Different diameter holes from the same single size drill bit??

[guninhand]

I bought an aluminum mold from NOE (great product) as well as a mold themometer. This mold already had a hole to take the temperature probe. Included was a #19 drill bit to adapt other molds to use the themometer temperature probe. Being ever cautious and patient, I used the bit on a scrap piece of aluminum first, and the probe was an exact slip fit, just like NOE claimed.
Using the same drill press and supplied drill bit, I drilled a hole in a steel mold. The probe is too big to fit!! I put the drill bit in a handheld drill and let it free spin in the just drilled hole, hoping it would open it just enough, but no luck. Anyone know why I got 2 different size holes from the same bit, or what I can do to open the hole I made. If the fit goes any looser than a slip fit the probe might be falling out while I'm casting. Going by my micrometer, the drill bit diameter is .16515 and the probe is .16510.

[I'll Make Mine]

Yep, twist drills cut inconsistently, usually oversize, and often with a three-lobe shape. This has been well known in machining circles for as long as there have been twist drills; if you want an accurate, round hold you need to drill slightly undersize and ream to final dimension. All these tendencies are magnified in softer material, too, hence why #19 is right in aluminum but undersize in steel.

Best I can suggest to open the hole in your steel mold "just enough" would be to get a piece of 5/32" (nominal .156") brass rod at the hobby store and a little valve grinding compound to lap out the hole in the steel until the probe just slips in. If the rod is too loose to start with, you can mash it (very) gently with a hammer or vise to make it just slip into the hole before starting to lap.

[guninhand]

Thanks IMM. It always amazes me how much I don't know. The valve grinding compound trick sounds like just the ticket.

[Calamity Jake]

The difference in the diameters is caused by the different materials your drilling into the bit takes more material out of the
aluminum then it does the steel.
Get some lapping compound and apply to the shank of the drill bit then hand lap the hole to the size you want.

[XWrench3]

Also, the amount of pressure you put on the drill and the speed can make a difference. As the drill "bites" into the material, it deforms the bit. A light pressure will open (deform) the drill less that a lot of pressure will. The sharpness of the drill can play a factor also. A slow speed and a heavy pressure will drill a hole larger then a high speed and light pressure, especially with a sharp drill bit. Because it is biting harder, the twist on the bit increases. That deforms the bit more under high pressure. The actual point of the bit still cuts the same size hole, but the sides of the bit, being twisted and deformed, will open the hole up. And unfortunately, not perfectly round. As stated earlier, a ream is the propper way to get a round, correctly sized hole.

[MtGun44]

+1 to out of round, usually three-lobed holes. You must ream if you want a precise hole.

I wonder what magical instrument you have that measures to 5 digits. Not a micrometer.

Bill

[Buckshot]

+1 to out of round, usually three-lobed holes. You must ream if you want a precise hole.

I wonder what magical instrument you have that measures to 5 digits. Not a micrometer.

Bill

.0", .00", .000" does pretty well for most of us. .0000" Actually the purview of precision grinding, precision sliding fits and more accuracy then most of us need in our day to day routine. .00000" temperature controlled environment, equipment and stock. Liquid honing with machines that cost more then our real estate, in environments a central AC won't maintain. .00005" is 50 millionths, and not something any of us would derive any benefit of using. If someone gets better accuracy with a slug sized to .30075" vs .30073" God bless'em and I'd like to see pictures of their reloading bench. :-)

..............Buckshot

[I'll Make Mine]

.00000" temperature controlled environment, equipment and stock. Liquid honing with machines that cost more then our real estate, in environments a central AC won't maintain. .00005" is 50 millionths, and not something any of us would derive any benefit of using. If someone gets better accuracy with a slug sized to .30075" vs .30073" God bless'em and I'd like to see pictures of their reloading bench. :-)

Never done it with slugs, but a few years ago I made a telescope mirror. That's a surface accurate to a few millionths (better than a quarter wavelength of sodium light) and smooth to a millionth or so, over an eight inch diameter circle, made with handmade tools, on an old metal 55 gallon drum in a basement, and measured with handmade optical tools that were invented in the 18th century.

No, probably not possible with lead -- the metal moves around a lot more than glass -- but for applications that actually need that level of accuracy, one doesn't need millions of dollars worth of equipment and climate control, one just needs the techniques to control what needs controlling and measure what needs measuring.

[MtGun44]

Optical measurement is sometimes amazingly easy, but you are "measuring" the effect of
a true parabola versus a nearly true parabola, usually starting from an approx spherical
surface. I have done a mirror, too, although I never finished the figuring - it was at school
and as a Navy kid, we moved before the final figuring was done. Darn it!

Certainly it is precision manufacturing, but not quite 'precision measurement' in the sense that
you will get a dimension value.

Just pulling the author's leg a touch about all those digits. Agree 100% with Buckshot. We
have a precision measurement dept at work and they use a laser interferometer for the best
measurement, but of course, if you don't control the temps this is meaningless.

Bill

[theperfessor]

Odd thing is that when drilling metal the hole is always bigger than the drill bit, when drilling flexible plastics the hole is often smaller than the drill bit. The plastic can flex away from the bit, which is why drills used in plastic will overheat pretty easily and get dull on the outside as much as on the cutting edges.

[country gent]

Okay driling with a twist drill requires the correct speeds and feeds for the actual material being drilled. Aluminum is ran faster than steel steel is slower than brass. Lead, relief and point angles are actually diffrent for difrent materials, why a store bought drill grabs in brass and plastics. Clearing chips correctly makes a big diffrent also. Chips and swarf remaining in the hole can act as an abrasive. Proper cutting lubes are also important. Black sulfar oil, water souluable oils, wax and soaps all will work some better than others in certain materials. A drill with the point sharpened of center will cut a oversized hole. A old drill with the lands worn and sharpened correctly will actually cut an undersized hole. You see this in bigger drills that do get removed from service when the should. In aluminum beeswax for cutting oils a farily high rpm and feed works well clearing chips every .100-.150 of drilled depth. In steel Black sulfar oil and slower speeds and feeds clearing chips every .050-.100 of drilled depth works well. Brass dull drill ( less relief angle will keep it from grabing) and widenn point angle light oils or wax for lube. High speeds and feeds, clear chips every .100 of drilled hole depth. If you dont keep chips cleared frequently you will never get accurate consistant holes. There are hand gauges that measure point angle and center of web when sharpening they are only 10-12 dollars and worth the investment.A reamer is not a cure all ran incorectly a reamer will cut oversized also. The old tap magic ( smellled like pepermint) was notorious for making taps drills and reamers cut undersized. It also discolored some metals. Lard Black oil beeswax lanolin soaps ( think the waterless hand cleaners) are good cuting fluids, along with the store bought cutting oils.The Machinists Handbook gives alot more information for this and actual angles for diffrent materials.
In most shops for a truly precisson hole the hole is drilled on location undersized then Jig ground and or lapped to finish size location. Reaming is done at times more so in "production" scenerios. We had a set of burnishers for finishing holes to high levels of finish and precisson also. These were adjustable and very accurate give correct lubes speeds and feeds. Lapping honing is very accurate and easily done but time consuming also.

[gray wolf]

I guess I'm way, way behind the curve with my yard stick.

[dragonrider]

You can get a reamer of any decimal size you need, MSC or Carr-Lane or other places.