# how much bigger does a  drill bit drill?



## Rcdizy (Aug 11, 2017)

If I want to drill a hole in stainless, and press fit a 3/16,  O1 drill rod (.1875) into it....

What drill size?  numbered, lettered, decimal?


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## Ed ke6bnl (Aug 11, 2017)

my guess is a number 13 drill .1850 lot of times drill will drill oversize holes that may work for you. but lets hear from some real pros.


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## Ulma Doctor (Aug 11, 2017)

+1 on the #13 bit


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## Bob Korves (Aug 11, 2017)

Drills do not make holes to an accurate size.  Drills do not make round holes.  Drills do not make straight holes.  All that said, the #13 might do it for you, depending.  It also might end up too tight, or might possibly end up loose, depending...  A drill one size smaller, #14, followed by an appropriate reamer for the desired press fit, would make for a lot less guesswork and a lot more confidence in a good outcome.  You should also carefully measure the O-1 rod diameter with a trusted micrometer or compare it with a 3/16" dowel pin.  O1 is usually within .001" of nominal size, but is definitely worth checking, especially at the end of the rod.  Check it at several clock positions, too.  In a hole of that small size, there is less room for tolerance stack up if you want a specific fit.


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## chips&more (Aug 11, 2017)

Using a drill bit for holding correct hole position and or for making a specific hole size and or making a round hole is a crap shoot.


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## Rcdizy (Aug 11, 2017)

The #13 was too loose still,   Size smaller too tight.

The rod is excactly .1875

So in a perfect world,  what reamer?

Would locktight work in this type of application?


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## Wreck™Wreck (Aug 11, 2017)

Excellent question Dr. Suess, .1865


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## mikey (Aug 11, 2017)

I would bore it. Right size, straight walls and you can decide the tolerances for the press fit.


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## RJSakowski (Aug 11, 2017)

In my experience, drill rod is often slightly smaller than nominal size.  How much oversize a drilled hole is depends a lot on the condition of the drill.  +1 on the undersized hole followed with a reamer.  The size depends on the class of fit you require.  Are you going for a tight slip fit, light press fit, or heavy press fit?


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## Bob Korves (Aug 11, 2017)

chips&more said:


> Using a drill bit for holding correct hole position and or for making a specific hole size and or making a round hole is a crap shoot.


Chips, you said the same thing as me with a lot fewer words... 



Rcdizy said:


> So in a perfect world, what reamer?


Totally depends on the actual rod size and the desired fit.


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## Bob Korves (Aug 11, 2017)

Read your original post.  We have no idea what you are making or what it will be used for.  Only a rod to push into a 3/16" hole.  Some things want to be put together pretty much permanently and resist becoming loose.   Others want to be relatively easy to take apart.  We need at least a feel for what you are making, even if the actual intended use is highly classified...


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## Wreck™Wreck (Aug 11, 2017)

mikey said:


> I would bore it. Right size, straight walls and you can decide the tolerances for the press fit.


You bore sub .200 holes regularly?
If so you are my hero.


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## mikey (Aug 11, 2017)

Wreck™Wreck said:


> You bore sub .200 holes regularly?
> If so you are my hero.



Sure - a Sherline lathe and some Micro 100 carbide bars will bore that hole probably faster than it takes to type it.


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## Bob Korves (Aug 11, 2017)

I am sure I could bore a small hole with a small boring bar in a small lathe or mill, but the real question is if I could get the press fit right...

Edit:  Just need a Moore jig borer...  or jig grinder...


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## Desolus (Aug 12, 2017)

Bob Korves said:


> I am sure I could bore a small hole with a small boring bar in a small lathe or mill, but the real question is if I could get the press fit right...
> 
> Edit:  Just need a Moore jig borer...  or jig grinder...



I just turned a shank for a tool I'm making on my sherline lathe to .3750 on the dot, over the entire length of the shank... seeing as how boring is exactly the same operation im going to go ahead and say that you could in fact get the press fit right...


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## mikey (Aug 12, 2017)

Bob Korves said:


> I am sure I could bore a small hole with a small boring bar in a small lathe or mill, but the real question is if I could get the press fit right...
> 
> Edit:  Just need a Moore jig borer...  or jig grinder...



