Counterboring in wood

Make a template with a hole in it from a scrap of ply, then use it to guide your router with a guide bush fitted. You can put radial cross hair lines around the hole to make it easy to line up with the centre of the existing hole.

Not sure what 'standard' sizes counterbores come in, I only have one.

I think what you want would be described as "M6, XXmm x 15mm cut diameter", where XX is the total length of the bit, sounds like a long bit isn't required in your case?

As for the larger size you're after, I mainly see even M numbers, so you might be out of luck, unless a 1/4" shank by 7/8" cut diameter is close enough? Might have to buy from USA?

I recently needed to make some 65mm diameter by 17mm deep pockets for some castors. I 3d printed a template and then used a router bit with a bearing on a 1/4 trim router:

I could have also drawn a circle on a piece of scrap and jigsawed out the hole, but I needed a friction fit so used the 3D printer to do it accurately (turned out I actually needed the pocket to be 66mm so I 3D printed a second template)

You can also do it with a Forstner bit - I'd draw a circle centred on the hole and then ensure you get the Forstner started within the circle. But you'd need to buy two bits, while you only need one router bit (and that'll come in handy for other jobs).

Theo

1. Does the counterbore need to be *accurately* centred on the hole? As long as it's large enough to deal with a little offset then I suspect that absolute accuracy isn't essential (presume you'll be plugging the counterbores ... ?) If the counterbore needs to be accurate than use 6,, (and 7mm) pins to locate the router, fix or mark its position and change to your 15/20mm bit, plunge to depth
2. A flat bit (the sort with teeth at the edges to reduce tear-out) in a drill press?
3. Use a forstner bit to make a hole in a piece of ply and then use this (held with double-sided tape) to guide it for the counterbores?
4. make a template and use a smaller bearing-guided router bit?

If 't'were me I would use a centring pin to locate and then use a forstner bit.

If you have not drilled the through holes yet then counterbore the hole first with a flat bit if accuracy is not essential for the depth then a piece of tape on the flat bit set for depth required will suffice. Then using the centre point depression left by the flat bit drill the through holes. If accuracy is important then use a forstner bit using the same process.

Thanks, I hadn't thought of a template.

I have a total of some 40 holes to counterbore on a sheet and I feel this would take an inordinate of time to clamp etc for each hole.

I could take a sheet with the counterbore holes all the way through and use that as a template. The issue is I don't really want many large sheets for templates and each design may undergo future changes.

Thanks for the idea.

I have been searching for similar and the closest I got was a set like this:

The counterbore is intended to keep the fixture flush with the ply. Not much depth is required. Accuracy isn't that important except for aesthetics. While it won't be visible, it will be from a QA POV and obviously my view!

This needs to be a quick operation and I have 20 in each size.

I did come across these flat bits with pilot:

It is important that the holes are made with relative precision. Therefore the holes must be made first by a CNC machine.

The reliefs can be made by the CNC machine, except due to long file lengths the machine can't cope with the relief has to be made in 2 passes. I thought the quickest way would be to manually make the counterbore using the hole as a reference.

These are the closest I've got in style I was thinking:

Perhaps braze a plug on the end end of one of these:

to go into the holes and then use standard flat bits with the centre spike sitting in the tube. The tube will help with centring and prevent damage to the hole. Alternatively, buy one of the cheap CNC routers (but from another reply I see you may already have one).

I just tried a search for "forstner bit with pilot" and several appeared

- if you replace the pilot drill with a pin you (I think) have what you need.

I've just drilled a trial hole with a 20mm Forstner:

Theo