PDA

View Full Version : Why do drill bits always drill oversize.



David S Newman
10-09-2008, 03:50 PM
Just needed to drill 5/8 inch hole in a casting. Stepped it up gradually to the 5/8 but it was still 8 thou over, as it happens it wasn't a disaster as it was well within what I needed, but why cannot a 5/8 drill drill 5/8 more accurately ?

RobbieKnobbie
10-09-2008, 04:00 PM
Because the one you're using isn't ground correctly.

garyphansen
10-09-2008, 04:08 PM
You need to use a reamer if you want an exact size. Gary P. Hansen

tattoomike68
10-09-2008, 04:19 PM
A good bit will drill D + .005" if ground right. In plastic they will drill undersize.

motorworks
10-09-2008, 04:34 PM
Hi
If you get a chance pick up the book "Deep Hole Drilling"
reprinted by Lindsay.
Great book on drilling and boring holes!!

boslab
10-09-2008, 04:47 PM
slightly different land length as a result of offset during grinding, drill under and ream or drill under an take out with a core drill [3 flute]
mark

pcarpenter
10-09-2008, 04:54 PM
Since no one is really addressing the "why" of the original question, I will take a stab at it with my limited knowledge.

First, I will assume you are talking about a typical two-flute twist drill. If so, they involve two cutting flutes and a center web. Unevenness in the grinding of the flutes or web will cause the bit to walk side to side. In addition to that, if you think about the cutting action, you always have a part of the bit rubbing on a bit of the uncut metal as the cutting edges do their work. This is especially true if an inadequate amount of relief is ground into the bit.

If you think about the hole being drilled by a tapered-point bit of any sort, at any given time, if you stopped the drilling, that conical bottom of the hole is not even. One part of it (the part just in front of where the cutting edge stopped is higher than what was immediately behind it. Forces on that cutting edge want to push the bit back into that deeper cut-away portion and given that a typical bit will be quite flexible for its diameter....it will probably do so. This tends to make a sort of orbital action relative to the axis of the bit...and a hole bigger than the actual drill diameter.

Often, a shorter, "screw machine length" bit or even one made from a broken bit, will produce a tighter measurement since flex is reduced.

The short answer is yes...that's why we don't use drills for really precise holes...but there are lots of degrees of necessary precision and even reamers are not perfect.

Paul

lane
10-09-2008, 06:27 PM
#1 A drill will not make a precision hole.

#2 If a good fitting hole is needed ether ream are bore to size.

#3 If the drill chuck that is holding the drill has bad jaws and drill is not running true hole will be big.

#4 If machine is not rigid enough hole will be big
.
#5 Normally a hole is to insert a bolt through so big is good.

#6 Average clearance for bolt is 1/32 over bolt size up to 1/2 inch bolt.More for larger bolts and less for number screws 00- 10

SGW
10-09-2008, 06:49 PM
The problem is almost certainly asymmetrical sharpening of the two flutes, so the two flutes have different chip load and the drill gets pulled (or pushed) off center.

Stepping up to final size can give almost to-size results, but a drill tends to want to follow an existing hole and if the hole wandered in the stepping-up process that could throw off the final result too.

mark61
10-09-2008, 06:58 PM
Even though drill bits are metal they probably do flex, twist and torque and bow under use scraping the hole sides behind the cutting edge.

mark61

x39
10-09-2008, 07:27 PM
A properly sharpened drill bit can be made to cut exactly to size by stoning a slight radius on the corners where the lip meets the margin. Known as a drill reamer, it is used in the same manner as one would a conventional reamer. Mold makers of old often kept entire indexes of drills prepared in this manner. Woe betide the apprentice that violated their sanctity.

wierdscience
10-09-2008, 09:12 PM
#1 A drill will not make a precision hole.

#2 If a good fitting hole is needed ether ream are bore to size.

#3 If the drill chuck that is holding the drill has bad jaws and drill is not running true hole will be big.

#4 If machine is not rigid enough hole will be big
.
#5 Normally a hole is to insert a bolt through so big is good.

#6 Average clearance for bolt is 1/32 over bolt size up to 1/2 inch bolt.More for larger bolts and less for number screws 00- 10

#7 Drills are seldom ever perfectly straight or concentric with their shanks,especially S&D drills even top name American drills.

bobw53
10-09-2008, 11:02 PM
I'll put in my two cents and then you can all think I'm crazy. I learned this from an old British machinist that pops up here once in a while, he gave me a 30 year head start in two years, if you want to know who he is, start a thread on spiral bevel gears, he'll be all over it.

