Does anybody know if it is possible to get a set of tumbler gears (or
any other approach) for a Rockwell 11x36 Lathe to let you cut metric
threads? I remember reading a mention about it somewhere, but cannot
remember where or find it again.
To get true metric you will need a 127 tooth gear somewhere in
the gear train. In many cases you can get very close [better
than the lead screw can cut] by using a geat set that
approximates 1.27:1. [several possible ratios]
Problem with cutting metric threads with an inch lead screw [or
inch threads with a metric leadscrew] is that you cannot
disengage the halfnut or you will loose lead/registration. You
will have to stop the lathe, back the tool out, and reverse the
motor to move the tool back for another cut. If at all possible,
it is better to use a tap/die, possibly in a holder in the
An old-fashioned accessory that is almost a necessity for doing
this is a spindle hand crank. Threading is entirely manual, no
power applied. see
you do this I suggest removing the spindle drive belt as the
crank will do major damage to anyone in the way if the motor is
turned on while it is attached.
Follow-up to my own post:
Some of the possible gear combinations for approximating the
ideal 127:100 gear set [1.27:1] are:
85/67[1.2687], 66/52[1.2692], 70/55[1.2727], 71/56 [1.2679],
80/63 [1.2698], 85/67 [1.2687], 94/74 [1.2703]
There are many others.
A lot will depend on what gears you have, what gears you can get,
and how much space is available in the lathe.
For home shop use, plastic [nylon/delrin/minlon] gears should be
adequate. You should be able to get the #teeth and required
pressure angle as a stock item [do a google search]. You may
have to carefully enlarge or bush the bores and/or file a keyway.
If you bond the gears together [epoxy? Dowel pins?] you can most
likely skip the keyway. If you are lucky, Emco will have a brass
bushing with the right id/od and possibly the right length. If
not, time to machine one.
Let us know how this turns out as metric threads are becoming
Either 100/127 or 60/127 or 50/127 ... whatever your lathe calls for.
For lathes which call for 100 or 50, 37/47 will approximate all
conventional and model maker's metric pitches to 0.020% error (for all).
For lathes which call for 60, 60/53 will approximate most conventional
and some model maker's metric pitches to 0.016% error, and most model
maker's and some conventional metric pitches to 0.046% or better error.
The "better" can be achieved by a combination of not using any
transposer at all, and by using additional stud gears.
One problem is ...
1) coarse pitches are based on 0.25mm increments (0.25, 0.5, 0.75 ...
2) fine pitches are based on 0.05mm increments (0.25, 0.30, 0.35, ... mm)
... so an approximation which is good for 0.25mm increments is not
necessarily good for 0.05mm increments.
An approximate transposer which is intended for 0.25mm increments also
happens to deliver the same low error on 0.25mm and 0.5mm, but not on
any of the others below 1.0mm.
Regardless of which transposer you employ, you are likely to require up
to eight stud gears in addition to the transposer.
Rockwell sold a metric tansposing gear set, with seperate models for their
10", 11" and 14" lathes. The 11" lathe was P/N 25-630 - you might lucky and
find one on Ebay. Good luck, though. I've been watching for a similar kit
for my Clausing 5900-series lathe and the last two brought over $700 each
and you will probably see similar prices for a Rockwell kit.