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View Full Version : Why are my holes to big?



ahidley
07-29-2012, 10:27 PM
I've drilled 10s of thousands of holes and the last few, for some reason, are alot bigger than the drill.
Here is the details: Drilled some Q size holes for some helicoils. The holes were drilled with a brand new bit. The holes were so large that the helicoil would fall into them... This was in aluminum.
I drilled another hole in a one inch diameter Steel rod. It was a .375 drill bit and the hole was .407 in diameter when done. The drill bit seemed to drill fine. I didnot see any wandering and the chips comming out were nice and were the SAME from each flute. I did not predrill other than a center drill just to locate them accuratly. I indicated the runout on the chuck holding the drill bits and it was only .001 These were NOT split point drills but the end resulting hole seems to be excessivly over sized. Any thought as to why? I am having a hard time beliving its because I didnt predrill....

Lew Hartswick
07-29-2012, 10:34 PM
You did check the bits with a mic. didn't you? With that "lack" of runout and
more or less equal chip from the two flutes "is a puzzlement" as a once and
good movie actor said in the "King an I" .
...Lew...

Mike Burdick
07-29-2012, 10:39 PM
Twist drill bits drill holes. If you want "accurate" holes drill undersize hole and then use a reamer.

tdmidget
07-29-2012, 11:00 PM
What kind of machine? As Lew asked, did you measure the drill(not"bit", it's a drill)?

cameron
07-29-2012, 11:28 PM
Did you indicate the runout on the chuck body or on something held in the chuck?

JoeCB
07-29-2012, 11:41 PM
An off center point (unequal lips) is the usual cause of oversize drilled holes
Joe B

Mcgyver
07-30-2012, 12:08 AM
An off center point (unequal lips) is the usual cause of oversize drilled holes
Joe B

+1. That is likely the problem, a poor grind is 98% of reason for oversized holes. Whats the make of the twist drill? Just because its new doesn't mean its perfect

Unless extreme, an eccentric action won't much matter; the drill will flex and overcome it...but if the lips aren't identical, the long one determines the hole size and Shorty is just along for the ride.

btw, properly done, drills will drill within .002". I agree use a reamer when it matters, but it's always impressed me just how good drills can be when you sharpen them properly

lakeside53
07-30-2012, 12:35 AM
"It was a .375 drill bit and the hole was .407 in diameter when done"

32 thou? A full 1/32 over? that's HUGE.... without trying I get maybe 5 thou oversize with machine screw length, and sometimes bit more with jobber. What brand are you using? Hand held drill or drill press/mill?

gnm109
07-30-2012, 12:53 AM
Twist drill bits drill holes. If you want "accurate" holes drill undersize hole and then use a reamer.

That's it. They seem always to dtill a few thousandths oversize. Part of it might be that the drill chuck has some runout - even good ones have a thousandth or two.

Another issue might be that used, resharpened drills are not ground on center. Thus, they may wobble when started.

As Mike says here, I will always drill undersized and then finish with a reamer or a boring tool if possible.

J Tiers
07-30-2012, 01:15 AM
An off center point (unequal lips) is the usual cause of oversize drilled holes
Joe B

You'd think.........

BUT

That does not seem to correspond with the OP's statements.....


I didnot see any wandering and the chips comming out were nice and were the SAME from each flute.

Normally, if the drill has unequal flutes, it will also have rather unequal chips. With a hole size problem of 32 thou, you'd suspect that the off-center point would be clearly visible to the unaided eye, and that the chips would be very obviously different.

I'm puzzled unless something the OP said is in fact not the case, even though he thinks it is..... But WHAT it is, I haven't a clue.... he seems to have covered the obvious areas of problems, and as I recall from other posts, he is no dummy about tools.

J. R. Williams
07-30-2012, 01:15 AM
You should use a "spotting drill" to start the hole. A "center drill" has a 60 deg point and the drill is usually 118 deg. With a sloppy spindle the drill can walk before it gets a good start.

dp
07-30-2012, 01:30 AM
Regarding your drill, something you are sure of is wrong. Drills have few characteristics so it should be fairly light work to isolate the one that is creating the problem. I'd include the chuck and arbor in the search, and also think about what was going on when this change occurred.

