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Old 06-03-2007, 07:15 PM
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What Size Drill Bit???

I'm fabricating my own 4-link and I don't have a drill chart for the size I want to drill. I got to drill out the ends of the rods for a 3/4" 16 fine thread tap, what size drill bit do I need? Any help would be greatly appreciated!!

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Old 06-03-2007, 07:34 PM
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http://www.newmantools.com/tapdrill.htm

Looks like 11/16"
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Old 06-04-2007, 04:39 AM
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Yea 11/16" I found it on an internet search. Thanks cucumber.
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Old 06-04-2007, 06:53 AM
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Another way to find the tap drill size is to minus the pitch from the diameter.
i.e, 3/8-16 thread, .375 - .0625 = .3125 or 5/16
1/4-20 thread, .250-.050 = .200 or 13/64

to find the pitch divide 1 by the number of teeth per inch. Dan
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Old 06-05-2007, 04:26 PM
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here is one more (like you need another)
3/4-10 21/32 - -
3/4-16 11/16 -
these are off the web as well.
Joe
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Old 06-06-2007, 06:51 PM
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Thanks guys you do overdo yourselves but that's great and I thank-you!!
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Old 06-06-2007, 07:40 PM
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no ones over doing themselves, the sheer amount of things there is to know about threads would blow any ones mind. pitch diameter, basic P.D., measurement over wires, best wire size, u.n.f, u.n.c, u.n.s, metric, acme, whit worth,class 1A,2A,3A, or 1B,2B,3B, helix angle formulas, lead, imaginary cylinders passing through the thread for the purpose of measurement, the list goes on and on and on.
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Old 06-06-2007, 08:00 PM
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Quote:
Originally Posted by leldai73
no ones over doing themselves, the sheer amount of things there is to know about threads would blow any ones mind. pitch diameter, basic P.D., measurement over wires, best wire size, u.n.f, u.n.c, u.n.s, metric, acme, whit worth,class 1A,2A,3A, or 1B,2B,3B, helix angle formulas, lead, imaginary cylinders passing through the thread for the purpose of measurement, the list goes on and on and on.
LOL.
Yeah ... that over-simplification of thread is one of my pet peeves.

I was at a truck repair shop the other day, and a couple of young guys had just finished constructing a 1" hydraulic hose for a track hoe (where the length HAS to be exact).

They've got a 1" Male NPT on one end and a 1" 37 JIC female on the other. The shop foreman is livid ... and their excuse is "I thought it was just all 'bolt-thread'!"

Needless to say they ended up with about 20 ft of wasted hose that will never fit anything. Sure, some will say that they can "hack it up" and re-use it ... but that seldom happens in the real world, right?
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Old 06-07-2007, 04:46 PM
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Generally,When I plan to tap something, I take the tap over to the window where the light is better and look carefully at the tap. Most say on the side what size bit to use.
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Old 06-07-2007, 05:41 PM
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Quote:
Originally Posted by 61bone
Generally,When I plan to tap something, I take the tap over to the window where the light is better and look carefully at the tap. Most say on the side what size bit to use.
Yep thats right unless of course they are made in china. This is one of my favorite pet peaves (threads that is) and finding the good quality taps and dies is getting harder thanks to places like HF.flooding us with there garbage.
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Old 06-07-2007, 06:11 PM
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on the subject for low quality taps, i wanted to return a broken craftsman tap the other day because all craftsman hand tools are guaranteed for life, and i would consider a tap a hand tool. but the friendly folks at the local sears disagree. a tap is classified as "a bit" and is there for not covered under the guarantee.
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Old 06-07-2007, 10:57 PM
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A Machinerys Handbook is worth it's weight in gold for stuff like that. I have the 26th edition and it's more than 2600 pages. It has everything you ever wanted to know about threads, drills, heat treating, welding, machining, gears. You name it, it's got it.
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Old 06-08-2007, 07:28 AM
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Quote:
Originally Posted by hemi43
Another way to find the tap drill size is to minus the pitch from the diameter.
i.e, 3/8-16 thread, .375 - .0625 = .3125 or 5/16
1/4-20 thread, .250-.050 = .200 or 13/64

to find the pitch divide 1 by the number of teeth per inch. Dan
Brilliant! I must admit my first reaction was entirely negative, as I've always used a chart. The connection between pitch and thread depth didn't hit me at first, but, of course, the pitch would be very, very close to double the thread depth.

This little tip is particularly handy when tapping for metric, where the pitch is given as part of the thread specification.

Here's another little tip an old timer gave me when I was starting at Chrysler back in the fifties:

The nominal diameter of a "numbered" screw thread, in thousandths, is equal to the number times 13 plus 60. For instance, for a number 10 screw, the nominal diameter is 10 time 13 plus 60 or 190 thousandths.

I wish there was a similar relationship for numbered drill bits.
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Old 06-11-2007, 02:14 PM
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Metric is the easiest to figure out. just take away the pitch and you have your tap drill. ie, m10X1.25 = 8.75. Dan
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Old 06-11-2007, 02:18 PM
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Quote:
Originally Posted by BillyShope
Brilliant! I must admit my first reaction was entirely negative, as I've always used a chart. The connection between pitch and thread depth didn't hit me at first, but, of course, the pitch would be very, very close to double the thread depth.

This little tip is particularly handy when tapping for metric, where the pitch is given as part of the thread specification.

Here's another little tip an old timer gave me when I was starting at Chrysler back in the fifties:

The nominal diameter of a "numbered" screw thread, in thousandths, is equal to the number times 13 plus 60. For instance, for a number 10 screw, the nominal diameter is 10 time 13 plus 60 or 190 thousandths.

I wish there was a similar relationship for numbered drill bits.
Tap drill sizes are all based on a 75% contact area. The reason that the pitch works so well to figure out the tap drill is because national threads are based on a 60 degree tooth patern. BA threads are 55 degrees, so I don't think that this rule of thumb may work for those. Dan
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