His rule was that you never stick a drill down a hole that is bigger than 1/3 the diameter of the drill you are drilling with. I've proven it to myself multiple times, sneaking up on it seems like a good idea, it just doesn't pan out, and if it does, you just wasted a whole bunch of time.

Now, my extension of that and things I've learned. 5/8 drill, probably not a split point, and if I wanted it on size I would have pre drilled probably something in the .200 range, enough to clear the chisel point, whatever was in the machine is fine 3/16 or a A, whatever.

Another thing, if the drill enters the hole on the tip or the edges, its going to wander off, you want the drill to first contact the material on the cutting edges.

Feed, a light feed will help stay on size, .002 to .003 a rev, so not overly babying it, but definitely not pushing it to its full metal removal potential.

Drill quality, crap drills are just that and false economy, even for a guy having fun in his garage, you don't need the best, but you really don't need the worst.

As for the uneven flutes, yeah its important, but by keeping the feed light, correct pilot and minimal runout, I've drilled some very accurate holes, easily within .002 with handsharpened drills that are favoring one flute over the other.

Now to the disclaimer, drills are not the way to get accurate holes, but I'm not afraid of a .003 total tolerance, where the drill is right about on the minimum.

Paul Alciatore
10-09-2008, 11:46 PM
I have to agree with most of the above. But I find another factor on my mind when drilling. Chips. A drill used for a hole that is more than two or three diameters deep will have the flutes completely filled with chips. These will rub against the sides of the hole and scratch and enlarge it.

If you are in a production situation and need speed, then you may have to put up with this. But, if you want the best hole, you should peck. Clean the flutes and add fresh cutting fluid each time the drill comes out of the work. Not the fastest way to drill a hole, but it does produce better finishes inside the hole and probably a tighter fit.

Just my $0.02s.

SDL
10-10-2008, 01:57 AM
Just needed to drill 5/8 inch hole in a casting. Stepped it up gradually to the 5/8 but it was still 8 thou over, as it happens it wasn't a disaster as it was well within what I needed, but why cannot a 5/8 drill drill 5/8 more accurately ?

It can be more Accurate but drills are designed for hole based tolerance stsems so - 0 to plus a bit

the bit depends on

Drill type and condition,
Machine and chuck condition
Rigidity etc

Dormer will promise from H12 -0/+.0071 (so 8 thou over is close)
to H8 giving -0/ +.0011

See http://www.dormertools.com/sandvik/2531/internet/s003591.nsf?OpenDatabase.


You pays yer money and takes your chice.

Steve Larner

Rustybolt
10-10-2008, 07:09 AM
Paul. The "rule of thumb" for production drilling was 1st pass 3 times drill diameter.2nd pass 1 1/2 times drill diameter. Every subsequent pass, no more than the diameter.
Anybody who has run screw machines will tell you, the more pull outs the better because the chips will definitely enlarge the hole.
When drilling a hole, even in a drill press you can make up for an unevenly ground drill if you center drill the position first and follow bobw's advice.

SGW
10-10-2008, 08:55 AM
It can help to plunge a multi-flute end mill the same diameter of the drill for a short distance; the end mill will cut a straight and to-size hole (assuming the spindle rotation is concentric) which can guide the subsequent drill.

A.K. Boomer
10-10-2008, 09:33 AM
I have to agree with most of the above. But I find another factor on my mind when drilling. Chips. .




There ya go, Many of the things mentioned can attribute to some degree but the most simplest solution is if you want a hole to stay as close to the size of the bit as possible then keep it clean, Paul stated that its the chips rubbing against the side of the bore and I believe some of this to be true but I also think the majority of the enlargement is actually happening 180 degree's from there -- where the drill flutes are getting pressed into the side of the bore from the chips on the opposing side ---

One more thing that I dont think has been mentioned, the amount of drilling pressure is paramount --- If retaining hole size is important but not important enough to use a reamer then use just enough pressure to cut, Drill bits are spiral wound, they are also spiral wound in a direction to scoop out chips and bring them to the top of the hole - All drill bits act as a torsion bar when drilling depending on the amount of pressure applied --- what this means is they will "fan out" due to the leading direction of the spiral, The more torsional deviation degree's (cutting pressure applied) the larger the bit gets in size.
I get very good results in drilling with keeping these two things in mind and also staying away from jobbers when possible....