Mike Burch
07-30-2012, 02:03 AM
And even if the drill is correctly ground, on my less-than-perfectly-rigid drill press the combination of table and cross-slide vice allows for a bit of wobble, so my holes are nearly always over size.

darryl
07-30-2012, 02:23 AM
I bet you could try a different brand of drill bit and the hole would come out on size.

Evan
07-30-2012, 02:39 AM
Totally agree with Joe and Mcgyver. A good bit will drill within a couple of thou. It's the flutes that matter. Not just the width of the flutes but also the front and back relief angles since if that differs the flutes will take off different amounts.

Willy
07-30-2012, 02:50 AM
I've drilled 10s of thousands of holes and the last few, for some reason, are alot bigger than the drill.
................
................

After that much success drilling holes I would venture to say that the OP has a firm grasp on what it takes to drill holes to the required size.

So obviously something in the equation has changed.
Different procedure, machine, chuck, or a change in the source of tooling, but something is not the same as in all those other successfully drilled holes.

Although we can suggest sources for the causes of oversize holes, it's hard for us to guess what has changed in the setup.
I would suggest that the OP look for any procedural changes no matter how seemingly insignificant they may appear to be.

This diagnosis or troubleshoot technique has always worked well for me when trying to determine what went wrong after all was well previously.
Analyze the situation....something has changed....otherwise the results would by the same as before.

dp
07-30-2012, 03:00 AM
After that much success drilling holes I would venture to say that the OP has a firm grasp on what it takes to drill holes to the required size.

So obviously something in the equation has changed.

First principles of observation tell us this and it is a given. But as I said, something he is sure of is wrong. That can include what we've been told. We are at the point in this where we need to be in the room where this is happening or the OP needs to rethink the circumstances.

tdmidget
07-30-2012, 03:14 AM
And we STILL don't know what machine. US drill or Chinese? Maybe a pic of the drill point? How was the workpiece held?

malbenbut
07-30-2012, 04:01 AM
Method for small accurate noles
Drill, Bore, Ream.
MBB

ahidley
07-30-2012, 09:09 AM
Ok, more info, or repeat of previoulsy posted ;)...... The bit, i.e. drill bits , were held in a milling machine.
the work was clamped in a milling machine vise. Yes it was SOLID and the spindle has no movement.
I indicated the runout at the point of the drill bit where the flutes actually cut. It was only .001 not .032/2 . .032 is a huge oversize in hole diameter.
Now I said "bits" because it happened twice. The first time was a Q sized drill bit used in alum for a helicoil. The hole was so big that the helicoil fell into the hole. The second hole, in steel, was drilled with a .375 bit and the hole ended up .407. Now I didnt predrill these holes. But I used a center drill to a depth of about 1/4 inch just to locate and start the hole. I do this because the very point of a drill does not cut and will cause a drill to walk around. this can cause a hole to be poorly located even though the spindle is dead on to the anticipated hole location. i.e. the drill will end up bending and cause a differant location and not a true, perpenduclar hole. Thats why I said I did not use split point bits. They will maintain size better. (like compareing a center cutting endmill vs one that is not) Now the bigger the bit the bigger the little spot is that does not cut right at the tip. I'll take some pics of the bits and you naysayers can respond. It'll be a few days before I can do it I think.

madman
07-30-2012, 09:18 AM
WELL heres a idea If youre measuring youre drill before using (highly recomended) and using youre trusty rusty vernier try closing the vernier and see if it is at zero?? Then re measure that drill and see what it is? Perhaps this is the Issue? Just trying to help Good Luck Mike

J Tiers
07-30-2012, 09:32 AM
Totally agree with Joe and Mcgyver. A good bit will drill within a couple of thou. It's the flutes that matter. Not just the width of the flutes but also the front and back relief angles since if that differs the flutes will take off different amounts.

