Measuring thread depth without wires - your views, please

[goldstar31]

Hi Guys,

An easy way to make some anvils would be to drill a suitably sized hole in some round material and thread it, and then mill just over half off the thread. This will leave you with as many thread forms as you would need and the correct radius for that thread size. When done they would look like those in the picture above.

I agree with Baron and would go on to relate a bit in my past when I recall something similar but halving the test sample - with a hacksaw and filing it up and using a comparator.


Thanks John

 

[holmes_ca]

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[HennieL]

HennieL: I recently ran into this same issue with wanting to know that I made a correct thread without guessing that I followed the procedure of feeding the compound slide the correct amount, as every description I read, I felt confusing.
If I start with a sharp 60 degree cutting bit, and feed the compound that is set to 30 degrees off axis (~29.5), and I start with the round OD right at the described hardware diameter, I should feed the compound in the exact distance of the pitch of thread, as this is one side of the equilateral triangle. Ignoring the crest and trough radii. This should be a perfect sized thread (I understand a nut with the trough radii per the specification, will not fit on this cut thread)

Yes, this bugged me as well when I started threading some years ago, but I have come to the conclusion that flex in the machine, and unknown tolerances, are to blame for the constant need to make at least two shallow (0.05mm) cuts in addition to the calculated number and depth of cuts. Two things to keep in mind, that has helped me get closer to the required thread depth than just using the 0.6134P figure:

• Don't thread on a bar that's at nominal diameter - look up or calculate the maximum and minimum values for the major diameter, and turn your bar to a diameter half way between these values - that puts you in the middle of the tolerance, and helps to achieve a good "in spec" fit.
• Because I mainly use carbide insert cutters with radiused tips, and the radiused tip touches the bar when you zero the cross slide, I add the "thickness" of the tip radius and half of the fundamental deviation (es) to the calculated or tabulated thread depth - that usually helps to get me to within two or three "cleaning" passes of 0.05mm increments.

Hope this helps you to get closer as well.

An easy way to make some anvils would be to drill a suitably sized hole in some round material and thread it, and then mill just over half off the thread. This will leave you with as many thread forms as you would need and the correct radius for that thread size.

Thanks BaronJ - will have to give this some thought....

 

[Mark Duquette]

You can use the shank of small drills to measure threads. If you know the size of the drill shanks it can be inputted into the formula to get the correct measurement. I have used this method in the past with good results.

 

[SmithDoor]

I started with drill bits
The down side is calculating the size.
When you buy a set thread wires it comes with chart.

I have Excel work sheet for odd thread not cover by chart.

Most have a problem holding the wires
I use two rubber bands to hold the wires so put the mic the thread. Tape does not work and then need to clean the air after trying.

Dave

You can use the shank of small drills to measure threads. If you know the size of the drill shanks it can be inputted into the formula to get the correct measurement. I have used this method in the past with good results.

 

[Richard Hed]

Hi Guys,

An easy way to make some anvils would be to drill a suitably sized hole in some round material and thread it, and then mill just over half off the thread. This will leave you with as many thread forms as you would need and the correct radius for that thread size. When done they would look like those in the picture above.

Oh ho ho, you mean I've been doing this wrong all the time? I was using "square stock" for threads. Does you have a photo of what you are saying?

 

[awake]

One trick for holding the wires is a dab of grease.

 

[BaronJ]

Oh ho ho, you mean I've been doing this wrong all the time? I was using "square stock" for threads. Does you have a photo of what you are saying?

Hi Richard,
Have a look at post 17.

 

[HennieL]

Oh ho ho, you mean I've been doing this wrong all the time? I was using "square stock" for threads. Does you have a photo of what you are saying?

Square stock, with a round shank turned on the one end to fit into the micrometre will probably be easier than trying to drill the round hole on a round bar...