Disclaimer --- Research this yourself as I dont claim to be a machinist nor have ever read any books on the matter and just use my own theory's of operation to describe what I believe is going on... But - so far so good:p

huntinguy
10-10-2008, 10:39 AM
You said you were drilling in a casting. First thing is castings are not flat and smooth as far as an entry point is concerned. As soon as the flutes touch the casting they will start to push away and since they will not start at the same time you have done two things. Started to drill oversize from the get go and the drill has nothing to stabilize itself against. If you are drilling into a cored hole the problem is amplified.

I have yet to find a perfectly uniform casting as far has hardness and density go. So now you have an unstable drill start and an uneven wear pattern on the drill. Not a good plan for on size holes.

Core drills and reamers have multiple flutes to help stabilize in the hole. Try it by drilling through thin material with a 2 flute, 3 flute, measure the holes and then go back through will a core drill and then with a reamer. Three flutes are pretty good though.

The radius trick suggested by x39 will bring a hole very close to size because it put more pressure on the radial cutting edge and the two edges are now pushing against each other. Basically you have made a wedge. I have found this works better if this is the second drill in the hole. That way you do not add in the effect of the drill point, chisel point, and other poor grind issues. If you get the radii two different sizes... well, lets just say I have made holes way larger than the drill.

lazlo
10-10-2008, 10:46 AM
#7 Drills are seldom ever perfectly straight or concentric with their shanks,especially S&D drills even top name American drills.

I'm not sure about that Wierd -- I use drill blanks for plug gauges :)

Good quality drills are ground from the solid: a centerless ground drill blank is fed into a CNC grinder, and the drill blank is rotated as the grinder gashes the flutes. The whole deal is drenched in flood coolant during the grinding process.

The concentricity is specified on the good drill sets -- I'll see if I can find it on Chicago-Latrobe's web page.

David S Newman
10-10-2008, 04:45 PM
I have been experimenting today drilling many holes in scrap metal and find that the last 1/16 th or so is accurate but the rest is oversize so it's got to be the rubbing of the drill enlarging the hole. I was using a Dormer bit in new condition drilling through 2 inches plus and only taking it from 1/2 inch to the 5/8. But as I said it was not a problem as the hole didn't need to be precise. Normaly I drill undersize and ream if it needs to be spot on. David

lazlo
10-10-2008, 05:02 PM
The concentricity is specified on the good drill sets -- I'll see if I can find it on Chicago-Latrobe's web page.

The concentricity spec on drills that are ground from the solid are around +/- 0.0001" for screw-machine length, and about twice that (2 tenths) for jobber length:

http://www.ipstool.com/Catalog/0006.pdf

wierdscience
10-10-2008, 10:38 PM
I'm not sure about that Wierd -- I use drill blanks for plug gauges :)

Good quality drills are ground from the solid: a centerless ground drill blank is fed into a CNC grinder, and the drill blank is rotated as the grinder gashes the flutes. The whole deal is drenched in flood coolant during the grinding process.

The concentricity is specified on the good drill sets -- I'll see if I can find it on Chicago-Latrobe's web page.

Drill blanks aren't fluted,heatreated and finish ground,so ya they would be straight:D

Ground from solid?Some,mostly small drills,the bigger ones are milled then ground,too much metal to remove otherwise.

About once a month I spin down the shanks of 10 or so 3/4 x 12" drills for a customer so they will fit a 1/2 chuck.The drills are either CL or PTD.They always have at least .003" runout in the shank.Once the shank is machined they run better than .0005".

S&D bits are worse yet with Vermont American being worse than the average Chinese import.

doctor demo
10-10-2008, 11:12 PM
S&D bits are worse yet with Vermont American being worse than the average Chinese import.


I don't think Vermont American meets the minimum requirement to be considered scrap iron.

Steve

J Tiers
10-11-2008, 12:10 AM
They drill oversize because there is only ONE way for them to drill "exactly" on their diameter, but 5 million ways for them NOT to.....

How lucky do you feel?

Chuck not on center, by some amount, drill sharpened "off", etc, etc ,etc......

Only a perfect drill, spinning exactly on-center, has a shot at on-size.

What someone above said, about getting a good hole even with an off-center drill, is true..... Properly managed, feed-wise, you can get the drill to act like an end-mill and "cut" instead of drilling....