But if he 'takes different amounts" the chips will not be the same..... one will be thicker, the other only a skim, or nothing. The OP states equal chips.

he's no dummy, so I tend to take his word....... but there is always the chance of anyone not seeing something correctly.

firbikrhd1
07-30-2012, 09:46 AM
True or not, I can't say, but a tool and die maker friend of mine once told me that twist drills can "unwind" due to being very small, having a thin web or having too much drilling pressure applied while drilling, all with the end result of a hole that is over sized. If this is true, perhaps one of those are the culprit.

cameron
07-30-2012, 10:18 AM
Perhaps the next person composing a post to inform the OP that he needs to drill and ream all holes from henceforth might spend a moment or so reflecting on the implications of doing that before hitting the send button.

GadgetBuilder
07-30-2012, 10:22 AM
Center drills aren't the best choice to start a hole when accurate size and/or location is needed. Their short length makes them rigid but they produce a starting hole whose angle is steeper than the following drill. This allows the tip of one lip on that following drill to catch and push the drill slightly off center just as you start drilling. A better choice for starting would be a spotting drill with point angle equal or greater than the target drill. I prefer spotting drills with a larger point angle than the following drill because it ensures the following drill will locate into the divot rather than having a lip catch. Carbide drills may chip at start of drilling; HSS seldom chips but is sensitive to the same centering issues:
http://pdf.directindustry.com/pdf/ingersoll-cutting-tools/hole-making-catalog/17869-11078-_93.html

Assuming accurate sharpening, split point drills tend to drill straighter than regular drills because the thrust required is much lower and, as you noted, excessive thrust can push the drill off-line.

John

Rosco-P
07-30-2012, 10:46 AM
If only the letter Q drill bit was making oversized holes, I'd suspect that the bit was mismarked. But the OP has given examples of two drill bits giving him similar issues. Same machine, similar setup? Either the manufacturer is having a bad run of luck or the problem is caused by one or more other factors.

cameron
07-30-2012, 11:03 AM
I agree the intended .375" hole is greatly oversize, but I'm curious about the Q holes. The fact that the helicoils dropped into the holes (tapped or untapped? ) suggests to me that the oversize was much greater. Can you tell us the actual size of these holes?

tdmidget
07-30-2012, 11:08 AM
[QUOTE=Rosco-P;787441 or the problem is caused by one or more other factors.[/QUOTE]
I'm betting on that too.;)

Rosco-P
07-30-2012, 11:20 AM
I'm betting on that too.;)

Readin that the OP had success with 10's of thousands of holes being (close to) on size, something recently has changed. Bad chuck, new chuck/arbor, bad (bent, loose) arbor, burr inside chuck on one jaw, loose spindle bearings, slop in the machine table, etc?

Euph0ny
07-30-2012, 11:57 AM
The bit, i.e. drill bits , were held in a milling machine.

Have you checked that the mill is in tram? If it's off, you will be cutting holes more oval than usual...

I would also try a few of your other drills - if they are all cutting oversize where they didn't before, something is wrong with your mill setup (chuck/collet, spindle runout, tram...).

Try putting the "suspect" drill in your drill press - does it cut oversize there too?

Just some thoughts...

[edditted for speling]

Toolguy
07-30-2012, 11:57 AM
The only thing I can think of that fits all the criteria (I think) is that maybe the drills were in the wrong bin and were picked out and used without actually being measured first. I have the Huot drill indexes that are drawers with several of the same size in each slot. It is a simple matter to get one out of or put one away in the slot next to the intended one. I have done that a time or 2. Now I pay more attention and also throw a caliper on any drill that the size of the hole matters.

vpt
07-30-2012, 12:02 PM
Wrong, incorrectly marked, or the like drill bit. Or simply just grabbed the wrong one for the helicoil. Or OP grabbed the wrong helicoil? Being that far out without seeing the bit wobble means the bit is the wrong size. Try another bit.

JCHannum
07-30-2012, 12:05 PM
A letter Q drill is possibly for an 8mm Helicoil. If the Helicoil falls through the hole, the hole is off by approximately the same amount as the 3/8" hole. This suggests the problem is machine related rather than drill related.

