Kind of courious what the correct size drill bit to use to tap a 3/8 hole would be. Just wondering what the rule of thumb is for sizeing the holeproperly befor tapping it. Im looking for a machineists opinion on whats recomended, thanks
you can get a chart, or on the tap it say sometimes. but there never "normal" drillbit sizes ussually in 16ths or 32th or 64ths but you can drill just under or at the size the of tap shaft diameter (under the thread size)
Thanks guys, i went in the garage and looked at the tap and there it is 21/64 ddduuuuuhhhhhh i feel stupid, but i usually do it on a daily basis.
Another rule of thumb is, a skosche bigger than the next size smaller bolt. Next size smaller than 3/8" is 5/16", which is 20/64....see how that works?
Machinery's Handbook will have EVERYTHING you ever wanted to know about threads and much more. For OD threads they are usually a few thousandths under nominal, but if your die is sharp you can get away with nominal - I've done it many many times no sweat. Now if your 3/8" OD part was substantially oversize things tend to get interesting fast. The other potential issue you might have been fighting was your threading die - it's common for these to be made from "carbon steel" - those are OK but don't last as long as a "High Speed Steel" die will. The HSS are usually quite a bit more money. Maybe your die was tired. BTW I wouldn't even bother with "Carbon Steel" taps - waste of money IMHO. The low end of the standard there is HSS - IMHO. There's better - all I'm saying is don't go lower than HSS for taps. Dies - most of us probably don't do enough threading to justify the extra $$$ for HSS on dies. I just threaded a bunch of 1/4-20 studs for some valve covers - I used grade 8 bolts for thread durability - it was pretty hard on my Carbon steel die, but did work. Probably most guys won't have the need to thread grade 8 and therefore the "carbon steel" dies will be satisfactory for a much longer time. YMMV
You definitely need the chart that shows tap drill sizes. Most tool room quality taps aren't going to tell the drill size plus many threads require other than fractional sized drills. You need a set of number drills and a set of letter drills in order to handle tapping properly.
interesting... 3/8's shoulda worked fine unless your die was junk or you were doing stainless and thats hard to thread.. 1/4 20 =13/64 drill 5/16 18 =17/64 3/8 16 5/16 on and on.. dave
For 3/8 16 you would use a 5/16" drill, all 16 pitch threads use a drill size 1/16" smaller. I believe a 3/8 24 would use a "Q" drill. Most tap drill size charts give a 75 percent depth of threads
I just ignored the 20/64ths deal.. I never will understand why the fuck they teach fractions in school anyway.. Im a tool maker fuck that 1/4 inch cause its .250..3/8 .375 etc etc.. About the only time i needed fraction was for a fucking 1/4 or 1/2 barrell. Dave
Sgtlethargic, were you useing a threading die or just a chaseing die? A threading die would be adjustable, should be opened slightly to rough cut the threads, then readjusted to finish them. A chaser or rethreading die will not be adjustable and will probably not cut a good thread very well. 3/8 stock is correct for 3/8X16 threads. I'm thinking the ROT for numbered course threads is number X 13 + 30 = tap drill. 8X13+30=134 for a 8 X 32 thread, anyone remember?
Adjusted properly and sharp a 3/8 16 die should have no problem putting a thread on a 3/8 shaft. One time deal you shouldnt have to adjust a second time . Now it like i said before its stainless then look out.. Thats shit will ruin a die in a heart beat.. Dave
Go to your local Fastenal store, ask nicely, and they will give you a decimal equivlent/tap drill chart.
If the guy wanting this info here had taken just a general shop class in high school he'd probably know about the tap/drill chart, letter, number and fraction drills, class of thread fits, screw pitch gauges, NF, NC, taper, plug, bottoming, adjustable dies, bolt marking, etc., etc. But unfortunately, most of these programs are being phased out by educators that think this information should only be taught at vocational schools . In the early 70's I taught High School Industrial Arts . First thing I did was give all my students a ruler test. This included a piece of paper with 20 lines on it and a 12" ruler with 1/16" marks. Measure the line and write the answer in the blank. Answers were bazaar. If I could get a kid to even get close, ie. 3 4/16" on a 3 1/4" line, I had a candidate for college. If they put 3 2/8" this was an honor student. If they did put 3 1/4" I had a potential Einstein. I never could figure out what the hell they were teaching in grades 1-8. It sure wasn't reading a ruler, fractions and reducing to the lowest common denominator 30+ years later it has to be worse. I cringe to think what'll be like in 20 more years . I recommend buying a basic metal working book for any aspiring car guy that missed out on this stuff in high school .
Oh yeah, you forgot to mention if this was a fine or coarse thread you're trying to tap. Each uses a different size drill. The chart will specify. I hope ya' don't ask a similar question when you're doing a 5/16" thread and again when you do a 1/4" and again when ........ get the idea where I'm going with this.
OMG dont even go in to engagement. thats WAY more info than is needed here. all youll do is confuse the situation more. with holding information can be a good thing some tiimes. 3/8-16=5/16' drill. 5/16-18=17/64" drill. 1/4-20=no.7 drill Those are the most common remember them and you'll be ok.
Well, MR. TOOLMAKER, why don't you go buy a set of DECIMAL sized drills? quote=jusjunk;2445988]I just ignored the 20/64ths deal.. I never will understand why the fuck they teach fractions in school anyway.. Im a tool maker fuck that 1/4 inch cause its .250..3/8 .375 etc etc.. About the only time i needed fraction was for a fucking 1/4 or 1/2 barrell. Dave[/quote]
If you don't have a tap drill chart, or if the size you need isn't on your chart, just remember that the OD minus the thread pitch (distance between thread crests, not number of threads per inch) gives you the tap drill diameter for a 75% thread. That's generally what you want, unless you're working with something really hard to thread, in which case you can go shallower. For 3/8-16: 3/8 (.375) -1/16 (.063) = 5/16 (.312). For 3/8-24: 3/8 (.375) - 1/24 (.042) = .333, which is close enough to a Q drill (.332). Your .332 Q drill's gonna make a .334 hole anyway! For numbered machine screws, as someone pointed out the diameter can be calculated by multiplying the number by .013 and adding .060". #5: 5x.013 = .065, +.060 = .125" From there, the formula for calculating tap drill size is identical. It works for metric too, just in case you happen to be working on somebody else's car. For an M8x1 thread, 8mm - 1mm = 7mm (.276), which is the tap drill size. Remember that metric pitches are called out directly as the distance between thread crests, as opposed to imperial pitches which are threads per inch. To convert millimeters to inches, divide by 25.4 or multiply by .03937. For pipe threads, use a chart. There might be a formula, but I don't know what it is. Andrew
I generally drill bigger than the chart says, for Pipe Threads,and Easy Outs. Break less stuff that way. And DON'T buy cheap No-Name Easy Outs.
Thats what i love about thie site, u can learn so much and still have a laugh at the same time. T hanks for all the input and the holes are all drilled and tapped and the body is starting to be shimmed
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