Joints for casement windows and frames

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Full time tool collector, part time woodworker
25 Sep 2011
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Now that I finally have a dedicated workshop, I am getting my act together to sort out the windows on our house (by which I mean make some better ones). I have recently bought a second-hand Hammer F3 spindle moulder to hopefully make a decent job of it. I think I have got my head around most aspects, however the bit that I have options on which will determine tooling is the design of the mortice and tenon joints. There seem to be a few different ways this is approached:
  • Single open mortice and tenon - I have seen these in some of the dedicated window tooling used for the frames
  • Double open mortice and tenon - this is what most of the dedicated window tooling seems to use for the casements and for some of them also the frames
  • Single haunched/franked mortice and tenon - this is the way most traditional window makers seem to go
  • Single/double loose mortice and tenon - such as the Fry router system uses, or if using a Domino
My options are open at this point as I've not started purchasing any tooling. I don't have a hollow chisel morticer and aside from this specific job I don't think I would make much use of one. I also don't have a Domino and I'm not inclined to get one, though I could go the loose tenon route with a router. Or I could go the open M&T route with spindle tooling.

Using a single open tenon joint would be preferable from the perspective of having less tooling/setup and minimal need for additional tool purchase. Mainly though I am interested in views/experience on the relative strength of the finished joints. I presume on the frame it makes less of a difference since the sections are larger and the frame is secured in the opening anyway, so the joint isn't doing that much. But on the casements I can see there is relatively little material in the joints at the corners of the frame once all the rebates have been made (I'm looking to make storm-proof casements so rebates on both sides of the frame) so the joint choice is a bit more critical. My concern is driven at least in part by my current windows, which have some badly warped casements (approaching 1" gaps to the frame in places). I am planning to use accoya for stability and longevity.

Whichever option I go down, I will be intending to use power tools/machinery. Some people love perfecting their hand woodworking skills and chopping out mortices by hand - if this is you then I admire your talents but I am not that way inclined. I would prefer to stick mainly with tooling for the spindle moulder, as I already have that, rather than having to buy additional machinery - but if it makes a big difference to the finished article then I'm prepared to explore that.

All advice and experience gratefully received!
Double open mortice and tenon - this is what most of the dedicated window tooling seems to use for the casements and for some of them also the frames

I've long used this for the frames and sashes of modern (friction stay / espag) dg windows, along with a Eurogroove for the hardware. However you don't have to buy dedicated tooling - it can all be produced fairly efficiently using generic tooling.

In essence the depth of a window joint is modest compared to say a door joint, hence the advantage in a double tenon. However in practice an outer frame once installed is subject to little stress so you might go single there. The sash is where the stresses are more present.

In brief, you tally the tenon positioning (as ever) across the work with the rebate / upstand positioning. I'd also tally the tenon widths and spacing with a suitable hollow mortice chisel - maybe 8mm / 5/16".

If sashes have an outer rebate as is common, run this last, after assembly.

Method of joint cutting: cut all to length, thus you have a square end on all pieces as reference face across all sizes of window. Mark one end out for the joint. Cut outer shoulders on the sawbench with a crosscut carriage. The rebate shoulder will likely be square, and the upstand shoulder splayed. Mortice once per joint, back at the shoulder. Bandsaw in to remove the rest of the waste. Hope that makes sense.

Whether to invest in dedicated tooling depends on the volume of work ...
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Ah - no morticer! You have a drill press that you could follow with a hand chisel, then bandsaw as above? Personally, I wouldn't use Dominos for external joinery - it seems like cheating, and what happened to haunches?

Buy a morticer then sell it on after ...
Thanks for your reply. So you use a double open M&T for the sashes and you find this produces a good strong joint?