Try a few more random sized holes to see if the oversize holds true. I have no idea what the cause might be, but it does not appear to be with the drill bits.

beanbag
07-30-2012, 06:06 PM
Just drop the drill shank back into the hole to see if it really is oversize or not.

beanbag
07-30-2012, 06:13 PM
A better choice for starting would be a spotting drill with point angle equal or greater than the target drill. I prefer spotting drills with a larger point angle than the following drill because it ensures the following drill will locate into the divot rather than having a lip catch.


I never saw a spot drill with an angle bigger than 135.

Carld
07-30-2012, 07:01 PM
This is getting more interesting with each post.

GadgetBuilder
07-30-2012, 11:45 PM
I never saw a spot drill with an angle bigger than 135.

Another excuse to build a drill sharpener... :)

vpt
07-30-2012, 11:52 PM
Just drop the drill shank back into the hole to see if it really is oversize or not.



You with your simple logic. We need to blueprint and diagnose this problem bit by bit from beginning to end. Figure out what drill bit are best, what coatings are bad, how heat effects hole sizes, and so on before we can just go dropping drill bits in holes.

tdmidget
07-31-2012, 12:12 AM
You with your simple logic. We need to blueprint and diagnose this problem bit by bit from beginning to end. Figure out what drill bit are best, what coatings are bad, how heat effects hole sizes, and so on before we can just go dropping drill bits in holes.

Yeah, like Roscoe said, it's "one or more other factors".:rolleyes:

metalmole
07-31-2012, 06:44 AM
What I would do is put a collet in my mill, install a sharp end mill of known size and machine a hole in a test piece and measure that and that would tell me if my mill was at fault, of course I would check the run out on the EM shank......

Toolguy
07-31-2012, 10:53 AM
In a drilling application an end mill will always cut oversize.

metalmole
08-01-2012, 01:12 AM
In a drilling application an end mill will always cut oversize.

Right, but if ya know what it should be u can measure it and know if your machine is on its Ps and Qs......I've cut many a hole with an EM for many different applications....

ahidley
08-02-2012, 08:37 PM
The drill bits are stored in a drill index, letter, number, fractions. I picked the correct sized drill.
The Q size was for a 5/16-18 helicoil. The Q size bit is a cobalt drill and the .375 is a HSS. Now the .375 I have re-sharpened a few times via a drill dr. (please don't jump on the drill dr as the culprit. I don't think that's the issue. We will see). The Cobalt is brandy new.
My dial indicator IS zeroed and not rusty LOL.
The mill is trammed perpendicular.
The Q sized holes were un tapped when the inserts fell in. I just said that to give the readers an idea of how big the hole was. In the same setup I drilled with the Q size and then removed the drill bit via the chuck key and placed the SPI tap into the chuck tightening with the key. The tap went into the hole without cutting any threads. Thus location of the tap was identical as to where the drill bit was.

So from what was posted here I feel I have two possibilities as to the cause.
1- The chuck jaw might have had a chip in it. This is after the fact and when I checked it for out of round I had to put the drill bit back into it. so that's a possibility.
2- I did use ALOT of down force to drill these, everybody is in a hurry. So I will try drilling a few more holes and use the least amount of down force as required and repeat with max down force and compare resulting hole sizes. Thus determining if the flutes are deflecting.

If the holes required reaming I would have drilled one or two sizes smaller and then reamed. But in this case the hole is sooooo much larger the ream would fall through without touching. So it's an important reason to find the cause.

ahidley
08-02-2012, 08:45 PM
OK, I tried a few more holes, this time in 1/2 inch thick steel. The .375 drill bit drilled a .410 hole and it did NOT matter if I leaned on it or babbied it. Both gave the same results. I then took some pics of the drill bits. For the ones herre to disect and report on. The pics are really blurery (sp) I'll upload a few of the best ones. I resharpened the .375 drill and now it will produce a .385 hole. Some thing must not be correct on the drill bits sharpened edges. I hope the smart guys can explain what is happeneing here??

TGTool
08-02-2012, 08:58 PM
OK, I tried a few more holes, this time in 1/2 inch thick steel. The .375 drill bit drilled a .410 hole and it did NOT matter if I leaned on it or babbied it. Both gave the same results. I then took some pics of the drill bits. For the ones herre to disect and report on. The pics are really blurery (sp) I'll upload a few of the best ones. I resharpened the .375 drill and now it will produce a .385 hole. Some thing must not be correct on the drill bits sharpened edges. I hope the smart guys can explain what is happeneing here??

