What's wrong with my threading technique?

[Paul Alciatore]

Yes, you should start with the proper OD and CLEAN UP THE BURRS after cutting!

And burrs will produce the exact symptoms that you are experiencing. The deeper that you thread the male thread into the female one, the more resistance will be experienced.

DEBURR YOUR THREADS!

NO.. don't reduce your diameter to account for burrs! Burrs happen on gummy materials. Set your starting diameter correctly (not technically correct I usually go a couple of thou less than max, but often just start at max and even have been known to feed the tool like a cutter to reduce the od after.), go to the correct depth (measure with wires or whatever), knock the burrs and a tad more off off with a file, clean up with cratex.

Threading external is very simple. You are fighting it way to much.

[J Tiers]

An internal threading tool has a different profile than an external one. You should have used an external insert for an external thread.

Examine the illustration. The external thread has crests that are narrower than the crests of an internal thread. By the same token, the external thread is shallower than the root internal one.

Dan.... look at your post.

The PD of the thread is smack dab in the middle of the sharp V theoretical thread ("4/8 H"). It's all laid out right there plainly.

The sharp V and the UN share the same PD. The UN has the thread profile modified from the sharp V, but not the PD.

The flat is 1/8 of the Height below the crest of a v-thread. that is the ideal form. The minimum major diameter is with the flat at 1/8 H below the V thread crest. The maximum is just below the V thread crest. But that is not changing the PD by that amount. The PD has its own tolerance. The variation in major diameter is a variation that only changes the flat location vs the V-thread crest from 1/8 H to something less. it does not imply a change in PD. the right PD is the right PD and comes first.

If you start with a diameter at the ideal flat diameter, you will cut a thread having the minimum major diameter.

The OP is having trouble with threads that are too large, and do not enter the nut.

So to eliminate troubles due to other issues, if he cuts a thread that has the minimum major diameter, which will happen if he starts with the smallest OD prior to threading, then if he can clean off just the burrs without cutting down the OD, he will have the theoretical correct major diameter. It will be a correct thread if he gets the PD correct.

If THAT measures OK but does not fit, then there is a different issue. With a correct male thread (much easier to measure), the nut must be at fault, tap or no tap.

We need to find out the actual PD of what he is cutting. odds are that it is going to be very different from what it would be if everything he thinks is going on really HAS gone on.

There is no way he cut an additional 0.060 on the RADIUS and it did not fall right through the nut, unless the OD of the blank, or the threads of the nut, are NOT what has been stated.

BTW, since he is using a plain V cutter, NOT a topping or full form cutter, the only issue is the size of the flat on the end. It appears that what he has will cut a considerably finer thread, so the root should be clear of anything that may interfere.

[PStechPaul]

One problem with threading on the back side of the piece is backlash in the compound, which can be as much as 0.060". If the gibs are just moderately tight, advancing the cut by cranking the compound into the work could cause the tool to back away from the axis of the work, causing a taper, and giving the impression that the tool is not cutting as much as it should. The indicator dial could turn quite a bit before the backlash is taken up and the tool cuts consistently. Backlash may act differently with force applied in a different direction from turning. However, it is the same direction (toward the operator) as boring.

[danlb]

Yes, you should start with the proper OD and CLEAN UP THE BURRS after cutting!

And burrs will produce the exact symptoms that you are experiencing. The deeper that you thread the male thread into the female one, the more resistance will be experienced.

DEBURR YOUR THREADS!

I've never had a burr tall enough to interfere with a properly cut thread. I can't imagine a .006 tall burr on a .031 thread. That's a lot.

OK, after lunch I will single cut a 5/16-24 thread in 6061 and measure the burr. I'll try to get photos too.

I did not bother with Jerry's post. I trust that eventually he will manage to rephrase what I said and get it right, but reading it is not worth the time.

