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07-04-2009, 08:36 AM #1
BEST general procedure for drilling holes?
OK, as you guys all I know I am new to machining (if you can call it that at this point) and I am trying to learn proper procedure for drilling holes. Lets talk about "regular" materials, nothing crazy. Just aluminum, steel and some stainless. I have a few questions about various different aspects here:
1: Starting. I was recently confused about the proper procedure for starting a hole, so after some searching, It seems that a center drill is NOT a valid method for piloting a hole, and that I should be using a spotting drill with an angle GREATER than that of the drill bit. Am I correct in this? Would it be the end of the world if I did use a center drill to get my holes started? I am on a major budget here and can not invest in tons of different length/angle drills. I am also learning for my own personal benefit, I am not a machinist.
2: Sizing it up. Whenever I drill by hand I always drill a small pilot hole, and final drill to the hole I need. It seems that this is not the proper procedure? What I understand is that I am better off just getting the hole started by the proper method above, and drilling with the correct drill size? Also, if a regular twist drill should not be used to enlarge a pre-existing hole, what is the proper method? I am not talking about going up a few thousandths, I mean like going from a 1/4 to 1/2 in hole?
I ask this on here because you guys know your stuff, and many older machine shop txt books I salvaged a few years ago suggest using a center drill for getting a hole started and now I question what these books tell me.
07-04-2009, 09:07 AM #2
07-04-2009, 09:42 AM #3
Would be helpful to know what you are drilling the holes with. A mill and drill press are very similar- but a lathe is different as it is the work that rotates.
The other thing we need to know is what level of locational accuacy and diameter accuracy are needed.
I must have missed the post on the center drill vs spotting drill. I will use a center drill- but only the tip. I have not measured them- but looks to be about 118. They are very ridgid. I will use them with a center puched hole at times- as the smaller diameter picks up the hole easily. I also use them like a center punch- but they give a broader hole- which is easy to pick up with the larger drill.
I also use spotting drills. Depends on the situation.
07-04-2009, 09:47 AM #4
Sorry, I was pretty much refering to mills/Drill Presses.
Also, As far as locational accuracy, lets say within .005? Diameter not quite as critical. More of a "good enough" mostly for tapping parts. I am more concerned with everything lining up correctly when I go to bolt things together.
07-04-2009, 10:43 AM #5
There have probably been a thousand times the number of holes started with a centerdrill versus a spotting drill. For super accurate hole location, you pick up the hole location via coordinates, then use a very short, large diameter drill (spotting or centerdrill) to avoid walking and flexing wen you spot the location. Then switch to the drill of your choice and proceed to make the hole.
For typical holes for bolts and structural work, just centerpunch the location, load up a drill bit, get it to where the drill doesn't move when you touch off on the punchmark and poke the hole.
Speed is very important to avoid breaking off or burning up drills. You have to maintain proper surface speed (which changes with spindle speed and/or drill diamter change) for the material you are drilling. Here is my simple rule of thumb... 500rpm for a .500" hole in steel using a HSS drill bit. Double this speed for a 1/4" bit (.250) and half for a 1" bit. You can interpolate sizes between, but don't get overly complicated with the figuring.That's a bit slow if anything, but better to take a bit more time drilling a few holes than to burn up your drills.
For aluminum, triple the above figures. For stainless, half them. For carbide, double again.
Speed burns, feed breaks. If the corners of your drill lips are burned, slow it down a few hundred and try again. If you keep wringing the drill off, kick the speed up and take it easy on the feed handle (or back off feed rate, if using powerfeed). Typical mistake with small bits is underspeed and too much feed. Conversely, most run the speed to high and feed to lightly on larger bits. The light feed is often required on lame little drill presses using 1/2" shank large diam drills, which stall the motor.
Run too fast and too light a feed on stainless and it'll work harden before you can say, "Oh Sh..." When that happens, you'll either need carbide or start a new part. Don't ever let stainless rub without cutting a full chip. Dull drill will do the same thing.
07-04-2009, 10:50 AM #6
If you have centre punched the hole location it is possible to get the final drilled hole with +/-0.005" of the center location. I use a small spotting drill so the web across the point is small than the center punch mark because then it self centers, if the drill is largish such as above 5/8" I may then use a larger spotting drill in the first spot mark so the spot drill depression is large enough to pull the drill to center.
