Drilling (specifically centre points) question

[sploo]

Once in a while I tackle a project where I need to use threaded inserts (like these http://www.screwfix.com/p/insert-nuts-t ... 0wodP08L5A) to make a knock-down item that's then put together using screws/bolts with a wide flat head (I can't find a link right now).

That means the part taking the (usually M6) screw needs a 15mm pocket for the head (cut with a forstner bit), then a 6mm+ hole (usually 7mm, for a bit of clearance). The part taking the threaded insert needs an 8mm hole for the insert, and if it's Type D (with a flange) it'll first need a 12mm pocket for the flange. Finally of course, the holes have to line up in the two parts!

What I usually do is put the parts together and drill through with the thinnest bit I can get away with (e.g. 2mm), then use that as a guide for the forstner drilled pockets and finally the holes. Problem is, I tend to find that drills don't "sit" in the 2mm hole very well, so I frequently end up with the hole not quite in the centre of the forstner pocket. The forstner bits usually (but not always) tend to line up OK with the original alignment hole.

If I drill through both parts first with a 7mm bit (not using the 2mm bit) then I stand no chance of getting the 8mm hole centred, and there's no centre for the fortner bits, so those holes go everywhere.

I have tried a number of different drill bit types, but never had much luck. Anyone got a better way?

 

[CHJ]

.....If I drill through both parts first with a 7mm bit (not using the 2mm bit) then I stand no chance of getting the 8mm hole centred, and there's no centre for the fortner bits, so those holes go everywhere.

I have tried a number of different drill bit types, but never had much luck. Anyone got a better way?

The threaded socket nuts have a slightly chamfered collar/flange, use a good countersink bit in the 8mm hole to clear enough for the flange.

 

[sploo]

.....If I drill through both parts first with a 7mm bit (not using the 2mm bit) then I stand no chance of getting the 8mm hole centred, and there's no centre for the fortner bits, so those holes go everywhere.

I have tried a number of different drill bit types, but never had much luck. Anyone got a better way?

The threaded socket nuts have a slightly chamfered collar/flange, use a good countersink bit in the 8mm hole to clear enough for the flange.

Thank. I suppose that would probably work OK - not a big deal if that pocket ends up a little wide as they're not seen when it's all screwed together.

The killer is getting the drill bit to follow the guide hole/point though. If find that even for cases where I drill a pocket with a forstner (no pilot hole) then often the subsequent drilling doesn't centre perfectly on the point left by the forstner.

 

[CHJ]

....
The killer is getting the drill bit to follow the guide hole/point though. If find that even for cases where I drill a pocket with a forstner (no pilot hole) then often the subsequent drilling doesn't centre perfectly on the point left by the forstner.

How many holes do you need to counter bore?
Can you not use a metal collar (thick washer in effect) located in your counter bore to drill your 6/7mm hole centrally.

 

[custard]

There are sets of forstner bits that have thin, replaceable twist drills in the centre to act as guides for jobs just like this. You start with a "pilot" hole through both workpieces using the twist drill, then load the twist drill into the appropriate sized forstner, after that you're golden.

Incidentally, if I'm installing an insert where it has to be dead on accurate there's a slow, but very precise method,

-take a shoulder bolt that fits the insert and hacksaw off the head
-pop a couple of nuts onto the bolt, then screw on the insert with the slotted section pointing down into the workpiece (this will help cut really clean threads, some people argue this is the correct way of installing an insert)
-lock off the two nuts at an appropriate height so the shoulder bolt is threaded almost right through the insert
-place the bolt in the chuck of a drill press with the workpiece on the drill table (this keeps the insert dead vertical)
-use a spanner on one of the locked off nuts to wind the insert home while applying a bit of downward pressure via the drill press

It's a faff but the results are very precise, it's what I do for very critical jigs and such like

Good luck!

 

[sploo]

....
The killer is getting the drill bit to follow the guide hole/point though. If find that even for cases where I drill a pocket with a forstner (no pilot hole) then often the subsequent drilling doesn't centre perfectly on the point left by the forstner.

How many holes do you need to counter bore?
Can you not use a metal collar (thick washer in effect) located in your counter bore to drill your 6/7mm hole centrally.

For the current job? Probably another 10. A perfectly fitting metal collar that's near the tip (so fits into the pocket just before the tip starts to drill) would be an ideal solution. That's a very interesting idea.

I have a CNC machine so I guess I could cut sets of various common drill bit + forstner bit rings (likely 6mm or 7mm internal and 15mm external). As long as one lasts for a few cuts it'd be worth the effort.

Some of these types of cuts only have a ~2mm deep pocket (the screws have a wide flat head), so a collar may not work. But, I suppose a tube that you pop into the pocket would also work, as that would guide the drill bit down the hole in the centre of the tube. With a bit of care, the collar could be made on the lathe rather than the CNC too.

There are sets of forstner bits that have thin, replaceable twist drills in the centre to act as guides for jobs just like this. You start with a "pilot" hole through both workpieces using the twist drill, then load the twist drill into the appropriate sized forstner, after that you're golden.

