A machinist is telling me that twist drills are tapered, so they get smaller each time they are sharpened. This is his excuse for drilling undersize holes. I thought it was just tip wear. Anyway we have been arguing about it for over a week, and believe it or not, its a lousy 5/16 inch jobber drill. I could just bring him one from home, but then I'd have to make him actually change it out...
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Are Twist Drills really Tapered?
- Thread starter rabtrfld
- Start date
- Replies 45
- Views 7,442
CarbideBob
Diamond
- Joined
- Jan 14, 2007
- Location
- Flushing/Flint, Michigan
Yes the margin on a drill is normally ground with a back taper.
Would be nice if they were not as then you could just feed them all the way through the centerless.
Instead you have to feed until the shank area just clears through, (making it straight) open up and kick back or remove the drill.
It is not a lot in most cases and the taper per inch depends on the drill diameter. Controlling this back taper is a PITA but a given if you make drills.
You can see this by accurately measuring the mounting shank which will be below the tip width.
This backtaper is to prevent the margin from rubbing or binding on holes with decent depth as the margin is simply circle ground unlike a endmill which is ground straight but has relief.
Reamers have the same problem and are also circle ground with a back taper but less taper per inch than drills.
Some manufacturers have their own in-house numbers for backtaper but most just go by the published aerospace standards.
So he is right sort of but you have to take a bunch off the tip to see real numbers as it is a straight taper per inch along the flutes that should be maintained.
Not sure what "undersized" here means, how the resharp was done but how can you blame the machinist?
Poked the hole, not big enough. Ok, process QC, something wacky.
I like both your sides of the week long argument and would maybe fuel it to better understanding of why and what is happening.
I would be remiss to say... A big Welcome hug to posting on the forum....
Bob
Would be nice if they were not as then you could just feed them all the way through the centerless.
Instead you have to feed until the shank area just clears through, (making it straight) open up and kick back or remove the drill.
It is not a lot in most cases and the taper per inch depends on the drill diameter. Controlling this back taper is a PITA but a given if you make drills.
You can see this by accurately measuring the mounting shank which will be below the tip width.
This backtaper is to prevent the margin from rubbing or binding on holes with decent depth as the margin is simply circle ground unlike a endmill which is ground straight but has relief.
Reamers have the same problem and are also circle ground with a back taper but less taper per inch than drills.
Some manufacturers have their own in-house numbers for backtaper but most just go by the published aerospace standards.
So he is right sort of but you have to take a bunch off the tip to see real numbers as it is a straight taper per inch along the flutes that should be maintained.
Not sure what "undersized" here means, how the resharp was done but how can you blame the machinist?
Poked the hole, not big enough. Ok, process QC, something wacky.
I like both your sides of the week long argument and would maybe fuel it to better understanding of why and what is happening.
I would be remiss to say... A big Welcome hug to posting on the forum....
Bob
sfriedberg
Diamond
- Joined
- Oct 14, 2010
- Location
- Oregon, USA
Short answer is: Yes, but almost certainly not the cause of your problem.
As you probably know very well, drills tend to make holes that are too big, not too small.
Makers of nice drills put a very small amount of axial relief on the flute margins, called "back taper". This is exactly like the same axial relief given to machine reamers. We are talking approximately 0.001" of diameter per inch of length for a garden-variety drill, increasing to 2 or sometimes 4 times that much for high performance drills designed for hard or tough materials. Also, a given drill design may use larger amounts of back taper for larger drills (over 1/2"). So unless your 5/16" drill has been half ground away and the holes are just barely undersized, back taper is not the cause!
If your guy is drilling holes that appear substantially undersized (like 0.005 to 0.010" or more undersized) because you can't get any larger plug gage into the hole (or a 5/16" shoulder bolt won't go through the hole), the holes are probably badly lobed. This could be due to a badly sharpened drill, a really slack machine, or just general ineptitude.
[Added in edit: I see CarbideBob types faster than I do!]
As you probably know very well, drills tend to make holes that are too big, not too small.
Makers of nice drills put a very small amount of axial relief on the flute margins, called "back taper". This is exactly like the same axial relief given to machine reamers. We are talking approximately 0.001" of diameter per inch of length for a garden-variety drill, increasing to 2 or sometimes 4 times that much for high performance drills designed for hard or tough materials. Also, a given drill design may use larger amounts of back taper for larger drills (over 1/2"). So unless your 5/16" drill has been half ground away and the holes are just barely undersized, back taper is not the cause!
If your guy is drilling holes that appear substantially undersized (like 0.005 to 0.010" or more undersized) because you can't get any larger plug gage into the hole (or a 5/16" shoulder bolt won't go through the hole), the holes are probably badly lobed. This could be due to a badly sharpened drill, a really slack machine, or just general ineptitude.
