Single Point Threading 101

Topics include, Machine Tools & Tooling, Precision Measuring, Materials and their Properties, Electrical discussions related to machine tools, setups, fixtures and jigs and other general discussion related to amateur machining.

Moderators: GlennW, Harold_V

User avatar
Harold_V
Posts: 20231
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Single Point Threading 101

Post by Harold_V »

earlgo wrote: Fri May 21, 2021 9:09 am Harold wrote:"...and you don't have the option of the thread relief."
Please explain to this ignorant mechanical engineer when it is necessary for no thread relief.
Alas, not being an engineer, I am not able. All I can say is that when I was in training, multiple parts that required a thread were not permitted a thread relief. As this was in a facility where a solid propellent rocket was built, I have to assume that the engineers in question had good reason for their decisions. It was not for me to second guess their work---it was for me to conform.

Bottom line here is that if one works to prints, where features are inspected, and there is no callout for a thread relief, it can NOT be added.

In my mind, this topic is akin to the one about working to tight tolerance. If one works as if there is no thread relief, the skill level required is developed. If not, then when it is required, it may be overwhelming. It's like playing the scales over and over, until it becomes routine. You'll never be sorry for improving skill level, but you may regret not doing so when faced with the need.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
User avatar
Bill Shields
Posts: 10459
Joined: Fri Dec 21, 2007 4:57 am
Location: 39.367, -75.765
Contact:

Re: Single Point Threading 101

Post by Bill Shields »

back when spark plugs had copper gaskets against the shoulder...a groove was a recipe for 'non-sealing' because the gasket could go non-concentric.

Yes...they the threads are rolled..and IIRC they now have tapered seats (been a few decades since I had one out.)

As opposed to my Sunbeam Tiger where it was necessary to take hatches off the transmission tunnel and reach in with a special tool to get the rear two plugs out.

and don't get me started on having to pull the engine mount bolts and jack the engine up to remove the valve covers so that the non-hydraulic lifters could be adjusted.
Too many things going on to bother listing them.
stephenc
Posts: 311
Joined: Sat Jul 05, 2014 6:13 pm
Location: youngstown ohio

Re: Single Point Threading 101

Post by stephenc »

I seen a hardinge lathe that was set up specifically for threading .
The carriage / tool retract stop had a little turret on it with different length buttons .

Say you had the lathe set up to finish your thread in 5 passes . on the first part you ran you set the stop turret to the first position . then it would automatically turn itself to the next button for each pass .

The different length buttons made up for the tool moving forward on each compound advance so the tool retracted at the same spot on each pass .
earlgo
Posts: 1794
Joined: Sat Jan 29, 2011 11:38 am
Location: NE Ohio

Re: Single Point Threading 101

Post by earlgo »

Harold wrote: "Bottom line here is that if one works to prints, where features are inspected, and there is no callout for a thread relief, it can NOT be added."
I cannot disagree at all with this statement. Critical industry is different than hobby machining.
IMHO anyone who cuts a thread up to a shoulder while making a hobby item, is over-killing the project.
At the Trade school where I learned gunsmithing, we were required to cut barrel shank threads up to the shoulder. Later in life the question arose and I got out my calculator and, if I remember correctly, determined that with 140,000psi chamber pressure (a blue pill load) one only needed about 2 threads in shear to hold the barrel in place on a Mauser 98 sized rifle. The students at the trade school were instructed to do abutment threading apparently because the arsenals did it.
My apologies for rambling off-topic.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
User avatar
Steggy
Posts: 1976
Joined: Tue Jun 28, 2011 9:19 pm
Location: JB Pritzker’s Hellhole
Contact:

Re: Single Point Threading 101

Post by Steggy »

Harold_V wrote: Fri May 21, 2021 3:19 pm
earlgo wrote: Fri May 21, 2021 9:09 am Harold wrote:"...and you don't have the option of the thread relief."
Please explain to this ignorant mechanical engineer when it is necessary for no thread relief.
Alas, not being an engineer, I am not able. All I can say is that when I was in training, multiple parts that required a thread were not permitted a thread relief. As this was in a facility where a solid propellent rocket was built, I have to assume that the engineers in question had good reason for their decisions. It was not for me to second guess their work---it was for me to conform.
A thread relief may lead to the formation of a stress riser, which can cause a fracture under conditions of significant vibration and/or load cycling. That would be the likely reason the rocket engineers forbade a thread relief.

Unbrako socket head capscrews (the strongest threaded fasteners currently available from stock) have what the manufacturer calls a "radiused root runout" (RRR) at the point where the (rolled) thread ends. On socket screws that are fully threaded, the RRR nicely blends the thread runout into the head's seating face, greatly reducing the potential for the formation of a stress riser. Naturally, correct countersinking of the mating surface is required to prevent interference between the RRR and part being fastened.