Incidentally, if I'm installing an insert where it has to be dead on accurate there's a slow, but very precise method,

-take a shoulder bolt that fits the insert and hacksaw off the head
-pop a couple of nuts onto the bolt, then screw on the insert with the slotted section pointing down into the workpiece (this will help cut really clean threads, some people argue this is the correct way of installing an insert)
-lock off the two nuts at an appropriate height so the shoulder bolt is threaded almost right through the insert
-place the bolt in the chuck of a drill press with the workpiece on the drill table (this keeps the insert dead vertical)
-use a spanner on one of the locked off nuts to wind the insert home while applying a bit of downward pressure via the drill press

It's a faff but the results are very precise, it's what I do for very critical jigs and such like

Good luck!

The forstner + twist drills are interesting, especially if you need to start a cut at an angle; though I find centering the forstner isn't usually the hard job.

I do often use the drill press (not running of course!) to help get an insert in, as it's a pig if one starts to go in at a slight angle. You're obviously using better quality inserts - all mine are the nasty ones with a hex recess for an allen key :wink:

 

[Bod]

Could you use a smaller pilot drill? (assuming you could find a long enough 1mm drill. http://uk.rs-online.com/web/p/twist-drill-bits/0792046/ shows them available)

Bod

 

[sploo]

Could you use a smaller pilot drill? (assuming you could find a long enough 1mm drill. http://uk.rs-online.com/web/p/twist-drill-bits/0792046/ shows them available)

Bod

That would be worth a try too - thanks. I have a couple of 1mm drills, but they're too short.

 

[iNewbie]

The Forstner bits Custard mentioned are these -

http://www.fine-tools.com/forstnerbohrer.html

 

[bugbear]

Could you use a smaller pilot drill? (assuming you could find a long enough 1mm drill. http://uk.rs-online.com/web/p/twist-drill-bits/0792046/ shows them available)

Bod

In a non-homogenous material (like wood) a 1mm drill bit isn't rigid enough to be trusted to follow a straight path, or even to enter where you want it to.

BugBear

 

[oakmitre]

I think getting a nice tight fit on the flanged insert is more important that the recessing of the screw head on the other side, in fact having a tight fit of the screw head is probably not desirable.

In the pillar drill, make sure you are using drill bits of similar length so you can change them without adjusting the table or the workpiece. Just drill all the way through with the small bit, forstner the recess for the flange, then the hole for the insert, then for your 7mm hole, go almost all the way through but not quite.As long as you peck the first 5mm initially when the drill bit lands on the surface - they will be concentric. If you pull the handle down in one motion, the drill will flex.

On the other side, forstner a slightly bigger hole than the screw head, when you use the 7mm drill on this side it will follow the path of least resistance and be pulled into the partially drilled 7mm hole.

 

[DiscoStu]

Not sure what your funds are etc but if you have CNC and this is a commercial project then could you make use of the new domino connectors? I assume you've seen them?


Sent from my iPhone using Tapatalk

 

[woodpig]

I'm not sure if these would help you. I've used them to to drill counterbored holes in metal.

http://toolguyd.com/dewalt-pilot-point- ... -favorite/

They are often called "bullet" drills over here. The nice thing about them is that they drill a perfectly concentric pilot hole (albeit not very deep) at the same time as the counterbore. Not seen imperial ones over here as in the picture, only metric.

 

[oakmitre]

Other option is to use a 1/4 trim router with a removable plunge base.The base can be screwed to some rubber backed thin laminate and clamped to the workpiece (even vertically). The base stays firmly in place whist the motor is removed to swap bits (unplugged of course). If you plunge a 1/4" hole all the way through, a 1/4" piece of silver steel rod in the collet can be used to align the base again on the other side.

Its fairly accurate, as long as you plunge slowly.

You can even use a fancy (not sure what it is called) roundover flatbottom bit on the screwside - although the one I have is 1/2".

 

[oakmitre]

I should mention that the plunging depth for the above router is 35mm, so for 2 layers of 18mm = 36mm you would have to recess first and set the bit very slightly below the router base before plunging through.

 

[sploo]

Lots of replies!

Forstner bits - yeah, the ones I have are Famag Bormax3 (without the drill in the middle). They're of a quality standard that if something doesn't work, it's you, rather than the tool!

The bullet drills are an interesting idea. Come to think of it - I do have one that looks like that, but not the right diameter for this project.

I have CNC, this isn't commercial, and I have seen the new Domino connectors . Sadly those connectors are a bit too large for this project, but I did consider them. I find Domino joinery much more accurate than this hole drilling malarky, but it wasn't the right tool for this job.

I've got it all together now, and used a hand plane to skim off any inaccuracies between pieces. That's what a plane's for right, to cover your mistakes

 

[Droogs]

Page 10 of the FAMAG 2015 catalogue has all the forstner bits with centre drills you would need sold as pilot guided bormax

 

[sploo]

Page 10 of the FAMAG 2015 catalogue has all the forstner bits with centre drills you would need sold as pilot guided bormax

I already sold one kidney to afford the set I've got. I need my other one for my whisky habit :wink: