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Reason for various drill bit speed for different drill bit size?

  1. Apr 6, 2013 #1
    In the shop I was taught that , for a larger drill bit, use slower speed, vice versa

    I would like to know for what reason this is? I understand by using slower speed for bigger drill bit and vice versa, we can maintain the same/similar velocity at the outer edge of the drill bit, (omega * r = v)

    but for what reason do we want a certain drill cutting edge velocity to be in a certain range?

    I first thought it was the material strength, harder the material, faster the speed , but it didn't make sense to me since the machine would just output whatever torque require to keep the drill bit at constant velocity while drilling, so the material hardness shouldn't have anything to do with the speed.

    so is it for safety? chip removal rate? if not, what?

  2. jcsd
  3. Apr 6, 2013 #2
    or, come to think of it, if I stab a knife in some materials, from experience it does seem that if i try to cut through the materials(if it's hard) faster it would require a larger force, so maybe there are some viscous effect in material cutting? or friction?

    if so, does that mean if in some weird material with super low internal kinetic friction coefficient, it would makes no different if i cut it faster or slow?
    Last edited: Apr 6, 2013
  4. Apr 7, 2013 #3


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    I'm not a machinist, but I have used most metal working devices from lathes through drill presses, shapers, surface grinders, etc..
    As far as I know, the speed of any tool is governed by how much material is being removed in any given time period. A big drill bit takes off a lot more than a small one, so slowing it down keeps friction heat, tool wear and tear, and other nasties to a minimum.
    For the record, if I have to make a big hole I start with a small bit and then incrementally increase it until I reach the desired diameter. Also, remember to oil the hole if you're working in metal. I cuts down on heat and helps to auger the chips out of the way.
  5. Apr 7, 2013 #4

    so it has more to do with reducing frictions? since bigger bit has larger contact area (per second) with material so if we use the same speed as small bit it will get heated up more is what you are saying?

    Should I also be concern about "jamming" or should i say "jacking" the drill bit if i am trying to drill too fast or is it not usually a concern?

    thank you so much for you reply!
    Last edited: Apr 7, 2013
  6. Apr 7, 2013 #5
    It's simply so you don't burn out your drill bit. The friction of drilling creates tremendous heat and if your bit gets above a certain temperature it will lose it's temper and suddenly become a softer material. Then it will lose its sharp edge and create more friction making the bit and workpiece even hotter. If that goes on long enough the bit and the part can actually get red hot and end up welded together. You don't even want to get a fraction of the way there, you want your bit to last long enough to pay for itself in material removed.

    The smaller the drill bit the less material the outer edges encounter per revolution (the circumference of a small hole is obviously smaller than a larger hole). The smaller the bit the more you can jack up the RPMs without creating more friction per revolution.

    As far as material goes, the harder the material the slower your RPMs have to be. Hard materials create more friction.

    Danger mentioned lubricant. What lubricant you use, and how it's applied, modifies all this. In the last shop where I worked the parts were flooded with a flow of this dedicated special lubricant the whole time you were cutting or drilling. That allows for faster feeds and speeds. If you're dabbing oil on manually with a brush, you have to go slower on your RPMs and feeds.
  7. Apr 7, 2013 #6

    got cha: so bigger drill bit, more contact with material, more heat, so slower speed.

    my friend said that if you go too fast you could "jack" up the material, just because if the speed was too fast then there's more material you are scraping, so more bonds need to be broken per second and a higher resistive force/torque for higher speed, is it true?

    thank you for reply!
  8. Apr 7, 2013 #7
    In the shop they used to call this "crowding the drill". That's when the removed material gets jammed up in the flutes and doesn't emerge from the top end. This is a problem because the material getting sheered off by the cutting edges at the tip has nowhere to go. The bit will just ride on the bottom of the hole not cutting anything, and the material crowed in the flutes will rub against the sides of the hole, heating everything up and ruining the finish, and possibly ruining the dimensions of the hole (making the diameter inconsistent).

    Preventing crowding is not so much a matter of speed as good lubrication and backing the drill out of the hole periodically to "clear the bit", i.e., letting the material that's starting to crowd the flutes get flung off. Generally, the deeper the hole the more times you have to clear the bit.

