How much taper on sliding dovetails?

[Just4Fun]

I cut a tapered sliding dovetail today for the first time (apart from a couple of practise joints before tackling my project work pieces). I obtained a very good fit and I am very happy with the result but the process of getting there was less satisfactory. Trial fit, remove, fettle, re-fit, repeat, repeat, repeat ...

One thing that was obvious during this process is that I had insufficient taper. The idea of the taper is to make the joint easier to assemble but my joint had a lot of friction quite a way before the joint was fully together. I needed more taper. However from my test joints I decided that a pronounced taper exaggerates the effect of any slight error and it is easy to have the "tenon" piece slide too far through the housing. So my question is how much taper should I use? My joint was 40cm wide and I tapered it from 22mm to 19mm. I have to cut a similar joint on the other end of my shelf so I will try a more pronounced taper there.

An alternative explanation for my problem is that my joint is binding on the bottom of the housing, or on the shoulder. I believe my joint is a good fit there but I am not sure how I can categorically eliminate that as the source of my problem. Any tips?

 

[MattRoberts]

Are you judging this after applying a lubricant like a paste wax?

 

[ED65]

I think the ideal taper is 'enough', which isn't of much help but it seems there are no rules or formulas for this. And it's sure to vary with species/wood harness, the harder the wood the more you could get away with a very shallow taper.

An alternative explanation for my problem is that my joint is binding on the bottom of the housing, or on the shoulder. I believe my joint is a good fit there but I am not sure how I can categorically eliminate that as the source of my problem. Any tips?

Can you see bruising or burnishing? Look at a shallow angle if necessary for shiny spots.

If there's nothing you can make out you could maybe chalk the housing and see where you get ruboff. If you don't intend to glue the joint home you could use graphite which would make it easier to spot.

 

[dzj]

If the "tenon" piece is cupped, then you usually need more taper to make things fit.
From 19mm to 22mm might be OK for hand tools, but for a router, I'd say it's a bit too much.

 

[Eric The Viking]

There's a set of three sliding dovetails across the back board(s) of our grandfather clock, circa 1805.

I don't know what the wood is (200 years of grime on top!)., but I think it's Elm, and the cross-pieces (tenons) are some sort of darker wood - probably not mahogany but might be - nothing I can identify as British native (straight grain, might be Walnut, but unlikely).

At one level I can see why the maker did this - they lock the three back boards together pretty well, but wood movement is an issue. They're not obviously loose, but there are chunky gaps between the boards now (around 1/16 "- 1/8"). It makes me wonder how useful they would be if one board moved more than the other two - shrinkage changes the angle of the sliding dovetail.

It's hard to tell, but I think the boards were cramped up, the housing cut, and then the "ribs" hammered across, after which the ends were sawn off and finished to roughly match the other end. It's tidy but not neat, IYSWIM. Although the visible side of the clock is veneered, the back boards seem to have no glue at all, and the ribs aren't anchored either, merely held in by friction (which surprises me slightly).

All this is behind the clock, on the outside of the case, but unseen. The ribs are roughly 1 1/2" wide and 1" thick (including the "tenon" part. Looking at the ends of the housings, they seem to taper by roughly the same amount as those of the OP - somewhere between one and three degrees. I do wonder if the intent is that each board can move independently, without greatly messing with the overall dimensions. If they were rubbed joints, this wouldn't be true.

I'd be very interested to know from the acomplished makers on here if this joint has still has general application, and if so, for what, nowadays?

E. (reading with interest)

 

[Racers]

+1 for the hammer it in and saw off the excess method.

I made a trivet from 8mm oak for the kettle and teapot to stand on, so it needed some support so I did two sliding dovetail splines underneath.
Trim slide in repeat and cut the ends off when it comes out of the other side, I did do it all with hand tools.

Pete

 

[custard]

We tend to think of a tapered sliding dovetail as a super precision joint, one of those show-off flourishes to awe the apprentices.

But I suspect it's more an ingenious and practical solution for dealing with cupped components. The wedging effect being used to pull everything flat.

In today's world of heated workshops, kiln dried timber, man made boards, and four sided planers, there isn't much use for a tapered sliding dovetail. But in a previous hand tool environment it would have been a quick answer to utilising less than perfect boards. And because of the joint's practical, problem solving nature, I doubt it was executed with micro precision regarding the angles, just off by a couple of degrees, bang it home, then flush off the overhang much as Pete suggested.