[Added in edit: I see CarbideBob types faster than I do!]
All that said, the amount of taper behind the cutting edge/drill point should not be enough to drill under sized holes!!
First of all, if you re using a "standard jobber length drill" probably not precision holes. In my experience, those drills are going to go at least .001" oversized, and that is when it's going good. So 5/16" Jobber Drill I would say .314-.316" is probably average. 5/16" Jobber is about 4 1/5" OAL, and 3 3/16" flute length, I would guess that in 3 3/16" there is no more than .0015 total taper. So .3125-.002=.3105", and with the average drilling .002" O.S. back to .3125", plus hand regrind is going to make the oversize worse.
Are the holes smaller than that??? Dude picked up the wrong Drill, no biggie right. I mean shit it's under---re-drill them. Unless this person has some Micro Managing Bureaucrat looking over His shoulder wondering about every detail of the work.
R
First of all, if you re using a "standard jobber length drill" probably not precision holes. In my experience, those drills are going to go at least .001" oversized, and that is when it's going good. So 5/16" Jobber Drill I would say .314-.316" is probably average. 5/16" Jobber is about 4 1/5" OAL, and 3 3/16" flute length, I would guess that in 3 3/16" there is no more than .0015 total taper. So .3125-.002=.3105", and with the average drilling .002" O.S. back to .3125", plus hand regrind is going to make the oversize worse.
Are the holes smaller than that??? Dude picked up the wrong Drill, no biggie right. I mean shit it's under---re-drill them. Unless this person has some Micro Managing Bureaucrat looking over His shoulder wondering about every detail of the work.
R
jhruska
Titanium
- Joined
- Jul 5, 2009
- Location
- Munster, In. USA
The other tapered part of the drill is the web.
It gets thicker close to the shank.
John
It gets thicker close to the shank.
John
eKretz
Diamond; Mod Squad
- Joined
- Mar 27, 2005
- Location
- Northwest Indiana, USA
Yes they are definitely back tapered in most cases. Also check the actual diameter of the drill he used. I've seen heavy wear on the lands cause problems like this as well. Barring that, maybe he did grab the wrong drill. How far under size are we talking here?
Forrest Addy
Diamond
- Joined
- Dec 20, 2000
- Location
- Bremerton WA USA
Yep. Good twist drills are back-tapered a few thousandths from the lip so the margins don't drag.
Most drills drill a trifle oversize, even new drills that are precision machine pointed. You might get a hole that appears undersize but if you check carefully you may find the hole tri-lobed - especially in tough copper allows. Your machinist may be blowing smoke..
Most drills drill a trifle oversize, even new drills that are precision machine pointed. You might get a hole that appears undersize but if you check carefully you may find the hole tri-lobed - especially in tough copper allows. Your machinist may be blowing smoke..
WizardOfBoz
Diamond
- Joined
- Sep 30, 2006
- Location
- SE PA, Philly
Forrest,
What's the solution for tri-lobed holes? Carbide drill? Neutral cutting edge? What else?
Thx
adh2000
Titanium
- Joined
- Dec 21, 2005
- Location
- Waukesha, WI
A drill bushing helps a lot, a whole lot. But then you need a jig to hold the bushing and things get complicated.
K
keithmech
Guest
reaming is about the only cost effective way
of getting round/sized holes.
of getting round/sized holes.
michiganbuck
Diamond
- Joined
- Jun 28, 2012
- Location
- Mt Clemens, Michigan 48035
yes thy are.. if not they would seize up as they wore to a point end taper..Often we made such at .0002 per inch..so rarely would that harm a drill specification..End mill are made straighter.. this is Ok because they are cutting on the sides so they do not lock up.
Now it all makes sense.
Glad I thought of that. I think I have a phobia of micing hardened objects with the pristine new tools I bought for this job, because you should see the worn out stuff in the shop, they all read different and every year I shake my head watching the cal lab come in and tag them again (yes I know how to zero a mic, I did a lot of theirs). I'm mostly worried about the narrow tips of the calipers, I won't use them at all. The best thing is my counterfeit Mit that was so cheap I don't value it at all so I can ID anything with gusto and throw it away after a year.