    While it's true the higher the RPMs the faster the bit will crowd because you're removing material faster, that's not why you use lower RPMs for larger bits. As I said before, larger bits cover more circumference per revolution and get hotter for it, experiencing more friction per unit time. Lowering the RPMs for larger bits lowers the friction experienced per unit time and keeps the bit cooler. The cooler the bit stays, the sharper it stays. Crowding is taken care of by lubrication and clearing the bit, not really by low RPMs.
    You're welcome.
  9. Apr 7, 2013 #8

    Wow, that cleared my confusion. completely, you sir have taught me a valuable lesson!

    thank you again, I hope one day I will be as knowledgeable in machining as you.
  10. Apr 7, 2013 #9
    My first job in the machine shop was drilling three holes in a mass of aluminum parts. It took three days to finish them all. Many thousands of holes. You learn a lot about drilling holes with practical exercise like that.
  11. Apr 9, 2013 #10


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    Zoob, since you mentioned feeds, maybe it would be cool for you to elaborate upon that. I know about it in practice, just going by feel, but I don't think that I could possibly explain it as well as you can. All that I can think of to say is that you have to be careful not to apply too much downforce for whatever speed you are using.
  12. Apr 9, 2013 #11
    Optimum speeds and feeds are determined by engineers for all combinations of cutters and materials. You can get little charts of feeds and speeds for common cutters vs common materials. The balancing act they're trying to acheive is to make the cutter last for an economical amount of work at an economical pace.

    I was a manual machinist and, like you, got to the point where I did everything intuitively. The charts assume a new, sharp tool. More often the foreman hands you some battered old regrind. So, you watch your coolant and your chips. If your coolant is boiling off too fast you reduce feed or speed, and if the chips are getting brittle or blue at the edges, you reduce. Otherwise you try cranking things up incrementally. Somewhere there's a zone where you're removing material as fast as possible without overheating things. Generally, for any given depth of cut, the faster your cutter is going on the mill without problems, or faster your part is turning on the lathe without problems, the faster you can try to feed.

    If a person is programming a CNC, though, they have to know how to do it all by the charts. They can't go out and test each cutter that will be used and see what speeds and feeds they like for it. They program the recommended ones.

    Then there's the matter of finish, where feeds and speeds have nothing to do with avoiding heat. Generally the finish cut is fed into the cutter on the way slow side in order to leave a smooth finish on the part, and, since you're also removing very little material, so you can crank up the speed on the lathe or mill spindle without overheating things. The degree of finish they want is in the print. Some parts can be left pretty rough, others have to be very smooth. When in doubt, we always erred on the smooth side.
  13. Apr 9, 2013 #12


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    Thanks. That was informative to me, and I'm sure even more so to the OP.
  14. Apr 10, 2013 #13
    You're welcome.
  15. Apr 11, 2013 #14
    Yes I did! Thank you very much to you both!
  16. Apr 13, 2013 #15
    Excellent advice here from the older guys who've been doing this for a long time.

    I'd just like to reinforce the point that you're cooling your bit, not lubricating it. The idea is to vaporize the coolant, hence draw heat away from the bit and workpiece. If your out in the field drilling something in a hurry you can do it faster if you keep spitting on the point of contact. Drilling steel with and without coolant is quite noticeable on a mill.

    On the other hand, factories with "big" manual lathe operators would take very heavy cuts, enough to throw sparks like fireworks even with coolant. The profit of getting the part manufactured and out the door far outweighed an abused toolbit.

