Need Advice on Drilling and Drill Bits for a Project


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Old 01-05-14, 12:25 PM
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Need Advice on Drilling and Drill Bits for a Project

Hello

I am trying to build a little contraption for a home project and part of this build requires me to drill 1 inch diameter holes in multiple slabs of 1/2 in thick steel.
Not sure what type of steel it is but it's just your average run of the mill half inch by 4 inch wide by 18 inches long slab of steel.

My problem is that I have used my good drill bit set to step the hole size up to 3/4 inch. I did not have any larger bits so I went to Harbor Freight and purchased a set of Black and Demming large size bits (Chinese made)

The 7/8 inch bit and 1 inch bit just spin at the top of the hole. They don't cut in. I am using a squirt bottle filled with automatic transmission fluid as lubricant (is that ok?) I have been using a high end Milwaukee corded power drill to do the drilling. It has worked fine at drilling the holes up to 3/4 inch but the new bits won't do anything to the steel slabs.

I sure the bits I got at harbor freight are low end Chinese crap but they said they were black oxide coated and made of high speed steel so I thought they would do the job?

My question is: What is actually the proper type of bit to cut 1 inch size holes in 1/2 inch thick steel? Would a regular bit work if it was not black oxide but maybe titanium nitride coating? Should I get a cobalt steel bit or carbide tipped? Or should I be using an annular bit, or an end mill bit?

Thank you for any advise you could pass along.
 
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Old 01-05-14, 12:53 PM
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A couple of things may be happening.
You may has work harden the piece from over heating. If so you may have to go with a cobalt drill bit.
No telling how those bits where sharpened. Look close at this picture and notice how the back side of the cutting surface is ground down so it's not causing friction as it cuts to keep it cooler.
Silver & Deming And Reduced Shank Drill Bits | MSCDirect.com
You need to be drilling at a really slow speed with a bit that big, and really should have been flood cooled not just a squirt once in a while. That's asking a lot of a hand held drill to be drilling a hole that big.
A high speed steel drill bit is just barely harder then the steel your trying to drill.
 
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Old 01-05-14, 01:07 PM
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I swear by my cobalt bits but I'm apt to swear at the cheaper drill bits

How slow can you set your drill? You might want to enlist some help to keep the bit oiled. I usually use burnt transmission or motor oil sometimes thinned with kerosene ......... but I'm cheap and it's usually handy Personally I couldn't imagine drilling that hole without using a drill press - don't have enough hands

almost forgot - welcome to the forums!
 
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Old 01-05-14, 01:14 PM
P
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I would try different drill bits. A sharp bit will cut and possibly cut so well you'll have a hard time holding onto the drill and may need a side handle. When you get a new bit oil it up. Push as hard as you can and turn the drill slow. I use cutting oil but even motor oil can work. You'll see it smoke but with that large a bit, turning slow you should be able to keep a puddle of oil in the area to cool & lube the bit.

Still, what you are doing is very difficult. A 1" hole with a hand held drill in 1/2" thick material is really pushing it. Long before that point I'd be clamping the plate in a mill or at least using a magnetic drill press. If you are not able to bore the holes reasonably easy I'd consider looking up a local welding or metal fabrication shop. They could quickly open the holes up for you without risk of breaking a finger when your drill grabs.
 
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Old 01-05-14, 01:25 PM
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I second PD's suggestion to hire it out - you might get it done for the price of a new cobalt bit.
 
 

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