lathe question

what size hole do I need to drill to tap a 1/4 nc thread?
Also, just so I'm sure - I am pretty new with the lathe, when tapping I feed the tap into the piece just like it was a drill bit? The part is turning under power and I feed the tap in with the tailstock?? thanks-

 

1/4-20 tap drill is a number 6. To tap on a lathe if you want to put the tap in the tail stock you must unlock the tail stock from the bed and let the tap pull the tail stock forward. Doing it with a 1/4-20 tap could be a bit edgy as they break pretty easy, make sure the the lathe ways are clean and well oiled. I would really suggest doing it by hand.

Rex

 

I'm no master machinist by any stretch of the imagination, but I have to agree that doing it by hand is the way to go. Usually, I'll mount the tap in the tailstock, and get it started in the part (a complete thread or two) by turning the chuck using the chuck key in one of the sockets and pulling it towards me, rotating the whole chuck, while simultaneously turning the handwheel on the tailstock. Then I unlock the tailstock and continue pulling the chuck key, obviously putting it in the next socket when necessary. Use plenty of cutting fluid.

 

good taps have a hole centered in the square end. I use the tail stock and center to hold the tap up against the piece then turn the tap by hand whild feeding the tail stock to keep it centered. Get a better feel of the pressure exerted on the tap that way.

 

thanks for the info - to do it Lobucrods way you have a tap handle on the tap then??

 

Yes. You'd have to, to keep the tap from turning with the work.

 

And--you desperately need to either go on line or get a catalog like Starrett with a chart of drill sizes for different threads, along with a decimal equivalent chart.

 

I've done some tapping on a Bridgeport...similar to doing it on a lathe I guess except you're working vertical. Usually I'll drill my hole and then leave it set up...and mount the tap. That way I know it's located accurately over the hole. I run the tap into the work and let it draw itself in...running at the slowest speed and I reverse the rotation after about a revolution or so. A little dicey with really small holes and taps, but on bigger holes it's easily done. The tap goes in centered and square...does a really nice job. They also make a "tapping head" that's used for tapping on mills and probably lathes, but I have never used one.

-Scott

 

Yes, as long as the squared end has a centered hole, shaped like an inside cone.


I am a machinist, have been for 26 years.

I do it using the chuck running at the slowest speed, the tap in the drill chuck in the tail stock, with the drill chuck tight enough to hold the tap, but loose enough to let it spin so it doesn't break.

I leave the lock on the tailstock comepletely loose to let the tap pull itself into the part as the chuck turns, i feed it in with the handle on the tail stock to get it started, then let the tap pull the tailstock towards the part.

Thats a fine line to flirt with, cause the chuck turning in low gear has some serious tap snapping torque behind it. Start out with the tap kinda loose in the chuck, then slowly tighten it until it stops spinning.

Get it started with around 3 or 4 threads cut, then finish it by hand, by putting a tap handle on the tap and tapping to desired depth.

NEVER TRY THIS IN A SHALLOW HOLE THAT DOESN'T GO ALL THE WAY THRU!!!!!!!!!!!!!!!!!!

 

The hole on the end is only on larger taps,usually over 3/8".

Most drill charts will say 13/64",or #7, for 1/4-20 IIRC.
I use #6 for most jobs,#5 on difficult to tap materials.

Make sure you countersink the hole BEFORE tapping.

With a decent quality tap,you should be able to
power tap with the tailstock no problem.
Just drill the hole deeper than the length of the flutes
on the tap,so the chips have someplace to go,and
use a slow rpm.

First choice is tapping compound,like Chromatap(sp?)
Motor oil will work,but not as well.

 

A decent T-handle tap wrench will have a center hole in the end,
to use with a center in the tail stock.

Works well with small taps that don't have a center hole in the end.

 

Most of the drill & tap charts I have show a #7 drill for 1/4-20.

One of them recommends a #8 drill.