If the drill lips aren't the same length it will cut oversize. You can imagine it mimicking a single flute cutter with the long lip rotating around the off-center point.

tdmidget
08-02-2012, 09:31 PM
Are these by chance zchinese drills? I have seen them , brand new, unrecognizable as a drill from the point end.

ahidley
08-02-2012, 09:32 PM
The Q is on the left and th e.375 is on the right. The Q is a split point that was never resharpened, its brandy new. In both pics the .375 cuts .410 holes.
http://i194.photobucket.com/albums/z80/ahidley/IMG_0225.jpg
http://i194.photobucket.com/albums/z80/ahidley/IMG_0225.jpg

http://i194.photobucket.com/albums/z80/ahidley/IMG_0227.jpg
http://i194.photobucket.com/albums/z80/ahidley/IMG_0227.jpg

vpt
08-02-2012, 09:41 PM
Those are the best pictures of drill bits I ever seen.

beanbag
08-02-2012, 09:58 PM
Blurry bits drill oversize holes. You need to "sharpen" them, lol.

sasquatch
08-02-2012, 10:40 PM
Just my opinion, but i'm pretty sure your'e problem lies in two spots.

The drill is not perfectly straight, (lots aren't,)

And one lip is not the same exact length as the other.

Grind Hard
08-02-2012, 10:44 PM
Blurry bits drill oversize holes. You need to "sharpen" them, lol.

http://instantrimshot.com/index.php?sound=csi&play=true

PixMan
08-02-2012, 11:01 PM
A properly sharpened drill of .375" diameter should drill no more than .002" to .003" oversize, if at all. If you get oversized holes yet the drill O.D. measures correct size and there's little to no runout at the spindle, it's a bad grind geometry on the drill. My growing collection of Titex and (some) Guhring drills cut very close to size. I regrind them with a Black Diamond No.2-B drill grinder and they work almost as good as new. I can't quite get the exact same split point, but other than that the machie makes a nice regrind on drills.

As for using center drills, I quit using those on drill holes long ago in favor of spotting drills. A 120 included angle for regular 118 drills is a good idea, but I most often use a 90 spot drill. With solid carbide drills it's important to sue spot drill of a greater included angle than the drill, but HSS and HSS-Co drills are tough enough that they rarely chip at the lip tips as a solid carbide drill would. There are 150 solid carbide spotting/pilot drills available for those 135 and 140 high performance drills.

Chuck K
08-02-2012, 11:59 PM
Can we start slamming the "Doctor" yet?

J Tiers
08-03-2012, 01:15 AM
Can we start slamming the "Doctor" yet?

I don't like mine (anymore) but it probably is not the problem..... so..... NO!

A bent drill, or poor setting in the collet will mess up the drill geometry.....

But if that's the real issue, what was that the OP said right at the beginning?



I've drilled 10s of thousands of holes and the last few, for some reason, are alot bigger than the drill.

I drilled another hole in a one inch diameter Steel rod. It was a .375 drill bit and the hole was .407 in diameter when done. The drill bit seemed to drill fine. I didnot see any wandering and the chips comming out were nice and were the SAME from each flute.

All this including more recent postings doesn't seem to be able to be true at one time.

jnissen
08-03-2012, 03:02 AM
I can almost imagine a situation where you mixed up the bits in the index and pulled the wrong one out! Assuming you really have what you say (the measurements should not lie) then the bits must be bent or you have unacceptable run out under load. I have an old drill press that indicates fairly neutral but start to torque on it and the indicator starts to show significantly more run out. Something could be going on that way.

beanbag
08-03-2012, 03:26 AM
There are 150 solid carbide spotting/pilot drills available for those 135 and 140 high performance drills.

Isn't there a HSS version? I don't feel like paying $28 for a tool whose only purpose in life is to make dents.

beanbag
08-03-2012, 03:28 AM
I think the op should go and buy a stub length American or European 3/8 drill and make some holes. Switch back and forth between this drill vs the one you have to compare differences.

