drilling 'actual' sized holes with HSS drill bits

If you do not want to ream, I would pre drill with a slightly undersized bit and make the finish pass with the fresh factory drill bit. If you get good results, continue to duplicate all conditions for all holes.

So what *is* your tolerance? And how are you measuring? And what is the problem if you get say a .178" hole?

When I want a precisely sized hole, I drill undersize and then ream.

When I drill, I know it is a fairly imprecise operation.

You will spend a lot more on a drill grinder setup then you will on the occasional reamer.

Anyway, it's a good question. I've seen a bunch of articles (in magazines, not on the web) that say to stone the trailing edge of the secondary relief angle behind the floozbub or something else. Always just with words, never a *good* drawing. Admittedly, this is an issue that cries out for a good piece of tech writing to communicate.

Grant

bill yohler wrote:

If you have a new drill from a good manufacturer and locate the hole with a punch it should come out withing a couple of thousandths of the nominal size. With a resharped drill done by hand or a cheap sharpener you could get anything.

John

Drill in scrap first and measure the hole produced.

Like Dan G said- use your drill as though it were a reamer. Just predrill first with a drill that's .005 to .010 undersize and then follow up with your final drill size.

"Same" being how close? If you want to create a hole that is sized within 0.0001 inch tolerance, you can't really do it by just drilling.

Maybe if you put a tolerance number on your hole diameter it would be easier to pick the best way to do it.

Jim

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What do you mean by "_same_"? There is no such thing as "exact" in metalworking (or anything physical, for that matter); you can only work to within a given tolerance. The smaller the tolerance, the more difficult it is to achieve. The tolerance required will generally dictate the tools and techniques needed to accomplish the task.

The thing to remember about standard drills is that they are *incapable* of producing extremely round holes of precise diameter (i.e., they do not operate to very close tolerances). In fact, drilled holes are often not even particularly straight. Even a new drill of very high quality will produce holes that tend to be trigon in cross-section and slightly oversized. (Note: if you are using a cheap drill, you might as well just forget about even getting close to your desired size.) The amount of oversize can be estimated at +0.004" to +0.006" per inch of drill diameter, but this can vary depending on a number of factors, including the geometry of the drill (how well it was made), the material being drilled, the rigidity of your setup, the concentricity of your drill with the spindle axis, etc.

The bottom line is, if you simply need to make a clearance hole for a bolt, then drilling a hole with a standard drill bit is usually fine. But if you need to produce a very round and precisely-sized hole (e.g., a running fit for a shaft), then you are using the wrong tool (for the finishing operation).

Regards, Michael

Hmm, you tried that? Or is this just what you imagine? In my experience, drills cannot ream. You can't use such a small step up in drill size, it won't cut, it just rubs and smears the metal. That's why there has to be reamers for reaming.

If somebody could manufacture a drill that would hold a +/- .0001" diameter and location tolerance they would become very rich. Every manufacturer would buy them to avoid second operations like reaming, boring, honing, grinding. With a good setup and drill I've seen them hold a +/- .0005" tolerance. Then again I work on vertical machining centers not drill presses. I still don’t expect to hold that so I make the holes under and do the secondary operation. I've never used a drill like a reamer. I prefer endmills or boring bars.

Jeff

ok, guys, thanks one and all.

more info: tightwad here hoping to drill a .177 or .178 sized hole without reaming to size, centered in end of a .250 OD piece of cold rolled mild steel round, in my lathe (this will result in sort of 'thin wall pipe' of sorts) withOUT 'springing for' a reamer. mid-term goal being to silver solder into this (drilled) hole the 'head' end of a (for all practical purposes) round brass pilot jet, OD of jet .177 inch, or maybe two ten-thou's over that. final intent being, to place a 'hex head' on other end of this same very short steel jet 'extension', to make it 'real world adjustable' (using a small quarter-inch drive socket) WITHOUT having to 'spring for' the special "single D" tool required, ordinarily, to turn (adjust) these same two jets (one per carb). that tool's a 50-buck item...

note the existing 'head' of the 2 pilot jets are each 'recessed into' a sort of 'bucket shaped depression' in their respective carbs (& said 'bucket' wall is approx .260 in diameter), making gripping the jets with vicegrips (or anything similar, eg: needlenoses, pin vise, etc) impossible. also tried cutting a 'straight slot' into one jet 'top' for a normal screwdriver, but, due to dimensions _and_ tightness of jets in carb body screwthreads, this was a 'poor choice'; reason being it removed (broke off) half the 'existing tiny head' area. so I then had to spend some nerve-wracking painful _hours_ of drilling jet center out while it was 'in situ' IN the carb, using easy-outs, penetrants, torch heat applications to carb body, etc....

truth be known, rather than silver solder, I'd prefer to taper the shaft of the brass pilot jets slightly (quarter inch per foot), then ream the 2 drilled holes (in the steel 'short hex head extensions') with the 'correct' section of a (shortened) #2 taper pin reamer, freeze the pilot, heat the 'new' steel pieces, and "shrink fit 'em" on/over the brass pilot ends, but, alas, the tightwad factor still applies & I got no correct-sized taper pin reamer here :-(

so, what's a poor boy ta'do?

Just drill the hole so your jet is a sloppy slip fit. .002 to .004 inches should do nicely. Place the jet in the hole and silver solder it up. You actually need the clearance for the solder to wick in and have some strength.

Erich

1) you don't need to get a 0.0001 tolerance hole to silver solder the brass part into the steel part. You could have a thou of clearance and it will work find. 2) if you absolutely feel that you need to manufacture the hole better than that, simply bore it to size using your lathe and a small boring bar. You can size it will within a thou that way. As mentioned you don't really want a press-in fit for silver soldering. 3) what kind of carb has an adjustable pilot fuel jet that is manipulated with a D-shaped tool? Most pilot fuel jets simply screw into the carb body and are swapped out if the size of the jet is not right. 4) the D-shaped adjusting tools are available for about ten bucks in most auto parts stores.

Jim

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I hope I'm not imagining it, I've done this a couple of hundred times in the past 26 years. Depending on the hardness of the metal and sharpness of the drill any material removal over approximately .002 cuts and does not abrade. Sure reamers do a better job and when the desired size reamer is available I naturally go that route but in pinch I've found this method works fine.

Isn't this exactly what gun drilling machines do?

I suspect there are three regions, (1) too small an increment to possibly work, (2) small increments that may or may not work depending on a lot of factors like drill sharpening, speed/feed, lubricant, etc., and (3) big enough increments that almost have to work. What the incremental dimensions between these regions are is the real question.

All I know is trying to go up one letter size (e.g., D=0.246 to E=0.250 to get a nice 0.250 hole) is too small an increment, at least in mild steel with the import TiN drills I have. Maybe two steps would work. But then you're missing the purpose of roundness, accuracy, and finish.

An old toolmaker's method is to pre-drill .010" - .015" undersize then rub the sharp corners off the outer edges of the finish drill before drilling at a reduced speed, high feed rate & a little oil. I have used this method for many years mostly for fitting dowell pins. I have never needed a reamer to do this.

Paul Cordell, Nth. Qld. Australia.