I don't know how often you make or measure threads but there is an important fact you've omitted. COST.
Tri rollers can't be used for threads with different diameters even if they have the same thread profile and pitch.
Just out of curiosity, have you set up and used tri rollers? I have. I also remember how shocked I was at the price for everything I needed.
Anyone know the complete price if you wanted to measure a 1-8 UNC? I'm guessing between $1,500 and $2,000. Am I too low?
Gordon
Yes, I have used tri-rolls.
In fact there is one of these:
http://www.johnsongage.com/pr_is_external3.shtml set up for .5000-20-UNF right now,
and one of these:
http://www.johnsongage.com/cat_cx_cx.shtml ( albeit dial indicators ) is set for .7500-16-UNJF on another machine.
The first part is a standard consumable item out of TI that is to fit mating units all over the world in the field, while the other is an aerospace
part.
For the first, I absolutely have to make sure that the thread is correct to mate with everything, the other I don't have a choice.
Aerospace absolutely REQUIRES quantitative measurement, no exception ( well there is an exception for small threads but I digress )
Now, a couple of things:
Thread rolls ( at least from Johnson Gage ) are pitch specific only. A 20 pitch PD and Functional roll can be used on everything from 1/4-20 and up.
IOW you buy the roll set ( approx $600 as I remember the last time around ) you are good to go on that specific pitch of any size. ( more on that in a bit )
Adjustable Tri-roll frames have a minimum and a maximum size they're capable of measuring.
The ones I've linked to can measure from .250 to 3" by design, but there are ones from 3" and up ( to a point of course )
Now all you need to purchase is:
1: A set-master that is specific to the size and pitch you're making. Cost is approx $250-350 depending on size and commonness.
2: A set block specific to the size-only ( such as 1" or 3/4 or .5625 etc ) to adjust the frame properly. ( approx $80 )
If you're trying to get away on the cheap, you may forego the last two items and use a jo-blocks to set the frame and plug gage to set the rolls.
Not as accurate, but as long as you've got the rolls you're good-to-go for free.
Now lastly, I am not advocating that everyone here go out and plunk down $4K or more for a frame set immediately and then buy all other
necessary accessories to measure threads. I certainly do understand that it is NOT a cheap way to measure.
At the same time I do take exception to the notion that a ring or plug gage coupled with a micrometer and an optical comparator is equivalent
to a tri-roll setup.
Sidenote: All of the above BS I've spewed applies to external threads only!
For internal measurements one could easily triple or quadruple the prices, and they ARE pitch and size specific.
I've only got 5 sizes, and it's $2K and up for each.
So, we do the Rube Goldberg method and swap back and forth within a few shops as-needed.