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There's two schools of thought about it, a lot of people simply mortice and scribe the bars, and a lot of people prefer a cross-halving joint with mitres like below:

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If you look at the free on line copy of Riley's Manual of Carpentry and Joinery page 384 shows two methods, one of which is suited to hand work.
 
Thanks for the replies.
Let me say firstly I am not a joiner! More a carpenter I guess. I do have a trend ovolo cutter with corresponding scribe and have used this in the past to good effect.
This is a big sash. Each sash is about 4'x4' and the bottom one needs to be replaced. Each has three rows of four glass panes.
I noticed that the one I took out has bowed on the horizontal bars and that's the reason for asking what would be the best joint.
 
If you look at the free on line copy of Riley's Manual of Carpentry and Joinery page 384 shows two methods, one of which is suited to hand work.
Thanks for this, new favourite book!
Fig.741 the scribed joint - there's a little bit of fat between the mortise and rebate in the past I've removed this so the bars butt between the feather 4-5mm ...is that ok?
 
I think that might reduce the strength a little but I can't see any serious drawback. I think that 740 might be quickest if you use a mitre guide block with the chisel to get the scribe profile.
Why not try a couple of test joints to see which is easiest?
 
The other thing that's puzzling me a s the full length glazing bars. Should these be vertical or horizontal. The one I removed has vertical full bars. However, I'm sure I read somewhere, perhaps goring's book, that it's better to have the horizontal bars full as they hold the weight of the Glass.
 
It's a better job if the vertical bars are continuos since its normal to pull the top sash down by pulling down on the meeting rail. The tenons on the vertical bars should preferably be through tenons with wedges at the meeting rail for the same reason.
 
The other thing that's puzzling me a s the full length glazing bars. Should these be vertical or horizontal. The one I removed has vertical full bars. However, I'm sure I read somewhere, perhaps goring's book, that it's better to have the horizontal bars full as they hold the weight of the Glass.

The vertical glazing bars normally run full length.
 
Yep, thanks for that. Just found my les goring book which indeed does suggest full glazing bars run horizontal but for double hung sashes they run vertically "to retain continuity and give support to the slender meeting rails." He also suggests the strongest joint is the scribed one.
 
Not all the information in any book is totally reliable. Sash Windows have been around for hundreds of years and there are many small regional variations in methods of construction. Just out of interest does the book you refer to suggest the meeting rails should be made of hardwood like the sill?


W
 
It doesn't mention material type in that chapter. Anyway, like you say there are many different ways of construction and I'm just happy to research and absorb enough information to choose the right one for me.
Thanks for all the advice it really helps.
 
Things are not so good. The ovolo profile is not the same as my trend scribe set. I have ground some cutters to form the profile and will cut rebates seperately. Without a scribe cutter I have to cut the joints by hand. I'm thinking about the halved scribe how do you cut these by hand?
 
If you are using the scribing shown in illustration 740. Are you familiar with the use of a mitre guide block?
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I've shown the block on top of a square edge but you would be clamping it to the edge of the moulded bar. Using the block as a guide for the chisel will give a shaped edge for you to cut with a gouge and chisel to the required scribe. I have to admit it's easier to demo that it is to explain.
 
Yep I understand the mitre block. Might have to sharpen my chisel! So I mitre to the centre point then use a gouge To cut the profile? Will experiment tomorrow.mthanks
 
Very nice. I prefer to use the mitre method as opposed to scribe because when doing multiple joints it can be set up on the panel saw and repeat cuts made.
 
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