Got some gear and some idea!

[zytooohParticipant @zytoooh]

Hi all.

I’m from the Teesside, I’m a model maker and also make a few tools, I built a small extension a couple of years ago as my tools out-grew the garage. I currently have a new sx2p mill with the gas strut conversion (so much better) and a Zyto lathe with the longer bed 18inch dead centre to dead centre, still on the original stand with some evidence of the treadle power but not fully formed, so my machine was electric from new but at the change over point 1950’s ?

A previous owner (probably one of several over the years) has replaced all of the change gears to Myford (better tooth profile and easier to obtain). I’ve recently made new half nuts out of a mystery bronze material as the original cast iron ones were loosing parts of their thread.

My question is regarding thread cutting, I have a copy of a myford gear chart which gives a huge selection of imperial threading options and a section covering a large choice of metric pitches. There appear to be lots of different Myford gear charts (I chose Myford as the zyto is similar in a lot of ways) but I’m not sure which is most appropriate chart to my situation. My zyto has an 8tpi lead screw. I’m of the generation where imperial was no longer the standard in school and have always been metric, so prefer to go metric when I can, but can cope with imperial where needed.

I have seen some mention of a 128 tooth gear and or a 63 tooth to achieve near metric pitches, but no mention of these on my gear chart even on the metric section, I don’t even know if I’d get the 128 to fit in the gear train. Sooo I was hoping some of you more experienced machinists out there could possibly guide me in the right direction for cutting imperial and metric pitches on my old Zyto. Most times it’s taps and dies as I’ve got sets of most types now and are quicker and easier in most cases. As the Zyto has a screw on chuck there’s no reverse, therefore I’ll have to rewind by hand when threading, not a major hurdle and I have fitted a threading dial from the 8tpi myford for imperial threading.

Hope my ramblings make perfect sense to someone, thanks in advance.

Peter

[Julie AnnParticipant @julieann]

First things first, welcome to the forum!

Metric gear cutting on an imperial lathe can be done with a 127 tooth gear as it is half 254, ie, the imperial/metric conversion factor. As you have spotted a 127 tooth gear is rather large and hence rarely fitted. Using a 127 tooth gear gives an exact conversion but a 63 tooth gear can be used as an approximation with other gears, as can other pairs. I don’t suppose metric threads were even a consideration when the Zyto was designed.

I will leave the details to others, who know what they are doing, as my lathe cuts imperial and metric threads just by twiddling knobs on the gearbox.

Julie

[JasonBModerator @jasonb]

Your best bet may be to use one of many online calculators. These allow you to enter the tooth counts of the gears that you have along with your leadscrew’s pitch. It will then produce a chart for the common pitches, listing the gear train and how close it will get you to the perfect pitch. You will also need to know what size gear fits onto the lathe spindle as that will be the first “Driver”

If the gears you have will not get you close enough to the common metric pitches you could buy more Myford gears or as they don’t take a lot of load the gears could be 3D printed.

Alternative is to forget the gears and fit an electronically controlled leadscrew or “ELS”

[duncan webster 1Participant @duncanwebster1]

I think 63t is used as a driver, not because it is approximatly half of 127

[JasonBModerator @jasonb]

depends on what pitch you want to cut, on my imperial leadscrew machine the 63T is driver for some and driven for others

As Julie says I have always though it was due to being closest to half of 127.

[Glyn DaviesParticipant @glyndavies49417]

I have a metric conversion set for my Myford and it doesn’t have a 127 driven gear – it has a 63t driver gear. You’d think that this would give an error when cutting metric threads of 126/127, which would be unacceptable. But it doesn’t. The trick is to use a 63 t gear as a driver in conjunction with a 8 tpi lead screw. The Myford set relies on the fact that 200/63 = 3.1746 mm, which is very close to 25.4/8 = 127/40 = 3.175 mm (.125″).

[BazyleParticipant @bazyle]

Creating thread pitches is all about prime numbers, the more available the merrier, especially for metric conversion.
Really the reason 63 works it to introduce some more primes ie 7x3x3 so you can instead use say 7×3 if your gear set can provide the other 3 elsewhere if needed. That’s why a calculator will help as it will find it for you.
Another favourite in some of the old tables is 76 because it brings in 19 and others use multiples of 11. However if you can fit a train of 6 wheels instead of just 4 you can get adequate threads for fixings from standard gears sets in steps of 5.

[Roderick JenkinsParticipant @roderickjenkins93242]

As Duncan says, the 63 is used as a driver to give a better approximation.  Myford used a 21 rather than a 63 in their tables.  The thing is, the tables produced by the lathe manufacturers for metric threads on imperial machines were developed by analytical means.  These days computers can juggle all the options in double quick time and give you a whole bunch of alternative combinations to give satisfactory results.  The table below is an example using just the standard set of gears as would have been supplied by Zyto (I used to have one).

[JasonBModerator @jasonb]

And here is the 63T being used as a driver and the driven. There is a fixed 40T on the spindle before the gears in the chart. 8tpi leadscrew. Position A is straight through the gearbox 1:1

[zytooohParticipant @zytoooh]

Hi folks

Firstly I would like to thank those of you who replied to my question and so quickly regarding metric and imperial threading on a Zyto. I now understand a little more and this explains the gear chart I have.

The imperial section uses 20 to 75 teeth in 5 tooth increments, plus a 38 tooth for one pitch in various combinations,

The metric section uses the same 20 to 75 teeth with the addition of some 21 tooth gears of which I have 2 as required for a few profiles.

I’ve also got a couple of 80 tooth gears, all I seem short of is a 12, 46 and an extra 40 to cover every pitch on the chart.

Most of these were provided in the purchase from the previous owner, so I think I have what I need. I now need to bite the bullet and ruin a piece of bar stock and give it a go.

Thanks again the help was really appreciated.

Peter

[Charles LamontParticipant @charleslamont71117]

Duncan Munro’s gear calculator is a very useful little program. I use it to work out gear trains that have pitch error far smaller than the leadscrew: https://metal.duncanamps.com/software.php

Roderick Jenkins, what is the leadscrew pitch used in your table in post #808946?

[Charles Lamont]

On Charles Lamont Said:

Roderick Jenkins, what is the leadscrew pitch used in your table in post #808946?

I should have mentioned that:  8 tpi

Rod

[Roderick JenkinsParticipant @roderickjenkins93242]

3.225 mm pitch😀

[JasonBModerator @jasonb]

Rod I just deleted my previous post as you were probably typing. I read it as Driver-driven-driver driven etc along the top. But it is all the drivers and then all the driven

[Charles LamontParticipant @charleslamont71117]

Aha! That’s what fooled me too.

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