Our holes are only a few thou under, but they are starting to jam the assembly machines. An outside vendor makes them to size, and he says he uses a 5/16 drill same as us. I brought a nice gage pin out from the lab, black oxide finish intact, the machinist set it aside and picked up HIS pin, which looks like a valve stem from a Rambler, and said "No problem. Pin fits." We make upward of a million of these per year and he's been on the same gage pin for years. The dial on his caliper is completely filled with oil (no harm in that I suppose except the oil is opaque), I thought it was specially made that way but it's an in-joke with the other guys. So when mentioned the out-source makes them to size, the response was, "his machines are better." True; our Acme-Gridley were in continuous use since the 1960s.
eKretz
Diamond; Mod Squad
- Joined
- Mar 27, 2005
- Location
- Northwest Indiana, USA
Perhaps you should get the machinist a new gage pin. Does his pin fit the holes while the one from inspection does not? Tri-lobed holes can usually be minimized by using proper hole starting techniques and as short and stiff of a drill as possible.
I hope you realize this is a forum of professional Machinists....right?
I know the solution, go out there and make the part the way you want it, then show this person how you did it, and how you inspected it.
Nice new pins with the black oxide still in tact mean that it doesn't get used. Shiny Micrometers are funny sign to me. Is this the blind guy complaining that the blind guy leading him is blind?
DMF_TomB
Diamond
- Joined
- Dec 13, 2008
- Location
- Rochester, NY, USA
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measure the drill of course with a micrometer. that took 2 seconds and you should be doing it anyway
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i have seen back taper of .001 to over .015" of course. carbide drills often have far more back taper. just measure drill at tip and by shank of course
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picture of long drills breaking cause pilot hole was .001 to .002 small and the long drills would vibrate and break. just redrilling pilots with new and bigger drill then the long drills drill ok everytime.
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drills are not for precision size holes. they can make -.015 to +.050 holes depending on actual size of drill and how badly they are sharpened. of course normally not that bad. -.002 to +.005 is more common. there are formulas to calculate for hole size over the actual size of drill but how do you calculate for badly sharpened drill ? so drill tip -.002" drilling +.002 over actual drill size making a on size hole to .000 but i wouldnt count on it. feed rate, coolant or dry, drill sharpened off center, drill runout, drill length stickout from collet. too many things each have a effect.
Attachments
michiganbuck
Diamond
- Joined
- Jun 28, 2012
- Location
- Mt Clemens, Michigan 48035
Perhaps the Holy Grail (nowadays)for modern drills the ISO pdf…
Guess it was written by engineers or professors so one would need a class to not be confused.
https://www.iso.org/obp/ui/#iso:std:iso:5419:ed-1:v1:en
Sect 3.18 Body clearance see figure -4 (X but is better shown in figure-3 INHO)
Sect 3.31 & 3.30 then looking looking at figure 5&6
The section reference numbers 3.31 and 3.30 look like Diameters… had those reference numbers been reversed 3.30 and 3.31 they would look like a drill with the numbers in proper order and show body clearance being a smaller number..
Section 3,31 back taper is explained. But there is no back taper at the drill point so not clearly shown…another sect number is needed to show back taper. Perhaps shown in figure 1. (3.31 and 3.30 shown on fig 1 in proper sequence big to small going to the right would have shown it plane as could be.
Yes some old salts will say that is good and good enough but it should be directed to the novice who is looking for the answers and so have nothing confusing.
The drill (PUN INTENDED) should be "keep it simple"..Or wait till after Chernoybl blows up then say " oh yah" now i see the problem.
Guess it was written by engineers or professors so one would need a class to not be confused.
https://www.iso.org/obp/ui/#iso:std:iso:5419:ed-1:v1:en
Sect 3.18 Body clearance see figure -4 (X but is better shown in figure-3 INHO)
Sect 3.31 & 3.30 then looking looking at figure 5&6
The section reference numbers 3.31 and 3.30 look like Diameters… had those reference numbers been reversed 3.30 and 3.31 they would look like a drill with the numbers in proper order and show body clearance being a smaller number..
Section 3,31 back taper is explained. But there is no back taper at the drill point so not clearly shown…another sect number is needed to show back taper. Perhaps shown in figure 1. (3.31 and 3.30 shown on fig 1 in proper sequence big to small going to the right would have shown it plane as could be.
Yes some old salts will say that is good and good enough but it should be directed to the novice who is looking for the answers and so have nothing confusing.
The drill (PUN INTENDED) should be "keep it simple"..Or wait till after Chernoybl blows up then say " oh yah" now i see the problem.
DMF_TomB
Diamond
- Joined
- Dec 13, 2008
- Location
- Rochester, NY, USA
.
i use carbide drills everyday the first 0.2" at new drill tip is on size and no taper. the rest of flutes have .015" back taper or .015" smaller at drill shank. got something to do with ability to drill over 30 ipm feed and to give a fairly smooth drilled hole surface. easily measurable with micrometer and actually is a requirement to measure each time drill changed as if drill -.001 i cannot use as longer drills do like going into a -.001 hole with out a lot of vibration
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