hole size for tapping

Re: hole size for tapping

Alan Zenreich

According to this chart, a 1/2" coarse thread hole requires a 27/64 hole

While 1/2" fine thread requires 29/64.

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

Re: hole size for tapping

Barry Irby

Google "drill bit and tap sizes".

1/2" by 13 requires 27/64"

1/2" by 20 requires 29/64"

More than I thought, too.

Re: hole size for tapping

Bill Tucker

Thank you - thank you. Once again, this forum just saved my butt! In case you're wondering, I'm fashioning a light stand out of pvc....if it works, I'll let you know ;-)

Thread depth

John K Jordan

Another factor sometimes not considered depends on whether you want 60% or 75% of full thread depth. From my best tap-drill selector guide: 60% is generally recommended (giving about 90% of the potential holding power). 75% is recommended for shallow hole tapping where the depth is less than 1.5 times the hole diameter in unhardened metal.

The numbers in most tables are for a 75% thread thread depth.

I like to use 75% for critical parts with high stresses and for softer metals. The 60% thread can be easier to tap.

If the 60% thread depth will work in your application, instead of a 27/64" bit you can use a 7/16" bit for a 1/2-13 thread, especially if you don't have the other bit on hand.

JKJ

Re: hole size for tapping

Bruce Pratt

Not quite sure of your specific application based on your comment, however, remember that plumbing threads, including PVC fittings are tapered threads (NPT), while bolts, etc. are straight threads (NPS). The tap geometries differ between the two types.

There was a fairly extended discussion about this, either here or on SMC, not too long ago; something about urn lids, if I remember correctly.

Re: Thread depth

Bill Tucker

John, the 75% might be better since the holes are being drilled and tapped into one of your Stubby stand bases. Thanks for the input and any further thoughts would be appreciated. If this works, I'll let you know. Wish me luck.

bill

Re: hole size for tapping

Keith Tompkins

Hi Bill, don't forget the cutting fluid, and don't attempt to tap the holes in one shot; make a cut, then back off on the tap to break the chip, then cut a bit more, repeating the process until done.

The other Jordan

John K Jordan

Bill, you must be thinking of the other John Jordan, the pro, the turning teacher, the nice guy. I'm John K Jordan, the retired amateur, the kindergartener teacher, the other nice guy! I can't afford a Stubby but am stuck with four wood lathes and a couple of metal lathes. I do a lot of fabrication though, for fun and farm repair, welding, machining, etc. and have tapped many hundreds of holes.

It's hard to evaluate without seeing the parts and getting a feel for the types and magnitude of the stresses involved, but I'm guessing using 60% or 70% threads should not make much difference in your case, in either mild steel or cast iron. The 70%s are a bit harder to cut but not a huge deal. If there is potential for significant vibration AND this doesn't need to be disassembled much, some thread locker (Permatex, etc) will help.

The biggest problem likely with threading is breaking a tap. However, unlike a tiny tap, you should not have any trouble with a huge 1/2" tap - that size is pretty strong. Just keep in mind that taps are hardened and generally brittle and breaking one off in a hole is a potential disaster.

High quality taps are stronger and will last longer but are much more expensive. I would not hesitate to use a cheap tap if I expected to only do a few holes.

As mentioned use fluid - tapping fluid is best but any oil or even kerosene should work fine. Make sure you hold the tap straight with the hole for starting. The non-bottoming taps have a long taper to help start the threads. If possible it is generally easier if you can position the work so you are tapping straight down - that makes it easier to keep things straight while applying force.

I apply fluid, apply down force, turn "just a little", then back off to clear the chips, then repeat. how much is just a little? Experience helps here but it may be 1/8 to 1/4 turn. Best to go by feel - turn until it suddenly gets tighter, then immediately back off some. Too tight, twist with too much force and you break the tap! Not enough down-pressure and the first threads might not be clean. If you hold the tap at an angle to the hole you can get into trouble.

Note that once the hole is started nicely you shouldn't need down force.

One of the best small investments I made years ago was a tapping "machine" - well worth it for someone who likes to build and repair things. It has a clamp for the work, it keeps the tap exactly straight, and has a big handle to turn while applying the proper force - makes things very easy! Somewhere I have a picture of my 3-year-old grandson sitting on the floor tapping a 1/4-20 hole in a 5/16" steel plate.

JKJ

Re: hole size for tapping

walta

I think your stand is made from thin walled square tubing In my opinion 1/2 coarse thread in that tubing will not have much holding power. Fine thread will be better but not much.

I think you would be better off drilling thru both sides of the tube and spread the load with a washer and a nut.

Walta

Re: The other Jordan

Bill Tucker

Glad to meet you and thanks for the advise.

Re: hole size for tapping

Bill Tucker

Thanks everyone for the good advise and yes, I did remember to use light oil on both the drilling and the tap. The bolts are in and holding. As to whether they will survive daily vibration is yet to been seen. If they fail, the only viable option would be to drill through the opposite channel wall and through bolt everything with washers. We'll see.

Thanks everyone,

bill

Stand is tubing?

John K Jordan

>I think your stand is made from thin walled square tubing

Yikes, if this is true it changes everything. 16 ga? The threads will not hold and a through bolt with a nut will likely bend the tubing when tightened unless the tubing is reinforced some way. Collapse can happen even with heavy wall sq tubing if tightened enough. Can you get to the inside of the tubing? If so, a piece of round pipe sized fit the bolt and cut to slide inside could support the walls.

If it's not possible to slide something inside, without seeing it I can imagine doing one of several things but they all require welding. One, the way they do this on a larger scale on heavy equipment: drill big hole through both sides and fillet weld a bushing that will fit all the way through, then drill and tap that or easier, slide a bolt through and use a lock nut on the back side.

Another thing might work: weld a heavy mounting plate to the tubing and drill and tap that, or stronger, cut out a section of the tubing and slide something to reinforce inside and weld it closed.

Seeing it might suggest better ways. Got pictures and dimensions? This is all guess work based on no data and a series of unsubstantiated assumptions!

JKJ

Re: Stand is tubing?

Alan Zenreich

Another approach for adding a thread to a blind surface (where there is no access from the other side) is to use a rivet nut.

This might not be necessary in this case. If necessary, he might be able to drill a hole in the other side of the leg that is wide enough to insert a nut on the end of a ratchet extension.

Rivet nuts

John K Jordan

>Another approach for adding a thread to a blind surface...

>is to use a rivet nut.

I can't believe I never heard about rivet nuts - I must be living under a rock. These look so useful I think I will order a kit. These would be great, especially in the smaller sizes.

The stand application here would require a big gun - most appear to go up to 1/4", some 3/8". The gun I found that will handle 1/2-13 threads was $220.

JKJ