A.K. Boomer
10-11-2008, 10:32 AM
There really not that bad --- good results can be achieved and for the most part counted on,
First and foremost lets remove most of the 5 million ways for error and start with a high quality drill that is of course sharpened properly,

Now - If the two flutes are ground correctly they will take the same identical "bite" on each side --- If a center drill was used this will ensure that the bit holds true to its start position..

If you baby the drilling process through the first stage and build up a nice stable wall all the way around the drill then you can count on this to hold you in a stable drilling position...

If you blast the hole with mist or flood and peck at it with a controlled light pressure you will take care of the two major errors in oversize drilling (granted you have equipment capable of doing proper work)
Those errors are chips and drill torsion/distortion...

Ask yourself this,
Whats the difference between a drill and a reamer? Besides the plunging capabilities of the drill both are basically the same thing except
drills have to be spiral wound in order to remove chips --- chips from all the material from plunging,,, Some reamers are spiral wound also although much less drastic,
It is in the fact that the drill is trying to do three jobs at once that there are problems in maintaining proper hole diameter --- Its plunging, its removing massive amounts of chips, and its Fanning out its leading edge spiral while doing so, this equates to the drill diameter getting larger while going through this, straight flute reamers dont have to deal with any of these factors and because their straight flute it means that the more torsional resistance they see the smaller they get both lengthwise and in diameter - this is opposite of the leading drill spiral - the more pressure applied the longer it gets and the larger its OD...

By recognizing the "why" you can then create a controlled environment in which the drill works, You cant do anything about the inherent drill spiral, but you can keep its load light while removing chips properly, you can at least achieve a hole that looks as if its been reamed and also have that nice popping sound when pulling out a gauge pin.

Just my mechanical two cents, once again -- not a machinist..

J Tiers
10-11-2008, 10:35 AM
Of course.....

But with ONE way to BE right, and many to NOT be..... which do you suppose is more common?

lazlo
10-11-2008, 11:42 AM
Whats the difference between a drill and a reamer? Besides the plunging capabilities of the drill both are basically the same thing except drills have to be spiral wound in order to remove chips --- chips from all the material from plunging,,, Some reamers are spiral wound also

That was my point about the concentricity AK -- a 3/8" endmill, drill, or reamer are all ground from the same centerless ground blanks and gashed the same way.

The difference, that Lane summed up perfectly, is this:


It is in the fact that the drill is trying to do three jobs at once that there are problems in maintaining proper hole diameter --- Its plunging, its removing massive amounts of chips, and its Fanning out its leading edge spiral while doing so

Paul Alciatore
10-11-2008, 01:18 PM
I have to express some doubt about this "spiral unwinding" thing. I am sure the drill will twist a bit from the drilling forces, but I doubt that it will translate into an increase in diameter. It is far more likely to express itself as an increase in length and even here, I would bet it is only a tenth or two even on a large drill.

As for the sides of the drill cutting into the walls of the hole, have you ever tried to get a drill to cut sideways. This is really difficult to do, even in wood. And will produce a lot of heat likely resulting in discoloration of the drill and work. Late on a Sunday evening, I tried to use this "technique" to rout some slots in wood once. Used a 1/8" drill as a "side cutter". It was black and I think most of the wood was removed by burning it out, not cutting. And this was a fairly soft wood. Not pretty. Drills are designed NOT to cut sideways and I doubt that much side cutting is going on when a hole is drilled.

SVS
10-11-2008, 01:47 PM
I wanted to call B.S. on "spiral unwinding" but Paul said it much more diplomatically, so I won't. Interesting bit of mental trig exercise though.

Peck drilling is well and good, but when do you end the peck? As long as two nice spirals are curling out of the hole I think you're golden, and you don't get the nice spirals without enough feed.

Scott

x39
10-11-2008, 05:48 PM
I had a nice little gig back in the late eighties/early nineties making extrusion tooling for the wire and cable industry. I routinely used drill reamers to produce holes held within .001" total tolerance in a variety of materials, including 4140, D2, 17-4 PH, 416 stainless, and inconel.

A.K. Boomer
10-11-2008, 10:28 PM
I wanted to call B.S. on "spiral unwinding" but Paul said it much more diplomatically, so I won't. Interesting bit of mental trig exercise though.


