Early Myford Super-7B – Metric Conversion??

[DMRParticipant @dmr]

Hi Flatline or Rich D,

The answer to your question is that the current Myford metric set will fit your box but you will need some more cogs to cut all metrics with any accuracy. The following is a write up I did many years ago and relates to Myford Nottingham. You need the current Myford gearbox book to relate to the following which is available on line or I can copy it to you if you PM me your e-mail address. Whilst I can copy the words, the tables come out wrong on here so I will need your e-mail anyway if you want them:-

Metric pitches using current Myford slotted quadrant and the old gearbox (Pre Dec 1956 and Serial No QC 2496)

For anyone who, like me came by an early Myford gearbox and associated leadscrew, the current selection of gears advised by Myford for the slotted quadrant seems unusable. Attempting to resolve any form of metric threading with Myford proved fruitless as they no longer support the unit and I did not even have the metric table inside the changewheel cover to go by. I found a note on the Web relating to the slotted quadrant needing four cogs instead of the current five, but could not find what they were. Delving deeper and talking with Mike Smith who put the difference between the two boxes so clearly in MEW 107; I resolved to sort out the situation. Mike told me what the four cogs were in the original set-up and I carried on from there. The fact is that the early slotted quadrant ratios are not as accurate as the current select, and the current selection is more fitted to the current box, as will be seen, but there is no “must use early selection with old gearbox”. Either can be used with either box, but the later set is more accurate.

To cut any thread, the current Myford gearbox has to run twice as fast as the old one: the output then being divided by two on external gears to the leadscrew, whereas the old box has 1:1 external gearing. The guts of both boxes are identical in terms of cogs and ratios. Myford do not even give that information out, but for a fee you can buy the current gearbox handbook and therein lies the key when you know what to do. It certainly is not necessary to purchase the slotted quadrant alone and an old set of (10 off) individual cogs. Buy the current complete set of quadrant and 12 cogs for less, unless you already possess some of the cogs. The root reason being that the current set cuts threads closer to the correct pitch in all cases.

 

All you have to then do is take the current tables for diametral, BA and metric threads and either:-

A). Slow the gearbox output by internal gearing by selecting a factor of two slower, or

B). Alter the changewheels on the slotted quadrant to halve the feed to the gearbox.

The problem with that simple solution is that for A) the gearbox may not go any slower, and for B). You cannot just up a 60 tooth at Gear ‘X’ to 120 as it will not fit the quadrant, or divide a 55 tooth on the tumbler stud by two for instance. Solution A works perfectly for BA threads, Module pitches and most diametrals; just set the top lever one notch down. The lever does a x2, x1, and x0.5 of the Selector setting for positions A B C respectively, so setting B instead of A or setting C instead of B halves the output feed rate. The remaining Diametral pitches, where the lever is already in position C, can be achieved by making the 35T into a 70T for even DP’s, and the second stud driver to half size for the remaining odd DP’s. The idler wheels will need to be resized to fit the slotted quadrant.

 

The real problem is the metric threads, where the current tables will result in twice the thread pitch with no change wheel alterations. That is to say, using the settings in the current tables for 4.00mm feed will result in an 8.00mm feed (4 TPI) instead which would probably wreck the gearbox. So no threads below 0.40mm are selectable and no ratios exist for such as 1.25mm, 1.75mm, or anything where a times 2 of the current tables misses it out. Presumably the finer metrics were not used 50 years ago when the box was made, but there must now be a demand as the tables go down that far. So some new cog selections are necessary and the following table does the job.

End of quote.

 

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