Another threading question

[ShawnR]

I know single point threading is discussed many times, on many websites, including this one, but I came up across something today that I am confused on.
Perhaps I just have not read enough.
I have a scope for which Vortex does not make a sunshade, which I want. I thought it would be good experience too. Threads are visible in the objective opening (that the lens retainer is held in with so they are not intended for a sunshade). After some emails back and forth, Vortex tells me they are a 45 x 0.6 mm - 6H. I have determined that the 6H is a "fit" and it is normal, if I understand correctly (ie not too tight, not too sloppy). I measure the opening to be 44.23 mm so I think the 45 is correct. Searching on line, I have found 45 mm threads but not in a 0.6mm pitch. My compound is set to 29.5 (no, I don't want to start a 29 vs 29.5 vs 30 battle) and it has been moved a total of 0.020" (0.504 mm)
But when I tried the scope onto the adapter (still in lathe), it is like it is just way too large. Threads won't even appear to start. I have sanded the final thread, touched the edge with a file, ran a 3M cloth over it. A 0.6 metric pitch gauge fits nicely into both threads so I believe I have the right set up.

My questions are, with no data that I can locate, how do I know when I have moved the compound enough? I suppose if I had a final minor dimension then I would do some math? Or have I just not found the data I need? I suppose they could be proprietary and therefore, no data available. Vortex did free up the size and pitch, but no other info. Or is there a rule of thumb that I should be aiming for (with regards to how far I move the compound)

I should add that I did grind the bit. It fits quite nicely (I think) into a gauge so I think it is ok, but maybe not good enough. It is such a fine thread, I thought I might grind a small toolbit rather than using my larger insert tooling.

Looking at the photos attached, it does appear to me that they are not "sharp enough" sooooo, maybe I just need to keep going with the compound.? But my questions still stands, ....how does one know when to stop? Especially without data @Brent H and I were discussing thread wires just last week (I said I should get some but have not needed them.....oops ) but I don't think they would help in measuring the required thread. Just my cut threads but I still don't know what I am aiming for.

Does any of that make sense?

 

[ShawnR]

After rereading this post, I am wondering....please correct me if I am wrong...

I kind of have the minimum as I measure the inside of the threads at 44.23 mm, correct? (sort of?)
Is this where I get into some trig to calculate the amount to move the compound? If so, and if I did it correctly, I have gone to a minor diameter of 44.5 mm (0.5 mm movement in compound) so it sounds like, to me, if I did any of this correctly, I need to keep going. Is that correct? And I am on the right track?

 

[PeterT]

Just looking at my go-to website, that particular thread is not listed. But that doesn't mean much, lots of threads are specialty. I just mean unfortunate because it spits out all the dimensional parameters you were asking about. I have some CAD methods that develop related thread cutting parameters kind of from base principles but I wont get to it for a bit.

Dumb question but are you working on a metric screw thread machine or using something 'close' on an Imperial machine? Are the pitches coming out as expected & you feel its juts more of a DOC issue? The other factor that comes into play especially on fine threads is nose radius

Metric screw thread: M Profile calculator

theoreticalmachinist.com

 

[PeterT]

I should stop & think before posting but here goes. This is a 0.6 pitch M3.5 thread, but I think you can compute depth of cut, min/max values on your larger nominal diameter using the thread profile specific values? If I made a brain fart I'll come back & edit

 

[Susquatch]

After rereading this post, I am wondering....please correct me if I am wrong...

Hey Shawn, I think you can forget norms when it comes to those threads. I could never make them work normally anyway. I made my own sunshade too. Normally, I buy them, but I couldn't get one for one of my scopes so I made it. It was a bitch to get right. So don't feel bad.

I did mine using plain old math. I examined the thread on the objective with a magnifying glass to make sure I understood the tooth profile. It was not a regular 60 degree thread - more like a half breed witworth. I did the best I could to grind a hss bit that would be generous against that profile. Loose is ok for this application because it comes tight to a shoulder and it doesn't carry a significant load.

I cut a test tooth profile on a smaller mandrel to make sure I had the right engagement and pitch. You can hold the mandrel to the threads to see if it's ok.

Then I measured the ID of the objective and added double the tooth depth minus 25% and cut my tube tenon to that OD. Then I cut my shoulder, and then cut the thread in two passes. With such a tiny little thread depth, I just plunged right in. No 29.5 to worry about. It fit looser than I would have wanted, but as soon as it hit the shoulder it firmed right up. I've never tried to make it better.

Next time, I might shoot for a slightly larger OD. It's easy to take more off if it's too tight.

Dunno if that will help. Cutting those stupid threads is not a science. But then again, they are not critical.

