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A.K. Boomer
10-13-2008, 02:24 PM
The recent drill bit discussion and all the doubting tomasses made me conduct my own test this morning as I had theorized in the other thread that a drill bit is like a piece of swarf, if you hold one end and turn the other opposing the direction of the wind the swarf will not only get larger in diameter, it will get longer also,

Welp, the proof is in the pudding

I chucked up a piece of scrap aluminum and took a .312" endmill and went down .250" and then walloed out .1" in each direction with an added .010" off to the sides, I then went down the center of the oval with a .312" drill to give the larger 1/2" test drill a nice place to bottom out and rest;

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC00434.jpg

Next up was bottoming out the half inch Jobber into the aluminum and "feeling" the elasticity of the bit while turning the drawbar with a Dork wrench, I got comfortable with how far I wanted to push the bit and then looked at the dork wrench reading - it said 20 ft. lbs -- so i then pushed it to 25, the bit held, and also self wedged all the way to the bottom of the aluminum.
I immediately knew Paul was way off in his prediction of a drill only getting 1/10 of a thou longer due to the fact that I could see my quill lever raise back up every time I applied torque to the bit.
I then took a static no load measurement of the drill bits diameter - it was .4988" and because the quill was being held down now I took a measurement of what judges the static state of the length of the bit -- its not of the bit itself, its just a number to show change if any, its just a measurement between the free floating quill and the quill adjuster nut at the bottom (because I only have 2 axis dro) That measurement was 2.444"

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC00436.jpg

Next up was holding the dork wrench hands free, an adjustable shower curtainpole was the proper tool of choice,

I proceeded to crank the R/T handle in the direction of what the drill operates in when being properly used (uhhh -ummm)
I cranked the R/T handle till the dork wrench read 25 ft. lbs
http://i146.photobucket.com/albums/r249/AK_Boomer/DSC00435.jpg

Now I went back and checked both measurements --- The drills flutes were now .4992" and the quill measurement was now 2.452"
The bit grew .008" in length and about a half thou in diameter.

Last but not least --- I then applied the mills hand brake very firmly --- this removed all elasticity from the dork wrench --------- I held it and relieved the rotary table from its tension It went from 335 degree's to 347...

The torsional deviance was 12 degree's there was approx. 3 full spirals in the length of this jobber ---- Conclusion; the bit got both larger and longer because these spirals got straightened out some -- kinda like a piece of swarf does when you apply the same kinda loads to it:D

NickH
10-13-2008, 04:47 PM
So is a dork wrench a torque wrench for those in some way challenged?
:D

ckelloug
10-13-2008, 05:04 PM
What an excellent experiment and write up. Good job!

Fasttrack
10-13-2008, 05:12 PM
LOL I like the "Dork wrench"

Thats pretty neat. I missed that discussion in the drilling an over-size hole thread.

Scishopguy
10-13-2008, 05:13 PM
Boomer....Makes perfect sense to me. A surface hardened spiral has all the makings of a torsion spring if you think about it, and what does that spring do when you flex it. Yep, change dimension.

lazlo
10-13-2008, 05:17 PM
Nicely done AK. :)

NickH
10-13-2008, 06:20 PM
The analogy of a two flute drill bit to a spiral of swarf is not valid, you are comparing a flat bar with a twist along it's long center axis to a coil spring, the two are logically and topologically different.

One could argue that the twist drill is effectively a series of thin flat plates welded together like the steps of a double spiral staircase (or the double helix of DNA), with no good logical argument that twisting straight should increase diameter.

Interesting bit of practical work but the diameter measurement would need to be taken in a proven repeatable manner at multiple marked points along the shaft, it may have been but the write-up lacks the detail to show if any good method was applied or not, more detail please,
Regards,
Nick

Rustybolt
10-13-2008, 06:34 PM
OK, Nick. How about a tortion bar with two grooves in it? Because under the right load that is exactly what it acts like. A drill under a torque load will break differently than a drill under a shear load.