But this just gets the hole fairly good on the center punch mark; laying out hole locations and center punching them to within +/-0.005" of the desired location is a bigger challenge and I am pretty sure my eyes and hands are not up to it these days (actually I doubt they ever where, I never trusted myself to better than +/-0.01 doing hand layout).
07-04-2009, 10:51 AM #7
You can just use good quality split point drills and forget the predrilling in most cases. Split points do not wander about, they just go where they are pointed, unless there is a center punch mark that is off-center. That will cause trouble. If you are using a vertical mill and have the part clamped down so the drill lines up where you want it, don't use a center punch. I don't have a DRO on my mill, so I use scribed lines and a centering microscope to get the part in the right place. You can just drill it to size with one bit. That is a great time saver. If you are worried about the hole being larger than your bit, then you can drill with one size smaller first and finish with a bit of the size you want. Use a reamer if you need even better size control.
Tiny drills usually do not have split points. In the mill, I will use a center drill to start a dimple on my scribed lines. The reason is that a center drill has a relatively fat shank that will not flex as the small point starts to cut, even though the point is not split. The small drills without split points do tend to flex and wander about before they start to cut, so they will often start off to one side of the mark.
If you are wanting to drill into a center punch mark with the work free to float on a drill press table, then you are not being fussy about hole location or probably size. The split point drill bit will still do the job without using a spotting drill.
07-04-2009, 11:30 AM #8
This wouldn't apply on a machining center with carbide tools but that isn't what we are talking about here. These "simple", "general" pointers always start debate because there are always exceptions.
I also admit I alway use center drills to start holes on a lathe because they are handy, right there in a stand. I think they tend to start better without pushing off when you try to start on the tit of a faced part at zero sfm. No matter how good of lathe and lathe tools you have there is some type of very small iregularity right at dead center.
07-04-2009, 01:09 PM #9
Using a spot drill with greater-than or equal angle to the drill is critical with standard twist type solid carbide drills, less an issue with HSS drills. The main reason is tip chipping on the carbide drills should the two leading edges of the lips dig in hard at the beginning of the cut. The HSS has a much higher transverse rupture strength than carbide in most cases.
I have often used "just the tip" of a combined drill & countersink type of center drill, no problems. I've also used 90 degree spot drills with both 118 and 135 degree HSS and cobalt drills with no problem. I rarely use the combined drills now unless it's for their true intended purpose, making center holes for shaft work between centers.
07-04-2009, 01:44 PM #10
Thanks everybody. My second question. If I am going to be using a spotting drill, assuming Im using the correct angle, Is it critical that I use the exact same size? I mean, would I be able to get away with owning 2-3 center spotting drills and just varying the depth to which I drill in order to get a "close enough" start on the drill point, or do I need to start the hole to its full size in order for them to be effective?
07-04-2009, 02:49 PM #11
Procedure for drilling holes
First off, it depends on what machine you are working & what tolerance you must keep. For lathe-you must center drill w/ the largest one feasible then proceed to drill & ream if necessary. For drill press-layout, center punch, center drill w/ the largest one feasible then drill & ream if necessary. Hold your work in a decent vise & clamp the vise to the table. Vertical mill-Center drill w/ the largest one feasible then drill & ream if necessary. If the hole is too small & an associated center drill would have too small & delicate point, then use the largest spotting drill feasible
07-04-2009, 03:57 PM #12
You can 'get away with' two spotting drills, 1/4" and 1/2". Use 1/4" for any drill size up to 1/2" or so and use 1/2" for any dril size from 3/8" upwards. You really only need to spot out to a diameter about two or three times the web thickness on the drill. You can of course spot deeper so that the spot is larger than the drill and creates a chamfer on the final hole; this is useful for tapping.
07-04-2009, 08:24 PM #13
07-04-2009, 09:29 PM #14
The "best" method?
Use sharp, quality, drills.
Always use lubricant.
Start the hole on center.
Use the proper feed; don't 'worry' the hole.
If you have to pull the drill out to clear chips, use a magnet or air to clear the hole; don't recut chips.
That's the critical list. The choice of center drill, spotting drill, predrill depends on the machine, the setup and the work. On a drill press I'm going to use a six inch center drill cause I'm going to follow it with a jobber length drill and I need to get the drill in and out. On a mill I can use a spotting drill with a 1/2" shank for rigidity; then I'll follow with a stub drill if posible. You've got to be thinking forward as to what operation follows next. I'm not going to use a stub spotter in a collet if the main drill is a jobber length held in a chuck. That would mean cranking the knee, which is to be avoided unless I missed my workout that week and need the exercise.
