Weird drilling conundrum

[loose nut]

I needed to drill or preferably ream a hole of.210" down the centre of a part on the lathe. I don't have a .210 reamer but I did have a number drill of .209" so I assumed it would be "good enough" since drills usually go a little oversized. I carefully step drilled the hole finishing it with the .206, .208 and finally the .209 drills. OK bit over done on the drilling but I wanted it as close as possible. Unfortunately it didn't come out right, I can run a .209" drill through it bit a .209 gauge pin won't go through, the closest that will is the .206" pin. It turns out that the inside is sort of hour glass shaped, .209 on either end but only .206 in the middle.

I would have thought that was impossible. Anyone know how I pulled that one off?????????????????????????????

[ezduzit]

...Anyone know how I pulled that one off?????????????????????????????

By not using the right tool.

[old mart]

I think you might have to bite the bullet and invest in a reamer. If and when you do the reaming, make sure plenty of cutting oil is used. A possible reason for the bad result was in creeping up to the size, drills don't like taking tiny cuts. Your best bet with a drill would be a solid carbide one, but it would cost as much as a reamer.

[eKretz]

Yep, wrong tool for the job. A drill is not intended to make a precisely cylindrical hole. It is intended to rip steel out of the way, it's only intended to be used as a roughing operation unless you're making clearance holes or tapping the hole.

If you need a precisely cylindrical hole, you should ream, bore, hone or lap it, depending on the level of precision/accuracy needed.

[George Bulliss]

While not the right tool for the job, sometimes you do what you have to do and I've used drills in place of reamers many times for odd sized holes, though these were typically for holes under .100". I run the drill just like you would a reamer, with a very low speed, plenty of oil, and a little material left to cut. For your .209" hole I would have left .005" - .012" to clean up.

Not the best way to do it, but you can get away with it. These holes were for ejector pins in plastic injection molds. The holes had to be good; many of these molds shot materials that would flash though a .0005" or less gap and with pins under .050" things bend quickly if it's not a smooth fit.

[Mcgyver]

Anyone know how I pulled that one off?????????????????????????????

space time dilation of the rotating part and the increased gravitation field in the middle of the part.

How are you measuring it? If with a pin I would suspect the observations are from the hole being less than straight than a dia. in the middle that is smaller than the drill

[Doozer]

I suspect you are not measuring hour glass shape with gauge pins
just a crooked hole.

-D

[Dave C]

If this is a one part job, turn a hard wood dowel to .210, sand one end undersized, cut some shallow grooves on it and load it with valve grinding compound. Lap the choked bore to to size. If more parts are needed, buy a reamer.

[754]

The less a drill cuts, the less likely to go oversize.
your first mistake...no test hole. .....I would try the .209 in a 3/16 hole see what you get .
if its a through hole , and you find right size bearing ball , you can size it .. or try making a hard bullet to push through .. being a small hole your push pin may fold up.

[mickeyf]

Probably not what's happening here, but I can envision an hourglass hole being created when the drill bit "wobbles" as it could when improperly held in a 3 jaw drill chuck, between two of the three jaws rather than properly centered.

[reggie_obe]

space time dilation of the rotating part and the increased gravitation field in the middle of the part.

How are you measuring it? If with a pin I would suspect the observations are from the hole being less than straight than a dia. in the middle that is smaller than the drill

Nope.
Worn chuck jaws so tight that they were compressing the material in the middle.
Influence of the Northern Lights magnetic field on non-magnetic objects.

[BCRider]

Nope.
Worn chuck jaws so tight that they were compressing the material in the middle.
.....

That's a good likely cause too.

I'm going with options for crooked hole or the walls being thin enough that as the drill dulled it stretched them out and then it sprung back in. And adding the jaw distortion as a good third alternative.

If you need only the one item I rather like the suggestion for opening up the center with the wood (or maybe aluminium) lapping.

[darryl]

If the hole is well enough centered, you only need to get the size correct all the way through. If you can find a rod of the correct diameter, just make a D bit on the end of it and clean that hole out. This is a case where it would be very handy to have some hardening powder on hand. Even a mild steel will absorb the powder and give you a hardened cutting edge. Pretty easy to do with just propane.

I realize that's not a very fat piece of rod being just over 3/16 diameter, so it will be flexible. But it will work, I've done it. Another option would be to modify a carbide tipped drill bit to get that exact size, and ream with that. These drills usually have the carbide a little wider than the shank, so you have the option to grind just enough away to end up at your size. These are not made for cutting steel, but with a good sharpening they can be made to work for a while. Because the shank of a suitable drill bit would be narrower than the hole, it will follow the wobble- so may not be as suitable as the D bit.

[Paul Alciatore]

If by hourglass shaped you mean as viewed from the side, I doubt that is the case. It is a lot more likely that you have a three lobed hole so the drill bit, with two flutes does fit, but a solid pin does not.

As for correcting it, ream or lap as suggested above. Or make a one flute bit by grinding that single flute on just the first inch or less of it's length. This is different from a D bit in that it will have a full circumference for most of it's length and about 3/4 of it for that first inch where the single flute is created. An actual D bit may drill this hole properly from scratch, but I do not know if it would correct your three lobed hole.

Be aware that any of these corrective methods will still leave the wider lobes that your drill bit created, but the gauge pin will fit.

[tom_d]

[QUOTE=loose nut;n1921305]..........................I wanted it as close as possible.
/QUOTE]

That's what you have done, and successfully, too. There's simply a limit to the accuracy obtainable when using a drill as a reamer.