 

[AndyT]

First, a spot of disambiguation, as they say in Wikipedia.
I think the OP is talking about a joint cut on the ends of a broad shelf and in the uprights of a bookcase or similar.
Eric has mentioned a different but interesting joint, where a batten (with a tapered wedge shape) is used to reinforce a wide board made up of narrower pieces.

Taking the bookshelf end joint first, I note that Just4Fun is being ambitious here - 40 cm (8") is on the wide side and will need more precision than a narrower build would.

I had a go at these in a bookcase project a few years ago and posted pictures here.

The shelves are only about 140mm deep, but I still had similar trouble with the joints.
These somewhat fuzzy photos show the size of the taper (sorry, I can't remember the exact size, but it was probably about 1/8" less at the narrow end).

I followed the same sort of procedure as Just4Fun, with plenty of test fitting and careful adjustment with a chisel. Tapers are difficult - a single shaving can take the joint from too tight to too loose. A bit of cupping on the width would help!

However, my real problem came later with the glue up. What I had not anticipated was that adding glue does two things - it lubricates (briefly, until it grabs!) making assembly easier but it also swells the fibres, making it harder to close the joints. I ended up getting a bigger hammer to persuade the joints home. And also planing off some slight overhangs afterwards. (The non-squareness really doesn't show in the finished item.)

You do have the option of not glueing - which would allow you to use wax to lubricate - but you wouldn't want to use wax and glue.

There's also a variant, with a plain housing across most of the width, and a short tapered section at the front. I had a go at something similar on a small scale here.


The second sort of sliding dovetail, used on a batten in things like old clock cases is more forgiving - this is the one where you can make the wedge over sized and trim off the end. I can't see how you could do that on a shelf end, unless you were willing to plane a lot off the shelf width.

Picking up on Eric's question of the use of this joint, I can add that it was widely used to make large wooden panels for oil paintings. It seems to have been more of a German/continental usage than it was in the UK.

Chris Schwarz has written about how it can be used on something like a table top, where the extra thickness at each end provides a convenient place to fit tenon-in-round-hole legs. (I can't find where at the moment.)

I'd also suggest a look at a historical essay (which shows some specialist saws and planes used) in TATHS Journal No 2, available here page 29 of the pdf.

Also, I just found this from Charles Hayward, as republished by Lost Art Press:

https://blog.lostartpress.com/2018/01/1 ... -housings/

 

[custard]

If there was ever a prize, for the best answer to a forum question, then my vote would go to Andy T for this one!

 

[Just4Fun]

Wow, what a response. Such a wealth of information. I thank you all. To answer some specific points:

@MattRoberts
I have not applied any lubricant. I thought about it but I am in a sort of Catch 22 situation. I have the option of using glue on the final assembly but if I use wax or something I would no longer have that option. I was afraid of messing up the joint to the extent that I might be glad of some glue, so I couldn't use wax until I knew the joint would fit well without glue, by which time I would not need the wax.

@ED65
Good point about the wood species making a difference. I am using pine so perhaps the worst wood for a shallow taper.

I have checked for bruising etc and as far as I can see there is nothing untoward, so thanks for the tips.

@dzj
Good call. I checked the shelf and there was some slight cupping. Nothing too serious but maybe this made my narrow taper more problematic.
I am using hand tools, not a (power) router. I am surprised that the 19-22mm taper might be too much for a router approach. I don't understand that, but then I don't use a router so what do I know?

@Eric The Viking
Interesting joint on your clock. I think that is different to what I am doing though. I suspect your clock joint would be easier because the ribs could be made over-long and trimmed off when hammered home, as you suggest was done. I can't really do that on my shelf because there are 2 ends, both of which have to fit. I did make the shelf over-wide in case I made a complete mess of the first end, but the 2nd has to fit correctly because if it went through too far the first end would no longer fit.

My initial thought with the joint on your clock is that the panels are able to expand or contract and the ribs will slide to suit. Thinking more about it I am unsure because I can understand how the rib could move out but I don't see how/why the ribs would ever move back the other way. Over time I would expect the rib to work its way out. Does it look like it has done that?

