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using a follow steady, with bearings, or rolls, be sure that they won't mess up your thread : tool is harder than material, and a bearing is harder than the tool, as a general, lets say, so even if you cut it correct, on the other side, where the steady makes contact, it will deform your thread more and more, as your cut goes deeper
such effect sometimes may pass unoticable, so .... as a consequence, for fine work, like smooth od, or fine pitch threads, etc, use bronze inserts on your steady, thus soft materials, that won't damage your parts; off course, with bearings, damage is at a low level, but consistent, thus it may be there always, progressing, at low scale, thus without noticing it...
if this effect is discovered too late, and you still wish to keep that part, then you need to deviate from standards; this allows to save costly a part
to your issue now
make contact with at least 3-4 crests, and if you use, for example :
... 3 bearings, then consider contact length as 3*ul, where ul < crest length
... 3 bronze flats, then consider contact length as 3*ub where, as before, ub < crest length
* how a bearing is tougher than bronze, ul must be >ub, considering also your thread dimensions, and actual case; but, with bronze, also a continous loooong pad may be just fine
please remember :
... making contact only on crest is not a guarantee that deformation wont occur
... use contact on as many crestst as possbile; you can conside 3 consecutive, or 12, skip3, then 45 ( your tool should cut at 3 ); thus, many consecutive crests actually may lead to worse than only contact on far ends; actually, many consecutive, may reach a situation where contact is only on 1 crest, and only 1 is not stable .... so a multi contact pad, if not done ok, is as worse as a simple pad, that makes contact on 1 crest;
the issues with only 1 crest, is that, as cut goes deeper, also cutting force increases, thus more pressure on the steady ... simply material may bend only in that area, not much, but enough to give you inconsistency
if you wanna see it, simply use a dial, magnetic base on steady, and tip on part; even so, is possbile to deliver a part with the dial going crazy : rough it in crazy mode ( dial moves a lot ), than change to sharp insert and prefinish&finish in smooth mode ( dial moves minimal )
you may use standad tolerances to your favor, so to deliver the part with minimal machining time, like smallest tooth height, aka largest crest, and other triccks as well; they deliver time save, especially on long threads, with big pitch; 6 foot is almost 2 meters, so yes, is ok to look for any allowance, so to say
explanations where a bit rushed, but if you have questions, pls feel free to ask anything
i just remembered : sometimes the steady itself may buckle : check it (with a dial/etc), and reinforce it needed
i am more of a metric guy, but if you wish, i will look over those acme dimensions, and be back with dimensions for the contact of your steady / kindly