
These notes should be used in conjunction with this program: NthreadsP written by Geoff Carter (unfortunately his program to use with screwcutting gearboxes has been lost). Another another screwcutting calculator program, ready to use, is here. To understand how changewheels are arranged in a typical setup, study: geartrain.bmp Don't just attempt to use the programs, to get the best results read and understand these notes first . If your lathe has a screwcutting gearbox it is even more important to go steadily though the explanations and examples given. The programs have been written to help with the following situations: 1) If your screwcutting chart is absent  or to calculate a new one. 2) If changewheels are missing the programs will calculate alternative arrangements using just the gears to hand 3) To generate a pitch not on the threading chart 4) To calculate the missing gears necessary to make a complete set 5) To generate metric pitches on an English lathe without using a 127 tooth gear
Much detailed information can be found in the modestlypriced book: "Screwcutting in the Lathe". We always have copies in stock for immediate delivery. Please email for details.
WHICH PROGRAM TO USE: NthreadsP is the program for lathes with screwcutting by changewheels while BoxftrhreadP is for lathes with a quickchange gearbox (remember, the latter needs to be downloaded before it will work).
BACKGROUND: Calculations will show that with a set of 20 gears, it is possible to have 380 sets of 2gear arrangements, 29,070 sets of 4gear and 775,200 sets of 6gear: a total of 804,650 different combinations. By working out all of these possible geartrain arrangements you can set up gear trains that will generate threads with only a very minor or even zero pitch errors. It would of course be extremely laborious to calculate all these possible arrangements by trialanderror method with a pocket calculator  but a computer program is an idea means of determining, very quickly, the many possible setups.
THE GEARTRAIN: The lathes for which the NthreadsP program is written are the smaller British and American types such as Myford, South Bend, Boxford, Colchester, Atlas, Craftsman, Sheldon, Delta, Rockwell, etc. For lathes that have screwcutting by changewheels (and not a screwcutting gearbox) a typical setup is shown in geartrain.bmp. Briefly, a gear on the end of the main spindle engages with a tumblereverse mechanism (two gears on a pivoting arm arranged to reverse the drive) that has an output gear to driving either a "simple" train of gears or (as shown in geartrain.bmp ) a "compound set"  where two gears are mounted together on one or more of the shafts. To space the geartrain out and make it physically possible to build, idler gears are also sometimes included, but these, no matter how large or small, have no effect upon the final ratio. The gear on the end of the primary shaft (shown red in the diagram at geartrain.bmp) is the first DRIVER gear. It drives a gear on the 1^{st} stud (yellow gear) which is a DRIVEN gear. The adjacent gear on the 1st. stud (in this case a smaller gear) is a DRIVER gear (coloured red) that drives the gear fitted on the end of the leadscrew  which is, of course, a DRIVEN gear (shown in yellow). The intermediate idler gear on the 2^{nd} stud (shown green) has no practical effect, but is needed to space the gears apart to prevent adjacent gears clashing. The number of teeth on the idler gear has no effect on the thread produced. NB : Depending on the gears required to cut a given thread it will sometimes be necessary to fit the idler gear on the first stud and the compound set on the second.
"SIMPLE" GEARTRAINS: Frequently the geartrain can be simplified (no compounded gear required). The geartrain then becomes: Driver (gear on primary shaft) and Driven (gear on leadscrew). One, or both, of the two studs (it makes no difference) will be idler gears.
HOW TO USE THE PROGRAMS: Examine your lathe and see if it has a geartrain setup similar to that shown in the geartrain.bmp file. If it not, the program may be unusable. Check that one turn of the spindle = one turn of the primary shaft: if not, see "Kludge Factor" below. You may have a quick change gearbox/preselect gearbox that allows you to run the BoxftrhreadP program instead of NthreadsP. See point 2.0 1.0 Run the program by double clicking on Nthreadp.exe (you should have unzipped the program already by using WinZIP) from My Computer, Windows Explorer or use the run bar to open it. You can also create a shortcut icon to the program if you intend to use it frequently. 1.1 Make a list of all the loose gears that you have to hand. The number of teeth on each is entered into the 24 boxes available. Enter 0 in the unused boxes if you have fewer than 24 gears. Press "save gearset" to save the gears for your lathe again. It is only possible to save the gears for one model of lathe. 1.2 Find out what your leadscrew is by measuring it or looking up in your manual. It is critical that you correctly identify whether the leadscrew is Metric or Imperial and what its exact pitch in mm or number of turns per inch (t.p.i) is. Enter this in the box for the leadscrew, check to see that you have ticked the first box if you have a metric leadscrew. 1.3 Decide what thread you would like to produce, tick the inchthread required or metricthread required buttons accordingly and enter the pitch for metric/BA or the t.p.i. for BSP, UNC, UMF, BSW and BSF. 1.4 Enter a maximum error. This should be as low as possible; the best thread is the one with the minimum pitch error. Press the Calculate button.
If you have too many results gradually reduce the error until only the best results are shown. If you have no results, increase the allowable error and press calculate again.
