Measuring Wood?

[VicParticipant @vic]

Is not normally a particularly critical requirement. I’ve been looking at some wood thread cutting videos recently. The female thread is cut with the jig first and then it’s carefully measured to determine the starting diameter for the male thread. Calipers were used for the job with the instruction to “wiggle it a bit” to get an accurate measurement. The obvious problem though is that the calipers have a sharp edge so it wouldn’t be difficult to damage the newly cut thread or get an inaccurate reading. So my question is, apart from internal Mics (I wouldn’t want to have to buy two or three) what might be a better way to do this?

[Peter Cook 6Participant @petercook6]

For the precision necessary, wouldn’t a pair of inside two legged spring callipers do the job. Measure the thread a couple of turns in and once set rotate the thread to screw the callipers out for measurement.

[Michael GilliganParticipant @michaelgilligan61133]

Maybe fill the female thread with wax [or whatever] and when it has set, unscrew the  ‘plug’ …then you would have something proper to measure.

MichaelG.

[JasonBModerator @jasonb]

As you don’t really want the crests and valleys touching then it does not need to be massively accurate and what about the tip radius of the tool that is going to cut the male thread, that will have a big effect on how deep you need to cut to get the flanks right.

Cheap pair of callipers with the edges rounded over would protect the wood, just set them to zero in a known gap  and adjust the final reading taking that into account.

[JasonB]

On JasonB Said:

 

Cheap pair of callipers with the edges rounded over would protect the wood, just set them to zero in a known gap  and adjust the final reading taking that into account.

I did think of something like this. You can buy round tips to retro fit calipers as well. It’s a shame they don’t make calipers with rounded tips.

[BazyleParticipant @bazyle]

Two washers but with slots instead of just holes with loose nut an bolt to hold together. Centralise to insert into hole and then slide apart to bottom out in thread and lock. Unscrew from thread which will also show if you have even depth of thread. Measure with regular callipers.

[bernard towersParticipant @bernardtowers37738]

I find as a metalworker that I have to take that part of the brain out and insert the bit that says to the nearest 1/8″

[DalboyParticipant @dalboy]

There is a proper tool for measuring on woodturning projects when threading. Look HERE

[DC31kParticipant @dc31k]

A standard pair of internal calipers cannot measure diametrically across a thread. The tips would need to be offset by half a thread pitch.

Look at how external threads are measured using the two wire method and the three wire method. Calipers are loosely analogous to the two wire method.

[HuubParticipant @huub]

I tap threads in wood using (self made) roll taps. For me, that works better than using taps, single point threading on the lathe or thread milling. The drill diameter depends a bit on the hardness of the wood. MDF is predrilled using the drill diameter for steel, hard wood (oak) is drilled using the drill diameter for roll taps.
If you don’t have a roll tap, use a hex bolt to check the quality of the thread. A bolt gives the “same” result, a roll tap just takes less force to make the thread.

I don’t make outer threads in wood, but measuring the thread size should be as easy as measuring the thread size in steel using the 3 wire method. You don’t need a calibrated wire, just 3 straight wires (nails) of known and equal thickness.

[Vic]

On Vic Said:

On JasonB Said:

 

Cheap pair of callipers with the edges rounded over would protect the wood, just set them to zero in a known gap  and adjust the final reading taking that into account.

I did think of something like this. You can buy round tips to retro fit calipers as well. It’s a shame they don’t make calipers with rounded tips.

Vic, you mean something like these ?  Rounded tips that are in this case 0.250″ to add to the vernier reading. some were 0.200″ they would in most cases measure across several thread crests.

[VicParticipant @vic]

Thanks for the thoughts so far folks. And thanks Dalboy, I’ll take a look at the Sorby tool.

Here’s a link to what I’ve been watching. Fast forward to 10:00 minutes to see the measurement.

https://youtu.be/9pE92qYvmnQ?si=663vMTl8Ae6tqqLj

 

[Michael GilliganParticipant @michaelgilligan61133]

A good demonstration, Vic … thanks for sharing it.

Are you planning to make a lot of screwed lids ?

… looks like it would be a big outlay for just a few.

MichaelG.

[BazyleParticipant @bazyle]

This guy makes a really good video. Clear, concise, I wish he was a metalworker I might actually follow him if he were doing something I was interested in.

[JasonBModerator @jasonb]

A form of thread milling except the work moves in the helical path not the tool.

I suppose as a salesman he should make a good video, probably the type of guy who does demos at shows so has run through the same thing many times before

Back to the original question then I think a standard calliper will be fine as the tool only moves in increments of 5 thou then your starting point could be 10thou off. Also lined up by eye is never going to give a truely parallel thread so any slight inaccuracy in measurement is not going to matter.

Box will likely go oval when brought into a centrally heated house anyway so needs some slack in the thread!

[SillyOldDufferModerator @sillyoldduffer]

Bearing in mind that wood isn’t a precision material, at least 10x worse than metal, I suggest cutting wood threads is a fitting process, where the fit should be loose rather than tight.  Wood is also unstable compared with metal and doesn’t hold exact dimensions.

Metal supports high-precision and accuracy; wood isn’t “good enough” for that.   As wood is bound to be inaccurate, don’t overthink it.   In the video, a digital caliper is used to expedite the calculation and to mark out the size of the male, centring by eye.  The digital caliper implies much higher accuracy than is needed.  Could be done with a school geometry set compass and a rule.  Thereafter, hand held tools, no measuring, and a notion of ‘parallel’ that a metalworker would jibe at!  Crude, but effective for wood.  Note a tight fit is relieved with sandpaper and retesting, not by measuring.

Dave

 

 

 

 

 

[Michael Gilligan]

On Michael Gilligan Said:

A good demonstration, Vic … thanks for sharing it.

Are you planning to make a lot of screwed lids ?

… looks like it would be a big outlay for just a few.

MichaelG.

I’d like to make quite a few. The alternative is hand chasing which doesn’t appeal to me at all.

Another video. Check out the thin threaded rings.

https://youtu.be/PFLmfVJUG0A?si=XDdNsA1r69sWIttf

 

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