Why won't a cobalt 1/2" bit drill this mild steel?
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  1. #1
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    Question Why won't a cobalt 1/2" bit drill this mild steel?

    With great difficulty I used a cobalt bit to drill a 1/4" pilot hole in this 3/4"-thick steel (which I assume is mild, but which could be hardened). Now I'm trying to cut the finished 1/2" hole using a brand-new cobalt 1/2" bit and after many minutes of pecking and stopping to add water-based lube (which isn't boiling, so it's not a heat problem), I've barely gotten the full tip width into the hole.

    I've tried running at 620rpm, 1100rpm, and 1720rpm. I've tried applying more force and less force. Essentially, the bit is hardly cutting. Figuring I must be doing something obviously wrong, I just stopped, took the following video at 1100rpm (where you can see how easily I can stop the bit with pressure), and am wondering if someone can give me any ideas on how to more effectively drill this steel?

    Attempting 1/2" drill in mild steel - YouTube

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    1100 rpm is 144 surface feet per minute at the periphery of the drill. This is probably a little fast for an unlubricated HSS drill, even with cobalt. Also, your drill press looks woefully underpowered at that RPM. Are you stalling the motor, or is the belt slipping? It sounds to me like the belt is slipping, so you may need to tighten that up before you proceed.

    You should cut the speed of the drill at least in half. That would give you twice the torque to turn the drill. If I had to use that equipment, I'd use the lowest speed possible. I would use a cutting fluid of some sort, and would use significant down pressure.

    You are on the right path with the pilot hole followed by the larger drill.

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    Check how hard the steel is with a file.
    Tap it with a center punch, how much of a dent does it make? Does it raise a good ridge around the dent?

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    Your machine needs to handle more torque so you can push and make the drill CUT ......500 to 600 rpm is about right, .so either the motor hasn't the guts or the belt's slipping.

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    Not that I don't want to try and help, but fuck man, where do you start?

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    First thing is you're using a drill press with a too loose or otherwise compromised V-belt. The poor drill bit never has a chance to dig in and do its thing.

    Next - don't drill the pilot hole, not with that quality of drill press. Just get the drive mechanism straighten out, slow down the RPM, then drill, with vigor, until you start breaking through. Then relax the force on the handle so you don't chip the drill bit tip or stall the spindle.

    And use oil when drilling steels, not water-based coolant. Screw your lungs, just pretend it's the 50's again and everyone smokes...

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    Quote Originally Posted by Larry Dickman View Post
    Not that I don't want to try and help, but fuck man, where do you start?
    Heh. And another "Heh" to make the text limit.

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    Try using a cordless drill, it may be more effective than this drill press of yours.

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    OK, helpful points: Yes, the belt is slipping (way too easily), and so I just haven't been able to get decent torque to the bit at any speed. I'll tighten that up, run at the low speed, use oil-based lube, and see if I can get the bit to dig in.

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    Quote Originally Posted by Larry Dickman View Post
    Not that I don't want to try and help, but fuck man, where do you start?
    Aye. S'truth. Lets' try anyway.

    With the old rule of thumb that the pilot for a helical twist drill should be no larger than the width across the web of the 'follow' drill at the pointy end, is where I start MY drilling of holes.

    Bluntly, a 1/4" pilot hole for a 1/2" drill is way the FUCK too DAMNED large!

    Point Two: The pilot size it DOES need is not going to be easy to push through 3/4" of steel if you don't know your shit. Settle for a pilot hole but deep enough to bury the point of the larger drill. Trust it to stay on-course of its own willpower once it runs beyond the pilot hole. Decent twists drills do that shit every day, all day, everywhere, and often-as-dammit with NO pilot at all. That.... is why we CALL them "drills".

    Point three.. a helical twist drill makes just about as BAD a substitute for a reamer as one is likely to be able to lay-hands on short of cooking up a batch of Rotini. That - with a 1/4" pilot, 1/2" drill - is what is being attempted HERE, whether recognized and planned in advance, or not.

    Just Do Not DO that.

    And finally.. the less experience one has, the higher-grade of drills they should purchase.

    Cheap ones need an expert to "baby" them, and few experts will waste their time unless the "company" is being stingy-stoopid at buying crappy drills and they have no choice. DAMHIKT

    Better drills can save your ass. Not because they covet it. They don't give a damn about you at all. They last longer and stand more abuse simply because their Mommy and Daddy did not raise them to die young.

    And they charge a price for that, according.

    Pay that price. It will save you money, time, and ruined work.

    PS: Your alleged drillpress needs a fresh rubber-band between motor and spindle. Cheat. Use a proper belt. China will not go to war over that.

