Threads

[Gerry]

Sorry but this has been bothering me for a few days

Its NOT a criticism, just an observation, but the thread look very flat. There seems to be no peaks on each thread and that worries me.

To get the max strength from a thread it needs to be formed with a good profile. If the peak (or point) of the thread is flat there is something wrong.
So now we got to get in to taps and dies.
In general the dies with a hex outside are 'dressing' dies and low cost. That is they are designed to dress or reform threads. If you want to cut a thread from a piece of bar you use split dies. Thats the ones with a round outside and a slot in them with a screw that allows adjustment for diameter differences. I payed 20 bucks for a set of 1/2 inch UNC taps and another 12 bucks for the die. Now think about the number of different sizes you are going to use and the price of premium stuff is not that bad.
There are 2 types of threads. Cut and Rolled. Rolled are the strongest but not really achievable at home, So we go for cut threads. Now here is where its get to 'You get what you pay for'
I have a set or 3 of the hex dies and use them to run down thread that are a bit tired, and thats all I use them for!!!
If I am cutting a new thread on fresh bar I always USE a premium brand split dies such as Dormer. You will feel the difference when you cut the thread.I also have the advantage of a Lathe which means I can chuck up the bar and use the tailstock to centre the die on the bar which gives me a true and parallel thread.
If the thread 'wobbles' over it length... its NOT a good one.

There is also screw cutting, but thats another ball park all together

Spend a little it will pay you back in peace of mind

Gerry

 

[2old2fast]

hey G, that's one crappy looking thread.! Have you had any luck,[or the opportunity] to try and thread stanless rod? my old bluepoint [snap-on] set just won't get the job done.


dave

 

[ellis8500]

Here's a tip for straight threads in a flat surface. I use a drill block to get a perpendicular hole. Then, I drill a hole in a wood block [drillpress] that is same diameter as the tap or just a bit larger. Use the block to start the tap so it starts perpendicular. after a few turns, remove the block and finish the threads.

 

[Keeper]

I picked up a "Titanium coated tap and die" set that I use for most of the little stuff, works well for cleaning threads and making some on smaller items.

For the big stuff (1/2 or bigger) I picked up some nice Made in US taps, I also found that a good cutting oil helps. After screwing up a few 5/8-18 holes I went and picked up some cutting oil, made a world of difference.

Oh and it helps having a nice t-bar I snapped the one in the kit twice so I found a local shop that had some good German made t-bars. Sure does help when cutting those damn 5/8 threads!

 

[Gerry]

hey G, that's one crappy looking thread.! Have you had any luck,[or the opportunity] to try and thread stanless rod? my old bluepoint [snap-on] set just won't get the job done.


dave

Dave
Here one on stainless.

With split dies you can open the die up, run it down and then close it again for a final size cut. Use plenty of cutting oil always helps.
If you are turning the part first, make it slightly under size on diameter with stainless. Also when tapping stainless above 6mm diameter drill it 0.1mm bigger than the recommended tapping size. This was told to me by the engineering company I know who make their own all alloy billet V8 from scratch, so they should know

I dont find the set you mentioned very good. Again if they are the ones I think they are... designed for dressing a thread not cutting from scratch.
Gerry

 

[Old Rotor Flap]

As a sidebar here, I usually don't buy titanium nitrided or TiN for home use as they are more expensive.

I do buy them for use where the TiN coating have an advantage and that's in tool life. TiN extends tool life when controlling speeds and feeds such as in a CNC machine. Speeds and feeds can't be controlled very well in a hand drilling or threading operations.

 

[Gerry]

As a sidebar here, I usually don't buy titanium nitrided or TiN for home use as they are more expensive.

I do buy them for use where the TiN coating have an advantage and that's in tool life. TiN extends tool life when controlling speeds and feeds such as in a CNC machine. Speeds and feeds can't be controlled very well in a hand drilling or threading operations.

I agree with that. Good old HSS is enough for us home builders, so long as its good quality.
Gerry

 

[Gerry]

By the way that thread in the first post is not one of mine. Its one of a member that recently joined. The reason for my post was to try and give some advice and help. I hope that person can see we are trying to help not 'trashing' his efforts.
I remember my first threads (as in ones cut on a round bar) and they were terrible.
Gerry

 

[spike]

When you're talking thread dies, the die should be made of tool steel or high speed steel to actually cut a thread from scratch. Carbon steel dies are not hard enough to cut a thread but are sufficient in dressing a banged up thread. You can also fix a banged up thread with a thread file. As far as the flatted peaks go, which is the crest of the thread, it is not critical for the crest to be pointed. The actual grip of a thread is created at the pitch diameter which is a theoretical line down in the "V" of the thread. Most machined external threads are flatted as the outside diameter is generally turned undersize. On an internal thread the tap drill diameter normally creates a thread that is actually 65 to 75% of the full thread form which will produce a flatted thread crest. Once again the critical diameter is the pitch diameter not the nominal diameter.

