Making Tight Threads

[davidwdyer]

I am wondering about how to make threads that are tighter than what you might buy at the hardware store.

Obviously, a tap will cut the size it is. Being here in Brazil, I don't have ready access to taps other than normal size. I assume that there are undersized taps??? but I will have a hard time getting them.

On the die end of things, I imagine that cutting with the die "loose" (on the adjustable ones) would help some, or cutting it on a lathe a little over sized.

Any thoughts or help out there?

[gizmo2]

Well, there are different classes of threads on a number system, from 1 to 4. 1 is interference, and will need some force to make the connection. A class 2 will start by hand but only for a couple turns, then you'll need a wrench. 3 will hand fit smoothly, and 4 will spin up freely. You know, hardware store stuff. Taps have the fit on them somewhere. Some dies are adjustable, others are made solid and are by class. When doing gun work, the goal is a class two fit, and you achieve it by how you machine the threads on the barrel tenon.

[justanengineer]

The easiest way to do it, size dependent obviously, would be to single point thread the necessary hardware on the lathe. You could then take as many or as few cuts as necessary to get whatever thread interference you wanted.

[aboard_epsilon]

dies ..there are split dies ..

taps ..drill the hole undersize and it will be tighter ,

all the best.markj

[cuemaker]

Well, there are different classes of threads on a number system, from 1 to 4. 1 is interference, and will need some force to make the connection. A class 2 will start by hand but only for a couple turns, then you'll need a wrench. 3 will hand fit smoothly, and 4 will spin up freely. You know, hardware store stuff. Taps have the fit on them somewhere. Some dies are adjustable, others are made solid and are by class. When doing gun work, the goal is a class two fit, and you achieve it by how you machine the threads on the barrel tenon.

The above information is incorrect. Typical hardware store thread fits IF made properly are to Class 2.

The correct basic info you want is below...How you achieve these thread fits is better explained by others.

Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads.

Class 1 threads are loosely fitting threads intended for ease of assembly or use in a dirty environment.

Class 2 threads are the most common. They are designed to maximize strength considering typical machine shop capability and machine practice.

Class 3 threads are used for closer tolerances.

Class 5 is for interference thread fits...

[Mcgyver]

generally done one of two ways, adjusting the split die to the desired result, or...

machine and measure them. Know what class the tap is for, look up the tolerances and machine the male a couple of thou smaller. If you don't know the class, measure it if its a 4 flute, if three tap a hole and thread some plug gauges and measure them.

Measurement via thread wires or thread mic. Like a shaft properly fitting a bore, it's tough to arrive at a good fit by accident....get measuring those threads :D

[Shuswap Pat]

When I was teaching apprentices - I wanted to come up with a way that they could easily remember A was Extenal thread and B was Internal. I thought about it for a while, and it came to me.

NOW, I am politcaly correct, so in a mixed class (Male / Female) - it was 'B is for Bagel - you can put your finger (bolt) in the hole of the bagel (Nut)- simple, and easy to remember - Thank you 'Tim Hortons' the great Canadian Icon.

Let me back up a bit here, so most of my classes would be all Male - 20 something, full of testostrone - ready to take on the world - smartest guys they know.

I an case where we had to get it down into make simplistic terms. and grab their attention - I decided to use an animal - another Canadian Icon:

For the internal thread - 'B is for Beaver', and they never forgot.

Patrick

[Dr Stan]

I too recommend single pointing the threads to achieve the desired fit.

One can also buy taps to cut the desired internal fit, but they are not usually listed in most distributors catalogs. Going smaller on the tap drill is one way, but you need to be extra careful so you do not break the tap in the hole. :(

I'm not sure why you're looking for a tighter fit as there are other ways to make sure the fasteners stay together such as nuts with nylon inserts (Nyloc), thread locker (Locktight), etc.

If you're looking for more shear strength just go with a fine thread instead of a course thread or go to the next larger size.

If you're trying to achieve better alignment that is usually done by using dowel pins or similar design features.

[davidwdyer]

What I am actually thinking about doing has to do with tsmartin's idea of how to cut large dia. stock on a short lathe with a smaller through hole on the head and chuck.

I plan to make a cat head for the tailstock end of the lathe. Then make a kind of cheapo chuck/follow rest for the carriage. Basically I thought to bolt a flat plate where the follow rest is attached on my SB and fit three bolts approaching the center on it's face, sort of like a regular follow rest, except that they would grab the shaft and hold it while machining. These bolts would want to have a fine adjustment and very little, if any slop.

If you want to see more what I'm talking about, see some of the last posts on "shop made tools."

[camdigger]

David,

I don't see why something like this made with 1/2" or 12 mm hardware store threaded rod and coupling nuts wouldn't work.
http://i766.photobucket.com/albums/xx301/camdigger/Shop%20made%20tools/PB291373.jpg

If gripping force or "dialing in" is a concern, add a 4th bolt and revise the spacing to the corners of the plate.

[danlb]

Please correct me if I'm wrong...

A die does not necessarily cut the crests of the thread, if the screw is turned to a size smaller than the major diameter needed for the nut. Same for a tap, when the hole is bigger than the minor diameter. That means you can adjust the fit somewhat by adjusting the hole/rod diameters.