Baron, Richard, Ken (and anyone else...), how would you then go about calculating the actual lathe cut thread depth of the bolt you are cutting, using this threaded half-round anvil? One cannot measure the actual half-round thread depth with wires or with a "V" wedge, and I would think the only way to confirm "within tolerance" (but still not actual thread depth value) would be to use a go-no go gauge to check the threaded hole before cutting it open... or perhaps with a 60° cone turned to an absolutely sharp point - but then one might as well just use that cone and the flat disk, as per my original post... or one can cut the "round hole" with a new set of taps and assume it is half-way within the specified tolerances... hmmm, I will have to think some more on this

 

[Ken I]

Hennie, Typically I screw cut using a sharp point, go in at 30° and work to a truncated thread crest vis (R.H. Image) :-

I drew this M8 x 1.25 without reference to any tables so consider it illustrative.
I can do the trig but AutoCad is just so much less effort.
Once the sharp tip scores the O.D. you have your reference and can screw to depth - I normally keep a close eye on the diminishing crest flat as an indicator as I go.
Using carbide tips with radii (L.H. Image), the method is still the same but the trig gets a lot harder and therefore using AutoCad makes your life easier.
Also using Cad vs tables means you can adjust for anything not standard (i.e. what you have done {or want to do} wrong) such as tip radius, major diameter etc.
Regards, Ken

 

[goldstar31]

Hennie


Does 'touching the bottom' as you describe it actually matter?
I always thought that it was the fit on the flanks that is inportant.

I recall the writings of K.C. Hart( as Martin Cleeve) not concerned with the rounded crests. No it isn't in his book but in Model Engineer about his days as a freelance nut and bolt maker after his redundancy. He wrote to a customer on this--- and received no reply!

For myself, and I insist being a 'bean counter' and certainly NOT and engineer or anything remotely connected, making a Quorn tool and cutter grinder. Apparently, it sorts the sheep from gloats(?) and noted that Professor Dennis Chaddock( who incidentally made bits for atomic bombs), truncated the measuring thread on the leading bed bar. Not that I wish to cause 'trouble

 

[BaronJ]

Hennie
Does 'touching the bottom' as you describe it actually matter?
I always thought that it was the fit on the flanks that is important.

That was what I was taught and have always believed ! As long as the thread pitches and sizes match then the exact diameter isn't vital.

 

[goldstar31]

Hi John B
Thanks for the comments.
Thinking about the bottom of things, I have replaced the spindle Vee belt on the 'new' Myford with a link belt which certainly doesn't reach the bottoms of the pulleys.

Absolute so and so to keep removing the links as the thing is stretching. I 'invented' a little jig to twist the links and then found that I had a pair of curved, locking pliers- probably suture ones from my late wife- which does the job beautifully. Hell, how do you patent a notch in a bit of sheet metal

So back to 'screw cutting', well yesterday I was removing metal using the gear box and the power cross feed to cut--- A SCROLL

It's all Pythagoras really and so old, hardly worth re-inventing.

Keep well

N

 

[BaronJ]

Hi John B
Thanks for the comments.
Thinking about the bottom of things, I have replaced the spindle Vee belt on the 'new' Myford with a link belt which certainly doesn't reach the bottoms of the pulleys.

Absolute so and so to keep removing the links as the thing is stretching. I 'invented' a little jig to twist the links and then found that I had a pair of curved, locking pliers- probably suture ones from my late wife- which does the job beautifully. Hell, how do you patent a notch in a bit of sheet metal

With great difficulty !

So back to 'screw cutting', well yesterday I was removing metal using the gear box and the power cross feed to cut--- A SCROLL

Brave man ! Not something that I would like to attempt

It's all Pythagoras really and so old, hardly worth re-inventing.

Keep well

N

Stay safe.

 

[HennieL]

Does 'touching the bottom' as you describe it actually matter?
I always thought that it was the fit on the flanks that is inportant.

No, not actually, but...