07-05-2009, 03:30 AM #15
07-05-2009, 06:11 AM #16
One thing I think is important no matter what type of spotting drill you use, let it dwell for at least a few revs to remove tool pressure. With CNC I used a dwell to do this, I am convinced it gives you a more accurate hole location.
I only use center drills to spot using the 118 tip, center drilling up in the 60 degree angle in a HARD material will show you how hard that is on a drill when you hear it grind and screetch and maybe chip the corners off the drill, then use a proper spot and see the drill start without any protest.
07-05-2009, 07:04 AM #17
All good stuff above Jim, you mention .005", so heed Walter's, "I learned a long time ago to use a prick punch on the layout line then look at it with a magnifying glass. You would be surprised how far off you can be by "eye". Once the prick punch is in the right spot then follow up with a center punch and check again. I've nailed holes within .005" consistently with this method."
The purists will yell but MikeC is right, "There have probably been a thousand times the number of holes started with a centerdrill versus a spotting drill. For super accurate hole location, you pick up the hole location via coordinates, then use a very short, large diameter drill (spotting or centerdrill) to avoid walking and flexing wen you spot the location. Then switch to the drill of your choice and proceed to make the hole."
Walter, (and everyone else who cares) "walks" the prick punch across, using a magnifying glass to acquire the correct location, creating a larger mound of displaced metal on one side than the other. MikeC's "misuse" of the center drill, will be as effective at chamfering that mound off to center, as the purists spotting drill.
Most holes aren't of the deep "gundrill" variety. I have a 1/16-1/2 drill index of "shorty" drills", (and I keep them sharp). For say 5/16" and up, (smaller aren't stiff enough), I accurately punch mark, then "kiss" the rim of the center punched hole for equalizing chamfer, then drill on through. This for holes that don't require supreme accuracy. For that, you've got the wrong machines anyway, a tight Moore jig bore....... "Screw machine" drills are short and stiff, should you run across a batch for a great price.
Of machine horsepower: A guy with a 40HP, 18" column, 6' arm radial drill may insist, "drill the hole in one step, no pilot hole!" He doesn't mention his big machine, he knows most shops don't have one, he's a smart-assed snob and he knows it. "Na-na-na- naaaa" is dancing around in his wee little head.
Your machine says, "max drill size 3/4". You must have a 1" hole, do you:
a) Commit suicide, 'cause your task is impossible?
b) Frantically shop for the smart-ass above and farm it out?
c) Carefully spot and drill a pilot hole that is approximately the diameter of the 1" drill's web thickness?
The advise in this thread, some common sense and experimentation, will get you to your goal.
Edit, Willbird posted while I was writing this, yeah, "dwell" is what I was looking for.....
07-05-2009, 07:13 AM #18
Tank refered to countersinking if you have to drill into a curved surface.
This can be done using a center cutting endmill.
If you drill deep enough and use a drill bit the same diameter you will not need to center drill. The drill can only cut on the end, and this will be held in location by the sides of the hole left by the endmill.
If you find you have started a hole which os "off" in location you can use the endmill to drill an overlapping hole where it should be. The endmill will not follow the existing hole. It acts like boring bar. This can be a useful way to use an endmill with a chipped flute.
( let me clarify about the endmill not following the existing hole. This could happen if the endmill is not sharp, is not ground square to the direction of cut, and it is a long endmill. Boring bars an deflect also.
Not a part of basic drilling but at some point you ought to read up on "D bits". It is not hard to make your own to the size you need. They can be used a reamers- but can also be used to enlarge or to staighten an existing hole which curved as it was being drilled.
07-05-2009, 07:39 AM #19
(I know there's a pun in there somewhere:"Too many flutes spoil the "?"") but I'm really curious about this.
07-05-2009, 09:50 AM #20
"A standard drill will cut big unless the point of it is doing some cutting."
I'm going to have to take exception to this statement. A typical twist drill will make a bigger hole with the chisel point in contact than when re-drilling an existing hole. As an extreme example of why, take a drill with one lip sharpened .020 longer than the other and drill two holes.
One is started just by plunging in. Due to the long lip, that hole is going to be .040 oversize, until the point breaks through and the drill self centers in the hole. The last 3/16 -1/8 of an inch of that hole will be smaller, and very nearly the diameter of the drill. Once the point is not cutting, the margin will guide the drill straight.