J Tiers
10-13-2008, 10:47 PM
I do NOT believe the actual drill diameter increased a half thou. To stretch the web even a half thou sounds virtually impossible with the available forces.

You are looking at the geometry all wrong.

The "slots" cut in the drill are actually such as to make the cross-section look like a somewhat wierd-shaped I-beam...... one that looks a bit like a curved "bow tie" turned vertical.

The drill web is the I-beam web, and the "flanges" are the not-cut-away parts of the drill.

What I think is ACTUALLY happening is that the "I-beam" cross-sectional shape is being distorted.

If you imagine the "web" as vertical, the 'top right" and "bottom left' parts of the "I-beam" cross-section (the "flanges") are being twisted, so that the distance from one 'edge" to the other is larger.

The top right edge is being bent 'up", and the bottom left edge is being bent "down". That will increase the EFFECTIVE diameter, because the lips are being "pried outwards" due to torque.

Draw an I-beam cross-section, then draw another with the top flange tilted up at right side, and the bottom flange tilted down at left side. That will show what I think is actually happening.

If I had a scanner or a camera handy I could draw this.

It makes the EFFECTIVE diameter bigger, but does NOT require that the web be actually stretched by some as-yet-undiscoverable force.

Paul Alciatore
10-13-2008, 11:55 PM
Interesting experiment. Nice work. I am truely surprised by such a large increase in length. But, as I said, I was only spit-balling. The increase in diameter is even more surprising. You don't say where on the drill you measured the diameter: I would be interested in knowing.

This may explain why almost all drills are a few tenths on the small size.

Still, we were talking about holes that were several thousanths large, not four tenths. So even if all you say is true, it does not explain the common observation that holes are quite a bit larger than the drill size. In fact it still falls short by a factor of about 10.

NickH
10-14-2008, 03:19 AM
OK, Nick. How about a tortion bar with two grooves in it? Because under the right load that is exactly what it acts like. A drill under a torque load will break differently than a drill under a shear load.

Yes, that would be closer, most analysis, physical & mathematical involves looking at smaller and smaller sections or pushing an example to an extreme to consider possible effects. Does the OD of a torsion bar change under load?
Nick

J Tiers
10-14-2008, 08:08 AM
Still, we were talking about holes that were several thousanths large, not four tenths. So even if all you say is true, it does not explain the common observation that holes are quite a bit larger than the drill size. In fact it still falls short by a factor of about 10.

The torsion distortion I mentioned above has potential to go quite a bit larger than a few tenths or so, depending on drill size.............

Rather than actually stretching the web, it supposes only a springing and "prying outwards" of the flute lips. If you think about it, torsion would be likely to do that.

And the far more likely reasons for most over-size are to do with imperfect grind, and imperfect guiding by the point, so that the drill while in action, does not actually rotate on its own axis, but around an axis displaced from that of the drill itself.

A.K. Boomer
10-14-2008, 10:26 AM
The analogy of a two flute drill bit to a spiral of swarf is not valid, you are comparing a flat bar with a twist along it's long center axis to a coil spring, the two are logically and topologically different.

One could argue that the twist drill is effectively a series of thin flat plates welded together like the steps of a double spiral staircase (or the double helix of DNA), with no good logical argument that twisting straight should increase diameter.

Interesting bit of practical work but the diameter measurement would need to be taken in a proven repeatable manner at multiple marked points along the shaft, it may have been but the write-up lacks the detail to show if any good method was applied or not, more detail please,
Regards,
Nick

Thats why I said; "kinda like a piece of swarf does when you apply the same kinda loads to it"

I did check the bit in a variety of places and most all were the exact same,
there is another potential phenomenon - as the bit "fans out" it also changes its angle of attack where the flutes dig with a more aggressive pitch, I would also bet the entire bit goes through the wobblies as some area's are stronger than others, now that could really attribute to a walloed out hole as it would have to gain the clearance needed to function all while being moved up and down the bore,
I do know this, when I take it easy i achieve great results - generally, --- now I was drilling some D2 tool steel the other day --- you cant take it easy because it work hardens on the spot, you have to go for it, its a good thing the part called out for +/- .002" on the specs:p

All you guys bring up some good points, I really think there's many reasons that all add up, but I do believe with good control load and proper chip removal that if a high quality bit is being used that you can achieve great results with a material of good machinability.

