Size of splines?

[Ron Illig]

I am building a table base, which has 4 sections that each consists of three curved pieces joined with miter joints- see attachment. The pieces are about 2 inches thick. To assemble the miter joints I was planning on first gluing them together with some dowels or biscuits just to get them lined up accurately with only minimal strength (but hopefully enough to do further milling on the assembly). Then for real strength I would add splines – see dashed lines in sketch. To make groves for the splines, I thought I could create a simple table saw jig, probably guided by the fence, to hold the assembly on edge with the miter perpendicular to the table and push it through the blade. I would probably run through a second time with a spacer in the jig so I wound up with a ¼ inch slot. Here is my question, how deep do the splines have to go to get the strength needed for dining room table legs? With my 10-inch blade I can get about 3 ¼ inch deep, which gets me more than halfway down the miter at both joints. Is it safe to cut this deep? With my 8 inch dado set, I can get about 2 ½ inches deep. Is there some rule of thumb for spline depth or area vs. strength?

[Bill White]

I would think that , in this case, the most you can cut. Be sure to align the spline material grain lengthwise in the joint for max effect.
Bill

[Dave Zellers]

I'd want a thicker spline than 1/4", 3/8" would make me more comfortable. One more pass with your jig. Full depth (3 1/4") and make sure you use a blade that gives a flat bottomed cut.

[John TenEyck]

Why not use loose tenons instead? If you can only cut 1/2 way down at full depth with your TS your joints might be strong enough but certainly not optimal. With loose tenons you can cut the mortises closer to the bottom of the joint where the applied forces in use are higher. I'd use two loose tenons, 3/8" thick and 3 to 4 inches long. You could even make the mortises stepped, if required, to allow for the curve of the outside of the leg. If you want splines because of their decorative appeal, you could still add them afterwards w/o having to be concerned about their strength.

John

[Yonak Hawkins]

How about running your spline parallel to the joint ? That could make for a 6-1/2" wide spline. Just make sure the grain direction is across the joint.

Also, I agree with Dave about the thicker splines.

[Richard Coers]

That will be some job to balance that thing on a table saw. What size is it? Most dado sets are not good for really deep, basically ripping cuts, in hardwood. The outer blades have very little set, and you can expect a lot of smoke on a 3" deep cut. Do the jointery before assembly.

[Larry Browning]

How about running your spline parallel to the joint ? That could make for a 6-1/2" wide spline. Just make sure the grain direction is across the joint.

Also, I agree with Dave about the thicker splines.
+1 for this idea. This would take the place of a biscuit or dowel. It also almost assures that you will have perfect alignment. You could make the slot depth as much as a inch or so. That would give you a 2 inch wide spline that ran the full length of the joint. Not sure about the thicker spline though, I think 1/4" is plenty. Oh, wait, I take that back! I was thinking the stock was 3/4 not 2 full inches. Make the spline 3/8 or maybe even 1/2 inch. Or alternately how about two 1/4" splines in each joint? That would be really strong! Albeit a little more complicated to do.

[Ron Illig]

+1 for this idea. This would take the place of a biscuit or dowel. It also almost assures that you will have perfect alignment. You could make the slot depth as much as a inch or so. That would give you a 2 inch wide spline that ran the full length of the joint. Not sure about the thicker spline though, I think 1/4" is plenty. Oh, wait, I take that back! I was thinking the stock was 3/4 not 2 full inches. Make the spline 3/8 or maybe even 1/2 inch. Or alternately how about two 1/4" splines in each joint? That would be really strong! Albeit a little more complicated to do.

I agree that this is a good idea but then the splines will show. With the method I describe, they are totally hidden, which is what I am going for. I suppose I could stop the saw blade short of the corner (show side of assembly) when cutting the groves but that is reducing spline surface area again. I made a quick sketch comparing 1 inch deep parallel groves stopped short to my method and it sure appears to be a lot less spline "area".

[Ron Illig]

That will be some job to balance that thing on a table saw. What size is it? Most dado sets are not good for really deep, basically ripping cuts, in hardwood. The outer blades have very little set, and you can expect a lot of smoke on a 3" deep cut. Do the jointery before assembly.