    One of the things that makes a manual machine shop a religious experience is all that haze and odor from the coolant.
  17. Apr 13, 2013 #16


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    Now that I have COPD and use oxygen, I probably wouldn't enjoy that quite as much.
    You made a good point that brings us to scale. What you mentioned about the guys going balls-to-the-wall and cooking off their toolbits is acceptable for large pieces where precision isn't critical. The stuff that I worked on had tolerances down to 0.0001". Any appreciable wear on the bit could make a part unuseable. (I refer here to lathe work; drilled holes weren't usually more than about 0.001" accurate.) I should mention here as well that this was just high-school shop class; I expect that commercial work requires far more accuracy.
  18. Apr 13, 2013 #17
    Anything coming off a manual lathe that requires a "tenth" (ten thou) precision is going on an OD grinder to finish. Parts that critical will have a surface finish called out as well.
  19. Apr 13, 2013 #18
    I don't know if I should start a new thread for this, but since all the experts are here now, I would like to ask another question if you don't mind:

    I just got shop certified today and am ready to work on the machines, but there are lots of question that still need answered.

    for instance : Why is it that climb cutting for a mill is preferable than conventional cutting (the physics behind it). if someone can give me a good intuition on this it would be nice, after reading countless google results and wikipedia, all I get is that conventional cutting have poor finishing while climb cutting doesn't, but what is the physics behind it?

  20. Apr 14, 2013 #19
    Here's what I was told: With conventional milling, the cutter will try to dig itself into the part. With climb milling the cutter is trying to push itself out of the part. In the latter case the end mill has a tendency to remain pushed slightly away from the surface it has just cut as it "climbs" onto the next bit of surface it is going to cut, so it doesn't rub against it and mar the finish.
  21. Apr 14, 2013 #20

    that makes sense, qualitatively i guess, so you are saying the endmill pretty much is bouncing around the material "cut, bounced off, cut bounced off" so that's why it has better finishing? and the reason why this happens to only conventional cutting is because there are more resistance/material "behind" the trajectory of the cutter?

    I guess that makes sense.

    So why is it that climb cutting require more force? my TA just told me is because the cutter is cutting more material onset of the cut so it is like a collision that needs higher force to steady the velocity of the endmill. But if the force is a function of how much material is being removed per second, i really don't see how the same could not be said about conventional cut.

    UNLESS, the force is actually a function of rate of change of material removal rate. then it would make sense.
    Last edited: Apr 14, 2013
  22. Apr 14, 2013 #21
    I think another way to explain it is; on a conventional cut, the cutting edge makes contact directly opposing the motion of the workpiece. On a climb cut, the cutting edge makes contact perpendicular to the the line of motion of the workpiece. This tends to push the tool bit away. Also, I think the linear distance traveled by the cutter while in contact with the material is half that of the conventional way.

    The climb cut is slower and removes less material. But since it provides the nicest finish, it should be done on your last pass. In my experiences, aluminum looks like crap if not climb milled.

  23. Apr 14, 2013 #22
    No, there's no bouncing at all. The side cutting edge of end mills has a spiral configuration. That eliminates sudden pressure differentials. It is continuously flexed away from the part. The point of contact travels vertically from the bottom to the top of the endmill and the next cutting edge should engage before the previous one is removed such that contact is continuous.

    The kinds of cutters they use on horizontal mills might experience the kind of "bouncing" you describe on a shallow cut, but I never worked on a horizontal mill and couldn't say.

    I was never told climb milling required more force, and I don't know if that's true. Anyway, force is nothing a machinist ever has to calculate, and I don't particularly see what effect more or less force would have on finish. The bad finish of conventional milling seems just to result from the cutter rubbing against the surface it has already cut, smearing whatever small chips are around onto that surface.

    Once you actually do both climb and conventional milling it should become clear how the cutter is pushing itself away from the part in climb milling and trying to dig itself into the part in conventional milling.
  24. Apr 14, 2013 #23
    Ah, good idea! A picture.

    I had the same experience with aluminum: a conventional cut leaves an almost fuzzy finish. The same is true of steel, but it's not as bad.
  25. Apr 14, 2013 #24

    wait, what tends to push the tool away? isn't the cutting edge always experiencing a force from touching the material? I heard that climb cut actually require more force because the onset contact require more force to steady the endmill?

    also im working out the geometry/math right now to show the linear distance travel is half of that of the conventional cut (if this this true that means the max chip width of the climb cut is actually half of that of conventional cut?)
  26. Apr 14, 2013 #25

    forgive me but all these time i was imaging a flat blade cutter with sharp cutting the material (not spiral, but like a straight boss extrude in outside of the page)......well that changes everything, everything seems to make much more sense now, let me go process this.
    Last edited: Apr 14, 2013
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