Buying a morticer is certainly an option if I need one. I would prefer not to give up the extra space for a tool I won't often use; like you say I could sell it on afterwards although I do have a bit of a tool collecting habit so perhaps risky in that regard 😉 From what I have seen the best way to make the double joint is with a couple of groovers on the spindle, so that would be the extra tooling I would be in for if going that route? Already got a 96mm profile block, 125mm rebate block and two tenon discs on the shopping list...
So you use a double open M&T for the sashes and you find this produces a good strong joint?
Absolutely! Use waterproof glue. You can always peg the joint as well, if you want to.

This drawing shows my customary sections, though latterly I've started installing a double weatherstrip - one in the frame and one in the sash, offset one to the other with an air gap between them.

You will see that there's a bit more to it than just slotting - the shoulders are to be cut as well, and some are splayed ...

I part out the rebate material on the sawbench and reclaim the glazing beads from it.

The hardware determines a lot of the dimensions and is critical. For instance friction stay hinges come with different 'stack heights' - the drawing suits one of 13mm.

The sill extension suits cavity wall construction but in masonry walls I don't usually bother with it.

Oak is my preferred material. Any screws should be stainless.
If using Accoya you will need to design your frame and sash/casement finished stock sizes relative to the standard sizes of the sawn Accoya available.

If your going to run all you joints, whatever option you decide on, with a single spindle moulder you will need to make as many of the same cuts in one go, once you change any tooling configuration or fence settings it can be a pain to re establish, which may become a source of frustration, you'll need to do trials of the joints to make sure they will all fit together, I use a range of spacers/shims on the blocks and back stops on the fence to be able to switch back quickly.

The only thing I do different to @rogxwhit is slope the cill rebate @ 9 degrees and run a groove for the cill extension to locate in:

Thanks both for your replies. @rogxwhit - when you say there is more to it than just slotting, I am envisaging a tooling stack comprising a tenon disc to cut the scribe profile (in your diagrams, a 9 degree slope - which is the same profile I was thinking of) with a cutting diameter relative to the rest of the tooling to give the desired rebate depth, an 8mm groover and a second tenon disc with a straight cutter to match the groover diameter, with appropriate spacers. This forms what I think is termed the "scribe tenon". Then the matching "slot tenon" is formed from a pair of equal diameter groovers with a spacer between them (the spacer being the size of groover used in the "scribe tenon" - in your figures, all 8mm). As you have drawn, these are set up in the profile so that the edge of the scribe/slot aligns with the glazing rebate.

If I've understood this correctly, then I think the minimum tooling needed to do all the joinery on the moulder is:
  • 2 tenon discs, one to scribe and one to form the lower tenon cheek at the same cutting diameter as the groovers
  • 2 groovers (8mm as you have drawn seems about right for the casement/sash, can use these together to form a single 16mm tenon in the frame)
  • 96mm profile block
  • 125mm rebate block
As I plan to use accoya, I will need to use PU glue, which meets your waterproof spec.

@HOJ - yes, I was thinking the same as you suggest i.e. have a tooling stack to perform each operation in one pass to avoid setup fiddles. Having spacers to set the different tool set-ups without adjusting the spindle height is a great tip - I will take that one on board, thanks! To achieve the correct joint to the cill rebate at 9 degrees, I think the straight shoulder cutter in the tenon disc should be replaced with a scribe to match the cill profile when cutting the lower end joint for the jambs/mullions?

Will definitely be playing with some cheap whitewood to get the process right before I start working with accoya for the real job - that stuff is way too expensive for learning on...
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If it's a one-off job as intimated (given that there are several windows in it) I wouldn't have envisaged buying a window tooling stack! But I suppose it saves you needing a morticer. Morticers, though, are good for other work, whereas window tooling just does windows, more or less.

PU glue needs moisture to cure, & I've found it best to apply a very light spray of water to joint surfaces before assembly - it can be a bit iffy without.

Regarding a sloping rebate in the sill - I've sometimes dabbled with doing that, but keeping the rebate flat doesn't seem to do any harm in terms of water retention. I think that normally there's very little ingress, even here in wet Wales. Also the standard espagnolette keeps need a flat platform, if the sash is top-hung.

The weatherstrip slots & clearances in my drawing are to suit my norm of an angled flipper seal - other types may differ.

