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Old 08-08-2005, 12:38 PM
Join Date: Jan 2004
Location: Candler,NC
Posts: 331
solid carbide drill bits

I've got to drill through hardened 440C and I used a solid carbide bit. I drilled very slowlyand use coolant, but just as the bit broke through, it caught and broke. Understanding that these are very brittle (and very expensive), I was looking for some tips on how to prevent the breakage. This project is for a friend so he's paying the bit cost, but it could come up again and I sure don't want to break (no pun intended) my buddies bank. Thanks.
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Old 08-08-2005, 01:24 PM
Jerry Shorter Jerry Shorter is offline
Join Date: Sep 2002
Location: Wellington, Nevada
Posts: 331
Rick, You have to back up the part that you are drilling with something hard i.e. steel. Drill through your part but not the backup material, make sure that your part and backup material is clamped down solid onto the table. Also helps to use straight-flute drill bits. with carbide I use about 600 rpm for 1/8 dia bit, I use Tapmagic for a lubricant. I also use the micro-feed on my mill-drill.

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Old 08-08-2005, 02:05 PM
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Ray Rogers Ray Rogers is offline
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Join Date: Jun 2002
Location: Wauconda, WA
Posts: 9,844
I agree with Jerry, you said carbide but you didn't say if they were spade drills or straight flute. Get the straight flute type. The spade drills are limited in the depth to which they can drill.

Feed and speed is especially important to carbide drills (if I may judge by the number of them I have broken). I prefer to run my drill press slowly, about 250 rpm, use lubricant, and listen closely to what's happening at the drill bit. The straight flute bits seem much happier when you keep some pressure on them, too slow a feed is a killer. Back the drill out occasionally especially on deep holes and clear the chips. Deep holes are those that equal or exceed the diameter of the drill.

An aluminum plate is my backing material of choice. When the drill punches through the steel it can sink into the soft aluminum and doesn't seem to hurt anything.....


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Old 08-08-2005, 02:34 PM
Join Date: Jan 2004
Location: Candler,NC
Posts: 331
solid carbide drill bits

Thanks guys! I did buy the straight flute bits and have the aluminum for backing. Didn't use any backing the first time and I bet that was the major problem. Gonna try again this evening.
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Old 08-08-2005, 09:56 PM
Frank Niro Frank Niro is offline
Join Date: Jun 2002
Location: Blind Bay B.C. Canada
Posts: 866
Hello . I use solide carbide bits in straight flute and spade design very regularly since I drill all my holes for my folders after the metal has been hardened. You will have lots longer bit life if you turn the speed up full and not add cutting fluid. The bits get dull before they break when you use them this way. If you are just drilling holes that do not require accuracy, I recommend you go to masonary bits. I just completed a titanium frame lock with seven holes with a diameter of 7/32 on each side using four sizes of the masonary bits . They will do a lot more work for me yet. Hope this works as well for you as it does for me. Frank

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Old 08-09-2005, 02:14 AM
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polarbearforge polarbearforge is offline
Steel Addict
Join Date: Jun 2002
Location: Parker SD
Posts: 144
It's common for a drill bit to break in just the fashion you described. When a drill breaks through, it's not completely through, and has a tendancy to corkscrew the remaing little bit of material. Easing up on pressure or having a sacrificial plate below it helps out a lot.

For 1/8 inch, 250 rpm is way too slow, even 600 is too slow. For hard milling, I'd run an 1/8inch bit/end mill at about 1280 or a bit faster. As far as coolant goes, if you can flood it, flood it, otherwise it's better to run carbide dry. Proper speed and feed as well as constant pressure is the key for carbide.


Jamie Boley
Polar Bear Forge - Custom Blades
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