Myford S7: Define A Most Useful Sub Set Of Change Gears

[SteveW]

Guys,

Found myself cutting threads on my Super7 the other day using a web based thing I found a while back called: Screwcutting Gear Ratio Calculator At the time I took the precaution of capturing the web tool to file from where I can always find it. This program is particularly useful for cutting metric threads on an imperial lathe and I guess the reverse. Its just a question of choosing the combination yielding the smallest error and for short threads is well good enough.

This tool, accessed via a web browser, allows the user to define his/her set of change gears, lead screw pitch, and target thread as either TPI or in millimetres. On hitting the tit it produces a list and image of the required gear pack. In many cases it provides multiple gears sets as not every one actually fits, typically for the S7 when the first and second driven gears are small.

I found it useful once captured to file to edit the HTML to suit my lead screw and current change wheel set, it saves a lot of faff every time.

My Question: With the Midlands ME Show next week and access to countless new toys, including change gears, it occurs to ask here, given my current gear set of:

20, 25, 30, 35, 38, 40, 45, 50, 55, 60, 65, 70 & 75

what useful sized gear(s) is/are missing? I've already discounted the famous 127 toothed monster as with the program I can already get close enough to metric threads on my S7.

Years back I found and brought a very useful little coupled gear/pinion ideal for simple slow saddle feeds. Using all my gears starting with this one got me a very good if very slow to produce final finish.

Answers please before next Thursday.

[ettingtonliam]

Didn't Myford use a 63T gear instead of the 127T for metric thread cutting?

[uuu]

33 and 34 gears are useful for adapting an imperial gearbox to do metric threads - a much easier alternative to the fiddly-to-fit conversion set.

Wilf

[jasonb]

Yes a 63T is quite common on imperial change gear lathes to get closer to metric pitches being approx half of 127.

The only thing to watch with the various online calculators is that although they will show various permutations of gears to give your desired pitch you may not be able to physically get them onto the Banjo, that is where a 63T helps.

[millman]

Rather than using online calculators I purchased a book called “The gearing of lathes for screwcutting” published by Crowood press, author Brian Wood. It doesn’t tell you how to screwcut, it gives charts of gear trains for pretty near every thread you are likely to cut. It is particularly good for Myford lathes. I have no connection to the author, just a satisfied customer.

[johnthepump]

Oct 6, 2023 13:11:24 GMT uuu said:

33 and 34 gears are useful for adapting an imperial gearbox to do metric threads - a much easier alternative to the fiddly-to-fit conversion set.

Wilf

I have used these two gears on my Super 7 ever since Wilf told me about them and he kindly supplied a chart to go with it. When I had ML7 without a gear box I used 127t gear.

John

[halfnut]

Martin Cleeve's book "Screwcutting in the Lathe" (Workshop Practice Series, written toward the end of the Pleistocene era) contains the chart to cut all metric threads using only the standard set of imperial Myford gears you listed above, ie 20 to 75 in steps of 5, plus a 38. The conversion error is in the region of one thou per eight inches or so in most cases. One thou in three inches at the very worst. There is more error than that in your mass produced leadscrew, even when it was new. So no need for 127s or 63s or anything else exotic. And all the gear trains in his chart will physically fit together and work. I have used them successfully for many years.

The book is well worth getting if you are interested in such things.

Brian Wood's book focuses on lathes with QC gearboxes and does not contain the information that is in Cleeve's charts for non-gearbox models.