Grinding chipbreakers into drill bits?

I've recently noticed a problem with drilling in steel that I want to
ask the group's opinion on. The problem is the drill grabbing in the
hole, and it only happens in one situation (unfortunately, a common
one). The problem happens when I try to drill out an existing hole
which is larger than the web of the larger drill, which you should
avoid when choosing pilot drills, but in which I often don't get a
choice (if it's modifying an existing part). Since the web of the
drill does not regulate drill feed, the drill tends to "bite off more
than it can chew", take a large chip, and bind. This happens most
often when drilling in the lathe, and tends to spin the drill chuck
rather violently in the tailstock taper. I know this isn't just a
general taper issue, since it works fine when drilling a full-diameter
hole in the same material, same setup, etc. The drill bits are brand
new, very sharp American drill bits in the 3/8"-1/2" size. Feeds and
speeds, etc are not the issue, nor is the material any more unusual
than mild steel. The primary machine is a 12x36 Atlas lathe, but this
has happened in other machines in the past as well.
My main question is about chipbreakers. When dealing with cutters
that tend to "bite off more than they can chew", such as parting
tools, I have had good success grinding appropriate chipbreakers into
the top edges. Since I can visually confirm that the binding is due
to the taking of an inappropriately large chip by the drill cutting
edges, it would seem that this is an ideal solution. However, where
would I go about grinding a chipbreaker into the cutting edge of a
drill? I do have a set of drill bits with small vee grooves factory
ground perpendicular to the middle of each cutting lip, but I don't
recall if they were advertised as chipbreakers or some other purpose.
I can't actually try using them since the proper sized drill from that
set has long since been reground normally in the course of normal
My question is whether or not grinding such a groove into a normal
drillbit would solve the problem and, if not, if there are any other
suggestions that people have to solve the problem?
Thanks in advance,
Reply to
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Grinding a chip breaker in a twist drill may not be in your best interest. Here's a couple things to consider.
Drills are circular ground, so it's the margin that establishes the diameter of the hole you drill. If, in the process of grinding the chip breaker, you grind past the margin, the drill will cut undersized until it reaches the full diameter margin, and likely bind, although it's also possible it will simply open the hole to full diameter. All depends on how you grind the chip breaker, and if you end up with relief in the proper areas.
The second consideration is, depending on how you grind the breaker, you may actually be encouraging the hogging by increasing the rake angle. Rake angle on twist drills is created by the helix, and is already rather generous.
Are you familiar with the procedure used to blunt drills for drilling in brass, and even cast iron? If so, what would serve you well is to grind the drill in a similar fashion, decreasing the rake angle. Don't grind it until it's zero rake, just reduce the angle. That will discourage the drill from hogging and likely perform without the problem you spoke of.
Be mindful that when you grind your drill, you face the same problem as when grinding a chip breaker. You must not grind past the margin. It does no harm to grind more towards the web, removing only a bare minimum at the margin.
Reply to
Harold and Susan Vordos
If I understand correctly, you just want to open up a predrilled hole? If you need to go more than .010-.020 " (more than a reamer can handle) you should get a core drill.It's a cross between a drill and a reamer. No cutting point, 3 to 4 flute and will leave you with a straight and smooth hole.
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down to core drill. Clay
Reply to
How's your freehand grinding skills? What's the requirement for precision?
You could grind your own bullet point on the drill bit, to suit the existing hole, or you could grind a pilot on the nose of the drill.
Pretty common thing to find on an aircraft sheet metal guy's bench.
You can save yourself a lot of time that would be spend dickin' around with chipbreakers, by adjusting the rake of the cutting edge. Most often, the rake is too much, and adjusting it to cut a little more of a scraping action, will slow down the "bite" that it takes. When cutting brass, the edge gets a flat that is parrallel to the axis of the drill, effectively making a zero rake angle. You can adjust between the originall angle, and zero as cutting dictates.
Easier than that, though, is to arrange to have a piece of similar material behind the workpiece, so that the cut proceeds through the work without having to deal with the lips digging in.
Some ideas, anyways. Use the ones that you feel will work for you.
My life is too short to be muckin about trying to grind matching grooves on drill lips.
Cheers Trevor Jones
Reply to
Trevor Jones
This is a pretty common problem, even when drilling soft metals like aluminum. Simply grind a very small flat along the cutting edge of the drill so that the sharp edge formed by the flute and the end of the drill is made a narrow flat parallel with a center line drawn through the length of the drill.
