Drilled Hole Tolerances

The only type of twist drill that will cut to size is a solid carbide stub, in a mill rather than a drill press. And after checking that the chuck or collet is running true.

Edited By old mart on 24/04/2021 16:55:31

Strangely a drill can drill a hole that will be difficult to insert the drill in, so it will be on or undersize.

Mike

Standard twist drill is supposed to produce an H12 tolerance hole, but there are a whole lot of caveats with that. A brand new drill, at least from producers like Dormer, will have an accurately ground profile concentric with the shank. Hole size is assisted with the use of a guide bush held close to the material being cut, and flood coolant to keep the swarf away. This also supposes the drill depth is within the diameter/ depth ratio for the selected drill, and speeds and feed are correct. In an industrial setting correctly setup drills can drill thousands of holes within tolerance. Once we move to the model workshop with drills of unknown provenance who knows what the best bet is for the hole size.

I would expect the explanation has something to do with local expansion with machinining. A hot hole will be bigger than when cooled. Differential expansion of material and cutter will likely be part of it?

Therefore, if you want a hole that is very slightly less size than the cutter, heat the item, drill quickly and then cool? Difference will be small but noticeable by the tight fit.

sorry Howard but my experience of Dormer drills is very variable, some are good others are b awful, very obvious when put on my cutter grinder jig.

Check that both lips are the same length - I use a metric steel rule and a jeweller's loupe. I generally find that the Dagenham Reamer method (drill about 1/64 undersize then follow with a lubricated full-size drill at low speed) will produce a good accurate-to-size hole. I get occasional unexplained failures, though, so if I can I do the hole before I put a load of work into the piece.

Thanks for all the responses - it being a nice day have been totally tied up in gardening and then the first barbie of the season, so sorry for no reply until now.

Some very interesting replies, and a nice trick with the to size drill with the corners stoned off, might try that.

Have taken all that has been said on board and will rethink the method application in the morning, too tired now!

Chris

It's a lot simpler than that. Stone the tips of the cutting lips round and the flutes will close slightly. Sufficient to not be able to get the shank of the drill into the finished hole.

regards Martin

That’s interesting Martin, I think I encountered drilling a tight hole with a drill that was in poor shape, it had lost its sharp corners but the cutting lips were still reasonably sharp. Maybe the corners had been stoned off by someone in the know about your tip and a bit of use disguised the fact that it was deliberate. A very useful tip, I will file it away. Worth bearing in mind that the corners have an important effect on finished hole size and the importance of keeping a drill properly sharpened unless the hole is required to be tight.

Mike

Edited By Mike Poole on 25/04/2021 08:49:06

I've previously mentioned these "pilot point" drills, which have a central split-point pilot and something like a slot drill behind. I git my first set years back branded B&D, and a more recent set Elu. I find them excellent, they don't need a pilot hole (in fact it's positively bad), don't wander, and drill very close to nominal size.

I'd guess the pilot points will work well so long as they're held with good concentricity - but if the piloted part runs out, I would think it'll cut oversize and allow the full diameter lip to dig in slightly and flex the drill, with similar results to an asymmetrically ground jobber drill.

The impression I have is that the key bit of the game in drilling is to get the cylindrical land correctly supported in a hole of the correct size, and if the point is in any way suspect, make sure the projecting part of the drill is as short and as stiff as it can be, and feed the initial penetration slowly enough to take out most or all asymmetric flex and spring. You'll be able to see from the swarf if one lip is doing all the cutting, and once full diameter penetrates, you can generally see if there's more clearance around the drill than the cylindrical land can account for.

.

There have been some useful comments since you posted that, Mike ... but I think another explanation is that standard twist drills can produce three-lobed holes [more evident when drilling thin material] ... so the “diameter” is correct, but you can’t push the drill straight through the hole.

MichaelG.

Yes Michael That is one way a drill can cut. With the corner rounding it works better with a pilot hole close to finished size. I have not seen any indication of trilobing when I've done it.

I suspect that the standard tip angle has been chosen to balance the forces on the flutes for twist drills in steel for normal feed rates so the 'squeeze' and the 'flair' balance to give a correct size hole. It would be an interesting excercise to vary the tip angle and see what happens. I'm sure there are some clever cloggs out there who know far more than me about cutting tool design so perhaps someone can comment.

regards Martin

I used to shop around for the best price on grey case drill bit sets, 1 to 10mm in 0.1mm steps.

Eventually, I became suspicious and got the jolly old micrometer out for a quick measure.

Then I bought horrible, red plastic, cased sets by Dormer. 1-10mm cost me £193.62

I have become jealous of those red cases, I hide them in the cupboard under my bed leaving the old grey cased sets lying around in the shop for other people to use. Those have lots of empty spaces, unlike my beloved Dormers

Bernard, things have changed since we used them, original UK sourced were good as were Gurhing. The problem these days is getting tools with known provenance, I suspect a lot of so called named tools come from questionable sources.

Today I made a 1/2" BSW screw with a 9/16" square top and a 1/4" crosspin for hand-operation. I centred and drilled the crosshole 6,0 mm, then followed up with a little-used 1/4" Dormer as a Dagenham Reamer.

The cross pin was 1/4"" silver steel, miking up as about .2499/.2500" on a Mitutoyo Vernier mic.

I still had to turn the crosspin down to .2490" in order to bash it into the 'ole with a Thorex 712, and I don't think it'll fall out any time soon.

Edited By Mick B1 on 25/04/2021 16:19:42