 

[ShawnR]

I should stop & think before posting but here goes. This is a 0.6 pitch M3.5 thread, but I think you can compute depth of cut, min/max values on your larger nominal diameter using the thread profile specific values? If I made a brain fart I'll come back & edit

lView attachment 55524

Thanks @PeterT. I have never used such a site. I have looked on line but usually only at pdf documents. This thread, what I think it is, could not be found by me either.

The leadscrew is imperial but the gears are in place and I do not disengage anything when threading metric. I think the pitch is good. As for profile, not sure about that. I was happy I got the cutter to look cozy in the thread gauge. I did not consider anything else.

 

[ShawnR]

Hey Shawn, I think you can forget norms when it comes to those threads. I could never make them work normally anyway. I made my own sunshade too. Normally, I buy them, but I couldn't get one for one of my scopes so I made it. It was a bitch to get right. So don't feel bad.

I did mine using plain old math. I examined the thread on the objective with a magnifying glass to make sure I understood the tooth profile. It was not a regular 60 degree thread - more like a half breed witworth. I did the best I could to grind a hss bit that would be generous against that profile. Loose is ok for this application because it comes tight to a shoulder and it doesn't carry a significant load.

I cut a test tooth profile on a smaller mandrel to make sure I had the right engagement and pitch. You can hold the mandrel to the threads to see if it's ok.

Then I measured the ID of the objective and added double the tooth depth minus 25% and cut my tube tenon to that OD. Then I cut my shoulder, and then cut the thread in two passes. With such a tiny little thread depth, I just plunged right in. No 29.5 to worry about. It fit looser than I would have wanted, but as soon as it hit the shoulder it firmed right up. I've never tried to make it better.

Next time, I might shoot for a slightly larger OD. It's easy to take more off if it's too tight.

Dunno if that will help. Cutting those stupid threads is not a science. But then again, they are not critical.

Thanks @Susquatch I will attempt some of those suggestions tomorrow. Yes, a sunshade is available for a Vortex Diamondback "Tactical" but not the Vortex Diamondback (non Tactical). I missed that tidbit and ordered the Tactical one but there is about 2-3 mm difference in diameter. Vortex told me they do not make a sunshade for the non-Tactical version, hence, this adventure of thread cutting.

 

[RobinHood]

The height H of a sharp V-thread with a pitch of 0.6mm is 0.51962mm (Machinery’s Handbook, 29th Edition, Table 3, pg 1881. (Sharp thread is one without a peak / root radius and is only theoretical - the dashed outline in Fig 1. below)

If you are going to keep your compound at an angle, you need trig to figure out the infeed. I would just put a metric (or imperial and convert) dial indicator on your tool post to measure the H depth of the thread (infeed).

If you were to put the compound inline with the axis of the lathe, you could “side shift” the threading tool a small amount to make the thread fit after you have reached your depth H. You can’t do that if you leave your compound at an angle as the thread will get too deep before it fits.

 

[PeterT]

My compound is set to 29.5 (no, I don't want to start a 29 vs 29.5 vs 30 battle)

You neglected to say anything about a 29.x vs ZERO degrees HaHa. Seriously, at this size I cant think of a reason not to go straight in. I do it all the time. Less things to calculate & less dials to turn. Just finished some 20 TPI in stainless, it looks & fits just the same, if not a bit better. This is using an insert but HSS is the same. I find most of the finish has more to do with DOC progression & sufficient spring / finishing passes. Usually small pitch means smaller diameter & more deflection, but a big diameter + small pitch should be easy-peasy from that standpoint. When your project is done & threading fresh on the brain, give 0-deg a try just for the heck of it.

 

[Susquatch]

@ShawnR - everything @PeterT and @RobinHood have said is correct - for normal 60 degree threads. But don't take your threads for granted. They may not be normal. Don't forget to examine your scope threads under a strong magnifying glass. When you do this, you can also examine the roots and crests to see how they were formed too. The roots of the existing threads usually matter more than the crests cuz your tooth has to fit into them with clearance.

If your profile looks normal, you are golden! Just do as others have described.

BTW, most tubes are quite flexible. Watch out that you don't tighten your jaws too much! It will never fit if you do.

How long is your tube? And does it fit your spindle bore? If not, you will probably have to use your steady. Even though the diameter is big enough for the 3x rule of thumb, there isn't much strength in a thin tube. The steady adds a whole nuther dimension to cutting a small thread. You might find it advantageous to cut a tapered plug to hold the tube in the proper shape.

Lots of fun, when this is over, you can be very happy about what you did! It isn't an easy peasy job.

 

[Susquatch]

@ShawnR - one more thing.

If you find a decent flat black flocked paint, please let me know. I used regular Matt Black, but I would have preferred a bit of flocking.