A Sherline lathe has very precise leadscrews and their handwheels are very accurate. Dial in half a thou, take a thou off the diameter. With the right tools, cutting to tight tolerances on a Sherline is pretty simple because what you dial in is pretty much what you get.


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## Dr_Romeo_Chaire (Aug 12, 2017)

Boring comments aside, maybe the OP needs some guidance on what constitutes a light/medium/heavy press fit? (probably depends on materials involved also, right?)

I seem to recall pressing 0.125" dia steel dowel pins into 0.250" aluminum plate and using a reamer that was 0.005" undersize for a heavy press fit. It's been decades though and I've forgotten, plus I might've just been guessing back then -- so it would be good to know for future projects!


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## Tozguy (Aug 12, 2017)

Rcdizy said:


> The #13 was too loose still,   Size smaller too tight.
> 
> The rod is excactly .1875
> 
> ...



Reamer size already suggested in this post.



Wreck™Wreck said:


> Excellent question Dr. Suess, .1865



The lock tight (or super glue) should salvage the situation, choose the right one.


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## mikey (Aug 12, 2017)

Dr_Romeo_Chaire said:


> Boring comments aside, maybe the OP needs some guidance on what constitutes a light/medium/heavy press fit? (probably depends on materials involved also, right?)
> 
> I seem to recall pressing 0.125" dia steel dowel pins into 0.250" aluminum plate and using a reamer that was 0.005" undersize for a heavy press fit. It's been decades though and I've forgotten, plus I might've just been guessing back then -- so it would be good to know for future projects!



Fits are a huge discussion that cannot even begin until the OP gives us an idea of what the application is.


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## Tozguy (Aug 12, 2017)

To respond to the title question, how a drilled hole turns out is a function of several factors not the least of which is the quality and grind on the drill bit. It is not predictable. It might even be possible to end up with a hole smaller than the drill.


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## Billh50 (Aug 12, 2017)

1) If installing a "dowel pin" I would use a drill then ream to .0005 - .001 under, depending on material and situation.
2) Just drilling a hole for a pin is ok for roll pins.


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## BGHansen (Aug 12, 2017)

Like mentioned above (so not adding any help here), I'd spot drill for starters, then drill undersized by 0.010" - 0.015".  Then use an undersized or oversized (+/- 0.001" from 3/16") ream.  The oversize will give you a slip fit for separating the parts.  I've had very good success with my over/under set from Shars off eBay for a touch over $70 delivered.  The set includes reams from 1/8" - 1/2" by 1/16", probably the only reams you'll ever use.

Bruce


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## Bob Korves (Aug 12, 2017)

Desolus said:


> I just turned a shank for a tool I'm making on my sherline lathe to .3750 on the dot, over the entire length of the shank... seeing as how boring is exactly the same operation im going to go ahead and say that you could in fact get the press fit right...


Internal cuts are more difficult to hit accurately than external cuts.  They can be more difficult to measure accurately and the tooling is much less rigid, at least for small holes.  They are of course totally doable.


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## Desolus (Aug 12, 2017)

Given that the smallest cut you can take is directly proportional to the surface finish of your cutting tool (I took mine to a mirror finish for the last cut on that shank which was 0.007mm or about 0.00027 inch) the largest factor in how well you can finish a part out is machine repeat-ability, with a highly polished cutting tool you can take multiple 'spring passes' to virtually eliminate any deviations in diameter caused by a lack of tool rigidity. I'm learning that working on a small lathe on small parts is entirely different than forcing your way through material on a large heavy rigid lathe using rigid tools... I honestly didn't know what to do when I got my sherline, started cutting and realized just how floppy everything was... I don't have anything to accurately measure an run-out on an ID so I can't compare but I have bored small holes in watch parts using tools that I ground and polished on my faceting machine and the holes preformed as designed.