A.K. Boomer
10-14-2008, 11:09 AM
Still, we were talking about holes that were several thousanths large, not four tenths. So even if all you say is true, it does not explain the common observation that holes are quite a bit larger than the drill size. In fact it still falls short by a factor of about 10.



I was also surprised at the ratio, I took a stab at the length to diameter ratio back on post #35 of the drill thread and will admit I was wrong but I was off by about a 1 to 2 ratio not 10 to 1 ---------

"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"


I really expected to see a .001" gain or more out of the bits diameter,

Its a half inch jobber and the flute area is 4.25 long ---
it actually puts the length to diameter ratio at close to 2 to 1 --- that opposes my prediction and did surprise me - but thats how you learn...:p

Whatever the construction comparison that is used - be it a twisted I-beam or a piece of swarf, my point is is that spirals or coils on the outer parameter have an effect when torsion is applied -- they can grow the entire unit or shrink it depending which direction loaded,
I dont care how you try to explain it, the fact of the matter is is that the two flutes that are 180 degree's apart "grow or shrink" under torsion --- If this occurs then the bit will make the necessary adjustments in running clearance that it needs (I.E. the hole will get larger and longer) nuff said.

Paul Alciatore
10-14-2008, 06:53 PM
Perhaps we all learned something new here.

SVS
10-14-2008, 11:09 PM
Well, I think I'm still in the call B.S. crowd as to "spiral unwinding" being much of a factor on oversize holes.

Been thinking on the subject a little, went back and re-read the drilling chapter in Moltrecht's "Machine Shop Practice", and conducted a little experiment today.

Fired up my Cinci VerciPower 420-18 vertical bed mill and stuck a fresh high quality TIN coated metric bit I measure at .628" in the spindle with a CAT50 to 3MT adapter. Bit has 8" of flute and installed run out measured about .001"

Got a bunch of these bits at auction so I was willing to burn one up in the name of science. The Mill has 10hp on the spindle(20 and 30hp from the factory), and weighs 23,000 lbs. Used it on the experiment for the power and range of precise feeds.

Test subject was a chunk of 1 1/4" x1 3/4" CRS I've had floating around. Had enough oddly spaced holes I've never used it for an actual project. Has a lot more holes in it now.

Basic idea was to drill through holes (1 3/4" deep, nominal .628" dia,@ 100 fpm(605 rpm) varying only the feed, and then measure the result. No coolant, no pilot hole, no peck drilling, braced a file at an angle near the bit to keep curls under control. Steel resting on steps milled in homemade soft jaws in my Kurt vise. Kept at least one drill diameter untouched between adjacent holes.

Popped 16 holes at feeds ranging from .0009"/rev. to .446"/rev. (9/16"/min. to 27"/min. I was half hoping to test to destruction by twisting the bit off, but things were getting a little scary and I was starting to run out of power at the 27"/min. feed, so I quit there. The bit still looks remarkably good. After a couple holes I grabbed a non contact thermometer and shot the bit and the side of the bar adjacent to each fresh hole. Nothing very controlled about this part of the experiment, but the bit ran about 130F. on the fine feeds, 120F. on the mid range, and 140F. on the heavier feeds. Curls ranged from shimmery silver tin foil, to smoking blue chunks as the feeds increased.

Once the bar had cooled I measured hole diameter with a telescopic gage, transferred to a 0-1" mike. I split the lines for half thousandths. Measured every hole near half depth, and about 1/3 of the holes top and bottom as well.

Every hole measured between .632" and .635". No individual hole varied more than .001" for the three measurement, most were within .0005", and several didn't show any taper.