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    Quote Originally Posted by Erich View Post
    Check how hard the steel is with a file.
    Tap it with a center punch, how much of a dent does it make? Does it raise a good ridge around the dent?
    Well ... I can feel a ridge around the dent with my finger, but it's not what I would call a visible ridge:

    dsc06177.jpg

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    Quote Originally Posted by Monarchist View Post
    Better drills can save your ass. Not because they covet it. They don't give a damn about you at all. They last longer and stand more abuse simply because their Mommy and Daddy did not raise them to die young.
    Yes and no. Cobalt drills are more brittle than regular HSS bits, and so more prone to breakage, especially when used in sloppy drill presses by noobs (no offense, OP). If the larger drill that's being used has a split point, I'd go ahead and not use a pilot hole, correctly sized or no. The center of the drill acts as a point of rotation, rather than allowing the two main edges to bounce back and forth when they're not constrained (pilot hole).

    For the OP, I'd suggest slowing down, and as long as he can get the belt tight and has at least 1/3rd HP on call, he shouldn't have any trouble getting a decent 1/2" drill through that steel chunk. Larger than 1/2" is a crap shoot, I'd much prefer to use a Bridgeport or clone in that case.

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    You are not feeding I guess. Let's say you were using the 1720 and wanted about .001 per lip feed per revolution - which isn't unreasonable at all with the givens, though the speed is UNREASONABLE since it is 225 SFM.

    Get this now...it would be THRU that 3/4" in a little over 13 SECONDS

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    Quote Originally Posted by dbooksta View Post
    Well ... I can feel a ridge around the dent with my finger, but it's not what I would call a visible ridge:

    dsc06177.jpg
    You were expecting Mona Kea, maybe?

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    Quote Originally Posted by Monarchist View Post
    You were expecting Mona Kea, maybe?
    Look at the ridge under a magnifying glass. If you see a bunch of teeny, tiny telescope installations, it IS Mauna Kea. And if you had done it twenty five years ago, a very tiny me would be waving "hello".

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    Quote Originally Posted by Milland View Post
    Yes and no. Cobalt drills are more brittle than regular HSS bits, and so more prone to breakage, especially when used in sloppy drill presses by noobs..
    as if they were Tee-handled pipe de-burr hand reamers, yes. Straight holes, they JFW, work well, and last a VERY long time.
    as long as he can get the belt tight and has at least 1/3rd HP on call, he shouldn't have any trouble getting a decent 1/2" drill through that steel chunk. Larger than 1/2" is a crap shoot, I'd much prefer to use a Bridgeport or clone in that case.
    Doesn't look much like an option for the OP. Nor an AB5/S.



    He SHOULD be able to improve the performance of that drillpress. Marginal to begin with with or not, it is only a dumb 1/2 hole.

    My 75 or so year-old itty-bitty Walker-Turner benchtop DP does those in about half the time John cited. CTD HSS-Cobalt parabolics, dark-oiled with an ignorant chip brush..

    Serious PRESSURE put onto 'em. AND NOT nibbly-fiddly farting about to see how many drills I can chip before removing significant metal off the workpiece.

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    Quote Originally Posted by Milland View Post
    Look at the ridge under a magnifying glass. If you see a bunch of teeny, tiny telescope installations, it IS Mauna Kea. And if you had done it twenty five years ago, a very tiny me would be waving "hello".
    Time is close, though I doubt the dates would be a exact match, but had YOU looked DOWN the rented white PA-28-150 Piper Cherokee Warrior left-seater flying around it WELL below the peak - was yrs truly. An experienced "local" CFII in the right seat, of course.

    Safer that way, what with a seriously irreplaceable Wife in back with the camera.


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    Maonarchist is a little hard to understand sometimes but I think he may have hit on your problem. All that stopping the spindle and lifting the bit out plus the bouncing may have chipped your drill bit. A good cobalt bit is more for heavy duty production and may be slightly more brittle than high speed drills.
    If the bit is chipped maybe you can find a machinist to sharpen it for you. Buying a new HS bit and slowing the drill to the slowest setting with the belt tightened up may get you through it. Or maybe pay someone else with a heavier machine to do it. The suggested hand drill is another option if you have or can borrow a heavier one.

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    Maonarchist is a little hard to understand sometimes but I think he may have hit on your problem. All that stopping the spindle and lifting the bit out plus the bouncing may have chipped your drill bit. A good cobalt bit is more for heavy duty production and may be slightly more brittle than high speed drills.
    If the bit is chipped maybe you can find a machinist to sharpen it for you. Buying a new HS bit and slowing the drill to the slowest setting with the belt tightened up may get you through it. Or maybe pay someone else with a heavier machine to do it. The suggested hand drill is another option if you have or can borrow a heavier one.

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    so, you've gotten a few good tips here. But the most important one that no one has mentioned yet, TAKE OFF THOSE FUCKING GLOVES. that way you'll still be able to wipe your ass with either hand tomorrow.

    And, no matter what approach you decide to take, it's a safe bet that you've destroyed the tip on what appears to have started out as a piece of shit drill. Go get another one, and perhaps squeeze a couple extra bucks out of your wallet and get a decent quality one.


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