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## Bob Korves (Aug 12, 2017)

Desolus said:


> Given that the smallest cut you can take is directly proportional to the surface finish of your cutting tool (I took mine to a mirror finish for the last cut on that shank which was 0.007mm or about 0.00027 inch) the largest factor in how well you can finish a part out is machine repeat-ability, with a highly polished cutting tool you can take multiple 'spring passes' to virtually eliminate any deviations in diameter caused by a lack of tool rigidity. I'm learning that working on a small lathe on small parts is entirely different than forcing your way through material on a large heavy rigid lathe using rigid tools... I honestly didn't know what to do when I got my sherline, started cutting and realized just how floppy everything was... I don't have anything to accurately measure an run-out on an ID so I can't compare but I have bored small holes in watch parts using tools that I ground and polished on my faceting machine and the holes preformed as designed.


I don't see what "polish" has to do with it.  If you are speaking of honing the cutting edge, then yes, a sharp tool deflects less.  The amount of shininess has little to do with it.  At some point, mirror polishing will make the edge less sharp, and sharp is what is needed.  A shiny and dull tool will just rub.


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## Desolus (Aug 12, 2017)

polishing and honing describe an almost identical process, the use of abrasive to produce a precision surface. Polishing is typically only about making something shiny, and also typically to tighter surface tolerance. But in gem cutting which is my default when talking about making precision surfaces, polishing is about surface finish (100k - 200k grit typical) AND producing that finish on a very sharp edge consisting of two or more very very flat planes. I've been cut by many a sapphire... So I agree with you that typical polishing won't produce a cutting tool and instead a rubomax, but if done correctly you can get an incredibly sharp cutter that will cut shallow with no problem... you just can't take a deep cut with it or you ruin all your hard work...


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## RJSakowski (Aug 12, 2017)

A lot of information is missing here.    Is the a hole in the end of a piece of round, in a flat plate, or in an irregularly shaped part.  What is the machine; lathe, mill, drill press?  What class of fit? How deep is the desired hole?  There is a big difference on boring a 1/8" deep hole through a piece of plate and boring a 1" deep hole.

As to boring and turning being equivalent operations, I would say only on the condition that you were boring a fairly shallow hole.  For a hole where the depth greatly exceeds the diameter, the boring bar will be relatively flexible and subject to deflection.  Measuring the diameter of a hole is problematic as well.  A pin gage will only tell you what the smallest diameter is.  It conveys no information regarding roundness, cylindricity, surface finish, etc.  Determining the true state of a hole requires the use of something like an air gage.


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## Bob Korves (Aug 12, 2017)

Desolus said:


> polishing and honing describe an almost identical process, the use of abrasive to produce a precision surface. Polishing is typically only about making something shiny, and also typically to tighter surface tolerance. But in gem cutting which is my default when talking about making precision surfaces, polishing is about surface finish (100k - 200k grit typical) AND producing that finish on a very sharp edge consisting of two or more very very flat planes. I've been cut by many a sapphire... So I agree with you that typical polishing won't produce a cutting tool and instead a rubomax, but if done correctly you can get an incredibly sharp cutter that will cut shallow with no problem... you just can't take a deep cut with it or you ruin all your hard work...


The first thing that comes to my mind with polishing is a buffing wheel on a motor shaft and a stick of rouge.  That is not going to make something sharp, just shiny.  Thanks for the clarification of what you meant and how you achieve it.


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## benmychree (Aug 12, 2017)

I quite agree with the earlier writer who suggests the over/under reamers; I had a set at my shop (left them there) and every time I used them to fit holes for dowel pins, they made the proper fits; force fit for the part the pin is pushed into, and a snug fit for the other part.  Drills have no place in this sort of work, other than to make a hole for the reamer to enlarge.


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## Bob Korves (Aug 12, 2017)

benmychree said:


> I quite agree with the earlier writer who suggests the over/under reamers;


I have lots of nice previously owned reamers from many sources, and usually have the one I need for a job, but it can take time to find it.  They are sorted by nominal size, but many are really difficult to read, and I never know if they have been previously sharpened, so I have to measure or test them, and also pick one the right style for the hole type as well.  Sometimes have to hone it sharp, too.  That all takes time, lots of it.  I have to admit that a set of over/under reamers in basic fractional sizes like Bruce posted sure sounds quick and easy for lots of work.


