Making Tight Threads

[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.

[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.

[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.

[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.

[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.

[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.


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?

[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.

[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.

[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................

[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]

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



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.

[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

[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......

[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.

[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