Thanks for the tip about dado sets - I have never tried cutting deep with one. I too am concerned about the tablesaw process. I am thinking the jig needs to be pretty beefy and guided either by the fence or miter slots. If any one has plans or tips for such a jig, I would appreciate them.

[Larry Browning]

I agree that this is a good idea but then the splines will show. With the method I describe, they are totally hidden, which is what I am going for. I suppose I could stop the saw blade short of the corner (show side of assembly) when cutting the groves but that is reducing spline surface area again. I made a quick sketch comparing 1 inch deep parallel groves stopped short to my method and it sure appears to be a lot less spline "area".
I guess I do not understand why yours won't show. It seems to me that your method would actually show more of the spline. I must not understand your drawing. Does the outside edge not show? I am very confused now.

I used a slot cutting router bit to cut slots in the joint. The smaller diameter of the slot cutter allows the cut to extend almost to the end of joint and still be hidden.

[HANK METZ]

If the design allows for it, a half- lap joint instead of the butt and spline will give a massive glue area.

[Yonak Hawkins]

It seems to me, if it's a choice between the spline showing on the inside or the outside of the leg, showing it on the inside would be less obvious. One option could be to stop the saw cut short of the outside corner and the spline wouldn't show on the outside.

On the other hand, if you wanted to show the spline as a design feature, and if you were of a mind to, you could add a coordinated design feature replacing the glue with through dowels to hold the joint together and you'd have a table where the legs could be removed, if ever needed, without unscrewing the entire leg units from the top. Of course, this may make for a type of table you may not have in mind for your decor.

[Ron Illig]

I guess I do not understand why yours won't show. It seems to me that your method would actually show more of the spline. I must not understand your drawing. Does the outside edge not show? I am very confused now.

I used a slot cutting router bit to cut slots in the joint. The smaller diameter of the slot cutter allows the cut to extend almost to the end of joint and still be hidden.

The assembly I show in my drawing is one of four that join together at right angles to form the table base. The vertical or right side of my drawing gets glued to a square center post along with three others at right angles to each other- thus the splines I show are buried in the center of the base.

[Art Mann]

I Will tell you how I would do it but it requires a precision doweling jig like the Dowelmax. The material is 2 inches thick. That would allow for two rows of 3/8" dowels. That joint would be stronger than a spline as you illustrate it and would be much safer to do. I must emphasize that this will only work if you have a Dowelmax or similar. Cheap dowel jigs will lead to misplaced holes and a poor fit.Loose tenons such as you can achieve with a Festool Domino or similar tool will also do the job. You could do two rows of those as well. I would hate to try to cut the slots for splines on such a large piece with a dado blade without a very sturdy and elaborate jig from both a safety and accuracy standpoint.

[Larry Browning]

I Will tell you how I would do it but it requires a precision doweling jig like the Dowelmax. The material is 2 inches thick. That would allow for two rows of 3/8" dowels. That joint would be stronger than a spline as you illustrate it and would be much safer to do. I must emphasize that this will only work if you have a Dowelmax or similar. Cheap dowel jigs will lead to misplaced holes and a poor fit.Loose tenons such as you can achieve with a Festool Domino or similar tool will also do the job. You could do two rows of those as well. I would hate to try to cut the slots for splines on such a large piece with a dado blade without a very sturdy and elaborate jig from both a safety and accuracy standpoint.
I really have to agree with Art on this one. If you want to do this the right way with maximum strength, the Dowelmax type jig will do the job. Plus, you will get to buy a new tool, which will open up a whole new way of doing joints.

[J.R. Rutter]

If it were me, I would proceed as you first mentioned and assemble the joints. Then just make a series of 1/8" kerf slots spaced, say, 1/4" apart and glue splines in. This gives you massive glue surface area and good depth using the blade that you have.

[John TenEyck]

You seem to be hung up on using splines when loose tenons (or dowels) would be so much easier and stronger. That's your business, of course; just seems like a lot of effort for no advantage and lots of potential problems.

John