@custard
I think I can make a logical case for using this joint:
1. I want to stop the sides splaying outwards.
2. Being able to slide the shelf in after the main carcase has been assembled will simplify final assembly & glue up of a large & unwieldy piece of furniture.
3. I need a very strong joint. This is the bottom shelf on a piece that will move on castors attached to the underside of this shelf. The full weight of the piece and its contents will thus bear on the joints holding this shelf to the uprights. The load could include someone sitting on a seat included as the next "shelf", and at 90cm wide there could even be 2 people sat on it. Significant weight, anyway.
Of course, that is mere post-decision justification for my choice of joint. The real reason I chose it was "just4fun". I had not cut this joint before so why not try it?

@AndyT
As custard commented, this was a very thorough answer to my question. Thank you.

Your disambiguation is spot on.

Maybe 40cm is ambitious, but it worked out OK and I am chuffed with the result. If it had all gone wrong there are ways I could have recovered without ruining the piece so I gave it a go.

The joints in your bookcase look to have more taper than my first joint. They also look more complex to cut, being stopped and with a shoulder above and below the "tenon". The top side of my shelf is just flat. Maybe I am not as ambitious as you.

The historical essay is interesting. I'll read that through more thoroughly later. The Charles Hayward article is possibly an easier method without particularly specialised tools, but is not the way I did it. I planed a 1:6 angle on the long edge of a piece of 4x2 and used that as a saw guide on both the shelf ends and the vertical sides. To use it on the shelf ends I put the shelf upright on end in the vice so the end was horizontal, clamped a straight & square piece of scrap wood flush with the end, and clamped my saw guide to the piece of scrap. I convinced myself that using the same guide to cut both parts of the joint would ensure they would be a good match and it worked as well as any other method I think.

To update my progress, I have now cut the joint on the other end of the shelf and here I reduced the thin end of the taper from 19mm to 17mm. This joint was a lot easier to work with - but then I would expect that to be the case anyway having practised on the first end. Things are always easier 2nd time. If I do another such joint I might reduce the thin end of the taper by another couple of mm but I think I a getting to the "sweet spot" now.

One bonus question on this joint if I may. When one side of the shelf is flat and the other side has the angled & tapered cut, why do all the references put the flat side on the top? For my application, where the shelf is supporting the sides, this makes sense to me because the weight of the sides will bear on the square mating side of the joint. In a more normal situation where the sides support the shelf this configuration must make the tapered side of the joint support the shelf weight. It seems this would make it easier for any error in cutting the joint to make the shelf sit out of level.

 

[Racers]

Don't forget that modern glues make this joint virtually redundant.

I guess the angled part is on the bottom so the weight pulls the sides together.

Pete

 

[AndyT]

Don't forget that modern glues make this joint virtually redundant.

Pete

Yebbut only in the way that plastic spoons make wooden ones redundant - some of us like to while away the hours exploring the old ways! :lol:

 

[Eric The Viking]

Yebbut only in the way that plastic spoons make wooden ones redundant - some of us like to while away the hours exploring the old ways! :lol:

Despite the wonderful answer earlier, you get the wooden spoon for that one, definitely!




.

 

[custard]

Thinking some more about this I remembered an application where I still might use a tapered sliding dovetail.

Imagine a standard chest of drawers, with a pair of smaller drawers above a number of full width drawers.

Now think about the divider that separates the pair of smaller drawers, and how it will be jointed into the rail above it and the rail below it.

The rail above will be secured to the top, either direct with screws or via buttons, so it will be very strong and won't flex. But the rail below will have a tendency to sag under the weight of the drawers. So the drawer divider is effectively carrying the weight of the two smaller drawers, consequently you need really strong and reliable joints. The traditional answer is for the drawer divider to be jointed in top and bottom with either wedged through tenons or sliding dovetails. Going back to the OP's project I think he has the same choice, and personally I'd go with wedged through tenons for the shelf that will double as a seat.

In the case of the chest of drawers you'll need a clean front face for the sliding dovetails, gap free and perfectly flush, however you'll be more relaxed about the appearance at the back. A tapered sliding dovetail is ideal for this as you can make the drawer divider a bit wider for flushing off after assembly, and give yourself a bit of slack at the back end of the female mortices to guarantee a quick and easy assembly with maximum tightness at the front where it's visible.

If I get the chance over the next few days I'll cut a test piece to show the joinery and post some photos.