Maybe in aluminum for the #8 drill, but to me, it's still too tight.
The #7 drill works welll for both aluminum and most steels.
Makes for some good fitting threads.

Rex's recommendation of #6 strikes me as a good idea for some steel alloys - tough ones.
Even so, a common 13/64 - which comes in the larger fractional drill indexes works for most metals and may be a good way to go for the tough stuff.

Fwiw:
Drill diameters -

#8 = .199
#7 = .201
13/64 = .203
#6 = .204

If your tap is dull or has seen a lot of use it may pay to err -in a manner of speaking - on the large side.

As inexpensive as HSS (High Speed Steel) taps are, it pays to have several of them in the most commonly used sizes.
Especially so in the smaller sizes.
10-32, 10-24 etc.

Home Despair and similar places sell taps with the correct drill in one package.
A bit of a pain sometimes, but having extra drills in a size you use a lot is handy.
It doesn't add much to the cost in the smaller sizes.


Bruce is right on with his recommendation about getting a drill/tap chart.

Lotta times a 'real' parts house or old hardware store will have a dill/tap chart on plastic stock or coated paper that they either keep on the counter for handouts or have them under the counter and p*** em out on request.

Once upon a time I asked for one at a parts house that handled Starrat - *****in stuff - and they handed me a rolled up chart about 15" x 24".

 

been doing it since i was 13 and ill feed it in chucked up the tail stock all day long I always use a 13/64 for 1/4 20.. Have for years.. #2 for 1/4 28 .. 17/64 for 5/16 18 ,5/16 for 3/8 16 and on and on ... Feeding under power takes practice.. I tap holes with a tap in my electric drill a lot too.. Like I said practice...
Oh ya did i mention im also lazy..
Dave

 

http://www.nolansupply.com/bysubcat...ndle+Tapper%26nbsp;&type=False&specs=True i made my own by welding a handle like this to a piece of tubing about 10" long then i chuck a solid round bar in the tail stock that the tube slides over, the length keeps it straight and gives me lots of travel.

 

Inch Sizes - National Co****

----------------------------
Tap Drill
Size Size
----------------------------
#1-64 #53
#2-56 #51
#3-48 5/64"
#4-40 #43
#5-40 #39
#6-32 #36
#8-32 #29
#10-24 #25
#12-24 #17
1/4-20 #7
5/16-18 F
3/8-16 5/16
7/16-14 U
1/2-13 27/64
9/16-12 31/64
5/8-11 17/32
3/4-10 21/32
7/8-9 49/64
1"-8 7/8
1-1/8-7 63/64
1-1/4-7 1-7/64
1-1/2-6 1-11/32
1-3/4-5 1-35/64
2"-4-1/2 1-25/32
----------------------------
Inch Sizes - National Fine

----------------------------
Tap Drill
Size Size
----------------------------
#0-80 3/64"
#1-72 #53
#2-64 #50
#3-56 #46
#4-48 #42
#5-44 #37
#6-40 #33
#8-36 #29
#10-32 #21
#12-28 #15
1/4-28 #3
5/16-24 I
3/8-24 Q
7/16-20 W
1/2-20 29/64
9/16-18 33/64
5/8-18 37/64
3/4-16 11/16
7/8-14 13/16
1"-14 15/16
1-1/8-12 1-3/64
1-1/4-12 1-11/64
1-1/2-12 1-27/64
1-3/4-12 1-43/64
2"-12 1-59/64

or go here ... on line chart.

http://www.newmantools.com/tapdrill.htm

 

especially if you live more than 10 miles from the nearest hardware store! I learned that one again yesterday.

Great info here, timely too, as I'm teaching one of the robot team kids how to use my lathe....

 

One other safety precaution, esp. on small holes, is after each half turn, turn the tap backward a bit to break the chip. This will help prevent breaking your tap.

 

That's fine with hand taps.

For power tapping,you want a spiral point machine tap.
That's all I use for everything.

