That's cool Andy. I have done similar all the time. For external threads I bought a cheap 3wire set that has a chart with it and gives me all the calculations I need to use. It's just one simple formula and it tells me the closest wire size I should be using.
For internal threads I usually use tig wire. Most machine shops have a tig welder and you can bend that and stick it in the bore to measure the threads. Method 2 is make a test gauge of what size the part should be (eg male part in your case a dummy lathe spindle.). Using thread wires to size the thread. And then cut the backplate to that size. Regards Andrew On Fri, Jul 31, 2020, 10:26 PM stjohn gold <thesaint4...@gmail.com> wrote: > Hi Andy, > great post, thanks! It all goes to show that threads are complicated. Some > of those standards were written over a period of 20 years, that is no joke. > Nothing to physically test your fit against - brave! > > cheers, St.john > > On Thu, Jul 30, 2020 at 9:54 PM andy pugh <bodge...@gmail.com> wrote: > > > (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 > _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
