Any safe way to thread away from my chuck on a South Bend 10K?

As to downsides of threading upside down.

The lathe was designed with the intent that the cutting forces be directed downward, into the tool, though the toolpost, compound and cross slides, then into the bed-ways. Because of this, they designed the compound and cross slide dovetails with the flat, precise, oily, load bearing surfaces facing up, to absorb these forces with minimal wear.
They put the angled surfaces of the dovetails, and the gibs below, away from cutting forces. Since the angled surfaces and gibs are designed control alignment and sliding clearances, the designers decided to put them where they would be subject to less cutting forces and wear.

The bed-ways were put on top of the bed, for the same reason. Cutting forces end up transmitted into the hardened, precision ground, polished and oiled bed-ways.



When you cut upside down and backwards, the forces are imparted upwards on the tool. In the lifting direction. So, the cutting forces are no longer transmitted through the precision ground, polished and hardened surfaces, but are transmitted through the much smaller angled dovetail surfaces and gibs. (p=f/a or pressure=force divided by area). So, in that simple formula p=f/a, when you reduce the numeric value for the denominator (a), you increase the value for p. So the same cutting forces result in significantly more pressure on lathe parts, and it is transmitted into parts that are not hardened, precision ground, and scraped to hold oil.

Also, consider. The final point where those forces are transferred into the bed, is the gib on the underside of the front of the saddle. That force is transmitted with an upward vector, and with a much higher value for p, due to that gibs much smaller value for a. In most of your (and my) lathe's, that gib isn't hardened, and in some cases, as it wears, it's not adjustable, but may need replaced if it wears sufficiently that the saddle starts to lift while threading backwards.


Now, all that being said, I still believe those are very minor downsides for most hobby machinists. Because we're threading with low cutting forces, finer pitches, and infrequently enough to wear things out with those low cutting forces.

I wouldn't be a bit afraid to thread upside down for most stuff. But if I were going to be threading a few hundred 1 foot long rods with 4 pitch ACME threads, I wouldn't do it. Or if I had a small lathe with non adjustable and/or hard to replace saddle, cross slide and compound rest gibs, I might not want to use that method.

For some, perhaps the juice is worth the squeeze. Some circumstances may allow for it, but other circumstances may not.




Another consideration is lathe design. I don't know much about Southbend Lathes but I recall a post on practical machinist indicating that they do not have a frontside undermount gib to prevent lifting.

Quote from a post on Practical Machinist:

"The smaller SouthBend lathes, 9" & 10" at least, don't have anything at the front of the saddle to prevent lifting. Just a bolt on gib strip under the rear shears.".

Im not familiar with Southbend, and have no idea what, or where the "rear shears" are, but If this is true, those Southbend models and any others sharing that design, might not give the best results if one chooses to thread upside down using one.
 
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mickri said:
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mickri said:
I made a chuck keeper (wrong term?) that will hold a chuck in place if I have to run the lathe in reverse. It is a long rod that fits through the spindle. The chuck end is a close fit to the inside of the chuck and the outboard end is threaded. When it is snugged up I can not get the chuck to come off no matter how hard it try. Another forum member explained how this works. The two different threads, 8 tpi on the chuck verses 13 tpi on the keeper, work against each other preventing either from unthreading.
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This is a clever solution. Not sure of it being fail-safe, but certain at least chuck won't leave spindle. I don't have any threaded spindles that are powered; but can relay this. A right hand [not even large] end mill cutting a transverse slot can unscrew chuck on an indexer.......
 
Adding this, perfect for this thread [pun intended? IDK].
Juggling machine design, part requirements, available tooling is part of every project; to the point no clear way to distinguish what order to consider those. Regarding tool positioning though, offering this. Sorry, no clearer way to format this is known, even tried color coding to no avail. Initial breakdown starts with external or internal threads, then whether RH or LH.
I literally hold out a finger [OD] or thumb and finger [simulating ID], anticipated tool bit in other hand.

A - Threading tool upright, point toward lathe back
B - Threading tool inverted, point toward lathe back
C - Threading tool upright, point toward lathe front
D - Threading tool inverted, point toward lathe front

L - Direction of feed = Left
R - Direction of feed = Right
f - Forward Spindle Rotation
r - Reverse Spindle Rotation

To Cut Right Hand External Threads: Lf(A/D) Rr(B/C)
To Cut Left Hand External Threads: Lr(B/C) Rf(A/D)
To Cut Right Hand Internal Threads: Lf(B/C) Rr(A/D)
To Cut Left Hand Internal Threads: Lr(A/D) Rf(B/C)
 