I generally don't drill with resharpened drills. Whenever I screw up drilling, I usually end up with bellmouthed holes that are only oversize at the top. My best guess is that the drills u have have been sharpened "wrong" and the very tip is off center (what others said). Then the drill spins about a point off-center and makes a bigger hole. I would also hazard a guess that if you predrilled a hole a little bit larger than the web (say 1/8"), and then drill with this drill, the hole will be on size.

JoeLee
08-03-2012, 09:23 AM
Perhaps the next person composing a post to inform the OP that he needs to drill and ream all holes from henceforth might spend a moment or so reflecting on the implications of doing that before hitting the send button.

I don't think it should be nessicary to ream the hole before threading for the Helicoil insert. As stated by others, your hole should be with in a couple thou of the drill bit. Besides you should apply some LocTite to the insert and that needs a little bit of space, but you do have a problem keeping the holes sized. All the reasons I can think of have been mentioned.

JL....................

loose nut
08-03-2012, 11:03 AM
Try step drilling a hole. If it is OK or better that way then it is probably the drill itself but if it is off the it must be the machine.

People mention that it could be a bent drill but the OP has used many sizes and unless they are the really cheap Chinese ones they can't all be bent.

J Tiers
08-03-2012, 11:44 AM
Clearly the OP THINKS that everything he said is correct.... but SOMETHING is not.

Maybe the calipers he used were not zeroed..........

that is easier to believe than that the OP failed to see the drill whirling around like Spartacus' chariot hub, or that ALL the drills were made exactly 0.015 off center, or that ALL are bent the exact same, or that the machine ALWAYS has a chip in it s jaws that ALWAYS makes the hole 0.030 over.......

Maybe the chuck has a problem.

he did say he re-sharpened and it "slightly improved" the issue, but I don't think he said it "went away".

Chuck K
08-03-2012, 02:13 PM
Clearly the OP THINKS that everything he said is correct.... but SOMETHING is not.

Maybe the calipers he used were not zeroed..........

that is easier to believe than that the OP failed to see the drill whirling around like Spartacus' chariot hub, or that ALL the drills were made exactly 0.015 off center, or that ALL are bent the exact same, or that the machine ALWAYS has a chip in it s jaws that ALWAYS makes the hole 0.030 over.......

Maybe the chuck has a problem.

he did say he re-sharpened and it "slightly improved" the issue, but I don't think he said it "went away".

Anyone who has drilled thousands of holes probably has more than one 3/8 drill in his box. Try a different drill.

dave5605
08-03-2012, 03:18 PM
The info I found for a 5/16-18 helicoil says to use a 21/64" drill bit. That's .3281" a Q drill bit is .332" so it should be ok.

What has me confused is why we are talking about .375" drill bits and their resultant hole size of .410? No wonder the helicoil and tap fall right in.

As to the oversize drilling for any given size that's been pretty well covered. Bent spindle, bent drill, sharpened incorrectly, take your pick of one or more. How about the table not being 90 degrees to the drill bit/spindle axis.

Chuck up a piece of drill rod or some other suitable round thing and check for runout at its far end.

I have put in a few helicoils in the most awkward places with nothing more than a junk hand held drill motor and questionable drill bits and not had the problem(s) the OP is seeing.

ahidley
08-03-2012, 04:12 PM
Hummm. My chart says a Q size drill....
http://www.helicoil.in/pdf/HeliCoil%20Catalogue.pdf

dp
08-03-2012, 06:33 PM
This problem should be easy to test. Drill a hole half way through the plate. Pull the drill from the chuck and place it in the hole. If the faces are not the same length the drill will sit to one side of the hole. If that isn't the case then the center of the hole will not be conical which is allowing the drill to drift. That would indicate it is cutting cloverleafs (swirling).

ahidley
08-06-2012, 08:58 PM
Here is what I have learned after many holes and many sharpenings. There are three angles on a drill bit as per the machinery handbook. I cant type in the symbols as pictured in the book. So here goes. 1-We all kmow about the 118 degree point. That was correct.