Scott



You guys need to keep basic observations open,

Go to your shop, git yerself a piece of nice coiled lathe swarf, hold it in one hand and twist in opposite direction of wind, watch it grow O.D. size and also watch it grow in length drastically, yes this is an extreme example and the swarf is missing the center "beam" but just the same the results speak for themselves, now go in the same direction of the wind and watch it get both smaller and shorter...
Now take a flat piece with no spiral, twist it and not only will it shrink in length, it will get a smaller O.D. (this you will not be able to "see") This is the perfect effect for reaming as it will not allow for growing in size when more torsional distortion is applied , you want a mild shrinking in size when the torsion occurs - it not only keeps reamers from progressively binding till they snap, by giving them an exit they will re-engage the material by there own design - till there is no more torsional effect, this ensures that whatever the reamer size is the bore will also be - esp. if enough passes are made with the reamer to get it to relax.

As far as Pauls statement of saying he could see a drill bit growing in length but not in diameter -- thats an oxymoron, If its growing in length then that means the spiral flutes are unraveling if there straightening out them that means there getting larger... and as far as a bit growing in length only a tenth of a thou. thats about a 50 to one comparison failure --- try about .005" on a long half inch jobber,
How do I know? Try godzillaing a half inch jobber (many spirals) into deep 304 stainless and have your quill depth set,Go fast all the way to bottom and then when you hear nothing rubbing -- keep the quill handle loaded and raise the knee till you hear it re-engage the material, I can see some saying maybe half of this is the bit "self feeding" due to the plunge cutting edge -- fine, but that still leaves .0025" ------ Now ask yourself what the O.D. is going through...

Paul Alciatore
10-12-2008, 12:42 AM
You guys need to keep basic observations open,

Go to your shop, git yerself a piece of nice coiled lathe swarf, hold it in one hand and twist in opposite direction of wind, watch it grow O.D. size and also watch it grow in length drastically, yes this is an extreme example and the swarf is missing the center "beam" but just the same the results speak for themselves, now go in the same direction of the wind and watch it get both smaller and shorter...
Now take a flat piece with no spiral, twist it and not only will it shrink in length, it will get a smaller O.D. (this you will not be able to "see") This is the perfect effect for reaming as it will not allow for growing in size when more torsional distortion is applied , you want a mild shrinking in size when the torsion occurs - it not only keeps reamers from progressively binding till they snap, by giving them an exit they will re-engage the material by there own design - till there is no more torsional effect, this ensures that whatever the reamer size is the bore will also be - esp. if enough passes are made with the reamer to get it to relax.

As far as Pauls statement of saying he could see a drill bit growing in length but not in diameter -- thats an oxymoron, If its growing in length then that means the spiral flutes are unraveling if there straightening out them that means there getting larger... and as far as a bit growing in length only a tenth of a thou. thats about a 50 to one comparison failure --- try about .005" on a long half inch jobber,
How do I know? Try godzillaing a half inch jobber (many spirals) into deep 304 stainless and have your quill depth set,Go fast all the way to bottom and then when you hear nothing rubbing -- keep the quill handle loaded and raise the knee till you hear it re-engage the material, I can see some saying maybe half of this is the bit "self feeding" due to the plunge cutting edge -- fine, but that still leaves .0025" ------ Now ask yourself what the O.D. is going through...

A drill bit is NOT a coil and no engineer would ever model one as such. The web in the middle is there and it provides a lot of strength to resist any sideways deformation. It would be better to think of a twist drill as three rods, one in the center and two coiling around it. The three are fastened with radial (sideways) members that are closely spaced. When an unwinding force is applied, the main effect will be for the outside rods to straighten so the whole does get longer. This will place some stress on the radial cross braces and they will get a little bit longer (stress in "little"), but it is going to be a LOT LESS than the increase in length, perhaps 5% or 1% or less. For the overall lengthing, the basic extra length is already there in the coiled side rods because they are coiled. So they don't really grow: they just unwind. Actually they probably compress a bit, but they started off longer than the center one and even compressed they retain that longer length. But the radial members will be in tension. It would take a lot of tension to break a piece of HSS but we are not coming even close to that. Just consider, have you ever seen a drill bit split down the middle?. No, I am sure you haven't and neither have I. So we are well within the elastic limit. And according to Young's modulus for common steels, it would take about 175,000 PSI to stress a 1/2" diameter drill to increase it's diameter by 0.003". That's just about the limit for some of the best steels and is probably beyond the elastic limit. I think the drill would want to split down the middle at that level. But as I have said, I have never seen a drill fail that way. Most drill breakage is from tortional stress.

