Fixing a split chair seat

[Ted Harrison]

Hi gang.  I'm trying to join together a split chair seat that is made of cherry.  It's 1" thick.  The longest fine thread screws I can find say they will work on hardwood up to 3/4".  Would I be safe in using these or should I dig out my biscuit joiner?  I appreciate any input I could get on this.  Thanks alot.

[John Schaben]

If it's apart far enough to use biscuits, I'd be inclined to use a spline. If it is an antique or collectable, pockets will pretty much destroy any value from that aspect. Otherwise, a couple coming in from each direction should work.

[Ted Harrison]

Thanks for the answerback John.  I've got the chair torn completely apart so the seat is in two pieces accessable to a biscuit cutter.  It is not an antique, far from it.   So, I probably will spline it.  Thanks again.

[Ken Darga]

I'd recommend a ''spline'' joint vs biscuits.

 

Make a spline from 1/4'' baltic birch plywood would make a stronger joinery, and cut 1/2" wide.

 

Make the spline overall length, so the ends are 3/4'' in from the end of the seat---

so as to make the spline joint invisible, in the end product.

(You don't want to change the appearance from the original design, is the reason for making a ''blind'' joinery).

 

Machine a ''stopped'' dado (groove) in each seat half---make the dado 1/16" deeper, in each seat half, for the glue-up. 

 

Use glue sparingly in the dado---a glue injector is ideal for this purpose---spread the glue evenly in the groove, wetting all surfaces,  using a small brush.

 

Use Titebond III---(for a longer working time during the assembly and glue-up).

 

Use clamps, and pull-up the joint ''closed''---so there is no visible seam.

 

NOTE:  make a dry-run of the assembly before gluing-up and final assembly,

so as to see if all joints meet your expectations.

 

Some glue will escape thru the cracks during glue-up---

wipe off all excess visible glue ASAP, with a damped cloth, after clamping, before the glue starts to set.

A scraper can be used to go over the area, so as to remove all traces of glue on the exterior surfaces.

 

[Jay Boutwell]

Hi Ted, I guess how I would fix it would depend on why it split in the first place.  If it split because the grains are all straight in the area of the split and if the split actually damaged the wood back away from the split.  Is the split in an area where the wood was joined in the first place or is it a split from weak grain.  If there is any doubt in any of the above I would resort to using some long dowels.  My reasoning for this is that it would bridge the area of the split further back than that of a biscuit spline or even a screw.  This would be of benifit to you especially if the split is located in any area near a leg or the support area of the back. 

[Ted Harrison]

Hi Jay.  Thanks for the reply.  The seat actually split in half, front to back, nice and clean, real smooth.  I would venture to say weak grain.  The chair is nothing special.  Just your standard wooden dining room style.  I hadn't even considered using dowels.  Makes perfect sense though and it would make the seat stronger.  So, I guess I'll dig out my doweling jig and give that a try.  Thanks again Jay, I really appreciate it.

[Ted Harrison]

Thanks for the reply Ken.  I'm definitely going to keep this procedure as an option.  Very nice explanation of the technique.  Thanks.

Ken Darga said:

I'd recommend a ''spline'' joint vs biscuits.

 

Make a spline from 1/4'' baltic birch plywood would make a stronger joinery, and cut 1/2" wide.

 

Make the spline overall length, so the ends are 3/4'' in from the end of the seat---

so as to make the spline joint invisible, in the end product.

(You don't want to change the appearance from the original design, is the reason for making a ''blind'' joinery).

 

Machine a ''stopped'' dado (groove) in each seat half---make the dado 1/16" deeper, in each seat half, for the glue-up. 

 

Use glue sparingly in the dado---a glue injector is ideal for this purpose---spread the glue evenly in the groove, wetting all surfaces,  using a small brush.

 

Use Titebond III---(for a longer working time during the assembly and glue-up).

 

Use clamps, and pull-up the joint ''closed''---so there is no visible seam.

 

NOTE:  make a dry-run of the assembly before gluing-up and final assembly,

so as to see if all joints meet your expectations.

 

Some glue will escape thru the cracks during glue-up---

wipe off all excess visible glue ASAP, with a damped cloth, after clamping, before the glue starts to set.

A scraper can be used to go over the area, so as to remove all traces of glue on the exterior surfaces.

 

[David Shipp]

Type II yellow glue which provides a bit of water resistance, (see the chart provided by Kreg) is stronger than the wood it is gluing if the wood is clean and the joint is clamped and allowed to set. The purpose of dowels and biscuits is to align the joint and other than in a case where there is additional slitting as suggested by Jay, above, Otherwise,  the dowels biscuits splines etc do not add any extra strength.   For all those who wish to argue this point, please first properly glue up some scrap wood and then after a day or two, try to break the joint. It won't break where the glue joint is.  That said, if there is a flat service and you can get your jig and kreg clamps  attached, the kreg screws will make alignment easier and faster after you apply the glue. 

[Ted Harrison]

That's a good point Dave.  Thank you.

David Shipp said:

Type II yellow glue which provides a bit of water resistance, (see the chart provided by Kreg) is stronger than the wood it is gluing if the wood is clean and the joint is clamped and allowed to set. The purpose of dowels and biscuits is to align the joint and other than in a case where there is additional slitting as suggested by Jay, above, Otherwise,  the dowels biscuits splines etc do not add any extra strength.   For all those who wish to argue this point, please first properly glue up some scrap wood and then after a day or two, try to break the joint. It won't break where the glue joint is.  That said, if there is a flat service and you can get your jig and kreg clamps  attached, the kreg screws will make alignment easier and faster after you apply the glue. 

