Copyright (C) 2002 the ToolJunkie

THIS SOFTWARE IS PROVIDED FREE OF CHARGE AND "AS IS". NO WARRANTIES ARE MADE, EXPRESS OR IMPLIED, AND THE AUTHOR DOES NOT WARRANT THAT THE SOFTWARE WILL BE FIT FOR ANY PARTICULAR PURPOSE. THE AUTHOR WILL IN NO EVENT BE LIABLE FOR LOSS OF PROFITS, NOR INCIDENTAL OR CONSEQUENTIAL DAMAGES. YOU USE THIS SOFTWARE AT YOUR OWN RISK. THIS SOFTWARE IS NOT SUPPORTED IN ANY WAY BY THE AUTHOR.

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PROGRAM OPERATION

Run the program and follow the on screen prompts. It is possible to enter a single desired thread in imperial or metric, or, to print both imperial and metric charts of common thread sizes. The bottom value on each chart has intentionally been chosen well out of the range of any gear set to find the finest possible thread. On slower computers, the program may take some time to generate each gear setting as all possible gear settings are tested to determine the optimal one. The more gears you have, the longer it will take.

For your convenience, imperial and metric threading charts for the standard gear set is available here.

INTERPRETING THE SETTINGS

The output from the program will display the desired thread, the actual thread for the displayed setting (which is the closest possible match for the gears available), the gear settings and the amount of error.

Virtually all imperial threads are possible with zero error. With the default gear set, the errors of many common metric threads under 0.001. I expect this is good enough for many applications. The full Myford 7 Series metric set performs upwards of 5 times better. I would be interested to know how these tolerances compare to the screwcutting tolerances of the lathe.

The settings are: First stud (furthest from the lead screw), second stud then lead screw. The lead screw always has just a single gear. The stud's have either a driver/driven pair, or an IDLER.

In the case of the driver/driven pair, the first number is always the driver and the second always the driven. The terminology is a little confusing since all gears drive something, and all gears are also driven by something. However within the pair, only one drives the other.

In the case of IDLER, the stud should be fitted with a single gear of your choice. The number of teeth on a idler gear will not affect the generated thread. Choose an idler (or idlers) that spaces the gear assembly on the banjo so good contact with the drive gear is made. The factory threading chart would suggest that 65 is a good choice for the first idler, and when a second is required, 35 or 45. For a spacer to accompany the idler gear(s), use the smallest gear(s) left over. (If you lathe came with spacers, let me know. My machine has just a single spacer for use on the lead screw).

MODIFYING THE GEAR SET

The gear selection can be modified by editing the file Gears.txt. If you have the full Myford 7 Series gear set, add 20, 38, 70 and 75 to the default. If you have the Myford 7 Series metric set, also add 21 and 21 (i.e. list 21 twice to indicate two 21 tooth gears). If your lathe came with an incomplete set of gears, you can delete the gears you don't have.

If you're looking at purchasing or fabricating a new gear or two (21 tooth gears are a good candidates if you're looking for better metric threads), experiment by adding different gears to the set to see what results you get. If you're planning of fabricating gears, consider replacing the 20 tooth gear on the spindle with a 21 tooth gear and add a 21 tooth change gear. To try this out, edit Gears.txt and change the drive gear from 20 to 21 and add a 21 tooth gear to the set. Notice you would be able to get excellent accuracy for most metric threads, plus still have perfect settings available for nearly all imperial threads.

SPECIAL CASES

• The program doesn't assess the length of the assembly on the banjo. Consider the case where each of the 3 positions had a 20 tooth gear. The gear assembly wouldn't reach the drive gear. It may be possible that the program will generate complex settings with the same problem. If this happens, please let me know. One solution may be to fabricate a third stud to allow for an idler gear. This would reverse the lead screw direction but can be compensated for by reversing the position of the tumble reverse lever.
• The program does attempt to avoid collisions - i.e. invalid settings where the gears would touch each other. This code may not be perfect so if you find a setting that can not be physically assembled, please let me know. There is one thing to note though - the program assumes you have (or are willing to fabricate) small diameter spacers to pair with idler gears. This was done intentionally - I didn't want to discard a more accurate thread because the only free gear to pair with an idler was, for example, a 35 tooth.
• The Myford Series 7 full metric set performs quite poorly for certain metric threads (such as 2mm). I am not sure whether this is correct or whether it is a bug in the program. If anyone can prove the program wrong (i.e. provide a better gear setting than the program provides), please let me know.

Click to download: AdvanceGears 1.0 (25KB)

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