky Norton]

Hello Fellow Creekers,
I offered to help a friend (read: little to no compensation for this) who wants to build a 6' diameter circular hot tub out of northern white cedar. We will be using 2x6 rough cedar boards which I will plane on the inside for a final thickness of 1 3/4". Each board will have a 4.5 degree bevel and a tongue and groove edge (preferably V-groove).

For the tongue and groove, I plan to build a jig for the router table to keep the edge parallel to the fence to compensate for the 4.5 degree bevel. No problem routing the groove but I have a problem with the tongue. I can't find a router bit for the tongue that has a cutting length of more than 1.25", which won't work on a 1 3/4" board without making multiple pass which I don't want to do.

Does anyone know of a company that makes a tongue and groove router bit set that has a cutting length of 1.75" or more? Or... does anyone have any ideas on a better/easier way to do this? Please keep in mind that I am a relative "newbie" when it comes to woodworking so please don't hesitate to offer suggestions. I welcome the help.

Thank you for your help,
Sparky

[Kirk Poore]

Instead of going tongue & groove, you could groove both edges and make splines of appropriate size to fit the grooves.

Kirk

[Sparky Norton]

Thanks Kirk,
That would certainly help with alignment but the friend/customer feels that the tongue and groove would help with "waterproofing". He currently has a similar hot tub but wants a new one that will hopefully address some problems he has with it. One of the problems is that after the tub/wood dries out it loses a lot of water when he fills it. He feels that the tongue and groove will help reduce the water loss when filling. I don't know if it will or not but that's what he wants to do.

Sparky

[Tony Joyce]

Thanks Kirk,
That would certainly help with alignment but the friend/customer feels that the tongue and groove would help with "waterproofing". He currently has a similar hot tub but wants a new one that will hopefully address some problems he has with it. >>>>>One of the problems is that after the tub/wood dries out it loses a lot of water when he fills it. <<<<< He feels that the tongue and groove will help reduce the water loss when filling. I don't know if it will or not but that's what he wants to do.

Sparky

Therein lies the problem, as long as it is wet it will stay sealed, but when it dries out it will contract(shrink). This will always be a problem unless it stays full.

[Anthony Whitesell]

Am I reading this correctly? You have a 6" wide 1 3/4" thick board with 4.5 degree bevel on each edge. One edge will have 1 3/4" deep groove while the other has a 1 3/4" tongue.

What bit are you using to route the groove? Could it not also be used to route the tongue?

[David Nelson1]

Therein lies the problem, as long as it is wet it will stay sealed, but when it dries out it will contract(shrink). This will always be a problem unless it stays full.

That's the reason why wooden boats are kept slung in the lift till everything swells back again!

[David Thompson 27577]

.............................For the tongue and groove, I plan to build a jig for the router table to keep the edge parallel to the fence to compensate for the 4.5 degree bevel. No problem routing the groove ......................

I doubt that you'll find a router bit that will do tongue and groove joints on stock that thick.

Got a tablesaw? If so, it seems like a simple problem -- four setups to cut the tongues, one more for the groove. Plan to cut the grooves before the bevel. Cutting the tongues will cut the bevel at the same time.

[David Thompson 27577]

Am I reading this correctly? You have a 6" wide 1 3/4" thick board with 4.5 degree bevel on each edge. One edge will have 1 3/4" deep groove while the other has a 1 3/4" tongue.

What bit are you using to route the groove? Could it not also be used to route the tongue?

I think he's trying to find a router bit set that will cut tongue and groove joints on stock that is 1.75" thick. Not one that will cut a 1.75" deep groove.

[Matt Winterowd]

The spline would be no more or less waterproof than an integral tongue. If the water would not work it's way around the tongue, then it won't work it's way around the spline. However, I concur with the majority that this is just inherent to the design of a wooden structure. Don't you suppose that all wooden hot tubs, barrels, and boats would all be made with tongue and groove if it provided any advantage?

[Sparky Norton]

Thank you all for your comments and suggestions. They are all good. Let me address some of the questions.

Ideally I would like to create the tongue and groove on the router table. One pass for the tongue, one for the groove. It's looking like that may not happen.

