Using Comb Joints In Double Glazed Window Sash

[pollys13]

I read this recently, " Window Sash Using Twin Comb Joints.
Windows for centuries have been constructed using the tenon and mortice joint, a good joint suited to the design and weight of a single glazed sash window.
Double glazed sash windows however, being much thicker and twice the weight, place a lot of stress on the corner joints. To address this structural stress, we use, as standard, a twin comb joint.

Whilst similar in many ways to a tenon and mortice joint, the big difference is a much greater surface area for gluing. It is this factor that makes the twin comb joint much more suitable for a double glazed window, as ultimately a greater gluing area gives greater strength to a joint. "

How can I go about making suitable comb, finger joints for flush fitting window sash?
Are there suitable Spindle moulder comb cutters available or can I do a work around using slots cutters etc.
Any help and advice as always gratefully received,
Cheers.

 

[Woody2Shoes]

Hi - my 2d's worth:

I'd question the need for this kind of joint to be honest. Of course, a DGU is somewhat heavier than the old SG equivalent. However, if you install the unit correctly, using the "heel and toe" technique, the forces trying to trapezoidalize (!) your sash are not large. If the trad. style m/t joints are well made and glued with the appropriate glue than they work fine (or they do for me at any rate). I think that if the sash were particularly large then maybe there's a stronger argument for re-thinking the trad. m/t joint.

Cheers, W2S

 

[RobinBHM]

There is an advantage to a comb joint in as far as it creates a large glue large glue area.

Im not sure that the argument that a tenon and mortice has more mechanical strength, it has a disadvantage because the mortice has to be reasonable distance from the end so it means the tenon will never be very large. Also the mortice is close to the edge so that bit of short grain is not very strong, you certainly would not want to be banging wedges in hard. Having said that, an accurately made joint will be more than strong enough.

The main reason window manufacturers use them is because it means all jointing can be done on a tenoner with no need for morticing. It is not used because it is stronger -it may be but is not the real reason for using the joint.

We use comb jointing for all double glazed sashes as we have the tooling but for single glazed or slimlite then we often use mortice and tenon. Traditional single glazed casements have the tenon in the moulding so are more suited to morticing. Glazing bars are morticed in the same way.

I would not bother using a comb joint for windows unless you want to tool up for production.

 

[pollys13]

There is an advantage to a comb joint in as far as it creates a large glue large glue area.

Im not sure that the argument that a tenon and mortice has more mechanical strength, it has a disadvantage because the mortice has to be reasonable distance from the end so it means the tenon will never be very large. Also the mortice is close to the edge so that bit of short grain is not very strong, you certainly would not want to be banging wedges in hard. Having said that, an accurately made joint will be more than strong enough.

The main reason window manufacturers use them is because it means all jointing can be done on a tenoner with no need for morticing. It is not used because it is stronger -it may be but is not the real reason for using the joint.

We use comb jointing for all double glazed sashes as we have the tooling but for single glazed or slimlite then we often use mortice and tenon. Traditional single glazed casements have the tenon in the moulding so are more suited to morticing. Glazing bars are morticed in the same way.

I would not bother using a comb joint for windows unless you want to tool up for production.

No not getting into production mode, just my own replacements.
Thanks for putting me straight on this one.

 

[pollys13]

Hi - my 2d's worth:

I'd question the need for this kind of joint to be honest. Of course, a DGU is somewhat heavier than the old SG equivalent. However, if you install the unit correctly, using the "heel and toe" technique, the forces trying to trapezoidalize (!) your sash are not large. If the trad. style m/t joints are well made and glued with the appropriate glue than they work fine (or they do for me at any rate). I think that if the sash were particularly large then maybe there's a stronger argument for re-thinking the trad. m/t joint.

Cheers, W2S

OK cheers.

 

[PAC1]

The finger joint cutters for windows are quite specialised (expensive) as each finger is a different length and needs to match with rebates. So unless you are going into production not a cost effective solution.

 

[pollys13]

The finger joint cutters for windows are quite specialised (expensive) as each finger is a different length and needs to match with rebates. So unless you are going into production not a cost effective solution.

OK cheers.