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## benmychree (Aug 12, 2017)

Bob Korves said:


> I have lots of nice previously owned reamers from many sources, and usually have the one I need for a job, but it can take time to find it.  They are sorted by nominal size, but many are really difficult to read, and I never know if they have been previously sharpened, so I have to measure or test them, and also pick one the right style for the hole type as well.  Sometimes have to hone it sharp, too.  That all takes time, lots of it.  I have to admit that a set of over/under reamers in basic fractional sizes like Bruce posted sure sounds quick and easy for lots of work.


I think that the set that I bought was Eastern European, probably bought from KBC Tool; that was probably 10 years plus ago, the way time flies, likely more like 15 years.  That set came in a wooden block, like the Shars set pictured; I made hardwood blocks for all my chucking reamers and taps larger than machine screw sizes.


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## Silverbullet (Aug 13, 2017)

There are ways to make a drill bit drill bigger or smaller. By varying the cutting edge length and width plus off center of the tip. As for you hole being tight you can freeze the liner and press in or set in when it warms up it'll be tight . But it's up to you. Lots of ways to get the job done. The norm is spot drill and ream to size. Amount of most press fits on liners is .0005 to .001 . If I remember right.


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## Robert LaLonde (Aug 14, 2017)

How much over does it drill?  It really depends on a lot of factors.  Runout, speed, technique, length, type of grind, setup.  I've got a few sets of modestly expensive stub length screw machine drills that only get used when a hole has to be "pretty good."  With good technique they drill holes that are pretty good.  When a hole has be nearly perfect in size a reamer is ok, but for small sizes it still won't be any straighter.  It will follow the hole you drilled.  Chucking reamers tend to be long and flexible.  A reamer is not intended to straighten a hole.  A spotting drill followed by a a short properly ground screw machine drill followed by a reamer is going to get you as close as you can under most circumstances for small holes.  For anything large enough boring will get you very close as well, but you will still have some variations due to the tool geometry, tool flex, and machine guality depending on technique.   

One might argue that you can't get reamers in all the sizes you might like, but for the most part you can.  They just cost a lot.  Outfits like Harvey Tool make small chucking reamers in decimal sizes to cover a huge range.  Some outfits will even make custom reamers (or mills) for you.  

One thing to be aware of is a reamer ONLY sizes a hole.  It doesn't do anything else, and if runout or offset is bad it can make the hole oblong or bell shaped.  I believe this is why they tend to be long and flexible.  So they can flex to follow the hole.  If you leave it in the hole to long spinning it will still over size the hole.  I ran into this problem making brass micro end mill adapters yeas ago.  Gunsmiths run into this problem when chambering rifle barrels.  The answers is a floating reamer holder.  You still need to only have it spinning in the hole the minimum amount of time to size the hole, and not any longer.  

*To the original poster. * You already have a slightly oversize hole in a part I assume is otherwise finished.  If your tolerances for the pin position will allow it just glue it in with some Loctite sleeve and bearing locker.  They make atleast one formula that is intended to fill small gaps.  Your local hardware or auto parts store may not have them, but you can always order it from somebody like MSC or McMaster.  Maybe even Zoro.  If your are confident in your hole position and it needs to be more precise just turn a pin out of some over sized stock and press it into the hole.  I sometimes wind up with oversize holes for one reason or another.  Usually my own carelessness.  I'd rather spend ten minutes making a custom pin than half a day remaking a part.


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## jlsmithseven (Aug 14, 2017)

In my experience so far, drill bits drill bigger for me because:
1. They're not in a collet ring. Whenever I tried a chuck or anything else to hold it, they usually drill larger.
2. The hardness of the material matters. Speeds and feeds as well.
3. Don't chuck up too far or too little on the drill bit. Right around the end of the flutes should be good.


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## mmcmdl (Aug 14, 2017)

Do what us old timers do when necessary . Smack that oversize hole with an appropriately sized ball bearing and load it up with Loctite retaining juice . No-one would ever know the difference !