The four lightest feeds gave diameters of .632" to .633". The four heaviest feeds exactly the same. The eight midrange feeds showed two holes each at .632", .633", .634", and .635" in seemingly random pattern.

To sum up, my little experiment, as flawed as it may be shows this bit in this machine drilling this material will run .004" to .007" oversize(about what the books predict) and feed/torque applied doesn't change a thing.

Scott

A.K. Boomer
10-15-2008, 10:30 AM
The Mill has 10hp on the spindle(20 and 30hp from the factory), and weighs 23,000 lbs.

Scott


Well there's your problem, --- I dont think your mill was "stable" enough:p

Would have been nice if you drilled blind holes and kept track of the depth of the hole also, You at the very least covered testing results for all the variables that you created - but - I dont think its a real good test for what I was saying due to the lack of keeping the hole clean in the testing process, example, the lighter feeds produce smaller coils and or chips, If not properly flushed they have more of a chance of getting caught between the drills flute and hole, forcing the other 180 degree opposed flute into said bore -- Iv had some of my best results with flood coolant forced directly down the bore while drilling very mildly and pecking at it --- its something I prefer to do manually --- I know you can program a machine to do it for you and about mimmick any given situation - but iv had material that seems to put up more of a fight at different drilling stages, dont know if its inconsistancies in material or what, Dont think its just my imagination, My friends TM1 haas has a spindle load indicator thats dancing all over the place when its drilling at a predetermined feed/speed --- Would be interesting if we can set up the TM1 to feed at a set spindle load rating and see what happens...

Thats for taking a chance with your bit in the name of R&D:)

(I chipped the end of my jobber backing it out of the test jig:o )

SVS
10-15-2008, 09:07 PM
A.K.
I think it applies very well to the "spiral unwinding"/oversize hole theory-which I theorize is wrong because you base it on an invalid model.(swarf curl)

I object to your model on this simple basis:Solids under strain should not expand in two axis. I believe you claim in several place that a bit under torque gets longer AND fatter. Shorter and fatter or longer and skinnier I can buy.

Putting the wood to the bit at 27"/min feed produced the same diameter hole as 9/16"/min feed. I'm confident the bit was strained significantly at the heavy feeds, but the resulting holes showed no measurable difference.

The flutes were not plugging at any feed, ie: the chips/curls rolled smoothly out of the hole, were not ground up, and did not drop out the bottom of the hole after the bit broke through. Chip thickness varied greatly in direct proportion to feed, but hole diameter was very nearly constant. Chips were either affecting every hole, or none. You tell me.

Certainly I will concede that surface finish would improve with lube. Most of my machines run flood coolant. I use and like it for most work. For this test I felt it was an uncontrollable variable, and I wanted maximum drill bit abuse.

To conclude, I will grant that this test was not definitive. No replication, no double blind, one bit, one test bar, one machine. Test results were near the limit of what I could quantify. etc.

Certainly does not support "spiral unwinding" as a factor in oversize holes, and the process was sorta fun. Have you ever pumped a 5/8" bit through steel at 27"/min? Couldn't decide if it was more important to hide behind the mill or stay close to the E-stop.

Scott

A.K. Boomer
10-16-2008, 11:02 AM
A.K.
I think it applies very well to the "spiral unwinding"/oversize hole theory-which I theorize is wrong because you base it on an invalid model.(swarf curl)

I object to your model on this simple basis:Solids under strain should not expand in two axis. I believe you claim in several place that a bit under torque gets longer AND fatter. Shorter and fatter or longer and skinnier I can buy.





