I live in a 1930s house in the UK with all the original windows (single glazed). All the windows are wooden casements (although a few are "faux casements": they look like real opening casements next to them, but in fact don't open).

I'm planning to replace all panes with vacuum-glazing (a form of double glazing) panes and also add some bulb weatherstrip in a groove to help with both noise and heat insulation. A first set of vacuum panes are on order and will be delivered in March.

I've bought a router (Makita RT0700CX4, 710W) and am a total newb when it comes to routers.

1. Cutting a very narrow groove for bulb weatherstrip

The weatherstrip grooves (to be cut on the side of the frames, obviously) need to be about 3 mm wide. I think the recommended thing here is something like this. But I'm also aware of these "groover" bits, such as this. In the latter case I believe the router would be moved along the top of the window, with the cutting blade cutting into the edge. Which type should I use?

2. Machining the rabbets on the frames to accommodate the new panes

The thickness of the existing glazing is probably about 3 mm. The vacuum glazing panes will be 8.3 mm thick.

This means that the rabbets on the outside of the casement frames will have to be made deeper. But the area in which the new panes will sit will probably also be slightly wider, because the edge beyond the sight lines of the new panes is 10 mm all round (NB inevitably these panes are much heavier than the existing ones).

So what sort of router bit should I looking at in order to both deepen and widen the rabbet groove holding the panes in the casement frames?

Hope all the above makes sense...


Following the comment by FreeMan I contacted Makita UK. Just a note therefore about the shank gauges on bits used with this router. According to the Makita UK technician you should use Imperial diameter bits with this router (1/4" or 3/8"), despite what it appears to say in the manual. Pretty important.

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    Welcome to WSE. Your question could really benefit from a sketch of the cross section you are trying to achieve
    – Ashlar
    Commented Jan 30, 2023 at 15:10
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    Hi and welcome. "and am a total newb when it comes to routers" Well pause right now. I have three tips. 1, before you even touch a piece of wood (especially one from your windows!) with a bit, familiarise yourself with all aspects of router safety. Headsup, this should be a multi-day thing, don't expect to be up to speed in just an hour of online searching. 2nd, do numerous practice runs on scrap wood before you commit with any of your precious window pieces! 3rd, I don't think this is optional, I think you're going to need a router table. Don't despair about added cost [contd]
    – Graphus
    Commented Jan 30, 2023 at 18:45
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    ....you can make a bare-bones router 'table' for almost nothing and it'll be perfectly adequate for what you require. This can be nothing more than a sheet of plywood or MDF with a hole in it, and one straight piece of wood or metal as the fence. As ridiculous as this might sound this can do all that many users require for an amazing amount of time (no joke, some pros used a setup like this for years in a professional setting).
    – Graphus
    Commented Jan 30, 2023 at 18:48
  • @Graphus Thanks, you're talking my language. I said exactly that about extreme respect for this new item (safety) in a comment on bowlturner's answer. And about experimenting on waste wood. I have already started watching vids on "beginner router user". This is a very frightening piece of kit. Have seen a couple of vids already about making a basic table/jig, so will follow that recommendation. Commented Jan 31, 2023 at 11:54
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    @FreeMan Just heard back from Makita UK: the technician said "use Imperial" and said it shouldn't really suggest metric gauge shanks for the bits. Nice to know (!). Commented Feb 2, 2023 at 10:42

1 Answer 1


If I had more pictures to make sure I'm envisioning correctly all the parts you are discussing I'd have a little more confidence in this answer. But here it goes.

Routers are the easiest way to totally hose up a project if care isn't taken. you need guides or fences to reduce wiggle, wobble and slips.

I am assuming you will be taking the frames apart to do this because if you don't you'll have to use a chisel or something similar to take care of all the corners (for the panes, not the weather striping).

I personally would tend to use a groover style bit, if it can make the groove deep enough for your project because you have one flat guide surface built into it. And if you have the right size guide bearing, it's fairly 'easy' to do it right.

If you have a router table (and you can make a simple one from a piece of plywood and 2x4's ) then you can use the straight bit and a fence. Which can be very fast and easy, once set up.

In either case, I'd have some test pieces to play with to make sure you get all your settings correct before trying it out on your actual windows.

One last thing make sure all your pieces are marked inside/outside so that you run them all through facing the same direction. Because no matter how close to 'centered' you are, you aren't.

  • Thanks, very helpful. From initial reading I already have a very healthy respect for this new piece of kit: safety, "run-away router", etc. Re (inner) corners: it had occurred to me that this would be tricky: rather than disassembling the casement frames I'd assumed I'd have to do that by hand or with a Dremel-style detail tool: have some wood carving skills and tools. I have til mid-March to experiment on bits of waste wood before I'll be doing anything for real. Tx for the table recommendation. Any specific recommendation re the type of bit to expand the rabbets? Commented Jan 30, 2023 at 14:08
  • @mikerodent for the rabbets, especially if you aren't taking them apart, I'd us a 'groove' style bit, it doesn't have to be the same width as the finished rabbet, you can set the 'height' to widen 1 side and/or the other individually, running the bit around the inside. Makes sense what I'm trying to say?
    – bowlturner
    Commented Jan 30, 2023 at 20:02
  • yes, lot of sense, thanks. Commented Jan 31, 2023 at 11:51
  • "'groove' stile bit" or "'groove' style bit"? There's a difference...
    – FreeMan
    Commented Jan 31, 2023 at 16:26
  • @FreeMan I can't spell.
    – bowlturner
    Commented Jan 31, 2023 at 18:50

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