Routing technique advice please

[Roger Mills]

I'm about to fit some sash balances to some of my sash windows. In case you haven't come across them, these are springs contained within 15mm plastic tubes, and are used instead of conventional cords and weights for counterbalancing the windows.

In order to fit these, I have to make some channels in the outer vertical edges of the sashes. These need to be 16mm wide and 16mm deep, but with rounded bottoms so that they will slide nicely on the tubes.

I have invested in a rounded bottom router bit from Axminster (like this

with a 15.88mm (5/8") diameter and 7.94mm tip radius, which I'm hoping will do the job.

I have a few questions:

Should I use this bit for the whole job, or should I remove most of the material with a few passes of a circular saw first, and just use the router to finish off?

At what speed should I run this bit?

If using the router bit for the whole job, how many passes should I make

- presumably not just one full depth pass?

I'm familiar with the concept of moving the router in such a direction that the cutting edge is cutting into the material - but what happens when you're cutting a groove the same width as the bit diameter - so that you've got one leading edge and one trailing edge, so to speak?

Any constructive comments (preferably not "If I were going there, I wouldn't start from here"!) will be greatly appreciated.

[Andy Dingley]

Hog out the waste first with a 1/2" or 3/8" cylindrical bit, because they're cheap and you've always got one handy (or else buy one). If the cut is central and the timber is symmetrical, then you can make single-sided passes with a 3/8" cutter, guided from each face in turn. Then finish with your core box bit in one central pass. Single sided cuts are always quicker and easier.

1/2" to do it in one pass would be fine too, just be careful to always hold it against the fence, as any movement in either direction is a bad thing - but you've got a whisker spare each side before cleaning up. As far as the cutting forces from "climb milling" one edge, that's not a big deal. Mind you, practicing on scrap never hurts.

I would consider this worth multi-passing with the router, even with a bit change. I wouldn't consider it worth the trouble of setting it up on the saw. Certainly not if it's blind-ended trenching.

This depends on the bit diameter, also on the timber, the cut depth depends as much on the router.

Assuming a router solid enough to handle the cutting forces safely (anything 1/2" shank with a brand name) then this isn't a "big" cutter and so you can run at 20,000 - 25,000 rpm quite easily. This might be top speed on your router, sometimes a bit less on a small machine.

I'd probably cut 1/4" at a time. Multi-depth passes are quick.

[John Rumm]

with a 15.88mm (5/8") diameter and 7.94mm tip radius, which I'm hoping

If removing much material, then its usually better to use a straight fluted cutter to do the donkey work, since they are cheaper. The the final pass or two with you decorative bit or two,

For something small in diameter like that, either the maximum speed of your machine or close to it. (say if it goes from 1 to 6, then 5 or higher).

Assuming you have got all the don curved depth with a straight cutter, then you can do the rest in one pass.

Its not a problem generally. Have the fence on the router so that it is on the side of the cutter that is being driven into the work.

Sounds like you should be fine ;-)

[82045]

And watch out for any nails or other debris in the timber.

[Martin Bonner]

That's another *excellent* reason to use a cheap cylindrical cutter first - it can find the nails, rather than the expensive fancy cutter.

[Roger Mills]

Yes, indeed. Many thanks to you - and to everyone else who has replied with helpful advice.

I'm fitting the sash balances to a sash window which was previously screwed shut, and never had any cords, pulleys or weights installed. I have recently freed it so that it slides up and down - but there's currently nothing to support its weight when in the open position.

There may well be some remnants of rusted-in screws in the area I shall be routing - so the point about removing most of the material with a cheap cutter before risking the fancy one is well made.

I'm still toying with removing some of the material with a circular saw. the grooves *do* need to run for the whole height of the sash - and so doesn't have to be stopped at any point.

As one or two have suggested, I shall practise on a piece of scrap wood before attacking the sash.

[John Rumm]

A TCT blade in a saw will also probably survive contact with mild steel fittings is you take the cut relatively slowly.

[stuart noble]

Usually no problem if the nail is firmly embedded. If it's flapping around in a mortice joint or similar, you may lose teeth

[Roger Mills]

If there are any screw remnants left in, they will go from front to back

- while I'll be cutting in from the edge, so my cuts will be perpendicular to the screws. I was taking the view that a circular saw blade would probably suffer less damage than would a router cutter if a screw were encountered. I sense that you both agree?

[dennis]

I have a wood screw that is cut down its length after I hit it cutting some old wood on my SCMS. The blade is fine. I didn't even notice it happen.

[NT]

My sympathies. IME spring balances are dreadfully unreliable.