SVS, I never stated that both the swarf and the bit got to their destination in the same way, all I stated is the end results are similar -- and they are, BOTH the swarf and the drill bit get BOTH larger in diameter AND longer in length,
Thats my test results --- Is this the main contributor to oversize holes? Maybe not, maybe its a bunch of different factors --- Your answer to whether solids under strain can expand in two axises is of course yes, Esp. if they are solids comprised of certain geometric forms,The swarf proves that the external coil size can grow both longer and larger when torsion is applied, the drill bit proves the same although no where near the exaggeration -- If you want to know how the bit does it its not hard to imagine a double helix straightening out and getting longer, Now put some "T" tops on the edge of the helix ends,
Now your starting to describe JT's cross section example of the "I" beam --- now take that I beam and lean it to one side so the main beam goes from this --- l --- to this --- / but still keep the bottom and top supports (which I cant show) totally horizontal ---- the results are that if you measure corner to opposing corner of the horizontal supports one measurement will be much longer than the other -- and if you take the combined measurement between the two and average them out they will be the same measurements of horizontal corner to opposing corner of the perfectly vertical main beam I beam example.
This is how the drill bit gets BOTH longer AND Larger in diameter when torsion is applied,
Its simple geometry.

Now imagine the effect that this has in a cutting tool --- where the flutes are ground at a certain angle of attack, and now that angle has become more aggressive --- all while the bit itself being bigger in diameter, Is it a factor? of course it is, How much? I dont know,,, all I know personally is when I take it easy drilling and the machinabililty of the material allows I can get great results, it also seems that if I godzilla a hole the results are not as good...

I dont dispute your test results, But you simply cannot dispute mine, they not only obey all the laws of physical geometry --- they prove themselves in the test results applied...

SVS
10-16-2008, 01:20 PM
I think our definition of "solids" is different.

A helix is a mostly air, with a small amount of steel(swarf curl) spiraling around the circumference. Radial and longitudinal restraint is close to nil.

A drill bit is the opposite. Mostly steel with a helix of air spiraling around the circumference. Lots of restraint.

The I beam model has some merit, but only as an approximation. Look at the end of a drill bit and try to apply an I beam's reaction to torsion to it. You have to imagine a beam with a deep web, and short flanges. Then you have to remove the leading half of each flange and curl the trailing half to fit inside a circle and thereby approximate the lips and margin of an actual bit. Takes some rounding, trimming and rotational shift for it to be valid, and wa la, the diagonal corner to corner measurements you cite fly out the window. Torsional deflection of the lips would be roughly equal, and simply mean a particular point on the periphery of the drill would rotate a few degrees around a circle with respect to the other end of the bit while the equivalant point on the opposite side would rotate in the same direction and the distance between these two points hardly changes.

I submit drill rod as a more valid model for a drill bit than a swarf curl.

If you compress a cylinder through it's long axis it gets shorter and fatter. Apply tension it gets longer and skinnier. Apply torsion with no axial restraint it could get short and fat, could get long and skinny but damm sure wont get short AND skinny or long AND fat.

Now, if we imagine a drilling cycle on a machine with minimum back lash and hitting a very solid depth stop before the drill breaks through I can imagine the bit being shorter and fatter while under feed, and then when the depth stop is touched "unwinding" slightly and continuing to drill as it resumes it's resting length and diameter. (I think you are observing this effect, but confused on the timing, and cause)

BUT:

I believe it to be physically impossible for a bit to be both fatter AND longer at the same instant in time, as you claim.

Hugs and Kisses,

Scott

J Tiers
10-16-2008, 08:14 PM
The I beam model has some merit, but only as an approximation. Look at the end of a drill bit and try to apply an I beam's reaction to torsion to it. You have to imagine a beam with a deep web, and short flanges. Then you have to remove the leading half of each flange and curl the trailing half to fit inside a circle and thereby approximate the lips and margin of an actual bit. Takes some rounding, trimming and rotational shift for it to be valid, and wa la, the diagonal corner to corner measurements you cite fly out the window. Torsional deflection of the lips would be roughly equal, and simply mean a particular point on the periphery of the drill would rotate a few degrees around a circle with respect to the other end of the bit while the equivalant point on the opposite side would rotate in the same direction and the distance between these two points hardly changes.


if you imagine that BOTH cutting edges are biting into the hole, while BOTH trailing edges are NOT, as would be expected, then there exists a force acting to open up BOTH "flanges", so that they are somewhat parallel.