Stay away from spiral flute taps.
More expensive,and more fragile.
Ok for CNC on blind holes.

 

Thread cutting tap= #7 Thread forming Tap = #8. If you use a thread forming tap use the lathe. Thread forming taps displace material as opposed to cutting material. Is this a blind hole or a thru hole? Bottoming tap or taper? Maybe I am over complicating this....If you are going to hand tap use a thread cutting tap. If you use a thread forming tap you need to use the lathe or mill as the faster you tap the cleaner the threads. Just my .02
Fitysix

 

This hole could be either blind or through - doesnt matter. I would like to use the lathe to tap the hole as I am trying to teach a couple kids in cl*** (as well as myself ) how to do some things on the lathe. They are building a brake pedal pivot as per SRI's T plans. Yesterday they cut the material, faced it and center drilled it. Today I would like to drill and tap the hole for the bolt...

 

Material? Again you will find using a lathe you should look at the thread forming taps. Since it displaces material you don't have to worry about chip bind and I would use a taper end maybe 3 thread lead in. Don't forget a good coolant.
Fitysix

 

Teach them to tap by hand first.

 

And the advantage is ???

 

material is crs I believe, whatever your standard garden variety 3/4" solid round stock is - I know it isnt hardened like shaft material or something like that...

 

Here's one of many tap charts online: Hangitonthewall!

http://www3.telus.net/public/aschoepp/tapdrill.html

...with this:

https://home.comcast.net/~rexmill/decimal_equivalents/Decimal_Equivalents_Chart.htm

http://www.acmeindustrial.com/drillchart.asp

Also useful is a chart that shows inch, number, letter, and metric drill sizes in order of relative size...that gives you options if you need something a bit looser or tighter, and gives you an idea of possibilities if you don't own all the drill types...as in "what the hell is close to an "L" in size??"

 

My personal charts have patina instead of digits...two ancient paper charts back to back, sandwiched between two sheets of battered and scratched plexigl***, held together all the way around by aluminum aircraft rivets... I think it has to be from a WWII aircraft worker.
Have your shop kids fabricate a chart with durability and cl***!

 

Cleveland has a chart with different drill sizes
for each tap size,and the thread % for each.

Very handy.

For students,the basic chart is better.

 

Make them do it the hard way first, gives them experiance to "feel" how to tap. If you insist on threadformers, I think STP is the best lube, otherwise use sulphur based cutting oil for normal taps.

 

the only advantage to backing off like that is to break the chip up to easier remove the chips from blind holes. if it's a through hole or a hole that is deep enough to allow the chips to build up, and large enough diameter to blow or pick out the chips, get in there in one shot! breaking chips by reversing puts more stress on the tap than cutting does, and also creates a situation where the chip can get bound up in the threads. again, it matters what kind of tap you are using, hand taps are made to break the chip more often, and cut at slower speeds, if you have machine taps, they cut better at high speeds, high meaning the lowest speed on a lathe vs. turning by hand. i've tapped many thousands of holes in my life and by far i've broken the most taps while reversing to break chips.

 

I would at this point suggest hand tapping ...just to get the project done . Use a good tap (HSS) with a good lead in. 1/4-20 is a rather large tap from what I am used to (4-40 and the like). If you want to for-go the chip problem ***ociated with blind or thick material thru holes use a high helix tap. If the point of the lesson is to lathe threads then do your homework and practice. You need to know your lathe very well and know your speeds and feeds. If it is a blind hole make sure your lathe will stop on a dime as any dwell at the bottom of the hole will ruin your threads. All the info given is valid and should be taken into consideration, Lathe setup, coolant etc....
Good luck and have fun. Just for kicks...on my CNC I can turn a 4-40 thread forming tap at 1200 RPM cutting taps usually break at around 80 RPM in 6061 aluminum stock. Lathe equals thread forming tap .
Fitysix