[Jay Boutwell]

Not trying to be argumenative but to qualify why I suggested a dowel over other methods is for adding strength to the wood area around the split area.  It is true a split can be glued and it will be strong providing that there is no additional weak grains in the immediate area.  From experience, splits in chair seats where there is alot of flat surfact that is having its edges pushed against by a constant pressure, such as a chair bottom, the cause of a split is most often the results of a grain failure.  Also from experence from working wood, I know that chair bottoms are built using multiple pieces laminated together to form a wide plank in which to make a seat.  Unless the material is carefully selected so that the grains are strong and not running mostly in a straight line that material is the part that will fail.  To be strong they should be interlocking grains.

In this example the split is stated to be a straight smooth split and that tells me that it is a straight grain failure. The reason I suggested using dowel is to span across more of these straight grains in an attempt to bridge the area were there is most likely additional weak grains and due to fail again in another location.  True the glued area with most likely hold but what about the grain next to it?  It stands to reason that this same area will have more that just one or two grains running in the same direction and that will split out when pressure is applied long enough and frequent enough.  My reason for the dowel is just for that reason to suggest a long and perhaps a permanent cure to the problem. 

The two other gentlemen, who are quite knowledgeable, suggested a spline and there was mention of a biscuit.  Both are excellent and proper examples of a cure.  The dowel reaches across the area in a greater distance that either a spline or a biscuit. The wood simply split from fatigue and from the gain running in a straight line.  That straight line was the weak point. 

As for a spline, biscuit or a dowel acting only as an alinement and does not add strength is not totally acurate. any time you add a piece of wood that runs in a different direction that the other grain does add substancial strength as you have added a connection between all the grains that it bridges.  The same principle of glue laminated beams or even plywood.  It is the bridge between the grain of its opposing grain.  Now when I am talking about a dowel I don't exactly mean the short dowels you get with the doweling jig but I am talking about a dowel  that you cut to fit the situation.  If the piece has a width of 4 inches across the stock where the failure occurred I would use a dowel I cut to extend well in to the opposing wood even if it was a piece 5 or 6 inches long and would use enough dowels to complete a closure of the split and at least 4 to 5 dowels.  

If the failing piece of wood was extensive, although it would mean refinishing the chair, I would cut out the piece and replace it with a different piece.  The dowel here offers a method of a permanent repair and I would in no way resort to pocket screws as this actually does not fit the situation.  It is ugly and much very un- attractive  The methods of the spline, the biscuit or the dowel is one you do not see once completed.

As far as clamping it together and you have a problem using the bar clamps or other clamps there is a better suggestion that I will give you.  Electricians cary around with them one of the best clamps you could ever get your hands on.  It is the plastic electrical tape. I use it by the rolls for clamping objects that are odd in shape.  It is cheap and can fit about any shape.  The reason it works is that it has a  "memory" to return back to its origional shape after being streached.  Streach it tight around your object and it will get even tighter as it sits and begins to return to its origional form.

Glue up a board and true if done correctly it will not break at the glue line but will at the point nearby and usually will be  near the glue line .  This is why I suggest the dowel as it will bridge that weak point. 

David Shipp said:

Type II yellow glue which provides a bit of water resistance, (see the chart provided by Kreg) is stronger than the wood it is gluing if the wood is clean and the joint is clamped and allowed to set. The purpose of dowels and biscuits is to align the joint and other than in a case where there is additional slitting as suggested by Jay, above, Otherwise,  the dowels biscuits splines etc do not add any extra strength.   For all those who wish to argue this point, please first properly glue up some scrap wood and then after a day or two, try to break the joint. It won't break where the glue joint is.  That said, if there is a flat service and you can get your jig and kreg clamps  attached, the kreg screws will make alignment easier and faster after you apply the glue. 

[John Schaben]

Good points David, however, not particularly valid if the grain in the area of the break is inherently weak, as has been tentatively determined in this this case. This where the discussion on dowels, biscuits and splines come in. To reinforce the grain. I'm not a big fan of biscuits or dowels. Biscuits, IMO, are not physically large enough to bring much to the party, dowels, I have alignment issues with. Probably just me as I know others use them with great success. Splines can be made fairly thick and deep, and when the grain of the spline is run perpendicular to the grain of the project, incredibly strong. JMHO

David Shipp said:

Type II yellow glue which provides a bit of water resistance, (see the chart provided by Kreg) is stronger than the wood it is gluing if the wood is clean and the joint is clamped and allowed to set. The purpose of dowels and biscuits is to align the joint and other than in a case where there is additional slitting as suggested by Jay, above, Otherwise,  the dowels biscuits splines etc do not add any extra strength.   For all those who wish to argue this point, please first properly glue up some scrap wood and then after a day or two, try to break the joint. It won't break where the glue joint is.  That said, if there is a flat service and you can get your jig and kreg clamps  attached, the kreg screws will make alignment easier and faster after you apply the glue. 

[Ted Harrison]

Thanks to everyone for their suggestions on my chair fix.  I used some tips from everyone to finish it off.  I used Jay's dowel method to bridge both pieces together and yellow glue for the bonding medium.  Seat is as solid as a rock now.  Thanks again.