In that condition, both slanted parallel, they won't fit in the same size circle as when NOT drilling. The "limit case" (never remotely seen, but instructive), is an I beam with the flanges tilted over parallel to the web. Whether that is bigger than the hole it would fit in undistorted, depends on the "flange" width.

Against that effect is the stiffness of the thicker web, which will oppose that tilt, and tend also to "pull in" the "flanges".

I don't know which would actually "win", and be the dominant effect.

SVS
10-16-2008, 10:40 PM
J.
I see your point, IF "I" is a good model. My point is that a bit has effectively no flange on the leading edge of our I beam web, and it strikes me that "Z" may be a more accurate over simplification.

Scott

J Tiers
10-17-2008, 08:14 AM
"Z" could work....... same idea, I believe.

A.K. Boomer
10-17-2008, 11:10 AM
Now, if we imagine a drilling cycle on a machine with minimum back lash and hitting a very solid depth stop before the drill breaks through I can imagine the bit being shorter and fatter while under feed, and then when the depth stop is touched "unwinding" slightly and continuing to drill as it resumes it's resting length and diameter. (I think you are observing this effect, but confused on the timing, and cause)

Thats not the way it works --- lets say you were pumping that half inch Jobber through some 416 @ 25 Ft. pounds of torsional load, What it means is that the second that torque is achieved the bit grows .008" longer --- it also means that you can set your stop up all day long to the static position of the bit and be off your mark every time, The "hypothetical " extra length drilling that takes place does not continue after the stop --- in fact as soon as the stop is hit there is no more material left to keep the bit in its longer than static position due to the material itself being the negotiator of the torsional distortion of the bit in the first place, Yet the fact remains that the bit was longer at that moment and the deviance it was experiencing cannot be achieved by simply re-engaging the feed again and again --- So --- your statement of the drill continuing to drill after is not logical, in fact -- the drill grabs till the very end and then recoils back in a fury, and the extra degree's of torque distortion that the bit was under dont do a thing in making the hole deeper, Yes the bit will twist in the direction of drilling and twist an extra 12 degree's -- but it will do nothing but grab air as it gets .008" shorter whilst doing this.....



BUT:

I believe it to be physically impossible for a bit to be both fatter AND longer at the same instant in time, as you claim.




What part of my test results dont you understand? or is it that you dont think I know how to measure?, If so then why should I trust your judgment on the holes you drilled? do you know how to measure properly? :rolleyes:

This may be a tough pill for you to swallow -- but once again, drills get both fatter AND longer at the same time, your going to have to come to grips with that and find a way of dealing with it...

SVS
10-17-2008, 01:56 PM
A.K.
I'm not out to antagonize you, but I think you fell in love with bad models (swarf curl, and I beam) and have to keep revising it and throwing out non sequitors to defend it. Certain points I have made you skip over, while giving me a patronizing "It's simple physical geometry Dumbass". You'll be right on the money though when utilizing swarf or I-beams for hole production.

Your drilling machine depth stop analogy rebuttal implies that the drill is pulling itself through the work, instead of being pushed through, as I meant when proposing the thought experiment. Apples and oranges. May happen in soft material, or with a healthy pilot hole, but not hard stuff and/or no pilot.

Your example is analogous to torquing down a head bolt, and implies that the bolt stretches(elongates) as torque increases. 100% correct so far, but your theory goes further to effectivelyly claim the bolt would also increase in diameter which is incorrect. The bolt pulls itself down the hole getting longer and thinner as torque increases. Limit is reached when bolt fails by combined effect of tension and torsion.

I say a bit is not a bolt, you push it through the hole, and it may get minutely shorter and fatter while under feed, and could get minutely longer and thinner if it was self feeding, but never longer and fatter, or shorter and skinnier in the same instant.

You have every right, and even the responsibility to question my method and result. I've pointed out possible deficiences in my test, and that the results were at the limit of what I can measure. Would you like me to cut up my bar and Flat Rate it to you?