As for the depth of the hole arguement, I would be interested in the end play in the spindle bearings on the machine. And any preloading in those bearings that may be overcome. And the stiffness or lack thereof in the column and head. Those would be a lot more likely cause of a few thousanths increase in depth with hard drilling - the self feeding thing as you mentioned. I would still bet on only tenths of increase in the length of the drill, not thousanths, assuuming a 1/2" drill and normal drilling forces. And in any case, I do not think it is linked to the diameter on a one-to-one or even on a ten-to-one basis.

Now, I don't say the increase in diameter effect is not present, I just feel the numbers would be in the millionths of an inch range, not thousanths.

I admit that all of this is from theory. I would be very interested in any actual measured data on these distortions, under carefully controled conditions of course. And on drills, not on swarf, springs, or rods.

JCHannum
10-12-2008, 08:51 AM
Winding and unwinding notwithstanding, a drill does deform in operation. Use a smaller bit, 1/8" or so on a harder material and crank on the feed mechanism and watch it bend while drilling. It will do this in the hole and the flutes will enlarge the hole.

Drills and reamers are different tools and designed for different functions. A drill can be used to clean up a hole if sharpened properly, new etc. etc., but it is not made on size as a reamer is. Drill specifications are plus zero, minus something depending on size. They can be -0.001 for 1/8"-1/4" and -0.0015 for 1/4"-3/4".

Spiral reamers have a spiral flute for use in bores with a keyway to span the interruption and still give a smooth finish.

The flute of a drill is not for cutting. The most accurate, deep hole gun drills are single flute, with one cutting edge, the chips are removed by a flow of high pressure cutting oil.

Beyond the first few thousandths, the flute does no cutting. In many industrial situations, twist drills are not used due to expense and accuracy and insertable spade drills are used. The straight fluted holder is reused, and the insert can be accurately resharpened many times with simple equipment.

A.K. Boomer
10-12-2008, 09:15 AM
I admit that all of this is from theory. I would be very interested in any actual measured data on these distortions, under carefully controled conditions of course. And on drills, not on swarf, springs, or rods.



Thats all im doing too, maybe someone will have some more technical info ---- I believe the ratio that the diameter grows is more as compared to the ratio of length, just that length trumps it due to how much longer the bit is in comparison to its diameter to begin with, anotherwords - a half inch bit with a half inch depth under torsional stress would increase its diameter more than it would increase its length -- its why i gave the example with the jobber, the more length the more spirals the more unwinding --- now the overall ratio changes, length is kinda like piston ring end gap, to figure the needed end gap the ring must be imaginary uncoiled and straitened out, then the rings material used and the operating temp. is applied to find the expansion and then this is used to deduce the proper running clearance,

For a bit I would assume the necessary ingredients to be the bits material, the depth of the flutes and center design, how radical the "spiral" and a measurement of unraveling the spiral like straitening out the piston ring, then the degree's of a given torsion distortion from drill shank to drill tip can be applied to figure out how much the spiral went from its original position to trying to straiten out, and of course you can't use this info - because now you have to do the math of the battle thats taking place between the coil wanting to grow in both diameter and length and the center beam trying to hold it back...

Yup -- iv had enough;) id like to see some specs...

J Tiers
10-12-2008, 10:47 AM
I don't know that this theory stuff about unwinding is necessarily addressing the original question.....

The OP STEPPED UP the drill size in what seem to be small steps?????

Step drilling can only work if the drill is as rigid as an end mill, and the machine as rigid as a good vertical mill. Otherwise the two flutes will catch and 'run around" the hole, tearing it up and possibly making it oversize.

A stiff 'core drill" with three flutes will do the job in a good machine, but it is practically an end mill in stiffness etc.

The only ways I have found to work with 2 flute drills is to either

1) drill to the web thickness of the large drill, and then follow with final size drill.

2) round the "corners" of a drill to use it as a reamer, drilling 0.015" undersize as per #1 first.

#1 is OK if the drill is sharpened right

#2 works OK but is a fair amount of trouble. You need to not have a reamer of right size to bother with it.

Paul Alciatore
10-12-2008, 12:18 PM
I had a thought while reading the replies. If a twist drill would get wider due to untwisting, would a straight flute reamer get smaller?

Just curiosity.