Many windows are inset in a deep stonework reveal and don't need the sill extension.

I'm not a fan of slot vents.

Officially any window replacement is notifiable (to your LA BC dept). I think the fee's about £200 these days? But not everyone bothers ...

But it's good to make them regs-compliant anyway ...

An important aspect is fire regs for egress. You can easily find info online.
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Where to start.... HOJ: from your drg i assume you do not use friction hinges unless all sashes are top hung?
rogxwhit: Form your drg 1) how do you drain the hinge groove in the cill rebate? and where do you groove the sash for the weatherseal?
siggy_7: You say you finally have a workshop, but is it really a workshop without a chisel morticer? This is not an occasionally used machine, in fact a dedicated window tooling stack for making one set of windows for your good self is more of an expense not needed than a morticer. In my opinion comb jointed sashes are mass produced in big factories geared for maximum throughput. My storm proof windows have 5/8ths M&T joints in both frame and sash. I make small batch windows 48 weeks of the year, (odd one or two to a house full, in fact the next job is 20 oak stormproof casements with 38 sashes. You're right in stating P.U. glue for the sashes due to there being no mechanical fixing, but in the frames, where there is, you may use P.V.A. In accoya though I find that a spray of water from a pump action spray bottle in conjunction with a slow cure P.U adhesive (e.g. Wurth) as opposed to the rapid set builders merchants stuff makes for a stronger joint. Rapid set goes off before getting a hold on the acetylised radiata.
More great info and tips, thanks!

Regarding the question of whether it's a big investment to purchase the full tooling. As I see it, the rebate block, profile block and tenon discs are about the minimum I would need for this job anyway, and would have a variety of applications beyond just windows and doors (particularly as the profile block and tenon discs have exchangeable profile cutters). So really all I'm purchasing on top of that are the two 8mm groovers, which compared to the investment (and space) needed for a morticer seems like a reasonable choice. I could equally sell on the unused tooling once I'd finished with it as well, which is a lot less bulk to move in/out of the workshop.

@Nerminater, personally I think it can still be claimed a workshop without a morticer (I wasn't aware that it was considered such a staple tool!). Honestly, aside from window and door making I really can't see myself using one. I have done all my mortice and tenon joinery to date using routers, which I find works OK for most applications - though in this instance with so many joints to do I am put off somewhat by this method. At some point I plan to make a Pantorouter (Matthias Wandel's invention) which I think will serve me well for most other M&T applications. I made these doors several years ago, which are holding up fine, but due to the smaller section sizes and full glazing I think the window casements have a much harder life - and I'm not sure I feel comfortable using the same approach: Double workshop doors

@MikeK, yes it is a surprising amount of work. I've seen Olly's video series as well, which is excellent.
many (manufacturers) making windows can only do it fast enough and to regs using window tooling.
it's really straightforward to use. everything is cut to the size required( ie the head and cill are cut to the window opening as are the the uprights. there set once at the start. the window section sizes are given(allowing a 4 sider to produce lots of identical stuff)
the downside is you need a 7.5
HOJ: from your drg i assume you do not use friction hinges unless all sashes are top hung?
Most of my window work is for flush casements, I make very few EJMA or Lipped windows, so I slope my cill's, so water wont tend to sit in the rebate, I use "conventional" hardware i.e. hinges, catches and stays.

My storm proof windows have 5/8ths M&T joints in both frame and sash.
I have a set up for doing "finger" type joints, but use mortice and tenons on frames as most will have mullions and/or transoms, and casements when I have small order quantities.

Officially any window replacement is notifiable (to your LA BC dept). I think the fee's about £200 these days? But not everyone bothers ...

But it's good to make them regs-compliant anyway ...

An important aspect is fire regs for egress. You can easily find info online.
All critical points, you may need to prove "U" values, Part Q shouldn't apply, but egress is a potential pitfall, could end up with flying mullion's to comply, I had to rework a window recently, after Architect didn't specify but BC picked it up.