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derekg
10-25-2012, 06:29 PM
Hi Folks,

I'm back with another dumb question. I keep seeing the lathe stops and it makes sense to ignore the dial and just feed until you hit the stop. My question is about hitting the stop under power. Are the half nuts made to automatically open when the carriage hits something immovable or will that just destroy the lead screw and the half nuts? I told you it was a dumb question. And no, I haven't tried it :)

Thanks, Derek

Lew Hartswick
10-25-2012, 06:32 PM
There is a difference between "dumb" and "stupid". Of course you don't run a
carriage into a "stop" under power.
...lew...

lane
10-25-2012, 06:35 PM
DO NOT hit the stop under power.You will tare thing up . A stop is used under hand feed only. You turn of the power feed almost to the end of the cut with in 1/8 inch are less then just ease the carriage hand wheel up until you contact the stop. Again not under power.

bytewise
10-25-2012, 07:42 PM
Some lathes have an automatic carriage feed stop feature, Hendey for example. This is quite different than a clamp on the bed. It operates a dog clutch in the feed gear train and is very repeatable.
Hugh

oxford
10-25-2012, 07:48 PM
The 20" TOS at work will jump the power feed off when it hits the clamp on the bed carriage stop. This may or may not be the same thing bytewise is talking about with the Hendey. But like the others said, don't run into it with the power feed on.

lakeside53
10-25-2012, 07:48 PM
To answer the original question, your shear pin (assuming it's not been replaced with a nail!) will shear protecting your lead screw and nuts. This is supposed to happen only by accident, so.. it would be dumb to do it every time :)

uncle pete
10-25-2012, 08:13 PM
"Some" lathes are capable of doing so, but I kinda doubt you've got one Derek. Some of the Hardinge models and their clones can thread up to a shoulder at speeds that are unbelievable. A lot of the American, British, European and other country's lathes can do so using various methods on the heavier duty industrial and tool room lathes to kick the feeds out. On the higher end and better machines, the lead screws aren't ever used for power feeding. A properly designed and seperate feed shaft gets used for longitudinal and power cross feeds. That way the lead screw gets it's accuracy preserved and it's only used while threading. There could be the odd exception I guess, but overall only hobby grade and mostly at the lower end of the hobby grade scale will use the lead screw for power feeding.

I can sympathize about how tough it is to find answers at times when your first learning. Try www.lathes.co.uk, Maybe spend oh I dunno, 10-50 hours on that site and you'll come away with a fair understanding, and with luck a much deeper appreciation for some of the worlds best manual machine tool builders of their time. There's also some cheaply built ones shown also. But those machine tool write ups normally contain both the good and bad points for each machine. And how some of their products were built and operate. That understanding and even the history is well worth your time. It does take some time and dedication to learn even the bare minimum about all this. Once you've rooted through Tony's lathe section? You can then start on the mill section. After that? hold onto your wallet, your much improved and hopefully refined taste in machine tools won't ever match your bank balance. There really is such a thing as Swiss built machine tool porn.;)

If you'd like a few good book recommendations that I think would help, then feel free to PM me.

Pete

sasquatch
10-25-2012, 08:20 PM
Excellent post Pete!

Doozer
10-25-2012, 08:56 PM
OK, yes, Hardinge and Hendey lathes have single dog clutch threading.
Hardinge lathes have this threading with automatic stops you can set to make the thread feed drop out.
ALSO Hardinge lathes have friction clutches for the carriage power feed (not threading).
Hardinge lathes (HLV, TFB, HC etc.) are made to feed to the hard stop on the bed,
and then the clutch friction will slip. It is made to slip. The owners manual says this.
Otherwise I would not believe this. I know Southbend lathes have friction feed clutches.
The older ones knobs, the later ones, levers. Not sure if you can use them like that.
I think SB uses iron cone clutches (not 100% sure). I know Hardinge uses paper friction
clutches in oil, so maybe that is how they can recommend this.
Nardini lathes have a power feed that cocks like a gun mechanism, and there is a button
that will hit the bed stop that drops out the feed. These lathes work very well with this
feature. Real nice for facing cuts.
So the answer is that it depends on your lathe if you can feed hard into the bed stop.
If your power feed has a dog clutch or uses half nuts, then probably not.

--Doozer

justanengineer
10-25-2012, 09:18 PM
I know Southbend lathes have friction feed clutches.
The older ones knobs, the later ones, levers. Not sure if you can use them like that.


A much older mentor of mine told me that doing so is common on SBs, but also that it pretty rapidly destroys the clutches. Not sure if this is true or not, but his info in the past was always reliable and despite also owning much newer/nicer lathes he always kept his 9A.

