Change Gear Setup

Hi all I would appreciate some help. I have a Myford Long Bed Super 7 with a quick change gearbox and also have a 1481/1 Metric Convesion Set with a chart that clearly shows the gear train and lever positions for all metric pitches. Can anybody assist me with how to calculate or provide a chart on how to cut additional imperial threads and what position to set the levers (A,B & C and 1 to 8) other than the 8 to 56TPI that I can cut by default using the quick change box as none was supplied with the lathe documentation which I purchased new in 1972. As an example I need to cut a 60TPI thread. All the setup charts I have are for the Std Lathe without the gearbox. I live in Australia.

Hello David,

I can help, but it would be more productive if you can list the additional pitches you would like to achieve. It isn't that I am keeping information to myself, far from it, but you will need to have a table of all the ratios the gearbox can provide and marry that information with the pitch you want to achieve to find a driver gear that will give it to you.

The maths is not hard, just time consuming to test the combinations and find a pitch outcome that is close enough to the required value.

In my book on the subject I already have listings for the following pitches if any of those are the values you are looking for, none of which are in Myford tables. These have all been calculated for use with the standard arrangement of fixed pin quadrant that the lathe is used for without having to install the metric quadrant at all. It preserves the fine pitch operation when the 57/19 tooth combination gear is reversed but it does need drivers with tooth counts that are not commonly supplied

Viz; 5, 6, 7, 15, 17, 21, 23, 25, 27, 30, 33, 39, 42 and 50 tpi

I hope that is helpful

Regards Brian

Hello again David,

I have played about with your example pitch of 60 tpi.

There are limitations caused I'm afraid by the physical gearing in the gearbox itself and 60 tpi is really at about the finest pitch it will deliver. However, all is not lost and without trying the effects of gearing reductions in place of the metric gearing set up, these are the results I came up with

First of all, using the regular fixed pin quadrant, the closest match I could find was with a driver gear of 22 teeth matched to a gearbox setting of 56 tpi. That will give you a pitch of 0.0164 inches [ 61 tpi]

By altering the parameters of the metric conversion quadrant, a driver of 29 teeth coupled to a leadscrew wheel of 65 teeth will give a pitch of 0.0166 inches [ 60.23 tpi]. Another value is achieved using more readily available wheels of 20 teeth as a driver and 45 on the leadscrew..The result is 0.01653 inches [ 60.5 tpi]

These values are obtained when the gearbox is set for 8 tpi

Beyond these results you will have to consider operating the metric conversion as a further external gearbox and test for new input gears matched to gearbox settings. It gets rather complicated I'm afraid

Regards Brian

David, Brian,

Looking at Brian's book, "Gearing of Lathes for Sscrewcutting", Table 4, for lathes with the Myford gearbox, , 30 tpi is achievable with a 32T gear on the mandrel and the box set for 40 tpi.

IF you could find / make a 64T wheel, you would then get 60 tpi at this setting..

This makes the big assumption that a 64T wheel would fit. (Table 1 shows a 66T wheel for producing Diametral Pitch threads, so it may be possible). BUT I speak / write without any practical knowledge.

Comments invited!

Howard

Hello Howard,

You have unfortunately fallen into the trap of ratios, otherwise nice try!

You actually need to HALVE the input gear size to achieve the necessary reduction to get a result of 60 tpi which means in this case fitting a 16 tooth gear in place as the driver. Without testing it I think the key to drive it will try to split the gear.

As a point of interest, a 64 tooth wheel would fit with my modified fitting to lower the banjo, in fact 80 teeth will clear the spindle.

Regards Brian

I believe the gears available with the metric conversion set are:

28, 30,35,40,45x2,50x2,55,60x2,63

According to my calculations the following gives an exact 60tpi:

Drivers: 28 30 35

Driven: 40 50 63

Gearbox set to 14tpi

Would anybody care to check?

I don't know whether the gears will actually mesh but it is probably worth a try.

HTH,

Rod

Hello Rod,

Yes, the calculation does agree, but as you say, they may not all mesh happily. Thank you for flogging through the workings

Regards Brian

Hello David, Roderick and Howard

Having slept on the ideas of yesterday, there is in fact a very simple general calculation method that will give David all the pitches [within reason of course] he might care to find

If we treat the Myford metric banjo as purely a means of introducing external gearing ratios and marry that with the ones available from the screwcutting gearbox, the formula becomes

Pitch in inches = R (outside) x R (box) x leadscrew pitch in inches, where R is the ratio in each case

Taking David's example of 60 tpi

P [1/60] = Ro x Rb x 1/8 Hence Ro x Rb is 8/60= 0.1333. Matching that with a gearbox ratio of 0.1667 [the value for 48 tpi] gives a nice simple result for Ro of 0.8

So, in this example, David needs to set up a 40 tooth driver meshing with a 50 tooth driven wheel,linked by driver(s) as necessary, select a gearbox setting of 48 tpi and the resulting pitch will be 60 tpi

So David it is up to you now. You have been silent thus far.

Send me a PM stating your email address if you would like a copy of the whole 32 values of gearbox ratio. I can't unfortunately post it this way, there is a compatability issue with my computer that I am unable to resolve

Regards Brian

Correction, The 40--50 gear set up above should of course read idlers as the link, not drivers. Sorry if that has confused things