I had to drill and tap a 5/16 hole in the king pin boos on my dropped axle. I used an 11/64 bit to drill a guide hole. Then I looked up the nominative size bit for tapping a 5/16 thread. Turns out you use an "F" bit. Most bits it seems to me are 11/64, 1/4, etc. What's the reason for "F". Why can't it be simple like 23/32?
thats just a letter bite, any machine shop has three sets of drill bites, fractional, letter and number, if you want the maximum strength, IE thread depth, you need to use the correct bite, if your not so worried about strength then use the next size up in your fractional bite set, and that would be 9/32".
Most good drill charts will have a few different drill sizes you can use, and then will rate the "%" of thread contact.
When you tap a hole, you cut away metal and leave the threads. "F" is .2570, if you use a 1/4" (.250) bit your hole would be too small to fit the tap in, if you go with a 17/64 (.2656), your hole will be too big. You won't leave enough metal for the threads of the bolt to fully engage. In short you really raise your chance of stripping out the hole. You shouldn't have a problem getting an "F" bit, but if you really can't find one, use a 6.5mm (.2559). As was posted above, you can use a 9/32" (.2813) bit, but then your hole will be close to .025" oversize. How tight do you need to tighten this bolt?
As above. the simple answer is that 1/4 is to small and 17/64 is to big. So they invented "F" to have a drill that was the right size.
******** 1/4" is too small, I use that size daily to tap 5/16" threads...it's a hell of a lot better than going up a size and losing thread engagement...your tap will easily cut .007 of extra material out (honestly, a 1/4" drill will almost always drill .005 oversize anyway unless you're really good at drilling with accurate equipment and brand new drill bits)...drill it at 1/4" and tap it, then sleep easy knowing it's as good as you can do...
Agreed, 0.007 is nothing. Its only 0.0035 per side, you should have no problem with your 5/16 tap. -Mike
IF you're tapping co**** threads, AND have a decent quality tap, you CAN get away with underdrilling. I wouldn't try it in 4130 with fine threads. Careful not to over-generalize...you'll have guys busting off ****py taps right and left ! Some tap drill sizes don't HAVE any fractional sizes close enough...
Better pray that the tap won't break!!! Do the right thing and get an "F" drill. Also borrow some Crisco from your better half and roll the tap in that stuff.. It acts like a lube for tapping threads. I've worked in prototype for the last 25 years and I've done just about every trick in the book. The Crisco does work!
I liked Crisco for steel, but I still prefer Ivory bar soap for aluminum...and breaking taps is 90% operator error, myself included...
Stay away from the 2 fluters... Those ****! Use a 3 flute starter tap and end with the 3 flute bottoming tap if your tapping a blind hole. Blow the threads clean with compressed air and your good to go.
I like 3 or 4 flute taps for hand tapping, the 4 flute ones are easier to guide in straight, but harder to turn once they get started...and I have never gotten a straight answer about why there's an E letter drill...
To drill a 1/4 hole!! Better question Rich. What is the reason for letter drills? They are goofy increments apart.
I always use a 4 flute when hand tapping, but never a 4 flute in a lathe or a mill. Breaks them every time. Two flutes are ok to use in a cnc or tapping head on a manual machine. -Mike
Wow. That's a lot more information then I anticipated. PREWARCARS4ME where can I find a chart like you mentioned. I went on the I-net and got the info which said use an F would be nice o have a more complte chart I think. I did buy an F bit. Two of them actually. If you ever try to drill a kingpin boss be perpared-they are tough. I'll tap it tomorrow and I'm pretty experienced at brearking taps. I done it enough times. Thanks for all the help guys.
Like everything else, it's online. Do a google on Machinist Handbook. Everything you need to know is there.
Blown, since you live in LHC, stop by Copperstate Nut & Bolt on Havasu Ave and ask them for a drill and tap chart. They will give you one that has all the percentages you will ever need on one handy card that fits in your toolbox. I put a magnet on mine and it is stuck to my tap tool box. If Dueces doesn't find one to post, I can scan mine.
another trick we use sometimes is to sharpen the next bit size down a little off center and the hole will come out .005-.020 larger than that drill size (drills come out a few thousandths over anyway). just another option If you only have one hole to drill and you dont want to buy a whole set of number or letter drills. good luck
Some of you guys think the folks that came before you just thought up letter drills and wire gage drills just to **** with you in the future? DUMB****S!
Sharpening drill bits off-center ****s, mainly because it won't give you a true round hole and often makes the hole triangle or hexagon shaped...I just did that trick today on the lathe by sharpening a 1/2" drill bit .055 off-center and it drilled .020 oversize...I was in a hurry, and the shop at work is one of the least equipped shops I've ever seen...it got me by, but it definately wasn't pretty tapping with a 3 flute 5/8-11 in a triangular shaped hole...and I was still undersize on the proper tap-drill size...it ****s working in a shop with zero tooling and under ridiculous deadlines, but it pays the bills better than unemployment did...
Been through that **** before... We had problems with not having the right size end mills, reamers, M6 x 1. taps and gage pins for checking hole sizes.
No problem Mopar man!! Glad to help out with whatever I know... And I don't know much.. I'm here to learn also from a great group of guys!
The Jalopy Journal