 

[Harbo]

I made this table a few years back for my daughter to fit a defined space.
To aid transportation I used sliding dovetails to fix the rails to the legs. Big mistake, the joints were not man enough and I finished up having to make new rails and use M&Ts.
Lesson learned.

Rod

 

[custard]

the joints were not man enough

I don't think that's the fault of the joint per se Harbo, I guess it's more a case of the wrong grain orientation. To be strong a sliding dovetail has to have the female part running cross grain, if it runs long grain then it'll just act as a wedge to split the fibres apart.

Nice table by the way!

 

[dzj]

In the case of the chest of drawers you'll need a clean front face for the sliding dovetails, gap free and perfectly flush, however you'll be more relaxed about the appearance at the back. A tapered sliding dovetail is ideal for this as you can make the drawer divider a bit wider for flushing off after assembly, and give yourself a bit of slack at the back end of the female mortices to guarantee a quick and easy assembly with maximum tightness at the front where it's visible.

If I get the chance over the next few days I'll cut a test piece to show the joinery and post some photos.

Yes, for dividers and blades/ rails, there's no better way of attaching them.
Sometimes lipping was applied over the SDs, so you could get away with a sturdy but aestheticly not 'spot on' joint.
Overhanging cabinet tops were also sometimes attached with sliding dovetails.

Another use for sliding dovetails is for attaching drawer fronts. With a router, a reasonably quick way to make a strong joint, particularly when combined with drawer runners.

 

[Just4Fun]

Going back to the OP's project I think he has the same choice, and personally I'd go with wedged through tenons for the shelf that will double as a seat.

That's an interesting thought, thank you. The seat is the next thing I need to tackle, and I am not committed to any particular joinery at this stage. I have been leaning toward more sliding dovetails but I could easily change that to wedged through tenons. I am curious how you would size these. The sides are 40cm wide and 24mm thick. The seat will be 30mm thick. So I am thinking of 2 tenons per side, each say 100mm x 20mm, with a wedge near each side of the tenon. Does that sound OK?

One thing I am unsure about is how this will cater for wood movement. The piece is almost all pine which will be painted. The seat will be oak, which will probably be oiled because it is from a recycled kitchen worktop that was previously oiled. I am assuming the pine will expand & contract more than the oak. This should not be an issue with an unglued sliding dovetail as it could move, but would it be safe if I use M&T joints?

 

[Sgian Dubh]

I am assuming the pine will expand & contract more than the oak. This should not be an issue with an unglued sliding dovetail as it could move, but would it be safe if I use M&T joints?

It's unlikely there would be a significant problem if you joined the two species with M&Ts, assuming you use material of approximately the same moisture content in each species when you do the construction, i.e., you don't have one at say 22% MC and the other at, say, 8% MC.

Your assumption regarding the species with the greater cross grain expansion and contraction in response to changes in moisture content is incorrect.

Scots pine, if that's the wood species you intend to use, has shrinkage factors of 6.9% tangentially, and 4.5% radially.
European oak, again assuming this is your chosen wood species, has shrinkage factors of 8.9% tangentially and 5.3% radially.
In other words, their shrinkage factors are relatively close to each other.

In both cases the shrinkage factors describe the likely or expected shrinkage as the wood moves from FSP (fibre saturation point, i.e., 30% MC) to oven dry, i.e., 0% MC.

In reality, you're likely to be dealing with wood that's already dried and when the item is made it will likely see in service conditions that would only cause the wood to expand and contract over a small proportion of the full range that wood could expand and contract, e.g., wood varying in MC from, for example, 8% MC to perhaps 14% MC. Slainte.

 

[custard]

The sides are 40cm wide and 24mm thick. The seat will be 30mm thick. So I am thinking of 2 tenons per side, each say 100mm x 20mm, with a wedge near each side of the tenon. Does that sound OK?

I'd have five tenons, each 24mm x24mm. The tenons would have to be square, or nearly square, as the wedges must run horizontal. Otherwise they could easily split the sides open. Wedging force must only press against end grain. This sketch explains the principle,

You would make the job look neater by containing the shelf/seat in a 3 or 4mm deep housing joint, personally I'd have a 24mm wide housing with 3mm shoulders on each side so that there's no risk of any splintered edges spoiling an immaculate appearance.

Regarding differential shrinkage, I agree with Sgian Dubh, it won't be a problem.