I would always complain that apprentices were taught to drill too fast and thread too slow they lacked the confidence to terminate the thread at a shoulder at 300rpm but had no qualms about ramming a 1" at 500rpm into a piece of steel dry.
YOU are a hobbyist there is no such thing as too slow. try shadow machining first. This where you leave the tool an .100-.500 away from the part and start at a reasonable speed say 150-200rpm (slower if need be) and practice you movements, engaging on the right division disengagement and pullout; returning to the start of the thread and finding zero, make your adjustments to your compound or cross-slide (depending on your methods); rinse and repeat. go slow enough and take light enough cuts to avoid using oil/coolant it can be quite dramatic for some learning to thread with smoke or flood obscuring their vision.
As your experience grows you will learn the most of the limitations you encounter will be that of the machine and not the operator.
 
Alter Machine said:
I would always complain that apprentices were taught to drill too fast and thread too slow they lacked the confidence to terminate the thread at a shoulder at 300rpm but had no qualms about ramming a 1" at 500rpm into a piece of steel dry.
YOU are a hobbyist there is no such thing as too slow. try shadow machining first. This where you leave the tool an .100-.500 away from the part and start at a reasonable speed say 150-200rpm (slower if need be) and practice you movements, engaging on the right division disengagement and pullout; returning to the start of the thread and finding zero, make your adjustments to your compound or cross-slide (depending on your methods); rinse and repeat. go slow enough and take light enough cuts to avoid using oil/coolant it can be quite dramatic for some learning to thread with smoke or flood obscuring their vision.
As your experience grows you will learn the most of the limitations you encounter will be that of the machine and not the operator.
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My first hundred or so times cutting threads were during "practical lab" exercises. The last semester for a degree in Mechanical Engineering Technology, taking the technical elective called Manufacturing Methods and Practices.

That's how the instructor had us practicing. Air cutting until confident. Then a scratch cut over layout fluid. He'd come around and check the pitch, and make sure the helix terminated in a circle at the right distance from the part face.

The goal was that the student could terminate the helix within a few thousandths of the specified distance from the face of the shaft, without the circular termination cut being too wide. 1 scratch cut pass, .005" deep, then 10 more passes @ the same dial setting over top of the scratch cut. There had to be 1 helix, terminating in 1 circle.

I seem to remember the acceptable width of that termination mark being unreasonably tight and pretty difficult to hit, with 10 scratch passes @ the same .005" deep setting.

It was kinda fun, competing to hit the narrowest termination mark, closest to the specified length of cut.
 
Having been single point threading for decades, those first passes in layout blue make all the difference. Not just pitch - hand combination, but tool clearance of holders to chucking, carriage with tailstock, the whole set-up. Pitch is a big deal, like why lathes geared for 2 TPI have 20 or 30 low RPM, not to mention the larger diameters involved.
Fortunately, [considerately?] machine engineers allow RPM changes without disturbing initial sync of lead screw. Get dialed in, bump it up.
Far as a mark being unreasonably tight and pretty difficult to hit, it's reasonable being apprehensive with new challenges. That is learning.

My post above, #43 lists each tool orientation - spindle rotation - leadscrew rotation for internal/ external threading, that apply equally turning to a shoulder, O-ringing, snap ring grooves, etc. Print wise, those have narrower yet allowances for error.
 
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The last semester for that program was a helluva lot of fun, but challenging too. One of the most challenging things I've done.

Engineering Mechanics, Mechanical Design 1, Mechanical Design 2, Manufacturing Methods and Practices, Machine Tool Design, and of course, "Industrial Psychology!". There were no practical labs for Industrial Psychology though :)
 
Separately, few things are more irritating regarding language, than free use of the term 'Technology'. More hype than fact.
Schools, advertising, general speech go wild, pasting it on anything that moves.
Ladies make-up? No, that's chemistry. Manual Machine Tools? No, simple mechanics and bare physics. No Iron Pants? No, just textiles. Kitchen appliances? No, electrical and electronics. You get the idea. Most don't even have technology involved in the manufacturing of same.
Farthest that Technology as a 'thing' reaches, in many cases, is merely development process.

But what about, lets say, stress and load tables of structural engineering? Or aircraft? Well, again not primarily. Those figures were established long ago by experience and slide rules. Technology has pared every spare ounce of material, that extra 10-15% everything had making it last forever.

Wish I had a dime for every "but Technology is critical or does so-and-so......" comment I answered "Ok, then how'd they build Golden Gate Bridge?"
Not to sound tech-phobic! Hell no, lol.

PS. Editing toward inbound posts during my essay.
Industrial Psychology? Good Grief! Spend more time worrying about those poor unfulfilled cubicle gophers!
 
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