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euta226
02-06-2018, 09:33 PM
Quick disclaimer, I'm very new to working with metal. I have a bolt that has a pitch of 32 SAE. The OD of the threads is 0.312 ". Seems like it is not a standard combination

My question is, how do I determine what size drill bit and tap that I need? The books I have don't go into enough detail and all the online tutorials I've found are of people stumbling through their explanations.

If you can recommend a site or book that is detailed and accurate, I would really appreciate it. Or if it's a lot more simple than I'm making it out to be, I would welcome any instructions you can give.

Thanks for reading and I look forward to hearing from you!

James

Doozer
02-06-2018, 09:42 PM
1"/32tpi=.031" pitch
.312" nominal dia-.031 pitch=.281" tap drill

-Doozer

lugnut
02-06-2018, 09:56 PM
Google is your friend. Dozens of charts available to use. Search tap vs drill bit size.

Mike Amick
02-06-2018, 10:44 PM
Look what apps are available for your phone. I have several on mine. Real handy

RichR
02-06-2018, 10:49 PM
Here's a nice chart from Little Machine Shop:
https://littlemachineshop.com/reference/tapdrill.php

ahidley
02-06-2018, 10:56 PM
It's a 5/16-32 extra fine thread. Drill a 9/32 pilot hole prior to tapping. That is for a 75% thread which is standard for steel threads.

Download an app for your phone as I just used and this info is easy to get.

Dam I posted my first error of the year. More to come

Doozer
02-06-2018, 11:10 PM
I'm a 21st century digital boy
I don't know how to read but I've got a lot of toys
My daddy's a lazy middle class intellectual
My mommy's on Valium, so ineffectual
Ain't life a mystery?

-Brett Gurewitz

adatesman
02-07-2018, 12:03 AM
7 posts in and no one suggested Machinery's Handbook, which has all of this in handy chart form as well as an explanation of how to calculate it for oddball threads?

BCRider
02-07-2018, 01:35 AM
7 posts in and no one suggested Machinery's Handbook, which has all of this in handy chart form as well as an explanation of how to calculate it for oddball threads?

He would have to buy one. And for a lot of us even the used ones are rather costly.

And there's so many tap/drill charts online. Or even just in the URL/search line of the browser I just typed in "tap drill size 5/16-32" hit enter and Google came back with the answer with no chart needed.

The interwebz is just making all this way too easy... :D

James, when you want something specific like that start with just that sort of key word statement. More often than not it'll give you the answer without having to even read down through a tap drill table. And certainly not need to fumble through any tutorials. Things that are this specific need specific questions.

That size isn't just "fine", as in 5/16 NF either. It's one of the extra fine series and you won't find it on just any old tap/drill chart. It came from THIS SITE'S CHART (http://www.lincolnmachine.com/tap_drill_chart.html). And that's a rather good one because it has many of the NS and NEF series as well as mixing in a lot of the metric info with fractional drill sizes. Rather worth printing a copy or two of this one out for the future.

wombat2go
02-07-2018, 09:13 AM
BCR, That is a good one.
I will hang it above my bench, Thanks

DR
02-07-2018, 09:47 AM
if you really want to get the best tap drill size you need a chart that bases drill sizes on length of engagement and thread class.

With a 32 pitch the drill size is not going to be so critical. With coarser pitches and difficult material it makes quite a difference having the "best" drill size.

I'm too lazy now to check Machinery Handbook, but that would be my first place to look if I needed to know.

MushCreek
02-07-2018, 04:55 PM
Doozer told you all you need to know. Subtract the distance of one thread (pitch) from major diameter. Works in SAE or metric. For example, take a 1/4-20 tap. 1" divided by 20 equals .050". Subtract that from .250 (1/4" in decimal) and you get .200". Closest size is a #7 drill at .201". How about metric? 8 mm diameter by 1.0 mm pitch. 8-1 equals 7 mm. 7 mm is your drill size.

DR
02-07-2018, 05:54 PM
Doozer told you all you need to know. Subtract the distance of one thread (pitch) from major diameter. Works in SAE or metric. For example, take a 1/4-20 tap. 1" divided by 20 equals .050". Subtract that from .250 (1/4" in decimal) and you get .200". Closest size is a #7 drill at .201". How about metric? 8 mm diameter by 1.0 mm pitch. 8-1 equals 7 mm. 7 mm is your drill size.

That may be okay for casual home shop use. Not something you'd want to use in production tapping or in difficult materials where you're concerned with tap life. You want the largest allowable drill size that takes into account class of thread, length of engagement and percent of thread.