Spike

 

[RPM]

My self I prefer Cobalt tooling. It lasts so much longer, it is worth the extra money. Drills and taps last more than twice as long.

 

[Gerry]

When you're talking thread dies, the die should be made of tool steel or high speed steel to actually cut a thread from scratch. Carbon steel dies are not hard enough to cut a thread but are sufficient in dressing a banged up thread. You can also fix a banged up thread with a thread file. As far as the flatted peaks go, which is the crest of the thread, it is not critical for the crest to be pointed. The actual grip of a thread is created at the pitch diameter which is a theoretical line down in the "V" of the thread. Most machined external threads are flatted as the outside diameter is generally turned undersize. On an internal thread the tap drill diameter normally creates a thread that is actually 65 to 75% of the full thread form which will produce a flatted thread crest. Once again the critical diameter is the pitch diameter not the nominal diameter.

Spike

If you want to see the technical definition have some fun with this. The major diameter is part of the calculation and is important for the overall strength of the thread.
The screw should fail before the thread strips. For this, it is necessary the shear area of the threaded feature be at least 2 times the tensile area.

Where:

L[sub]e[/sub] = Thread Engagement Length
A[sub]t [/sub]= Tensile Stress Area
D = Major Diameter of fastener (screw)
p = 1 / Number of threads per inch (n)





Gerry

 

[spike]

If you want to see the technical definition have some fun with this. The major diameter is part of the calculation and is important for the overall strength of the thread.
The screw should fail before the thread strips. For this, it is necessary the shear area of the threaded feature be at least 2 times the tensile area.

Where:

L[sub]e[/sub] = Thread Engagement Length
A[sub]t [/sub]= Tensile Stress Area
D = Major Diameter of fastener (screw)
p = 1 / Number of threads per inch (n)



Gerry

Sorry Gerry, I thought you were threading a part for your T bucket. I didn't realize you were building a nuclear reactor! lol

seriously, the OD is important, however a sharp V is not. For example to obtain a sharp V on a 1/2-13 2A thread, you would need an OD of .500". The specs for that thread say you can turn the OD to a minimum of .487" which would produce a flatted thread.

Spike

 

[ellis8500]

Typical bolt threads do not come to a sharp point. SAE only specs are for a 75% to 80% thread pitch form. Actuall, the top of the thread is a bit flat since it only is 75% of the height of a perfect triangle. The internal thread is almost to a sharp V. This design provides a clearnce between the bolt and hole and creates a space for some amount of contaminant. One can find this info in the Machinery Handbook.

 

[Youngster]

I can see and understand what the fabricater was trying to do in the post #1. When I'm faced with this problem, I drill and tap a piece of round stock, drill a cross hole, and insert a lenght of thread. Then plug weld the tread to the tube. If I need a flat for tightening, I will add a jam nut to the thread and weld it in place.

As for tapping holes, this is my procedure. Once the correct hole is drilled, I locate a nut welded to a plate with the desired thread using the drill as a guide and clamp the plate to the work piece. Then simply run the tap thru the nut and work piece. I use a wax stick made by DuAll to lube the tap. Result is a bur free thread at a 90 from the surface. I have a whole set of these guide made up for all the most used threads.

Ron

 

[Gerry]

Sorry Gerry, I thought you were threading a part for your T bucket. I didn't realize you were building a nuclear reactor! lol

seriously, the OD is important, however a sharp V is not. For example to obtain a sharp V on a 1/2-13 2A thread, you would need an OD of .500". The specs for that thread say you can turn the OD to a minimum of .487" which would produce a flatted thread.

Spike

Just though I would throw it in.
I am not that 'fanatical' just wanted to show there is more to a simple thread than meets the eye. Its the simplest things that sometimes let us down.
Gerry

 

[Keeper]

As for tapping holes, this is my procedure. Once the correct hole is drilled, I locate a nut welded to a plate with the desired thread using the drill as a guide and clamp the plate to the work piece. Then simply run the tap thru the nut and work piece. I use a wax stick made by DuAll to lube the tap. Result is a bur free thread at a 90 from the surface. I have a whole set of these guide made up for all the most used threads.

Ron

Now that is a great idea. I will have to make up some of these plates. I ran into an issue with crooked threads when I was tapping the rod ends.

 

[Youngster]

For tapping tie rod ends, I have a piece of 7/8" id DOM with a piece of5/8" id, 7/8"od welded in it. I tilt my drill table and clamp the tie rod, drag link or hairpin to it and drill my pilot hole. For tapping I have another setup with an 11/16" jam nut welded to it as a guide. Haven't had a crooked thread yet, knock on wood.

Ron

 

[Ted Brown]

Gerry, you opened a can of worms here, hehe