Isn't the fit a matter of getting the thread form perfect ( exactly 60 degrees at the proper depth ) and the major/ minor correct?

But if you want the bolts with no slop, I think you should look at using a bolt with a shoulder that will act as a pin to provide a positive location.

Dan

[Mcgyver]

A die does not necessarily cut the crests of the thread, if the screw is turned to a size smaller than the major diameter needed for the nut. Same for a tap, when the hole is bigger than the minor diameter. That means you can adjust the fit somewhat by adjusting the hole/rod diameters.

Isn't the fit a matter of getting the thread form perfect ( exactly 60 degrees at the proper depth ) and the major/ minor correct?


right you are. When speaking of size or fit of thread, you're not talking the OD or ID - that just concerns % engagement. Its the distance between the V's that matters - commonly measured with wire's that register in the V's

For, example, if you took a 4 flute tap where you'd measured it across thread wires, tapped a hole, then turned a threaded section to be .002 less across the wires, you'd get a thread that feels very exact, smooth and tight. .001" over and its interference. We don't typically measure the tap (often 3 flute) and instead rely on a tolerance range as defined by the class of fit - you just look it up in the bible.

Most fasteners are rolled and in production you have to work within ranges of tolerances - these are made suitably sloppy both to facilitate low cost production tolerances and to have enough clearance that with a max sized bolt and min size nut they'll still go together with less than clean room environment ....and it doesn't much matter insofar as strength is concerned. Where a nicely fit thread can be beneficial is on feed mechanism; ie dynamic vs static.

[SGW]

I've always tapped a hole, then adjusted the split die to get the fit I want. A few test pieces are typically in order to adjust the die properly.

Or you can buy a slightly smaller tap based on what its "H number" is. The H number of a tap is the number of half-thousandths (i.e 0.0005") a tap is oversize. If you look in a big tool catalog you'll see for a given tap (say 10-32) you'll see maybe H1, H2, H3, H4, H5 with perhaps H2 marked as "most common size." If you want a closer fit, buy an H1 version of the tap.

The "most common size" changes as the thread size changes. A 2-56 may typically be H1, while a 5/8-11 may typically be H4. (I'm just guessing on these values, but the larger the thread the larger the "most common" H number will be.)

[Rich Carlstedt]

For oversize threads, using a split round die is the easiest approach.
Use a die holder that has a sharp point to open the gap on the die .
If you have a Geometric head, its a piece of cake.

For tapping, Try heating the part to 390 (F) [200 C] and tapping while heated. This will give you about .0005" tightness at room temp.
It will be even better if you can keep the tap cooled ..which is hard.

390 degrees will not cause a loss of temper on the tap

Rich

[tdmidget]

I am wondering about how to make threads that are tighter than what you might buy at the hardware store.

Obviously, a tap will cut the size it is. Being here in Brazil, I don't have ready access to taps other than normal size. I assume that there are undersized taps??? but I will have a hard time getting them.

On the die end of things, I imagine that cutting with the die "loose" (on the adjustable ones) would help some, or cutting it on a lathe a little over sized.

Any thoughts or help out there?

Are you sure that you can't get such taps in the 3rd most industrialized nation in the western hemisphere?

Those wizards among you who have given bogus info, read cuemaker's post again.
No, reducing the hole diameter will not change the class of fit, which is determined by the tap. It will ,however result in broken taps. The clearance of the thread is flank to flank. The crests have nothing to do with it.

[cuemaker]

No, reducing the hole diameter will not change the class of fit, which is determined by the tap. It will ,however result in broken taps. The clearance of the thread is flank to flank. The crests have nothing to do with it.

This is my understanding also, strictly speaking in terms of class's of thread fit. The best way I understand it is that the thread fits are "tolerances". The higher the thread fit, the tighter the tolerance between then bolt and nut.

But I am an arm chair quarterback here......

[danlb]

Are you sure that you can't get such taps in the 3rd most industrialized nation in the western hemisphere?

Those wizards among you who have given bogus info, read cuemaker's post again.
No, reducing the hole diameter will not change the class of fit, which is determined by the tap. It will ,however result in broken taps. The clearance of the thread is flank to flank. The crests have nothing to do with it.


Very true when talking of taps. The profile and location of the flanks are fixed when using a tap. When using an adjustable die and adjusting the diameter of the screw/bolt/rod you can match it closely to the thread.

Sidenote; It was not until i read this thread that i realized why i can never fit a tap into a die that it is supposed to match. The tap is supposed to remove enough metal so that it does not have an interference fit with the metal left by the die. If the tap fits in the die, that means there will be an interference fit because the tap is too small or the die too big. I guess that I have ruined a few dies by running a tap through them to clean out the burrs. OTOH, they were cheap dies if they had burrs in the first place.

Dan

[JoeLee]

The above information is incorrect. Typical hardware store thread fits IF made properly are to Class 2.

The correct basic info you want is below...How you achieve these thread fits is better explained by others.

Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads.

Class 1 threads are loosely fitting threads intended for ease of assembly or use in a dirty environment.