The fit on the flank is important as the force is transmitted through the flanks, and not through the crest and trough "bottoms". The only reason why "touching the bottom" really matters to me is that it is an easy way that one can directly measure the cut depth whilst cutting the thread. Most manufacturers of threading inserts publish lists of cutting depths recommended for their inserts (but they all differ from each other, as the dimensions and tolerances on their inserts vary from each other as well), and using a direct depth measurement would be the easiest way to use this data to confirm if you are deep enough. Measuring with wires is tricky and expensive if you want to use the "proper" wire thicknesses and tight tolerances. Using small drill bits are a lot cheaper, but require some calculations, one need to have access to the published thread pitch diameters, and using the three drills is still tricky.

My original intention with this post was to just share an idea that could make the measurement of thread depth easier and/or cheaper, in order to confirm if the thread that you are cutting is within the specified tolerances and depth before removing the threaded bolt from the lathe - at least more accurately than using a nut... This discussion has brought up some very interesting points and ideas, and I appreciate everyone's comments - hopefully this can also help other people to understand the science/technology of cutting screw threads on a lathe.

An easy way to make some anvils would be to drill a suitably sized hole in some round material and thread it, and then mill just over half off the thread. This will leave you with as many thread forms as you would need and the correct radius for that thread size. When done they would look like those in the picture above.

Baron, I made a "quick and nasty" 1.5mm pitch anvil this afternoon as per your suggestion, as a "proof of concept" of your idea, and made the following observations:

• One would have to make such an anvil for each diameter and for each pitch in each diameter. I initially thought that one would only need one (largest) diameter "half-a-hole" for each pitch, but as soon as I measured a smaller diameter bolt with the same pitch in the larger hole, it rotated away from the cone point on the other side of the micrometer and gave false readings.
• One would have to figure out a way to ensure that the wedge or cone on the other side of the micrometer actually aligns with a thread trough, and not with a crest (or hit the side of the thread) - any suggestions on this, other than making it only two threads wide??

O, here is a photo of the anvil (with a 1.5mm pitch in a MF14mm "half-a-hole") - will need to make it MUCH smaller when I make the real ones...

 

[L98fiero]

O, here is a photo of the anvil (with a 1.5mm pitch in a MF14mm "half-a-hole") - will need to make it MUCH smaller when I make the real ones...

Instead of using a tapped hole, why not use a commercial bolt, mill/ grind away enough to produce a flat surface and leave the crest of only two threads and you'd have basically the same as the commercial thread mics, albeit somewhat cheaper and again, you have to set the mic at zero for each thread. Each anvil would have a range of useful pitches just like the commercial ones.

 

[Tim1974]

Just cut the thread job dun lol omg

 

[BaronJ]

Hi Hennie, Guys,

To be honest, after thinking about it, a set of good quality thread gauges would probably do the same job. Realistically you only need to ensure that the thread on one half fits the other half properly.

Making anvils as I suggested is really no different to a thread gauge.

Using your method of a mic with a 60 degree wedge on one side and a flat plate on the other would work just as well !

 

[Woodster]

I use Mitutoyo Thread Mic's daily. You will need different anvils for various thread ranges. They have a 60deg Vee on the bottom and a 60deg cone on the top. Measurement is simple, as you just measure the pitch or effective diameter. The Vee should be free to rotate and you adjust the height with the bottom thimble to zero them.

 

[HennieL]

Just cut the thread job dun lol omg

Thank you for your exceedingly helpful and positive contribution Tim1974 - I trust it boosted your little ego somewhat

To all the rest of the people who actually did contribute towards making this an interesting discussion: I made a second half-a-hole" anvil with a M6-1.0mm pitch thread, cut by die (as was the first one), and it came out very nice - except that yet again the cone on the other side of the micrometer did not line up with the bottom of the thread, thus making it impossible to zero. I'm thus marking this one up as "not a practical solution", and will use the wedge and flat disk on all threads of 1.5mm pitch and larger, and an appropriately sized small drill in the "one wire" method for smaller pitched threads, as stated earlier in this discussion.

So, let's put this thread to rest - I certainly learned a lot more about screw threading that I learned during the 5+ years that I have been cutting threads with a lathe the way Tim suggested...

Hopefully someone else can also pick up something useful from this discussion.