Dan

[tom_d]

My two cents says the problem is the insert and the way it's being used. To cut internal threads in the usual setup the point of the insert faces the operator and is fed to the left to cut a right hand thread, and will have relief to clear the helix angle of the thread.

If I understand what you are doing, the insert is put behind the work, right side up, point facing the operator, lathe ran in reverse, the tool feeding to the right to cut a right hand thread. If this is the case then the helical relief on the insert is opposite what it should be, it's cutting on the blunt side, and probably not cutting as much as you are expecting.

I would like to expand on this and offer the following: http://www.korloy.com/newkorloy/eng/file/k16.pdf with attention to the chart on page 2; "Helix Angle Diagram". Also, has it been confirmed that the threading tool used is set at the correct center height? A little bit above center would cause flank rubbing and thread profile deformation.

[tomato coupe]

I would like to expand on this and offer the following: http://www.korloy.com/newkorloy/eng/file/k16.pdf with attention to the chart on page 2; "Helix Angle Diagram".

I'm glad you posted the chart, as it illustrates something no one seems to care about; a right-hand internal threading insert is not designed to be used in the manner the OP is using it.

[754]

He is running the insert wrong direction, without enough clearance angle on the insert, looks like to me.
Again, start with the basics, determine wether the lathe and you can cut threads, before you try some oddball setup..

[txfireguy2003]

Okay, so I went back to basics and cut a thread the normal way. My starting OD was a bit undersized at 0.310, but it worked anyway. I fed the compound (set to 30 degrees) in 10 thou on the radius and made a cut, then another 10 and made a cut. Threads starting to look pretty decent, so I tried the knob on them. To my surprise, it went on a short distance, maybe two or three threads before jamming. I continued to cut, taking 1-2 thou each pass, and a spring cut after each pass (sometimes I'd get a few more chips, others nothing) until I got to a total of 25 thou on the compound slide. I test fit the knob after each consecutive cut and with each pass, it went on further and further before jamming. Finally, at 25 thou on the cross slide, I was able to thread all the way to the shoulder. Still though, it was somewhat sloppy at first, then got tighter toward the end, but I suppose that's fairly normal, plus the slop could easily be in the female part.

What this tells me, correct me if I'm wrong, is that my machine is set up right, and I at least understand the concept. I've learned a lot of things from this thread, but I think I'll stick to the normal method for now, and maybe order a different threading tool and try the reverse method another time, with a proper male threading profile, in a left hand holder, running upside down. It's the same concept, allowing you to run away from the chuck but uses the correct insert, I wasn't aware they were different. The benefit to working on the back side is that the cut is on top of the tool, so you can see it better, but I guess a mirror would solve that with an upside down tool.

Sent from my SAMSUNG-SM-G891A using Tapatalk

[tomato coupe]

Okay, so I went back to basics and cut a thread the normal way.

What is "the normal way?"

Finally, at 25 thou on the cross slide, I was able to thread all the way to the shoulder. Still though, it was somewhat sloppy at first, then got tighter toward the end, but I suppose that's fairly normal, plus the slop could easily be in the female part.

Have you made another nut that is fully threaded, not blind? It is easy to do and could tell you a lot ...

[txfireguy2003]

What is "the normal way?"



Have you made another nut that is fully threaded, not blind? It is easy to do and could tell you a lot ...

Normal way is with the tool on the front, part turning toward you, threading toward the chuck. Compound set to 29-30 degrees, feed in with the compound, make a pass, out with criss slide, reset carriage to the end of the part, dial cross slide back to zero, feed in with compound, take a pass and repeat.

I have not made a through nut, nor purchased a commercial one.

Sent from my SAMSUNG-SM-G891A using Tapatalk

[J Tiers]

I'm glad you posted the chart, as it illustrates something no one seems to care about; a right-hand internal threading insert is not designed to be used in the manner the OP is using it.

That chart appears to be SNAFU.