I don't question your measuring ability or your honesty, but your method may have some errors you can't account for as well. The curtain rod holding the torque wrench may have flexed the mill overarm/knuckle, torque wrench socket may have messed with your draw bar, may have flexed the quill, aluminum test piece may have acted nut-like---- who knows????, but there is enough potential for error that I'm not going to build my life upon your result-or mine. If I absolutely must have a perfect hole I'll use a good bit with lube, care and follow with a reamer just like the books say.

I've enjoyed the debate, wish you well, can't think of anything new to say---though I'll try to if you don't give up your mad quest, and should really get back to work.

Scott

rdesign
10-17-2008, 03:33 PM
This one has puzzled me because I have done several drive shaft analysis where I get a result that was not intuative to me. I am very wary of FEA models and always try to check them against hand calc's and common sense, though it is hard in this case.

A bolt being tensioned, sure is going to stretch and by Poisson's ratio the diameter change can be predicted.

If a cylindrical member in in torsion and free to move along it's axis at one end It will shrink in length. This comes up in drive shaft design for me often..
The deformation scale here is greatly exaggerated..
http://i201.photobucket.com/albums/aa315/rdesign_photos/TORSIONDEFORMATION.jpg

The flutes in the bit definitely change things, not to say FEA is always right, but this model shows a diameter increase and a length increase!
Again, lots of scale exaggeration...
http://i201.photobucket.com/albums/aa315/rdesign_photos/TORSIONDEFORMATIONOFDRILL.jpg

for what it's worth,
-Jon

A.K. Boomer
10-17-2008, 09:38 PM
Thanks for taking the time to post Rdesign, For some reason the drill bit seems elementary to me, I look at all material as being elastic with varying structual shapes and then just imagine what happens when certain forces are applied, the effect is visually exaggerated in my gord and I can generally take a stab at a rough conclusion --- I already knew that the bit would get both longer and larger under free-floating torsion - I designed a test to silence the nay-sayers, proceeded with it - gave the results and still some will doubt, its all good but for me there is no doubt, just varying degree's of change under varying circumstances...

Pretty fancy graphics you posted:)

J Tiers
10-17-2008, 11:16 PM
Possibly the differing views are due to differing models.....

What happens in the middle of a shaft, or a drill, when the end is twisted, may not be 100% representative of what happens at the business end of the drill when the cutting forces are added in.

AK has gotten a bit assertive about "it's here, its fatter, get used to it"...... and his reported results indicate a small amount of that at some point back from the cutting edges.

But as I read the results, they don't come remotely close to explaining (alone) the oversize drilling issue..... a half thou on a 0.5" drill is an amount I would consider to give a very nice "perfect" on-size drilled hole....... if that was all that the error was.

Not only that, but you just would NOT intuitively expect to see much more from the drill, since the only part stretching much (radially) is the thinnest part of the web, the rest is considerably thicker. If you can get even 0.002 straight radial growth out of that short length, I'd be surprised. It would depend a lot on the exact flute shape, and width of the thinnest part of the web.

AK's test did NOT measure the tip, but had to measure a portion somewhat back of the tip, at least 0.312 back, if not more. I don't think he said exactly where he measured, but it is a fair statement that the measured part of the drill is not responsible (we hope) for much cutting, nor hole sizing.

If there is any significant effect on hole size, it should occur at the actual cutting edge. I have to question whether the flutes would do any real cutting at a half thou diameter difference, or even a couple thou, for that matter.... they are not sharpened, nor intended to cut. So any sizing should occur at the cutting edge primarily.

It is possible that he might have measured MORE increase closer to the tip for a variety of reasons.

I have a thought about a way to actually measure the tip under similar load conditions to actual drilling, and If I can figure out a workable setup, I will report the result. It will have to include some way of determining if the flutes are twisting (as I suspect they are, given the cutting forces) as well as radially growing. Possibly the diameter change at "front edge" vs "trailing edge" of the flute would be good enough to make a determination of which effect (twisting vs straight radial growth) is really responsible.