2-The angle that is perpendicular to the center line is a clearance angle. This allows the cutting edge to bite in. Approx 12 degrees

3- The angle described in #2 causes the little tiny edge to be formed at the very tip (point of drill) when using a Drill Dr.

If #2 is steeper this causes #3 to be longer in poportion. The longer #3 is the more wobble was caused. Thus causing a larger hole.

SO: The smaller #2 is, the smaller #3 is, and thus the less wobble produced.

Every bit I looked at had differant angles, #2 and #3. I looked at hundreds of bits. Most american made but some chiKong. Surly the chiKong wouldnt last as long but of the few that I found that had the correct angles, these cut the truer holes.

As for the Q bit making oversized holes. It appears as though the helicoil chart has an error in it. Thanks dave5605 for pointing that out. I think someone else said a F bit????????????

Everything is square, solid, not bent.. Thanks all for the input.

Chuck K
08-06-2012, 09:51 PM
Ok...so this begs the question....what happens when you sharpen the drill with the drill doctor?

Tony Ennis
08-07-2012, 04:13 AM
My uneducated guess is that it isn't a Q drill. A 13/32" bit makes a .40625" hole.

Jaakko Fagerlund
08-07-2012, 10:03 AM
BTW, is there even any one good reason to use those "XYZ" drills, why not just list the size as a decimal, as then you know exactly what you need and what you get without memorizing things or looking at a chart?

cameron
08-07-2012, 02:34 PM
BTW, is there even any one good reason to use those "XYZ" drills, why not just list the size as a decimal, as then you know exactly what you need and what you get without memorizing things or looking at a chart?

One good reason is, for example, that I can read "N" on the the drill stand without putting my glasses on. Might not be the case with " 0.3020" in small characters to fit in the same space.:(

ahidley
08-07-2012, 03:24 PM
Chuck K. I do have and use a drill dr. The issue is two fold. First I was using brand new bits. It appears that the angles differ from manuafcturer to manufacturer as I stated above. If you can look it up in the machinery handbook and you'll understand the angles that I'm trying to descrbe above. Second, the drill dr has an adjustment to increase + or decrease- the angle perpendictular to the drills center line. This angle allows the cutting edge to bite in more or less. This adjustment is located on the left side of the drill dr and is used when indexing/locating the bit in the plastic chuck. Its needed on a drill dr because every company grinds the flutes with differant twists.

Chuck K
08-07-2012, 08:42 PM
"I do have and use a drill dr."

I'm just yanking ur chain a little. I know that a lot of people use and like drill doctors. My experience was very inconsistent. Probably due mostly to a lack of patience. It did give me incentive to learn to sharpen drills with my grinder. My drills don't always drill exactly on size, but they are sharp and they're close enough for the stuff I do.

Bill736
08-07-2012, 10:26 PM
I'm reminded of the first set of Chinese drill bits I ever bought , probably 20 years ago. The bits looked very nice , but drilled oblong holes. It wasn't obvious to the eye what was wrong with them, but I sent them back and got a refund.

PixMan
08-08-2012, 08:36 AM
BTW, is there even any one good reason to use those "XYZ" drills, why not just list the size as a decimal, as then you know exactly what you need and what you get without memorizing things or looking at a chart?

The main reason is tradition. The "letter size" and "wire gauge size" (also known as "number" drills) have been around for about 200 hundred years here, long before the metric measurement system came to be the more sensible standard. The lists of sizes, all between or below the 1/64's of an inch fractional size increments, where necessary to give us those minor but important sizes. I don't know the history of where the sizes were developed, but those lists give us far more increments than most metric drill sets.

A common set here is 118 drills, with 32 fractional (1/64" to 1/2" by 64ths), 26 letter drills (A-Z, .2340' to .4130"), and 60 number drills (#60 to #1, ,040" to .228"). The only size that's the same in a Letter size and fractional size is the "E" drill, which is the exact same size as 1/4". I have no idea why. The increments aren't perfectly even, but that often works out well. For all the years I worked on CNC screw machines, I had many instances of searching lists of both number sizes and of metric sizes to get the exact size I needed.

I think it's wonderful to have all these options, not confusing.