Diagrams 1 thru 4 seem OK, but diagrams 5 thru 8 are flat wrong, since the chart and the table do not agree on the rotation direction that is being discussed.

for diagram 5 the table SAYS counterclockwise, but diagram 5 actually SHOWS a red arrow for clockwise. 5,7, and 8 are likewise backwards as to direction.

That chart may not be as reliable as it appears from first glance.

[danlb]

Yes, you should start with the proper OD and CLEAN UP THE BURRS after cutting!

And burrs will produce the exact symptoms that you are experiencing. The deeper that you thread the male thread into the female one, the more resistance will be experienced.

DEBURR YOUR THREADS!

TL;DR There were burrs but it fit anyway.

Full story.

I just single pointed a 1 inch long 5/16-24 (5/16 UNF) thread with unknown aluminum.

I turned the shaft down to .3105 to start. All measurements were with my caliper since extreme accuracy was not called for. I verified that there was no taper. I turned a run out groove .027 deep at the chuck side.

I used a TCMT21.50 coated carbide insert. Compound set perpendicular to the lathe axis. Infeed via cross-slide. About 50 rpm.

Multiple passes were starting with .010 and ending with a spring pass. . Nothing came off the spring pass.

I measured the diameter again. It was 0.3150. Wow. That's 0045 bigger than when I started. Would it interfere? Good question.

I rummaged in my screw and nut bins for a suitable screw and found I had none. I eventually found one in the 12-24 bin. It looked right but I verified it was really 5/16-24 by threading it onto a tap. It went on without cutting into the nut. The tap measured .3155 across the cutting edges.

It fit well. No wobble. No binding.

Paul is correct. There will be burrs, even if they are too small to see. I did not expect that. They should be cleaned up.

[oxford]

OP, it is time to order a commercial nut and bolt or some sort of quality gauges. You state you are not a hardware store but you could have had this stuff already delivered from McMaster since this thread has been started.

Using a “nut” you made is fine in some applications but you state your final application is gunsmithing and I am guessing customer work. Getting a bolt handle you threaded to fit a nut you threaded is one thing but what happens when that customer wants a different bolt knob but a new commercially one now doesn’t fit properly because you made it “fit” a random know you made.

[tomato coupe]

That chart appears to be SNAFU.

Diagrams 1 thru 4 seem OK, but diagrams 5 thru 8 are flat wrong, since the chart and the table do not agree on the rotation direction that is being discussed.

for diagram 5 the table SAYS counterclockwise, but diagram 5 actually SHOWS a red arrow for clockwise. 5,7, and 8 are likewise backwards as to direction.

That chart may not be as reliable as it appears from first glance.

A typo in the chart does not invalidate the diagrams, which are self-consistent.

[tomato coupe]

TL;DR There were burrs but it fit anyway.

Full story.

I just single pointed a 1 inch long 5/16-24 (5/16 UNF) thread with unknown aluminum.

I turned the shaft down to .3105 to start. All measurements were with my caliper since extreme accuracy was not called for. I verified that there was no taper. I turned a run out groove .027 deep at the chuck side.

I used a TCMT21.50 coated carbide insert. Compound set perpendicular to the lathe axis. Infeed via cross-slide. About 50 rpm.

Multiple passes were starting with .010 and ending with a spring pass. . Nothing came off the spring pass.

I measured the diameter again. It was 0.3150. Wow. That's 0045 bigger than when I started. Would it interfere? Good question.

I rummaged in my screw and nut bins for a suitable screw and found I had none. I eventually found one in the 12-24 bin. It looked right but I verified it was really 5/16-24 by threading it onto a tap. It went on without cutting into the nut. The tap measured .3155 across the cutting edges.

It fit well. No wobble. No binding.

Paul is correct. There will be burrs, even if they are too small to see. I did not expect that. They should be cleaned up.

Your experience isn't too surprising considering the low (4 sfm) surface speed. Carbide tooling isn't real happy about that and it likely wasn't cutting very cleanly.