If AK repeated the test and measured the leading and trailing flute edges, instead of only the leading edge, it might give a little better picture of what is going on.

For the FEA, it is not clear to me how the load was applied, nor is it clear from the illustration how much increase was seen, nor whether the size increase was a straight radial increase or an effective increase due to a tilting distortion of the flutes under load.... Depending on how the load was applied, it might or might not be representative of actual drilling.

A.K. Boomer
10-18-2008, 10:16 AM
JT your post is a good example of level headed reasoning, My minds eye tells me that indeed the tip of the drill would increase in diameter even more so due to the fact of it not having leading edge reinforcement - but I have to add not much, I also dont believe and will state again that any of this is enough to make the difference that we see in size deviation but will once again add that its most likely one of many contributing factors --------- I could see it adding up to more than the sum of its measurements if the measurements deviated in a linear fashion throughout the length of the drill shaft, anotherwords if the torsion creates a wiggly worm effect throughout the length of the drill then I do believe the side flutes are fully capable of cutting out extra material to make room for themselves --- after all, there not just "guides" they have a very sharp cutting surface and a proper rake and chip removal system to achieve these results, I have to conclude here also though that I fit this back into the "contributing factor" category due to SVS's test and how the extra torsion applied did not mimmick this theory ---
Its all for learning and its all for fun, JT -- I would be interested in hearing how you would try to measure the drills tip whilst its under the torsional load, I have to admit im not seeing that one - or at least not seeing it done accurately -- for the record I will also repeat myself that I took 3 basic measurements from top to bottom of the drills flutes, it was not just done in one place.

J Tiers
10-18-2008, 10:47 AM
I thought of putting a couple of pins in a block, so that they would fit into the flute against the cutting edge but leave the outside of the drill free.

Probably one should use a soft material such as aluminum, so that the forces would tend to be simulated due to the edges taking as much as possible the same forces at same places as when cutting.... The main problem is the outside has a higher speed, and longer "lever arm" so probably the higher reaction force.

One would almost have to drill a hole in order to form a 'standard chip edge" and then stop the drill and machine away the outside of the part, leaving the bottom of the hole in place to load the drill against. That would probably be the most realistic, but you would have to limit the torque in order that you don't 'continue drilling" as you load the drill.

Scishopguy
10-18-2008, 02:31 PM
Thats not the way it works --- lets say you were pumping that half inch Jobber through some 416 @ 25 Ft. pounds of torsional load, What it means is that the second that torque is achieved the bit grows .008" longer --- it also means that you can set your stop up all day long to the static position of the bit and be off your mark every time, The "hypothetical " extra length drilling that takes place does not continue after the stop --- in fact as soon as the stop is hit there is no more material left to keep the bit in its longer than static position due to the material itself being the negotiator of the torsional distortion of the bit in the first place, Yet the fact remains that the bit was longer at that moment and the deviance it was experiencing cannot be achieved by simply re-engaging the feed again and again --- So --- your statement of the drill continuing to drill after is not logical, in fact -- the drill grabs till the very end and then recoils back in a fury, and the extra degree's of torque distortion that the bit was under dont do a thing in making the hole deeper, Yes the bit will twist in the direction of drilling and twist an extra 12 degree's -- but it will do nothing but grab air as it gets .008" shorter whilst doing this.....


What part of my test results dont you understand? or is it that you dont think I know how to measure?, If so then why should I trust your judgment on the holes you drilled? do you know how to measure properly? :rolleyes:

This may be a tough pill for you to swallow -- but once again, drills get both fatter AND longer at the same time, your going to have to come to grips with that and find a way of dealing with it...

I can see very easily how it can be both larger in diameter AND drill a deeper hole. Simply put, the tortion exerted on the spiral drill bit will cause the cutting edges to lag behind where they would be if they were not being twisted, thus causing the increase in diameter. When the depth is reached and downward pressure is removed, the twist will snap back to the original orientation, but there will be a little overshoot, that will cause the bit to momentarily become longer. Every action has an equal and opposite reaction.