Since home shop guys sometimes use marginal quality taps and tapping setups the formula is not something I'd recommend. The Greenfield Screw Thread Manual is the best I've found for sizing drills for common NC & NF taps.

Richard King
02-07-2018, 06:14 PM
If your new then you need to read this book. https://www.amazon.com/Shop-Theory-Henry-Trade-School/dp/1169755895/ref=pd_cp_14_1?_encoding=UTF8&pd_rd_i=1169755895&pd_rd_r=MTF8NN8VPGRVG9FBEXQ0&pd_rd_w=IGDRh&pd_rd_wg=D0zGf&psc=1&refRID=MTF8NN8VPGRVG9FBEXQ0

It cost $34.50 and it will be the best buy you will ever read. Also buy a high quality Industrial Taps, not the Chinese one you can buy at the hardware store. You also need to make a tapping block that will help you keep it straight when you start it. Oh...there are starting taps, plug taps, and bottom taps. Depending on what and where your tapping you can drill the next size bigger . The books say a # 8 drill for 1/4/20 and in most cases i use a # 7 that is .010" per side of hole bigger. Also use a high quality tapping fluid. I find Bee's wax works good and Tap Magic. :-)

David Powell
02-07-2018, 06:29 PM
[QUOTE=Richard King;1159573]If your new then you need to read this book. [url]https://www.amazon.com/Shop-Theory-Henry-Trade-School/dp/1169755895/ref=pd_cp_14_1?_encoding=UTF8&pd_rd_i=1169755895&pd_rd_r=MTF8NN8VPGRVG9FBEXQ0&pd_rd_w=IGDRh&pd_rd_wg=D0zGf&psc=1&refRID=MTF8NN8VPGRVG9FBEXQ0[

I keep an older version of Machinery's Handbook in a drawer in the shop. Once you have some basic knowledge you will find the answer to almost any question, of sizes , fits, strengths etc you will ever face as a home shop machinist. However, it will not teach you the actual techniques needed to reach the necessary results. regards David Powell.

wierdscience
02-07-2018, 11:37 PM
1"/32tpi=.031" pitch
.312" nominal dia-.031 pitch=.281" tap drill

-Doozer

That's exactly right,no charts needed.:cool:

I will add that on thread counts greater than 24 tpi it's generally best to drill undersize and ream to the finished dimension before tapping.

Doozer
02-08-2018, 09:04 AM
Yup, simple formula to remember.
...But everyone is totally enamored with their own personal pocket Jesus.
And heaven forbid they should commit some formula to memory...
Too busy trying to remember important sports statistics.

-Doozer

mklotz
02-08-2018, 09:29 AM
Yup, simple formula to remember.
...But everyone is totally enamored with their own personal pocket Jesus.
And heaven forbid they should commit some formula to memory...
Too busy trying to remember important sports statistics.


Too true, Doozer, too true. Besides the formula to memorize should include the depth-of-thread percentage as well. See below...

************************************************** ****

Tap drill calculation...

TD = tap drill diameter
MD = major diameter of thread
DOT = depth of thread expressed as percentage
TPI = thread pitch expressed as threads-per-inch

TD = MD - DOT / (75*TPI)

Example:

MD = 0.3125
DOT = 75 %
TPI = 24

TD = 0.3125 - 75 / (75*24)
TD = 0.2708 (closest drill = "I")

With DOT = 65 %, we have

TD = 0.3125 - 65 /(75* 24)
TD = 0.2764 (closest drill = "J")

The more simplistic formula...

TD = MD - 1/TPI

makes the assumption that:

DOT / 75= 1

or:

DOT = 75 %

which is actually more than needed in many situations.

Suggested DOTs:

MILD AND UNTREATED STEELS 60-65
HIGH CARBON STEEL 50
HIGH SPEED STEEL 55
STAINLESS STEEL 50
FREE CUTTING STAINLESS STEEL 60
CAST IRON 70-75
WROUGHT ALUMINUM 65
CAST ALUMINUM 75
WROUGHT COPPER 60
FREE CUTTING YELLOW BRASS 70
DRAWN BRASS 65
MANGANESE BRONZE 55
MONEL METAL 55-60
NICKEL SILVER (GERMAN SILVER) 50-60

************************************************** ****

CCWKen
02-08-2018, 09:38 AM
Sticking with the suggested DOTs sure makes hand tapping easier too. I hate those made up charts. I think they were invented by tap makers to sell taps. :D

Jim Williams
02-08-2018, 09:48 AM
Nothing magic about determining drill size. In an emergency, hold a micrometer or caliper up to the thread and eyeball the minor diameter. You will be generally close enough. If any doubt, you can always run a trial in a bit of soft scrap.