Bruce-in-Bangkok (Note:remove underscores from address for reply)
Reply to
Bruce in Bangkok
I have a 13x36 lathe and have never had the problem of the chuck arbor spinning in the tail-stock taper when drilling as you describe. I modified my chuck arbor by putting a protruding radial pin in the arbor right behind the chuck. It engages a notch milled in the mouth of the tailstock ram. This pin prevents the arbor from spinning in the taper. A good fitting taper prevents the arbor from being pulled out easily. The drill feed rate is controlled entirely by the tailstock hand wheel in my experience. I have often opened up holes as you describe without problems. Of course, the drill torque load is now carried by the tailstock antirotation key, so expect a little extra wear there.
Reply to
Randal O'Brian
Grind the cutting edge flat for zero rake as Bruce suggests will tame those large drills fine. Works well in the harder steels too.
Reply to
Your problem is the lathe, not the drill bit. The only way this can occur is if the holding chuck of spindle has fore aft play or play exists in the tail stock allowing the drill chuck to jump into the work. If you are using the correct MT drill chuck mount, it will have a tang on the end to prevent this rotation. Repair the lathe by removing the play and leave the drill bit alone. Steve
Reply to
Steve Lusardi
What is happening is that the rake on the cutting edge of the drill is actually digging into the bottom of the hole and pulling the drill bit into the material being drilled. If there is only a taper to oppose this "pulling" then the chuck or drill bit will be pulled out of the tail stock.
This is not at all uncommon when drilling soft materials or using a large pilot drill.
Bruce-in-Bangkok (Note:remove underscores from address for reply)
Reply to
Bruce in Bangkok
Bruce, That can only happen if your tapers are worn out/ not locking properly. Of course this does happen in a mill, but you have draw bars to counter that. It should never happen in a lathe, because the forces are not high enough. If it does, the most likely culprit is excessive back lash in the tailstock screw. This tendency can be somewhat countered by tightening the tailstock spindle drag/ lock lever. Abusing the grind on a drill bit is a bandaid, not a cure. Steve
Reply to
Steve Lusardi
the key thing you added, above, was "drawbar" - OP doesn't seem to have one - adding a drawbar might solve the problem, but most metal lathes I'm familiar with don't have a way to add a drawbar to the tailstock. Boring rather than drilling will also fix it
Reply to
William Noble
Got it. I have used zero rake (or at least minimal rake) drills before in sheet metal and have had good sucess. I believe that those will work well. That small change shouldn't be difficult to make with my bench grinder.
I have read about core drills, and undoubtably they are the best way to go, but out of my budget for the few jobs that need it. Modifying an existing drill seems like the ideal solution.
About my lathe--while my tailstock taper and associated parts do need a bit of work (it is a 60+ year old lathe, with the original drill chuck shank), it's more than that. The fact is that I can visually see the drill bit taking an excessive chip. If it weren't the taper slipping, something else would give instead.
Thanks to everyone for their comments. I'll let everyone know how the project turns out.
Reply to
Top posted to continue the sequence of messages.
Sorry but you are wrong about a taper holding, come rain or come shine. I have had taper shank tools fall out of hand drill presses, radial drills, vertical mills and lathes due either to chatter or the rake pulling the drill into the work faster then the feed rate. That is the reason vertical milling machines almost always incorporate a draw bar - to hold the taper shank tool in place.
To illustrate the forces involved try this. Drill a 7/16" hole in a piece of 1/8" thick commercially pure aluminum using an air or electric powered hand drill. Now change to a 1/2" drill and try to enlarge the existing 7/16" hole to 1/2" through only half the thickness of the aluminum. Odds aren't that you won't be able to do it (at least not without considerable practice) as the rake angle digs in and pulls the drill into the metal with considerable force.
Finally a drill is just another cutter and rake and clearance angles are determined by the material being cut, cutting speed and the condition of the machine being used.
Bruce-in-Bangkok (Note:remove underscores from address for reply)
Reply to
Bruce in Bangkok
Just a small comment. You probably don't need to grind a very wide flat on the cutting edge. Then when you are finished with ther project it is not a big job to regrind the drill to return it to the normal rake angle.
Bruce-in-Bangkok (Note:remove underscores from address for reply)
Reply to
Bruce in Bangkok
I have the same problem with my 40+ year old lathe, but only with some combinations of feed pressure and size ratio. Simply switching to a different size drill bit usually fixes it. I get good results opening up the hole in 1/4" steps and then feeding the final bit which may be removing only 1/16" quite slowly. For some reason taper shank bits work better for me than a chuck. Maybe having less rotational inertia they don't amplify slight chattering.
Jim Wilkins
Reply to
Jim Wilkins
Okay, will do.
I have a few taper shank drills, but I hardly ever use them anymore because the were cheap imports (all I could afford), and the American straight shank drills I bought at a closeout sale of a machine tool supply store cut so much better, it far outweighs (at least as far as I can tell) any advantages of my particular taper shank drills. Now if only I had...
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