Have some kind of tool made specifically to measure, bought wrong and never looked close at it.
> (Or "Why do I always take 4 goes at a fit with G76") > > I recently had the occasion to think harder than normal about threads, > and especially about their sizing and fits. > Threads were one of the very first things to be standardised and made > interchangeable, largely through the work of Josiah Whitworth. And it > turns out that they are one of the more complicated things to > standardise. > The reason I was thinking about this was that I was trying to make a > lathe faceplate for someone a few hundred miles away. I know that his > spindle nose is 2 1/4" BSF. (ie, one of Whitworth's threads) but have > nothing to use for a trial fit. > > Whitworth threads have rounded thread crests and roots. ISO metric has > flat crests and rounded roots, AN threads had flat roots, but that > interfered with British rounded crests, so the Unified standard > emerged which has flat crests and rounded roots. > > For a nut and screw to fit together there needs to be clearance > between the thread flanks and also clearance between the roots and > crests of both halves. This means that the roots of the internal > thread need to be at a larger diameter than the crests of the screw, > which also means a smaller radius or smaller flat. Similarly the minor > diameter of the screw needs to be smaller that the through-hole of the > nut. > > The flank clearance is assured by specifying different "pitch > diameters" (or "effective diameters") for the internal and external > threads. The pitch diameter is defined as the line through the thread > where there is exactly as much air as metal. (ie, where the width of > the thread is half the pitch.) > > Here is the data for the screw I was making, as an example: > Size 2 1/4" BSF Internal Thread > TPI: 6 > Major Dia Min: 2.250 > Effective Dia Min: 2.1570 > Effective Dia Tol: 0.0137 > Effective Dia Min: 2.1433 > Minor Dia Max: 2.0769 > Minor Dia Tol: 0.0403 > Minor Dia Min: 2.0366 > > It is interesting that there is no limit to how large the major > diameter of the internal thread can be. Presumably this means that it > can be perfectly sharp. > > The "Effective Diameter" is the important measurement when inspecting > threads, but isn't trivial because it is a measurement of an invisible > feature. > > There are special thread measuring micrometers with a V anvil and > point for measuring pitch diameter. And some maths is needed to > interpret the reading. However each micrometer can only measure 3 or 4 > specific pitches. > A more accessible way to measure threads is with the "three wire > method" where three short rods of known diameter are placed in the > threads. Two on the top in adjacent threads and one at the bottom. The > measurement over the wires is then taken with a conventional > micrometer. > > Using this method the pitch diameter can be determined using some > mathematics, here is an online calculator that I found: > https://www.cgtk.co.uk/metalwork/calculators/screwmeasurement > > But, 1) I needed to make an internal thread and 2) I needed to make it > before cutting it. 3) It isn't entirely clear what assumptions such > calculators are making. > > So, here is another way. > > Firstly, it is possible, but even more fiddly, to measure an internal > thread using ball bearings and an adjustable parallel. I was measuring > quite a large thread so could use 3mm balls and a fairly big parallel. > > I drew the required thread in a CAD package, and used tangent circles > to represent the balls. The pitch diameter in the drawing was set to > the mid-point of the numbers from the standard. > > I set the thread angle in the drawing to 55 degrees. I probably > shouldn't have; the perpendicular angle that defines the thread is a > little larger than the angle along the thread. > > alpha = arctan(P / pi.D). the effective angle is roughly 55 - 1.4 degrees. > > And this turns out to make quite a difference: > For 55 degrees and a min pitch diameter of 2.1433in the parallel would > read 49.01mm > For 53.6 degrees the parallel should read 48.976 > > But, the main point of this drawing was _not_ to work out how to > measure the thread but how to make it. > > The crest radius on the male thread is 0.53mm on this size thread. The > nut root needs to me smaller. I was using a Seco insert, and so had > access to the data table, saying that it was a 0.5mm tip radius. > > I then drew the root of the internal thread at this radius, and > measured the diameter that such an insert would bore at the nominal > pitch diameter. > > Then at the machine, I used the threading insert to bore its own plain > hole, and touched it off. I could then use G76 to thread out to the > major diameter of the thread from the CAD drawing, being fairly > confident that this would put the pitch diameter where needed. > > In the end it looks like I ended up on the large size, but inside the > tolerance (helix angle?) which is probably where you want to be when > fitting to a thread that you can't test to. > > -- > atp > "A motorcycle is a bicycle with a pandemonium attachment and is > designed for the especial use of mechanical geniuses, daredevils and > lunatics." > — George Fitch, Atlanta Constitution Newspaper, 1912 > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