Class 2 threads are the most common. They are designed to maximize strength considering typical machine shop capability and machine practice.

Class 3 threads are used for closer tolerances.

Class 5 is for interference thread fits...

From what I remember..... H3 is the standard fit. Anything over becomes a looser fit and anything below 3 becomes tighter. For taps up to about 5/16" each number designation is about .0005. Larger taps like from 1/2" and up each number designation is about .001.
This chart somewhat verifies it.

JL.....................

Pitch Diameter and Pitch Diameter Limit
http://images1.mcmaster.com/Contents/gfx/small/a2cga8s.png?ver=28479208
http://images1.mcmaster.com/Contents/gfx/small/a02c-abouttapsd1-gis.png?ver=29254913

Pitch diameter is the distance across the tap from the points where the width fo the thread equals the width of the space between the teeth. This is the reference point for determining tolerance (pitch diameter limit).
Pitch diameter limit is the tolerance above pitch diameter. For a looser thread fit or for materials that may shrink, select a higher pitch diameter limit. Pitch diameter limits are shown with the letter H (D for metric taps) followed by a decimal or decimal range in the table below. Most pitch diameter limits provided in this section are for Class 2B thread fit.

[lazlo]

From what I remember..... H3 is the standard fit.

I'm not sure that matters anymore. The sh!tty Chinese hardware you get at the big box stores is somewhere around Class 1.

You have to special order quality fasteners nowadays.

[JoeLee]

I'm not sure that matters anymore. The sh!tty Chinese hardware you get at the big box stores is somewhere around Class 1.

You have to special order quality fasteners nowadays.

I agree 100% but you won't find sh!tty chinese taps that come in different classes of thread fit and yu won't find good taps that do come in different classes of thread fits in a hardware store. I'm talking good quality taps, Cleveland, Union Butterfield, etc. etc. and others that you would buy from an industrial tool supply place. My point was that I think Cuemaker was wrong in his listing that class 1 was a looser fit, and class 4 is a tighter fit. I think he had it backwards.

JL................

[elf]

Are you sure that you can't get such taps in the 3rd most industrialized nation in the western hemisphere?

Those wizards among you who have given bogus info, read cuemaker's post again.
No, reducing the hole diameter will not change the class of fit, which is determined by the tap. It will ,however result in broken taps. The clearance of the thread is flank to flank. The crests have nothing to do with it.

You may find the tolerances for threads are specified in both pitch and major/minor diameters. Changing the hole diameter will change the fit for the major/minor diameter but not the pitch.

[deltap]

I have used screws supplied at work that have an interference fit out of the box. On close inspection they are not round but sort of three lobed external threads. They appear to be rolled threads but I'm not sure how they are made. They may be intended to roll a thread in a drilled hole in fairly thin material but do have an interference fit on a nut. Maybe these would suit your purpose.

[davidwdyer]

David,

I don't see why something like this made with 1/2" or 12 mm hardware store threaded rod and coupling nuts wouldn't work.
http://i766.photobucket.com/albums/xx301/camdigger/Shop%20made%20tools/PB291373.jpg

If gripping force or "dialing in" is a concern, add a 4th bolt and revise the spacing to the corners of the plate.


That is pretty much what I am trying to do. Is there some way to see a bigger pic?

[Don Young]

Since final adjustment has to be made with the nuts or other locks snugged up anyway, I don't think thread fit would be a concern.

[camdigger]

That is pretty much what I am trying to do. Is there some way to see a bigger pic?

PM me an email and I will try to hunt up a pre-compression image.


FWIW, that is the home built steady for my Taig lathe.

[gizmo2]

Thanks Cuemaker! You set it straight without casting dispersions on my heritage... I was going from memory and had it just backwards. Happens a bunch any more. This old age stuff is not for the feint of heart.

[cuemaker]

My point was that I think Cuemaker was wrong in his listing that class 1 was a looser fit, and class 4 is a tighter fit. I think he had it backwards.

JL................

In terms of fasteners and thread fits.. I do not have it wrong.

In terms of taps and dies, to which your chart I think refers to, I have no clue.

[spope14]

H2 and H3 are the common tap fits for class 2 threads. H1 will give a slightly finer fit if the screw is made properly.

If you can single point the thread, go to Machinery's Handbook and look up class three thread pitch diameter tolerances, as well as the major diameter tolerances. Run these to the high limits and you get a pretty good feeling thread or can even tighten it up. I have not been able to find this chart on-line, but if someone out there knows where it may be, post it. You may also be able to find the class 4 and five interferrence fits.

You can "kind of" mess this in with a split die, but thread fit is the fit of the "V"'s, not the shaft and hole when done correctly, and even interferrence threads are made to be assembled and disassembled.

Do NOT mess with tap drill sizes too much but to maybe go towards an 80% total thread height. Much more than this and taps break - as noted, or tapped threads start to be "galled" (I hope this term is correct, but they are pretty beat up and torn) ndue to a lack of chip clearance and material deformation, yes tapping does deform material when the hole is too small because even fairly well made taps do not have a completely sharp "V" bottom thread form, but have a bit if a radius at about the 80% to 85% theoretical thread height.