J Harp
10-25-2012, 10:05 PM
On the smaller and cheaper import lathes, if you are threading and hit a shoulder or a solid part such as a stop left clamped to the ways, maybe the shear pin will save the half nuts. If you are using the power feed which is driven by the lead screw with a keyway, you are out of luck.

I had the pleasure of brazing up and recutting some teeth on the small gear which is driven by a worm on the lead screw on mine. I had checked that the cut would end before the carriage got to the headstock, then had to move the tool for some reason, and forgot to recheck. Of course I was watching the cut when Bang! it was too late.

oldtiffie
10-26-2012, 12:59 AM
I don't use stops.

I use the dial on the carriage feed hand-wheel as it forces me to concentrate instead of assumimg a stop and its setting are OK.

dp
10-26-2012, 01:12 AM
Some people have put after market half-nut releases on their carriages but unless it is an over-center fast acting release, the half-nuts will release slowly, presenting incrementally smaller engagement to the lead screw, and this will accelerate wear. A spring loaded sear type release will ensure fast release - and hopefully, positive release as well. Not paying attention when the expensive parts come into contact and pile up stress exponentially will ruin your day and your machine.

If you search around the internet you can find some decent carriage auto-stop mods.

big job
10-26-2012, 06:00 AM
Thats why I made mine audible. Carriage contacts spring- 9vt batt.- old smoke detector
speaker that gets attention instantly especially comming to the end of threading. And it
is on 24/7 . Two years same battery. Works for me so I can concentrate more on what
Im doing instead of watching a fixed stop. As said hand is quicker than the eye, Hearing
also is quicker than the eye.

Doozer
10-26-2012, 08:46 AM
big job-
Never thought about a switch actuated buzzer stop,
but I like it and it makes sense. Simple genius really!

I thought of using a small Bimba air cylinder as a quick
release for the feed. Dennis is right about a rapid
withdrawal of the half nuts or feed clutch, else wear
will ensue. That is why I like the Nardini arrangement.
It is spring loaded, and the feed lever cocks like a gun.
Actually the downfeed on a Bridgeport cocks the same
way. This assures rapid release and positional repeat
ability. Beware, lots of bad power feed trip plans (ideas)
on the net. Know what to look for, the fast release.

--Doozer

lakeside53
10-26-2012, 11:39 AM
My lathe has a overload clutch on the power feed. I don't use it for any routine purpose, but it's got my attention a few times. I also have a magnetic clutch that stops the spindle in a heatbeat. I fitted a contact switch on the stop to actuate the magnetic brake for threading "real close". Works great. Saved my butt a couple of times when turning right up to the chuck, and I screw up.

oldtiffie
10-26-2012, 07:39 PM
Perhaps some need a bit of time on a largish turret or capstan lathe where there are often more stops than you would imagine.

They are great "engine" lathes and for screw-cutting just (quickly) change the half-nuts and the lead-screw!!

Speed changes are "on the run" and they have great clutches and brakes!!

http://www.oldengine.org/members/diesel/ward/ward11.htm

derekg
10-30-2012, 05:54 PM
Thank you all for the replies. Just getting back to this and really appreciate the info. I've learned some valuable things;

1) Lew has anger issues :)
2) *Some* lathes have disengagement systems, but mine doesn't
3) There is a failsafe on some lathes in the form of a sheer pin (going to research that now)
4) Never run under power to a hard stop
5) An audible buzzer is one of the coolest things I never thought about, but will be incorporating.

Many thanks, Derek

uncle pete
10-30-2012, 06:57 PM
Some more for your list. Run up against a stop hard enough and it can preload the half nuts and leadscrew engagement making it very hard if not impossible to release the half nuts at all. It's an amazingingly expensive sound listening to a machine you own trying to eat it's own parts while the motor slowly winds down to a stop when or if that failsafe fails. Some of the cheaper lathes use a plastic gear that strips a few teeth off instead of a shear pin. OEM shear pins are always made and priced like their made from unobtainium. And it's normally impossible to find out from the factory that if their aluminum it's usually 6061 T-6 aluminum, I'm pretty sure that's what my Emco uses anyway.

Pete

Forestgnome
10-30-2012, 07:03 PM
Thank you all for the replies. Just getting back to this and really appreciate the info. I've learned some valuable things;

1) Lew has anger issues :)
2) *Some* lathes have disengagement systems, but mine doesn't
3) There is a failsafe on some lathes in the form of a sheer pin (going to research that now)
4) Never run under power to a hard stop
5) An audible buzzer is one of the coolest things I never thought about, but will be incorporating.

Many thanks, Derek

Just to add to this, some lathes use sacrificial gears instead of shear pins. My Sheldon does. Not something you want to test (ask me how I know).