It's the maximum size piece of stock that you can feed through the headstock. Working this way is often convenient - if you want to make lots of little parts, you poke out an inch or whatever of round stock, machine it, cut it off, then poke out a bit more to make the next one. Also if you want to make somehting long and thing that would flex in cutting, you poke out a little bit, machine it, poke out a bit more and machine that, etc.
You can of course hold much larger workpiece in a chuck. But you have to cut them to a reasonable length first - which means you need a saw, and you waste material.
A greater than 1" spindle bore in combination with a spindle nose adapter for 5c collects (which max out at just over an inch feedthrough capacity) is a great thing to have, as this makes the re-chucking much more repeatable than using a self adjusting 3 jaw chuck, and much faster than manually adjusting a four jaw.
One word of warning - don't put long thing stock through the headstock that will stick out very far on the back side of the headstock. It will soon start whipping around, and the more it bends the more force on it, so it will soon be bent perpendicular to the spindle, spinning around as a deadly flail and trying to shake the lathe off its base...
It's the hole through the spindle. A nominally 1 inch bore will likely just allow a 1 inch round bar through it, allowing you to make parts on the end of a longer bar, then pull the bar forward and make another, where you would have to cut off a bar long enough to both grip it and make a part, leaving a stub that you cannot use for another.
Bigger is generally better. IIRC 1 3/8 inches is the minimum bore required for the use of 5C collets in a spindle adaptor.
5C collets are the cheapest and most common collets available for use in a lathe, generally speaking.
What others said..especially about the 5C collet system. I just put on the air collet closer that I seldom use so that I could more accurately machine some small bar and was reminded how great a collet can be.....point being, if you have a choice and are buying a lathe, consider getting one that'll take collets in addition to the standard chucks. 5C collets can be a really handy and accurate system for holding smaller things being machined. It's also a lot safer to work close to the face of a collet than it is to work close to the face and jaws of a spinning hundred pound chuck. Makes a lot less noise if you screw up also :)
Oh yea...beware the numbers given for through-hole, especially on foreign metric machinery. 1" may actually mean 25 mm which will not pass a 1" bar.
With regards to use of the through-hole, we do an operation that finishes the ends of long rod material. We can't chuck a 15 foot bar of
7/16" material in the lathe any way but through the spindle to machine the ends. Yes, you gotta be REALLY careful because the 13 feet hanging out the back will whip if you turn your head..even if you think you've got it supported right. This kind of thing is just what the through hole is designed for.
You would be surprised at how low a speed at which a bar will commence to whip. I heard an awesome noise one day, stepped back, and saw the tailstock end of our Weiler toolroom lathe repeatedly jump up and down several inches. Dad was finishing a taper on the end of a one inch stainless bar, and things got out of hand. The bar turned about 30 degrees and did its best to trash a set of steel shelving. When I laughed, his buddy was shocked that I would find humor in such a horrible accident.
I said, nobody got hurt, the machine is okay, he knew better, and if I got caught doing something that stupid, he would have torn me a new one =)
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================== Single spindle screw machines have what the call a stock tube to take care of this problem. Basically a piece of pipe the same length as the stock that sticks out of the lathe spindle on two (heavy) stands that hold the pipe straight and level at the same height as the lathe spindle.
These makes a *LOT* of noise in operation, especially with hex or square stock. This is why the old machinists that either ran or worked in a shop with screw machines tend to be deaf as a post. [Eh -- speak up -- your mumbling again...]
One trick is to glue a piece of heavy wall rubber hose inside the pipe. This will [help] cushion the stock flopping around inside the tube and help dampen the noise. Putting a few gobs of grease on the stock when it goes in the tube/hose will also reduce wear.
If you are going to run stock longer than the lathe spindle these are a necessity. Depending on the size of the lathe and the size of the stock, you may find that you will need to make up some sort of bushings for the spindle bore to keep the stock from flopping around inside the spindle and causing inaccuracy.
Unka George (George McDuffee)
...and at the end of the fight is a tombstone white with the name of the late deceased, and the epitaph drear: ?A Fool lies here, who tried to hustle the East.?
Rudyard Kipling The Naulahka, ch. 5, heading (1892).
Id be happy to sell you up to 8 Vanco bar air powered bar feeders. I own these personally and can make you a good deal.
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