Can you provide a URL where that pdf is located, Mike? I can't seem to find
it, and there is no Save Target in this hack/spitWindowssucks crapVista
Mail.

I saw a shop-made QCGB maybe 5 years ago, but don't remember where or what
machine it was for. It was an open style gear selector added on to the left
end of the lathe bed.

--
WB
..........
metalworking projects
www.kwagmire.com/metal_proj.html


"Michael Gray" wrote in message
news:G5t5l.1460$%v1.205@edtnps83...
On Sat, 27 Dec 2008 08:47:31 -0500, Wild_Bill wrote:

Well, there are positive points to consider, Michael.. the mini lathe
has many of the desireable features for lathes that the bigger models
don't, it just doesn't have a quick change gearbox.


I knew I'd seen this recently:

www.finger.de-web.cc/info/Drehen/WebQCGB.pdf

a home-made, effective-looking QuickChange Gear Box for the 7x lathes.
Haven't tried it yet, but it does look do-able.

Mike in BC

On Dec 27, 4:31*pm, "Michael Koblic" wrote:
...
I am still trying to get my head around the fact that a lathe cannot cut
both SAE and metric screws. Why? The screwcutting is a function of matching
the carriage travel to the number of spindle turns. As this is achieved by
gears, should one not be able to achieve a more or less infinite combination
to match both systems?
Michael Koblic,

The ratio between millimeters and inches is 25.4000000000, by
definition of the inch. The prime factors are 254 are 2 and 127, which
means that somewhere in the gear train a 127 tooth gear is needed. The
16 pitch one for my lathe is 8" in diameter, not easy to stuff into
the limited space available..

47/37 = 1.2702702..., not perfect but compact and close enough for
many uses.

To calculate gear ratios for threading you find a gear combination
that turns the lead screw at the same speed as the spindle. Usually
this is any 1:1 ratio. Those gears will give a thread the same pitch
as the leadscrew. Then for any other thread pitch you find gears in
the same ratio as the thread pitch to the leadscrew pitch. For example
my little lathe's leadscrew is 16 threads per inch. To cut 32 TPI I
need to halve its speed with any 2:1 pair that fits the available
space. One thread pitch equals half a turn of the leadscrew.

I made a spreadsheet with all the threads my larger quick-change lathe
will cut, and the corresponding metric modulus, none of which came out
even. Next I referenced a cell with 127/100 or 127/120 etc in the
formula, which shows the effect of those two compounded into the gear
train, one driven and the other driving the next gear. Some of the
thread pitch cells then displayed standard metric modulus values. 120
and 127 gave the largest number of fine lens threads so I bought them
from Boston Gear.
http://www.bostongear.com/products/open/change.html
IIRC 100 was better for standard metric screw threads.

Then that laser optics job stopped abruptly and I haven't cut a metric
thread since. My little gold and flat black precision mechanisms went
into the company museum.

Jim Wilkins

On 2008-12-27, Michael Koblic wrote:

[ ... ]

I am still trying to get my head around the fact that a lathe cannot cut
both SAE and metric screws. Why? The screwcutting is a function of matching
the carriage travel to the number of spindle turns. As this is achieved by
gears, should one not be able to achieve a more or less infinite combination
to match both systems?

One can use certain gear ratios (100:127 is the ideal, smaller
ones come close) to provide a conversion between metric and imperial
threads.

*But* -- during most threading, it is common to use half-nuts
in the apron -- engage them to start cutting the thread, disengage and
quickly crank the carriage back with the large handwheel, then re-engage
to cut the next pass of the thread.

However -- there are multiple points at which the half-nuts can
be closed, and only some of those will retrace the previous cut (which
you want to do -- only deeper). For this, there is usually a "threading
dial", usually on the right side of the carriage, though I have seen
them built into the apron and flush with the top of the carriage.

Anyway -- the dial tells you when to close the half nuts to
retrace the same thread path.

