I went the 2 x 21 tooth changewheel route and the Myford manual lists
the gear train setup for these.
It's not *exactly* accurate in the pedantic sense, but accurate enough
to more decimal places than we can measure
If you have a look at John's/Gert's shop on ebay - marypoppinsbag -
he sells these and includes a laminated copy of the setup chart with
Hi, are you saying I can use 2 X 21 tooth gears as well as the 127 tooth or
instead of it? I don't have a metric setup chart so need to know what
number of teeth on which gear, and where each gear goes. Perhaps theres a
web page with this info.
You can use the 21T gears instead of the 127T gear. I had neither with
mine when I got it and decided to go this way as it saved me doing any
simple calculations which I was bound to get wrong.
I've scanned in the 2 pages from the manual here, and this will tell
you how to set it up.
Someone else on the list has en entire Super7 manual scanned in as a
PDF file on their web pages. I can't remember the link offhand, but
I'm sure that someone will soon post it up for you.
The Super 7 manual is accessed in the Files Section of Yahoo's
There are a lot of other 'Myford' notes in the lathemod bits.
Sorry, I haven't been well enough to help put more on board.
There is a huge amount which is in Model Engineer- but not available to
newcomers who have bought Myfords.
Again, a lot is available in book form and I would suggest that you buy
" Screwcutting in the Lathe" by Martin Cleeve -Tee Publishing
" Model Engineers Workshop Manual" G.H.Thomas- also fromTee
After spending lots on your Myford, these are almost compulsive reading
and will add immensly to your enjoyment!
You can use either the 127 wheel or the two 21's to achieve metric
Myford recommend the two 21's and their metric chart reflects this.
The 127 is the only way to get truly accurate metric threads as 127 is
the lowest common denominator of 254, actually 1/2 and if you write
254 as 25.4 you will see where this figure comes from.
Advantages of using the 127 are true conversion but at the expense of
having to run without the guard as the 127 is too big to fit in.
Purists will say the 21's are not accurate as they don't work out
exact but the error in parts per inch is probably less than the
manufacturing error on the leadscrew allowing for wind, tide and the
curvature of the earth.
One point to note on whichever method you use, is that you can't
disconnect the half nuts whilst screwcutting metric threads on an
imperial leadscrew as the thread dial is useless because it only lines
up once in God knows how many turns.
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I think it helps to have a motor you can reverse. Mine doesn't which is
a real pain (I think if I dismantle it, reversing should be possible
but can't be bothered as I write...). I tried cutting an M8 thread on
mine and then hand winding the lathe back but the threads still didn't
line up. To be fair, wasn't using a proper tool and was trying to cut
silver steel which apparently isn't so easy to thread...
I've just come in from the shed where I was using a 63t (3x21) gear from the
myford conversion set to cut a 16x1.5 thread. The reason that I came in was
that I hadn't let the Loctite, that I'd stuck the part onto a mandrel with,
set properly. Had to clean it off and glue it back together, with stronger
loctite. I'll leave it overnight before starting again.
The part is a gland with rounded edges for a cable to go through, because I
couldn't find a grommet the right size.
If you use back gear to cut the thread then it is simple to disengage the back
gear to wind the mandrel back. A handle in the back of the mandrel is good for
the turning. It is a good idea to back off the tool before winding back,
otherwise the slack in the gear train can cause the tool to rub hard enough to
chip the edge. DAMHIKT!
Forgive me if I'm teaching grandma to suck eggs, but you need to
retract the threading tool from the cut before winding back to start
the next cut, as the backlash will be in the wrong direction and the
tool will foul what you have already cut. Then when you get to the
tailstock end of your thread, go a little further, advance the tool to
the new cut depth and start threading 'in air' for the back lash to be
taken up before the tool gets to the begining of the cut proper.
Does that mean if you put a carriage stop at the free end of the thread you
could use a tdi with a metric thread/ipmerial leadscrew by
1. winding the carriage back until it meets the stop, then
2. waiting for the tdi to indicate, then
3. engaging the leadscrew
Or is there a reason this doesn't work ?
Most US indicators have gears which have a pitch circumference of 4 inches
(32 teeth for a 8TPI screw). The dial makes one revolution for 4 inches of
carriage (or screw) movement. This should work for a 0.8MM thread pitch even
without the stop since 127 threads at 0.8MM pitch equals 4 inches.
You can get a good idea of what is required if you imagine the threaded work
to be very long and both the work and feedscrew to be stationary. From one
point where the threads match up, it is 127 threads on the WORK to the next
match up point for all metric pitches.
You could use a carriage stop by opening the half nuts, returning the
carriage to the stop, reversing the work and leadscrew to the starting
position, and then re-engaging the half nuts. The only problem is being able
to tell when the work and leadscrew are back to the starting point. Some
sort of external half-nuts on the leadscrew probably could be made to work.
A device to count 127 revolutions of the spindle also could do it, I think.