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how much bigger does a drill bit drill?

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benmychree

John York
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#31
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.
 

Silverbullet

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#32
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.
 

Bob La Londe

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#33
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.
 

jlsmithseven

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#34
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.
 

Bob Korves

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#36
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... :busted:
 

Highsider

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#38
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.
 

f350ca

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#39
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
 

bfd

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#40
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
 

Bob La Londe

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#41
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.
 

jlsmithseven

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#42
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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