Narrow countersinks?

[Eric The Viking]

I'm really frustrated.

I was trying to use a very short length of T-track, 3/4" size, to fix an issue with my bandsaw table. It's going to be surface-mounted on one edge of the table.

So I drilled a mounting hole and carefully tapped it to M4, and put a countersunk machine screw in to hold the T-track temporarily in position. The second hole was done by drilling through both the T-track and the cast iron together, so that the T-track stayed aligned correctly (even if the location of the hole wasn't spot on). This also worked a treat.

I was reaching for the M4 tap again when I realised what a twit I've just been: the first hole I used was a pre-drilled and countersunk hole in the T-track. It came with them already drilled at regular intervals.

The second hole, on the other hand, has no countersink, and it seems I can't make one as I don't have a countersink that is narrow enough to reach down through the open slot in the T-track (about 8.2mm across) to make it!

Any machine screw not countersunk properly will get in the way of the square nut that needs to slide in it. So no countersink on the hole = can't fit the T-track to the saw :-(

So I rummaged around and found my smallest snail-shell. That will just about slide along the track and I can get the point into the drilled hole, but I can't rotate it! I even wondered about reducing its diameter, but I think it would end up in two pieces. That idea isn't going to fly.

D'oh, as Homer would say. ](*,)

Can someone please point me at a source of supply for small (i.e. narrow) countersinks?

Failing that I can always freehand grind the point of a twist drill, but the result probably won't be pretty, and it's a lot less controllable too. Actually I can use the depth stop on the drill press for that bit, so it may be possible...

TIA

E. (feeling idiotic).

 

[NazNomad]

Could you not just put your normal sized countersink in the drill and hold it against a grinder to 'narrow' it?


...or...

Can't you slide the countersink bit down the track before mounting it in the drill?

 

[Eric The Viking]

Tried the second - slides along but can't rotate.

Was just looking for another narrow-ish countersink I think I have (might have a lead now: Interpol has been very helpful...). It will almost certainly be too wide, but it's the old multi-fluted type so can be ground.

With a snail-shell, although the finish is lovely, you can't make it any smaller than the hole drilled through it!

Edited earlier post: track is approx 3/4" and slot is about 8.3mm wide (narrow part).

 

[novocaine]

regrind a standard drill bit to the correct angle.

 

[sunnybob]

Just drill out the upper part of the T track to the width of the countersink bit. That small an area isnt going to impinge on any safety aspect of the track.

 

[Eric The Viking]

It's not being used that way, sadly it's for clamping.

Found the other countersink: It's a gnat's under 10mm.

So I've got choices: either lose 1mm off the radius or regrind a 7.5mm drill bit (have a few). I'll have to get by without a clearance angle but that may be a good thing.

 

[Rorschach]

I would re-grind an old drill bit for a one off job like this.

 

[ColeyS1]

What about a self countersinking screw

It's not massively quick, but freewheeling it the hole with a screwdriver created a countersink

It did make the aluminium quite warm though :lol:
Coley

Sent from my SM-G900F using Tapatalk

 

[sunnybob]

Grind off the bottom of the clamping bolts till they slide over the screw head.

 

[Eric The Viking]

Spun the narrowest conventional countersink I have against the grinder - worked reasonably well except that the battery in my drill died and it started to oscillate a bit as it slowed down.
The result is something that looks a bit like the rotor of a Wankel engine, but it did work.

Thanks all.

E.

PS: I think Coley's Idea would have worked too.

 

[SammyQ]

Eric, have you tried ordinary centre drills like wot they use on lathes? I freqently do and they cut most metals "like buttah" and leave a nice finish. Skanked in most drills, there should still be enough 'throw' to clear your metal edges.

Sam

 

[DTR]

The angle on a centre drill is 60*, a countersink is 90*. In this application it will probably be fine, but it will mean removing more meat than a 90* countersink would.

 

[Eric The Viking]

I'm aiming to tidy-up my mutilated countersink (it works but scuffs _very_ slightly on the edges of the slot - just needs to be round rather than a 'tricorn hat' shape).

And if I coke that up, Plan (b) is to do what Rorschach and Dave suggest and re-grind an 8mm drill to 90deg, and use that (have loads of suitable blunt-ish 8mm drills).

Cheers, E.

 

[AndyT]

If the job can wait a while, I have a few old style flat countersinks for use in a brace. I am pretty sure one or two would be small enough.

 

[xy mosian]

There may be something useful in here.
https://www.amazon.co.uk/Countersink-Ti ... ery+driver

More here.
http://www.ebay.co.uk/itm/3Pcs-90-Degre ... 0005.m1851

HTH xy

 

[Eric The Viking]

Thanks all. I think I'm good-ish.

I need to finish the job and get the bandsaw back together - as AndyT knows, there's a tablesaw waiting for after that

I'm hoping the bandsaw mods, and possibly the TS ones are worth a write-up. Trying to take pics as I go...

 

[Inspector]

A better solution than the centre drills that SammyQ suggested is a spot drill, at least that is what I know them as. They come in several angles (100, 120 degree etc.) and are normally used in machining to just start a hole for a drill to follow. When drilled a little bigger than the drill going into the hole it is nicely chamfered for tapping threads. A couple examples below.

https://www.cromwell.co.uk/shop/cutting ... e/39151101

https://www.cutwel.co.uk/twist-drills/h ... tre-drills

Pete

 

[Eric The Viking]

Thanks, yes those would work, too.

I do always chamfer the hole too before tapping - it makes a big difference.

 

[SammyQ]

Thanks Dave and Inspector; I'd neglected the detail of those centre bits. Think I'll invest in a couple od spot drills though.

Sam

 

[DTR]

Think I'll invest in a couple od spot drills though.

Sam

Likewise, I never knew they were available in 90*