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## Bob Korves (Aug 14, 2017)

mmcmdl said:


> Do what us old timers do when necessary . Smack that oversize hole with an appropriately sized ball bearing and load it up with Loctite retaining juice . No-one would ever know the difference !


Good idea, but never admit doing it...


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## mmcmdl (Aug 15, 2017)

Bob Korves said:


> Good idea, but never admit doing it...



Who ? Me ?


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## Highsider (Sep 5, 2017)

It is impractical to attempt to use a drilled hole for a press fit, or any kind of an interference fit.    Drilled holes are not precision in any way.    If you think different, drill a hole, re-sharpen the bit and drill another hole, then compare the diameters of the holes.  
Your .1875" hole size requires only around 0.00005" (5 one hundred thousandths of an inch) interference for a press fit.  I'd drill it out to what ever closest undersized bit  I had available, and then carefully (with lots of cutting oil) ream it out with a 3/16" reamer.   Heat the S.S. piece in an oven to at least 500-600 f.  (Hotter = better) If the drill rod will slip in, position it and let it cool.  If it won't slip in, polish the end of the drill rod down just a little and try it again, until it does lock up when cool.   If the reamer cut the hole large enough that the drill rod will slip in cold, then go to the lock-tite.


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## f350ca (Sep 5, 2017)

Bob Korves said:


> The first thing that comes to my mind with polishing is a buffing wheel on a motor shaft and a stick of rouge.  That is not going to make something sharp, just shiny.  Thanks for the clarification of what you meant and how you achieve it.


I use a hard felt wheel and green cutting compound to hone my wood chisels and plane irons after sharpening them on a water stone. The edge is shiny but also makes a razor seam dull.

Greg


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## bfd (Oct 11, 2017)

a drill bit will do the opposite of what you want. if you want oversize it will drill right on if you want on size it will drill oversize. if you want a drill to drill real close to size then take a stone and break the corners of the cutting edge. this will make the flutes do the sizing and will be closer to size bill


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## Robert LaLonde (Oct 31, 2017)

Everybody says "drill bits drill oversized" like its an absolute. 

 Last week I had to setup a bit of a finicky job.  I milled a mold cavity in aluminum, and then had to setup to drill an 1/8" angled hole out of the cavity for a core pin.  I made a guide plate, set the mold in an angle vise and took it over to the mill drill to fdrill it manually.  Because I was not drilling into a flat surface I started the hole with an extended 1/8" end mill.  Then I swapped out to an 1/8" x 6" aircraft bit to drill the hole.  I figured it would be oversized, but the media that goes in the mold doesn't tend to flow into very small gaps.  That's handy for venting, but its also handy when fitting hardware into castings I grabbed a piece of 1/8" teflon rod and it wouldn't got into the hole.  In fact a .125" chucking reamer wouldn't go into the hole without removing little material either.  I wound up using a .126" (.001" oversize reamer) in order to feed the teflon core pin into the hole. 

Afterwards I measured the bit, and across the flutes it came in at .124.  It sure didn't drill much larger than that.  In this case I was actually counting on it to drill oversized and it didn't.  I have to say I was surprised.  The little mill drill actually is quite good for an import, (as a drill press) but I would have though some flex and runout of a bit that long and that thin would have wallowed out the hole some.  

As an aside note.  I was surprised the drill bit was that close in size.  If you have ever measured the flutes on small drill bits in a cheap index you will find the actual size is all over the place.  Usually they are off by couple thousandsths sometimes more.  This one was from a middle price set of import 6" aircraft bits.


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## jlsmithseven (Oct 31, 2017)

Speaking from experience, I've had the most drill bit wander about .012. This was because the flute was super long and my piece was a little uneven. Most of the time though if I had a flat enough piece and a regular drill bit in, it won't go over .004" bigger. I usually just use a drill size down.

-However, in the manual lathe, if you're drilling to start a boring hole, make sure you're setup is rigid. I once had to use a 7/8" drill bit that was super long and drill about 2" deep. I went down 2 or 3 drill sizes for the recommended boring drill, and it actually took my measurement right on what I needed for internal threads and I didn't even have to bore! So keep that in mind, the more rigid the setup, the better and more accurate.


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