The size and shape of drill holes

I had need to set some firm but removable pins into my current project. Specifically I aimed for 10mm of support for a total 30mm of pin into a blind hole in 15mm hot roll that had been skinned and cleaned to 14mm.

So I drilled to 12mm spotting a test hole start with a spotting drill then straight to a 6mm drill bit. The pin showed unacceptable rocking. Miked the silver steel pin and it's dead nuts 6.0mm The drill bit shank mikes at 5.80mm.

If I drill the hole with a bit labelled 5.5mm then overdrill with the same 6.0mm drill bit then i end up with a hole where the 6.0mm silver steel is too tight. - as in it goes in with a gentle tap but is a bugger to pull out

I'm sure old hand's knew this but it was a bit of a revelation to me. As a short blind hole the use of a 'proper' reamer or toolmakers reamer wouldn't work?. I did consider just milling a flat along a piece of the rod and then reaming with that but found that just inserting and rotating the rod a while burnishes the hole satisfactorily for a good but finger-removable solution.

I repeated the test with both unused drills and used drills to same result (cobalt jobber drills from the same batch)

For completeness the holes were drilled on the mill with table locked.

It was late so i haven't followed on my miking other drill bits for shank size compared to labelled size.

So the question (finally): Is the discrepancy just that the final skim cuts to size as opposed to a one-stage drill hole loading causing some unrecognised vibratory load or is the shape of the hole affected - as in one approach it's more circular than the other. And if drill shanks are thinner than the hole they cut then what percent and in what sizes?

pgk

If originating a hole from solid, any imbalance in length or angle of cutting lip will have a greater leverage and thus a greater effect in forcing the ends of the lips off-centre and causing the drill to cut oversize.

Contrariwise if an existing undersize hole is being enlarged to size, there's only a little material for the loaded lip to dig into and lever a deflection, and as soon as the other lip is in the hole, even this tendency is controlled.

Drills normally have a reverse taper on them, so the shank behind the flutes is usually undersize - although 0,2mm undersize is about 10 times what I've usually measured.

Hi you can use a chucking machine ream to finnish blind holes as they are parallel and cut to size.

David

... or make a D-bit from your 6 mm silver steel.

Russell

As JS pointed out several years ago, for drilling to size in one go spade bits are most accurate, then four/six flute, then ordinary Morse twist drills.

The most expensive drills you will find have carbide spade tips and flutes behind to clear the swarf.

These examples are pretty close to four-flute geometry.

And even then if the spade bit is not ground evenly each side all bets are off as it will just pivot around the off ctr point and cut to twice the radius of the longest edge.

My late Father, an expert toolmaker, taught me:

• don't measure a drill by its shank
• seldom will any drill produce an accurately sized hole
• always finish with an approptiate reamer to get an accurate hole.

This good advice has stood me in good stead for over 50 years.

Eric

I considered a D bit but unless one was after a flatter bottom to the hole I couldn't see the benefit over the simpler? option of putting a longitudinal flute down the silver steel and reaming (if one was going to take that option). Either way you'ld need a pilot hole close to size. If one wanted a fully flat bottom then the d-bit would have to be less than half the diameter and bored out?? I also considered running a 6mm endmill down the hole for a flatter bottom and testing the hole size. Again both those options would need more swarf clearance operations and it's not just a single hole I'm after. Since just forcing rotation of the rod in the sl tight hole solves it then that's simpler still (this time)

I misread the mike of the drill shank. It was 5.9mm not 5.8mm <embarrassed>

Thanks for the other comments/knowledge guys

pgk

Still seems a little undersize. The shank on my Dormer 6mm drill is 5.94mm, ie, 0.06mm undersize. The taper is proportional to drill size; a 3mm drill shank meaures 5.97mm, ie, 0.03mm undersize.

For close fits for 6mm pins I'd drill 5.8mm and then use a 6mm machine reamer. If that's too loose you could try a 5.99mm reamer.

Andrew

As per the workshop tolerances thread. I don't have the beneft of such an array of reamers or even of reamers of perhaps close enough tolerance (cheap chinese set - I think they're H8 and through reamers anway).

If it came to it i could happily manage with marginally undersize holes and polish a micron or two off the in-hole end of the pins with no reduction in function - or invest in some cheapish 5.9mm jobber drills to see if they'd come out used directly.

It's the differences between hobbyists on the economic side of pratting about for a functional end result compared to commercial quality stuff.

pgk

Yep, that's it. I've had plenty to do with reamers when I was working in engineering, but I've not used one for hobby stuff in over 40 years. It would take a requirement to make several tens of identical well-finished and sized holes to justify one for me.

Yes, obviously the grind needs to be done well, but spade bits avoid the tendency of flutes to grab.

Four flute drills centre better than cone centre drills.

So a spade drill with a four-flute edge has the best of both worlds and is the most consistent.

Why not file a notch in the end of a pin (I assume they are hardened) and use it as a reamer into a just undersize hole?

Neil

I did make the note that I'd thought about putting a flute down a section of pin and using it as such. (they're not hardened yet) bt turned out that i can just burnish the hole with intact pin when step drilling to size from 5.5 to 6mm

pgk

Not wishing to hijack this thread (I think the answer has been given), but since the problem of unequal cutting edges on spade drills has been raised, can anyone tell me why gun-barrel drill bits are like spade drills, but with one cutting edge about twice as long as the other? Hole size is relatively unimportant, because a reamer follows drilling, but straightness of the hole is paramount. Why should this geometry work so well?

Gun drills are an illusion to me. The trick is to get the balance of the cutting force, from the centre to the outer, so that it want s to stay on centre. So they have support lands etc to guide the drill. The sharpening of a gundrill to drill straight is quite specific for that drill, and material etc. Now days, there is a new very high speed drill, that out dates gun drilling, with a catch. It needs a very accurate spindle, and very high through coolant pressure to work.

Gun drills can be hand re sharpened , but twist drills, some can sharpen them really well, and others not.

The best firm but removable pins, are tapered pins. The reamer can be a simple D bit reamer to a taper that you can easily reproduce. You can also take a tapered pin, and make it into a D bit, assuming that you hardened it of course. Anything from 1.5 deg per side to 3 deg per side does work very well.

Neil

A drill is a roughing tool. If you want a precision sized hole, you need a reamer. Machine reamers from China are cheap as chips these days and generally OK quality for home use. Even if you sand your pins down to fit your drilled holes, they may loosen up rapidly in use as the ID of the hole will be rough surfaced, egg-shaped, tapered, or whatever else the drill bit decided to do as it wandered into the metal.

The problem of removing dowels/pins from blind holes was solved many years ago by having a thread tapped in the end of the dowel/pin. This tapped hole was used with an extractor so that the body of the extractor bottomed out before the thread part when the extractor was turned thus causing the dowel/pin to be pulled out. Dowels and taper pins are commercially available with the tapped holes . I’m not sure about the extractors through hobby shop suppliers but am sure some large toolroom part suppliers will have them but it’s not difficult to make your own. The extractor is a piece of steel with a threaded end protruding at one end . the shank can round , hex or square. The hex shank is best as it can be used with a standard wrench.

Dino,

Thanks for that - more new knowledge.

I had originally intended to use threaded ends to the pins to lift them from the holes but then decided to keep this simple (it's only something for myself) and I'm going with a simple cross hole at the top one could stick a rod through for a pull.

I looked up those tapped dowels.. cheap enough but as you say the commercial removal tool at £50 notes is a touch painful enough to go DIY if ever used.

pgk

There is often confusion about D-bits. A properly made D-bit has the end just more than half diameter. It then cuts on the end not on the side. It makes a very effective reamer.

Russell