On imperial lathes with an imperal leadscrew, this works only
with imperial threads.

On metric lathes with a metric leadscrew the threading dial
needs to have a selection of (typically) four different gears to engage
the leadscrew to deal with the different ratios which metric threads
require.

In either case, you *can't* use a threading dial for an imperal
leadscrew to cut metric threads -- even with the 100:127 transposing
gears.

And -- you can't use a threading dial for a metric leadscrew to
cut imperial threads even with the 100:127 transposing gears.

So -- the only way to cut threads on the other system (unless
you are cutting a fine enough and shallow enough thread so you can cut
it in one pass) is to halt the spindle before you reach the end of the
thread, perhaps use a hand crank locked into the spindle, to crank the
rest of the way to the end of the thread, back the tool out of the
thread, crank in reverse to the starting end, crank the tool back in
with the extra depth for the next pass, again use the power for a long
thread but being sure to stop it before you reach the end, and crank
again and again.

So -- you *can* cut metric threads on an imperial lathe or vice
versa with the transposing gears -- but is is a royal plain. So modify
the statement to read "You can't *conveniently* cut X threads on a Y
lathe" and it becomes true.

The only exception to this is on CNC lathes, where there is no
direct gear connection between the leadscrew and the spindle. Instead,
the spindle has an encoder which gives one index pulse per revolution,
plus a lot of incremental pulses ever few degrees. The computer handles
turning the leadscrew the right amount for each incremental pulse to cut
the threads. The computer then backs the tool out at the end of the
thread, and reverses rapidly to reach the starting point, and then waits
for the next index pulse to start the same thread pass again -- deeper.
So -- it is a mater of telling the computer whether you want to work in
mm or inches. (Oh yes -- and learning to program the computer. :-)

I hope that this helps. It would have been easier if you had
already had the practice of cutting threads on a manual lathe of one
system or the other, so you would know what you would be missing.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

"DoN. Nichols" wrote in message
...

snip

DoN, Jim and Wild Bill, thanks for the detailed explanation.
I watched this done on a DVD but it is clearly one of those things that one
needs a hands-on.
One day...

--
Michael Koblic,
Campbell River, BC

On Sat, 27 Dec 2008 19:29:29 -0500, Wild_Bill wrote:

Can you provide a URL where that pdf is located, Mike? I can't seem to
find it, and there is no Save Target in this hack/spitWindowssucks
crapVista Mail.

I saw a shop-made QCGB maybe 5 years ago, but don't remember where or
what machine it was for. It was an open style gear selector added on to
the left end of the lathe bed.

Here you go, Bill.

http://homepage.mac.com/bhagenbuch/m...ages/qcgb.html

If my Christmas dinner and goodies hadn't been sitting so well in my
tummy I'd have put it in in the first place, wouldn't I?

Mike in BC

Thank you very much, Mike. That appears to be the same one I referred to,
but I didn't have a mini lathe at the time.

I was able to get the pdf from that page in the new/improved/less
filling-tastes great version of hack/spitExplorer.

It's good to know that there are smart guys out there that figure these
things out, then share them.

--
WB
..........
metalworking projects
www.kwagmire.com/metal_proj.html


"Michael Gray" wrote in message
news:2HO5l.1659$%v1.1135@edtnps83...
On Sat, 27 Dec 2008 19:29:29 -0500, Wild_Bill wrote:

Can you provide a URL where that pdf is located, Mike? I can't seem to
find it, and there is no Save Target in this hack/spitWindowssucks
crapVista Mail.

I saw a shop-made QCGB maybe 5 years ago, but don't remember where or
what machine it was for. It was an open style gear selector added on to
the left end of the lathe bed.

Here you go, Bill.

http://homepage.mac.com/bhagenbuch/m...ages/qcgb.html

If my Christmas dinner and goodies hadn't been sitting so well in my
tummy I'd have put it in in the first place, wouldn't I?

Mike in BC