Rustybolt
10-12-2008, 12:49 PM
I don't think so Paul. You can get a reamer to ream slightly bigger or smaller by burnishing one side or the other of the cutting edge. If you try to enlarge a hole drilled too small for the reamer , the flutes will pack and it will cut oversize and ragged.
One of the flutes on a staight reamer isn't exactly cut the same distance from the others. This is to keep the reamer from chattering in the hole. Spiral reamers tend not to do this, they are spiraled to remove chips from a blind hole.

A.K. Boomer
10-12-2008, 10:55 PM
I had a thought while reading the replies. If a twist drill would get wider due to untwisting, would a straight flute reamer get smaller?

Just curiosity.


Paul, I believe so - even stated it in post #32;

"Now take a flat piece with no spiral, twist it and not only will it shrink in length, it will get a smaller O.D. (this you will not be able to "see") This is the perfect effect for reaming as it will not allow for growing in size when more torsional distortion is applied , you want a mild shrinking in size when the torsion occurs - it not only keeps reamers from progressively binding till they snap, by giving them an exit they will re-engage the material by there own design - till there is no more torsional effect, this ensures that whatever the reamer size is the bore will also be - esp. if enough passes are made with the reamer to get it to relax."

JRouche
10-13-2008, 01:58 AM
Oh I love this place. I only read the OP question and a few responses. pcarpenter seemed to have answered it on the seventh post. And yet it goes on, and on.. Hahaha.. I love it.

Taking about drill bits. Everyone engaged in the topic knows they arent precision bits just by their cutting method and design.. And they all want to say why.. Thats the best thing.. The why???

Its the WHY that keeps us looking for answers... Love this group of WHY thinkers.. Too many other sites and people are about the I dont care why, just give me an answer...

You WHY thinkers are the best!!!!! JR

darryl
10-13-2008, 05:38 AM
Why is that :)

I tend to thnk of the point of the drill bit like a center- it's tapered, so if you poke it into a hole that's smaller than the bit, it will center in the hole. With pressure on the bit, the point wants to stay centered. If one lip is longer than the other, it will sweep wide and the diameter of the hole will be twice the distance from the edge of that lip to the central axis of the point, as ground. If the hole has stayed round, the point has stayed centered in the pilot hole, but if the hole is lobed, the cutting forces have overcome the ability of the tip to stay on track.

It doesn't matter if there's a pilot hole or not, the point will be creating it and is still subject to straying off course if there's a sideways force that's too much. The only thing that changes is the magnitude of that force and the restraining ability of the material being drilled. From this description, it then makes sense that a pilot hole should be no larger than the width of the web. The more length of lip that contacts the material, the stronger is the restraining force tending to keep the bit centered.

It's my opinion that if the hole comes out round, then the point has stayed well enough centered, and the diameter of the hole is going to be twice the length of the longest lip. I agree that abrasion from the chips is likely responsible in part for a hole becoming wider at the entry, but also I believe that the deeper the hole goes, the more centering action is added by the non-cutting outer diameter of the bit (it's not an endmill after all), and that the hole will then become progressively smaller until the minimum size it will cut has been reached. If it is perfectly ground (to a reasonable tolerance) it will create two equal spirals which won't impact much on the wall of the hole, and the hole then will have a very minimal taper and can be close to the diameter of the drill bit.

I'm not saying that this is the absolute answer- I'm sure other factors play a part in the size of the hole a drill bit will make, but I am saying MHO of course, that the predominant factor is the improper grind. Uneven length of lips and relief angle differences.

ralphe
10-13-2008, 01:30 PM
I just measured the diameter of a 2" deep through hole that I drilled in a bar of 12L14 steel on my lathe. I had used a new 19/32" (0.594") bit manufactured in Minneapolis by Viking. I started with a pilot hole of of 9/32". The hole measures 0.594" throughout its length. It will now be reamed to a final size of 39/64" (0.6094") to fit the piston in the model Sterling engine that I am building.
In conclusion, I guess, all holes do not drill oversize and that the quality of the drill bit may be a significant factor in all of this. I only use Viking drills since they were recommended by "Thrud" some years ago.

lazlo
10-13-2008, 02:48 PM
In conclusion, I guess, all holes do not drill oversize and that the quality of the drill bit may be a significant factor in all of this. I only use Viking drills since they were recommended by "Thrud" some years ago.

Thrud is before my time, but Viking is a high-end industrial drill manufacturer -- very good stuff! I have a set of their cobalt drills -- they're superb.