If you can, choose a "simple" geartrain answer in preference to a "compound". 1.5 Once calculated you can write down the results or cut and paste by right clicking the results box. Left click "select all", right click again, left click "copy" and paste to another application such as a word processor or spreadsheet. 1.6 To look for another thread go back to point 1.3. If you have finished, press the stop button. 2.0 Instructions for BoxftrhreadP are as follows:
Run the program as described in 1.0 by running BoxthreadP.exe . 2.1 Proceed as point 1.1, then work out the gear ratios for the gearbox part of the lathe (see. BoxftrhreadP calculations) and make a reference table for yourself to relate the lever positions to the ratio. Press "Edit quickbox" to enter these ratios. Start by entering the number of ratios to consider in the top box and then type a ratio into each box starting on the top lhs and working down. Press "save current data" and "press to stop". Restart the program, press "edit quickbox" and check that all ratios are correctly typed and positioned in the white boxes. Check that all gears are correctly entered in the main window as per 1.1 above. 2.2 Proceed as point 1.2 2.3 Proceed as point 1.3 2.4 Proceed as in point 1.4. If you have many gear ratios the number of results will frequently be high; to save, copy and paste the results to another application as per point 1.5. Depending on the speed of your computer, and how many ratios you have, BoxfthreadP may take a little while to do each computation and require some patience. 2.5 To look for another thread go back to point 2.3. if you have finished, press the stop button.
THE CALCULATIONS (NthreadP): These are the calculations upon which the program is based and also for assistance with some calculations required by the user: FOR AN IMPERIAL (inch) LEADSCREW TPI produced = DRIVEN/DRIVER x DRIVEN/DRIVER x LEADSCREW TPI It does not matter in which order you place the driver or driven gears. FOR A METRIC (mm) LEADSCREW PITCH produced = DRIVER/DRIVEN x DRIVER/DRIVEN x LEADSCREW PITCH It does not matter in which order you place the driver or driven gears
EXAMPLE 1: (Imperial leadscrew, imperial thread required) Leadscrew turns per inch : 8 (1/4 inch pitch) No. of teeth on primary shaft gear (driver) : 50 No. of teeth on 1^{st} stud driven gear: 80 No. of teeth on 1^{st} stud driver gear: 32 No. of teeth on leadscrew gear (driven) : 60 TPI = 80/50 X 60/32 x 8 = 24 This will produce a 24turnsperinch thread
EXAMPLE 2: (Imperial leadscrew, metric thread required) Leadscrew Turns per inch : 8 (1/4 inch pitch) No. of teeth on primary shaft gear: 48 No. of teeth on 1^{st} stud driven gear: 50 No. of teeth on 1^{st} stud driver gear: 36 No. of teeth on leadscrew gear (driven) : 44 TPI = 50/48 x 44/36 X 8 = 10.185 TPI Convert to millimeters : Pitch in mm = 25.4 / TPI 25.4 / 10.185 = 2.494 mm pitch This equates to a 0.25% error against a 2.5mm pitch.
EXAMPLE 3: (Metric leadscrew, metric thread required) Leadscrew pitch: 2 mm (metric leadscrew) No. of teeth on primary shaft gear: 80 No. of teeth on 1^{st} stud driven gear: 80 No. of teeth on 1^{st} stud driver gear: 36 No. of teeth on leadscrew gear (driven) : 48 PITCH = 80/80 x 36/48 x 2 = 1.5 mm
EXAMPLE 4: (Metric leadscrew, imperial thread required) Leadscrew pitch: 2 mm (metric leadscrew) No. of teeth on primary shaft gear: 32 No. of teeth on 1^{st} stud driven gear: 36 No. of teeth on 1^{st} stud driver gear: 25 No. of teeth on leadscrew gear (driven): 28
PITCH = 32/36 x 25/28 x 2 = 1.5873 mm Convert to TPI : TPI in mm = 25.4 / 1.5873 = 16.002 TPI This equates to a 0.0125% error against a 1mm pitch.
EXAMPLE 5: (Imperial leadscrew, imperial thread required, "simple" geartrain solution) Leadscrew Turns per inch: 8 (1/4 inch pitch) No. of teeth on primary shaft gear (driver): 40 No. of teeth on 1^{st} stud driven gear: idler No. of teeth on 1^{st} stud driver gear: idler No. of teeth on leadscrew gear (driven): 80 TPI = 80/40 x 8 = 16 This will produce a 16turnsperinch thread using a "simple" geartain
KLUDGE FACTOR: Some lathes have their geartrain configured in a different way. In an attempt to make the program work with these a "Kludge Factor" is included where (for a variety of reasons) the primary shaft does not run at spindle speed. The Kludge Factor is, in essence, a factor that will be required on some lathes to obtain useful results. The BoxfthreadP version does not have the kludge factor facility but this can be multiplied into your gear ratios if using the BoxfthreadP version of the program. KF = Spindle speed/First driver speed (Change this to first driver/spindle in later version) or: KF = No of teeth on primary shaft gear/no of teeth on spindle gear (may not be true for all lathes) or: KF = Actual TPI produced/TPI calculated by NthreadsP with KF of 1 or: KF = Pitch calculated by NthreadsP with KF of 1/Actual pitch produced You can work out the Kludge Factor from any of the above formulae  and it should always be the same. On lathes with a twospeed primary gear drive use the BoxthreadP program with 2 ratios.
THE CALCULATIONS (BoxfthreadP): If you have a gearbox in addition to a typical geartrain enter the number of ratios, and the ratios which you will have to calculate, as follows. Ratio = No. teeth on Driver gear/No. teeth on Driven gear x No. teeth on Driver gear/No. teeth on Driven gear etc or: Ratio = Speed of gearbox output shaft/Speed of gearbox input shaft or: Ratio = TPI calculated by NthreadsP/Actual TPI produced 0r: Ratio = actual pitch produced/pitch calculated by NthreadsP See also "sample sets for common lathes"; please send more to Geoff Carter for others to use To use the program results work backwards from the answer to find the lever positions For most lathes the input gear to the quickchange gearbox is usually the "leadscrew gear"  but this will depend on the make and model of lathe.
FILENAMES The following files relate to the 2 programs NthreadP.doc This document NthreadP.exe Application nthreadp.txt Save gear set file (used by NthreadP and BoxfthreadP) Geartrain.bmp Drawing of typical geartrain

