Screw cutting in the minilathe

[Tomas Johansson 1Participant @tomasjohansson1]

Issue 204 has an article on Screw cutting in the minilathe. As I don't have the magazine, I thought, 'Hey, I can do this calculation of gears myself' as I need to make some odd threads for a pressure gauge adapter.

I am sharing my Perl script if anyone else wants to use it. It generates a tab separated file with all possible pitches from 0.4 to 3 mm. The file can be imported in excel for sorting.

Code is here

Edited By JasonB on 08/08/2013 12:16:32

[Tomas Johansson 1Participant @tomasjohansson1]

[BazyleParticipant @bazyle]

Welcome to the forum. Hope you stay for some non computer fun.

Do fill in your profile. Guessing you may be a programmer as Perl is uncommon as a language to offer this kind of support function. Must try it on the computer IT allow me to play with if I can remember how to.

[Sub MandrelParticipant @submandrel]

If anyone wants the excel spreadsheet I used, pleaee PM me. I used trial and error to get workable ratios and home in on the target pitches, but excel does all the calculations.

Neil

[Tomas Johansson 1Participant @tomasjohansson1]

Bazyle: I will stay here also for some mechanical fun. I realize that this forum does not normally address programming. I am not a programmer, but programming is just one of the tools I use professionally as an electrical engineer.

The Excel file my code produces is here: https://dl.dropboxusercontent.com/u/9887035/table.xls

It probably contains some combinations which are not feasible, but there are numerous combinations for most pitches.

Edited By Tomas Johansson 1 on 08/08/2013 13:49:09

[Russell EberhardtParticipant @russelleberhardt48058]

Nice one Tomas.

For those not familiar with perl you just have to save the text to a flie then type in a terminal, or at the command prompt, "perl filename" and it generates the file of thread pitches.

Russell.

[Sub MandrelParticipant @submandrel]

Hi Russell – you probably need a runtime version of perl installed too…

Neil

[Keith Wardill 1Participant @keithwardill1]

Plenty of screwcutting calculator for the minilathe in the internet, along with dicusion and theory. e.g.

http://littlemachineshop.com/Reference/change_gears.php

http://www.gizmology.net/changegears.htm

http://www.varmintal.com/alath.htm#Cutting_Odd_Threads

(Scroll down page for this last one – has downloads of WIndoze compatible VB program – much easier to use than messing about with Excel)

…….hundreds of others available.

[Sub MandrelParticipant @submandrel]

It is surprising how flexible a standard set of gears can be, if you seek 'unusual' pitches. I'll try to explain without any advanced maths.

Imagine a simple gear train that includes a 55 too drive and 60 tooth driven gear. Calculated pitch is slightly too great, and you need to reduce it. the reduction is 55/60 or 1:0.917

If you only have wheels with multiples of five teeth, you can change to a 60 driver and 65 driven. Giving 60/65 or 0.923. That's roughly a 1% change. give or take, if you have plenty of wheels in 5-tooth steps you can cover a range in roughly 1-2% steps (bigger steps with smaller wheels).

Regardless of the metric/imperial conversion this is where 63 (and the unbiquitous 57) come in handy. By splitting the 55-60 and 60-65 steps they allow you a whole series of smaller adjustments of nearer 0.5%.

In fact for gears around mid range and using a two step train (which allows two odd gears to be used if required) it becomes possible to get around 0.2% accuracy for many 'odd' threads. This is fine for fixings, and some basic measurement/leadscrew purposes.

Whether you use trial and error or some number crunching programme to find you ratios is irrelevant, you should still be able to get reasonable results with a set of 5-step gears a couple of "od un's".

Neil

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