I should note that the threaded portion of a socket shoulder screw (aka stripper bolt) does have a thread relief at the juncture with the shank. However, the relief's purpose has nothing to do with forming the threads—the relief allows the screw to fully seat on the mating part without a need for a countersink. While stripper bolts have high shear ratings, tensile ratings are lower than their socket capscrew cousins due to the relief.
———————————————————————————————————————————————————————
Music isn’t at all difficult.  All you gotta do is play the right notes at the right time!  :D
User avatar
rmac
Posts: 787
Joined: Tue Jun 26, 2012 12:48 am
Location: Phoenix, Arizona

Re: Single Point Threading 101

Post by rmac »

earlgo wrote: Sat May 22, 2021 8:51 am My apologies for rambling off-topic.
As the OP of this particular thread, I say no apologies needed. All kinds of interesting stuff comes up when these conversations wander a little bit (or even a lot).

-- Russell Mac
User avatar
Harold_V
Posts: 20231
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Single Point Threading 101

Post by Harold_V »

BigDumbDinosaur wrote: Sat May 22, 2021 4:23 pm A thread relief may lead to the formation of a stress riser, which can cause a fracture under conditions of significant vibration and/or load cycling. That would be the likely reason the rocket engineers forbade a thread relief.
True, that, and any approved thread relief at that facility was designed as a full radius, never a sharp corner. There was a compromise of cross section in favor of eliminating the potential for stress risers.

There's a potential for stress risers to originate at the roots of threads, too, which has been addressed with a radius (J series) at the root instead of a flat. Anything but a sharp corner is a huge step towards greater resiliency.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
User avatar
Harold_V
Posts: 20231
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Single Point Threading 101

Post by Harold_V »

rmac wrote: Sat May 22, 2021 5:12 pm
earlgo wrote: Sat May 22, 2021 8:51 am My apologies for rambling off-topic.
As the OP of this particular thread, I say no apologies needed. All kinds of interesting stuff comes up when these conversations wander a little bit (or even a lot).

-- Russell Mac
Yep! If folks don't get their shorts in a knot when others post related, but often somewhat off-topic comments, there's no end to the benefits for those who are seeking knowledge. I, personally, encourage folks to comment. I am not keen on those who think they are the only ones who should be heard.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
earlgo
Posts: 1794
Joined: Sat Jan 29, 2011 11:38 am
Location: NE Ohio

Re: Single Point Threading 101

Post by earlgo »

Harold wrote: "...and any approved thread relief at that facility was designed as a full radius, never a sharp corner.
This is very true. I looked up stress concentration factors in Ruark's "Formulas for Stress and Strain" 3rd edition McGraw-Hill Book Co., Inc 1954 LCCN 53-5173 and once again was confused. :lol: The upshot was that fatigue, ie repeated loading, has a lot to do with failure as opposed to static loading. In Table XVII example 11 is shown a cylindrical section, D, with an axial protruding smaller circular section, d, connected by a radiused interface, r. Depending on the alloy and the r/d ratio, the stress concentration factor for repeated bending varies from 1 to 2.2. For the same situation, the stress concentration factor for repeated tension varies from 1.4 to 2.1. This should be clear and obvious to the casual observer. :shock:
For us hobbyists, a square cornered cutoff tool should not be used to make a thread relief but use instead a cutoff tool with as large a radius as possible on the shoulder side of the tool.
BDD,I am willing to be corrected.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
User avatar
Steggy
Posts: 1976
Joined: Tue Jun 28, 2011 9:19 pm
Location: JB Pritzker’s Hellhole
Contact:

Re: Single Point Threading 101

Post by Steggy »

Harold_V wrote: Sun May 23, 2021 1:20 amThere's a potential for stress risers to originate at the roots of threads, too, which has been addressed with a radius (J series) at the root instead of a flat. Anything but a sharp corner is a huge step towards greater resiliency.
Stress risers are basically what brought down the world's first jet airliner.
———————————————————————————————————————————————————————
Music isn’t at all difficult.  All you gotta do is play the right notes at the right time!  :D
User avatar
Steggy
Posts: 1976
Joined: Tue Jun 28, 2011 9:19 pm
Location: JB Pritzker’s Hellhole
Contact:

Re: Single Point Threading 101

Post by Steggy »

earlgo wrote: Sun May 23, 2021 2:35 pmBDD,I am willing to be corrected.
Corrected on what?
———————————————————————————————————————————————————————
Music isn’t at all difficult.  All you gotta do is play the right notes at the right time!  :D
roundnose
Posts: 103
Joined: Thu Nov 11, 2010 4:21 pm

Re: Single Point Threading 101

Post by roundnose »

I do a somewhat specialized threading where as the requirement generally forbids using relief grooves, the threads pitch is usually 12 tpi v threads, but also modified buttress, on occasion.
The other requirement is the nut must go on without bind, with no perceivable shake. It must be done quickly using no extra gages.
Its hydraulics!
The easiest way to do that is with a machine that has leadscrew reverse, or very effective forward-brake-reverse, like the Axelson I am fortunate to be able to use, or in a sense drive, where I sometimes work these days.
Eliminating having to disengage the half nuts, ensures better threading accuracy from the effect of the error possible from the nuts camming in, and settling in. Also, the machine can be run faster then the halfnuts can reasonably be engaged, and disengaged, resulting in a better finish with carbide.
Makes it easier to get that free, but no shake fit, the hydro techs want, and the no relief groove.
With practice, engaging the halfnuts on regular lathes, and try to allow the carriage to travel a short distance before the tool engages, will help getting predictable results.
Post Reply