The stock is 1.75" thick but the groove only needs to be 1/2" or so.

The hot tub will be located at a camp in the woods, outside and will only be used a few times a year. I'm not sure if he keeps water in it during the summer months but it certainly needs to be drained at least once a year before winter sets in. I'm not sure if he has a reason for draining it during the summer. I will find out.

Perhaps I was looking for an easy/fast way to do this without having to make multiple setups on the table saw to make the tongue and groove cuts. I'm now liking Kirk's suggestion on making a groove in both edges and using a spline. It will also save $$$ since he won't have to buy a $100+ router bit set.

I'm open to more suggestions and in the meantime I will talk to my friend about doing something other than the V-groove that he wants to use.

Thanks again for the responses,
Sparky

[Ed Hazel]

why does he want to use a tongue and groove joint? The wooden water tubs that I have seen do not have a tongue and groove joint in them. Personally I would skip this step seems like it's adding a lot of extra work. What do you plan on using for containment rings? You also need to cut a groove in the bottom of the stave's to hold the bottom in.
Old school how to video on making a wooden tub (the tub part starts about a third of the way in)
http://www.youtube.com/watch?v=l2KJbRHO76s

[Chris Padilla]

Look up birdsmouth joint. There are firms that make router bits and an application like this is perfect for it...assuming the bits exist for your particular design.

[Richard Shaefer]

if you want a wood structure that's waterproof, T&G may not be the way to go. Neither barrels or boats are(were) built this way. Google carvel planking and you'll get the idea.

[Sparky Norton]

Hi Ed,
I don't necessarily agree with my friend's reasons for the tongue and groove but since I'm not going to help with the building of the hot tub, only the milling of the wood, I wasn't going to question his reasons. He feels it will add strength, help with alignment and leakage (when filling the dry tank).

Yes, the tongue and groove will add considerable work for me in the milling process but may help him with the assembly so I was willing to give it a try. He has some containment rings (adjustable bands) that he used on his previous tub and plans to reuse those. I planned on making a dado in the bottom of the staves for the floor of the tub.

I just got off the phone with my friend. He is open to using a spline instead of tongue and groove. I don't mind doing this for him if he feels it will help him in the assembly process and HE feels it will add strength. I'm not going to get into that debate. :-)

I haven't watched all the video yet, just started. It looks interesting. Thanks for the link Ed.

Sparky

[Sparky Norton]

I guess I could have done a little more "homework" before telling my friend I would help him. Thanks Chris and Richard for your comments.

[Chris Padilla]

http://www.youtube.com/watch?v=l2KJbRHO76s

Wow! What an awesome video. Now I'm going to have to find the whole documentary. Very, very cool!

[Sparky Norton]

Old school how to video on making a wooden tub (the tub part starts about a third of the way in)
http://www.youtube.com/watch?v=l2KJbRHO76s

Old school for sure but very informative and helpful. Thanks Ed.

[Chris Padilla]

http://www.duckworksmagazine.com/04/s/articles/birdsmouth/index.cfm

If you really want to dig into birdsmouth joints, the above is quite exhaustive! :)

[Harvey Pascoe]

Tongue and groove is definitely NOT the way to go as the tongue will swell and split the sides of the groove. Look at how boats and barrels are made with a simple butt joint which then relies on wood swelling to seal the joint. Beveled joints and heavy copper bands about 2" wide would be my choice.

That is a very tricky project you propose, and without some experience in that sort of thing I wouldn't try it. You are not only dealing with water but HOT water that will cause the wood to swell even more so that you run the risk of the tub self-destructing via excess pressure.

There is a reason why barrels and buckets are not straight cylinders. They are curved or tapered so as to transfer excess stress by causing the slats to slip transversely. With boat and barrel construction the amount of swelling is carefully calculated. Boat hull planking that expands can transfer upwards to the deck, but a round vessel expanding more than calculated will buckle outward unless designed to do otherwise.

[Kent A Bathurst]

Wow! What an awesome video. Now I'm going to have to find the whole documentary. Very, very cool!

Yeah, buddy - very cool. If you find it, Chris - let us know, OK?


thanks

kent

[Matt Winterowd]

You can purchase it from here: http://www.der.org/films/bens-mill.html