Similarly, drill a web-thick pilot hole and then run that same improperly sharpened drill through. You will find the hole is the right size, as there is no point contact to cause the lips to walk.
If your twist drill is properly sharpened, it will make no difference whether you use a centerdrill or spotting drill to start. Just remember when hand feeding that when entering the centerdrilled hole, you are cutting less material, so feed rate will need to be a little higher, until you hit the bottom of the start. Don't let the drill sit there and chatter and walk or your hole will indeed be funnel shaped. Go like you mean it. With powerfeed, it's just a matter of throwing it in gear and watching it go.
Also don't start your hole as big as the drill you intend to use. All you want the start to do is guide the drill so it doesn't walk until the margin (non-cutting outer edge of the drill flute) is in the work. 1/2 the diam of the twist drill is plenty. It'll start on target and the chisel point will go to the path of least resistance... the center hole.
"Your machine says, "max drill size 3/4". You must have a 1" hole, do you:
a) Commit suicide, 'cause your task is impossible?
b) Frantically shop for the smart-ass above and farm it out?
c) Carefully spot and drill a pilot hole that is approximately the diameter of the 1" drill's web thickness?"
Bob, yeah, you can worry your way through the work with a 3/4hp HF drill press, but the hole is probably not going to be very smooth or accurate. If it is stainless or some other work hardening material, you may indeed NOT get that hole made, and might scrap a part in the process. If you have a bunch of these holes to do, you are going to be losing money and/or wasting time. A properly sharpened 1" drill can bring a 1.5hp drill press to a full stop if fed at anything near the proper rate. A dull one is even worse.
Other problem with a small press is usually speed. Most cheapo presses have a low speed of about 350rpm and a top speed of about 2000rpm. Using my rule of thumb above, that means just a fuzz over 3/4 is as big a drill as you can run without overspeeding and burning the corners of the lips. Add frequent hangups and chatter due to low power and the very minimal feed rate, which causes the drill to rub and generate more heat, instead of burying in the work and shedding the heat into the surrounding material and chips... and you have a totally ruined drill in no time, as well as a hole that looks like it was cut with an axe.
If that same hole is to be drilled in aluminum, it would seem to be a lot better situation, but no... You now have to triple the 250rpm spindle speed. No problem, the HF press can run 750rpm with ease. 3/4hp at 750rpm is never going to pull a 1" drill at anything near a reasonable feed rate, even in aluminum. There just simply isn't enough torque to pull the cutting forces of two 1/2"+ long lips buried in the work.
I do indeed profess to be one of the radial drill snobs, lol. The ones I run and own are nowhere near 40hp, but even a 3hp radial down around 120rpm can REALLY move metal. Powerfeed means no rubbing and workhardening, no tired arm or uneven feed rates, and no snatching on breakthrough. I have a little cheapo press that probably does 90% of the work in my home shop, but when holes over 1/2" are required, I fire up the Fosdick.
At work, I have not used the cheapo press once since we got the 5hp G&L radial. I have popped holes as small as 1/8" and as big as 2" with that one. Also very nice to drop down low and power tap.
I have worried big holes with a cheapo press millions of times in the past, and the first big hole I popped with a radial ruined me for life. You have proper speeds to not burn up the drill in stainless, you have enough power to pull high speed cuts in aluminum, you have powerfeed to keep the drill loaded properly and prevent breakthrough grabs, and the machine is rigid enough not to flex under load. I have also noticed a marked improvement in drill life, as well. I can drill dozens and dozens of holes in steel and never have to sharpen a bit. The proper speed and feed rate result in less damage to the drill.
1" hole in 3" steel? 250rpm on the gearbox, .014" per rev. I prefer Morse taper shank drill for holes this big, as a 1/2" shank will just spin if it tries to hang up, even in the Jacob's 18N. Nail the work to the table REAL good, adjust the arm and head to location and set the feed stop so you don't hole the table. Spindle on, hit the powerfeed and stand there whistling with your arms crossed. About 55 seconds later, the powerfeed will kick out, you retract the spindle and shut down. I figure this could be a 55 minute job on a cheapo drill press, if everything goes right. The same hole in the same thickness aluminum is under 20 seconds. Stainless will take twice as long on the radial, and would probably never get finished on a cheapo.
If you don't have a 1" drill, you can farm it out to me and have a few dozen holes done and in your hands for less than the cost of the drill. If it's just one, I might take pity, be nice, and just freebie it.