 

[ShawnR]

Thanks everyone. That opens up a new avenue to explore. I know nothing of this "zero compound method" so off to research that. The piece I am working on is old pipe, but it is a practice piece. I will track down some aluminum tube or pipe when I feel like it is doable (by me). @Susquatch I tried a piece of plastic yesterday as a practice piece, and yes, put a plug in it to support it where the chuck clamped it. But the plastic would not mill very nice. @RobinHood Thanks for the diagram and input. When I ground the bit, I did not touch the tip. I forgot about the practical side of cutting. I will look at that today.
And, re-reading your post,....a light might have gone on! Soooo, any 0.6 mm thread will have the same depth? Whether it will be on a 6mm bolt or a 45mm lens hood? If that is the case, well, that just simplifies a lot for me!! I never clued into that rather obvious spec before! And I like the dial indicator suggestion. @PeterT good points, Thanks

 

[ShawnR]

@ShawnR - one more thing.

If you find a decent flat black flocked paint, please let me know. I used regular Matt Black, but I would have preferred a bit of flocking.

Not to derail this thread, but by flocking, you mean that crushed velvet look often used in woodworking projects and automotive glove compartments? I just did a quick search. I think you could buy that at a crafts store, then apply it to a freshly painted piece, no?

Black Flocking Kit

Each kit comes with 1oz Black Flock - 2 Ounce Color Matched Acrylic Paint/Adhesive - Applicator Give Your Project the Soft Velvety Look and Feel! Easy to use, If you can brush on paint you can flock your project. Uses include but not limited to, jewelry boxes, vehicle dash boards, bottoms of...

www.flockingunlimited.com

 

[Susquatch]

@ShawnR - yes. The diameter doesn't matter. You can cut that thread at ANY diameter. It becomes a custom thread.

I never cut tiny threads at 29.5 despite preferring 29.5. Just plunge in at 90. No math required. As @RobinHood said, it makes widening the thread easy peasy - just set your compound at 90 and dial some in.

Btw, over the years, I've seen lots of guys use cardboard tubes or ABS pipe. If you are shooting lots of rounds, the barrel heat becomes an issue so the scope tube can get pretty long! Some guys put a flat piece of wide cardboard right on top of the barrel.

 

[ShawnR]

Lots of learning, for me, in this thread. For anyone at the low end of the learning curve like me, I found this video that illustrates the various angles, pros and cons. I think for now, I am going to stick with 29 to 30 but will experiment with zero when working with softer stuff.

Thanks

 

[Susquatch]

For anyone at the low end of the learning curve like me, I found this video that illustrates the various angles, pros and cons. Lots of learning in this thread. I think for now, I am going to stick with 29 to 30 but will experiment with zero when working with softer stuff.

I know you don't want to open a debate on the 29 stuff, but I gotta ask you why not 90? The 29 stuff is what confuses most guys. 90 is dead simple.

Don't forget that I'm a 29.5 guy. So for me to suggest 90 is saying something. It's a tiny little thread you could cut in one pass if you wanted to. In this case, 29.5 means nothing!

0.6 mm pitch is just 24 thou. Taking the root and crest into account its under 20 thou. That's a normal cut in one pass. Besides, even if you do a cleanup or spring pass, you don't normally use 29.5 anyway.

That video you posted, is ok, but as you might note - it goes on and on and on and focusses in on the stair case effect. You don't have to think about that with a tiny little one pass thread. 29.5 is for multiple passes. This is just one pass - maybe two. It's like a test pass for bigger threads!

Too bad you didn't make it to the Toronto meetup. @thestelster did a nice demo that discussed this in detail. I was watching other attendees. They were understanding him. That's what matters! His demo was much better than your video. If I can find it, I'll post a link to @thestelster subsequent post on this issue.

 

[ShawnR]

I never cut tiny threads at 29.5 despite preferring 29.5. Just plunge in at 90. No math required. As @RobinHood said, it makes widening the thread easy peasy - just set your compound at 90 and dial some in.

I can see that now.

Btw, over the years, I've seen lots of guys use cardboard tubes or ABS pipe.

It was ABS pipe I was practicing on. It had the wall thickness that I thought might work but did not seem to do a very small thread like this very well. perhaps it was me or my cutter that was the issue.

 

[Susquatch]

@ShawnR

Here is that stuff @thestelster did at our meetup.

Thread 'Threading Basics from April 6 Toronto Meet' https://canadianhobbymetalworkers.com/threads/threading-basics-from-april-6-toronto-meet.10613/

 

[Susquatch]

I can see that now.

It was ABS pipe I was practicing on. It had the wall thickness that I thought might work but did not seem to do a very small thread like this very well. perhaps it was me or my cutter that was the issue.

Most likely it was distorting - bending under the cutting pressure, cuz I find ABS cuts pretty well with a very sharp cutter. It even makes ABS wires just like threading in steel does.

 

[Susquatch]

@ShawnR - another thing about ABS. I don't think it likes spring passes...... In a weird way, it's kinda like carbide inserts. You prolly have to go full depth in one pass.