Sizeing questions...

[thequintessentialman]

Real novice question here. When looking at mills and lathes, how can I tell what the largest practical workable piece of metal is? Learned the hard way with some other equipment in the past so trying not to repeat that mistake.

 

[markba633csi]

Kind of a hard question to answer, are you thinking cube/cylinder shaped pieces, or long flat sections? And also, what type of operations- milling, drilling, boring, turning?
MS

 

[Dave Paine]

Difficult to answer, too many variables. Get the largest machine you can afford and can fit in the shop.

For a lathe, the swing will be the max diameter of the work. However it will not be easy to machine on the outside of a piece of metal whose diameter = swing, since the carriage will not be able to move far enough back. You could face the diameter. Subtract a few inches from the swing for max outside machining. I have a 12in swing lathe. I can easily machine upto 8in dia. Not sure about 10in. I may be able to machine this, but have not tried.

For lathe bed length the specification is normally tip of the spindle to tip of the quill when retracted and the tailstock at the end of the bed. Impractical to be able to fit some work at this length since you need some method to hold the work, at least a dead centre in the tailstock, etc. Max work length is going to be 2-4in less than spec if using a dog in the spindle, dead centre etc. If using a chuck, then the length is reduced by the depth of the chuck, several more inches reduction.

For the milling machine, max height will be reduced by a milling vise if this is used to hold the work, also length of collets to hold end mills, or protrusion of drill chuck+ length of drill if wanting to drill. Many inches of height can be consumed.

The spindle centre line to the column is the max distance to drill a hole from the edge of a piece unless the piece can be offset to left or right on the table, and the table has sufficient travel.

 

[rgray]

Lathes usually have "swing over carriage" listed as well as lathe swing. The swing over carriage is the more realistic measurement like Dave mentions.

 

[mikey]

Difficult to answer, too many variables. Get the largest machine you can afford and can fit in the shop.

For a lathe, the swing will be the max diameter of the work. However it will not be easy to machine on the outside of a piece of metal whose diameter = swing, since the carriage will not be able to move far enough back. You could face the diameter. Subtract a few inches from the swing for max outside machining. I have a 12in swing lathe. I can easily machine upto 8in dia. Not sure about 10in. I may be able to machine this, but have not tried.

For lathe bed length the specification is normally tip of the spindle to tip of the quill when retracted and the tailstock at the end of the bed. Impractical to be able to fit some work at this length since you need some method to hold the work, at least a dead centre in the tailstock, etc. Max work length is going to be 2-4in less than spec if using a dog in the spindle, dead centre etc. If using a chuck, then the length is reduced by the depth of the chuck, several more inches reduction.

For the milling machine, max height will be reduced by a milling vise if this is used to hold the work, also length of collets to hold end mills, or protrusion of drill chuck+ length of drill if wanting to drill. Many inches of height can be consumed.

The spindle centre line to the column is the max distance to drill a hole from the edge of a piece unless the piece can be offset to left or right on the table, and the table has sufficient travel.

And to complicate things, the spindle bore size matters because that will allow a work piece to enter the spindle, thereby extending the working capacity of the lathe. The typical measurements you see, like 10 X 24 or 12 x 36, usually reflect the distance between the tips of dead centers at each end of the lathe.

 

[thequintessentialman]

Kind of a hard question to answer, are you thinking cube/cylinder shaped pieces, or long flat sections? And also, what type of operations- milling, drilling, boring, turning?
MS

No idea at the moment. I like fixing/building things, I've been fascinated by the idea of garage machine tools, and recently I've gotten interested in building firearms but don't want machines limited to just that. When I got my first table saw many years ago, I realized real quick I didn't do enough research or ask enough questions. Trying to avoid that mistake this time around. I'll dig in to these answers and see if I can refine the question.

 

[brino]

.....another thing to think about for a lathes maximum cutting diameter is spindle speed range.

For instance, when I cut steel workpieces with HSS tools I aim for ~80 sfpm (surface feet per minute).

To get that surface speed:
For a 2" diameter workpiece I want to run it about 152 rpm.
However, for an 8" diameter workpiece I want to run it about 38 rpm.

Bigger diameter work requires lower rpm for the same sfpm.

There are various on-line calculators and charts you can use:
http://www.carbidedepot.com/formulas-turning.htm
http://americanmachinist.com/speedsfeeds-conversions-calculator

For selecting machines, you might also check out these links:
http://www.hobby-machinist.com/threads/a-guide-for-selecting-the-right-lathe-for-beginners.25915/
http://www.hobby-machinist.com/threads/how-to-select-a-milling-machine-a-guide.30066/

-brino

 

[fradish]

Though I have never tried this myself, I think you might be able to buy yourself
a little more room when machining a large diameter work piece if you turn your
QCTP so one of the dovetails faces you, mount a left-handed boring bar in a holder
and turn with that. Now this would only get you a little more room on the diameter
and you probably could only turn for a few inches horizontally, but I think I have
seen a YouTube video where someone was machining the side of a large diameter
blank this way.

As others have mentioned, your work envelope is determined by the travels allowed
by your machine minus the tool holding (drill chucks plus drills, collets plus end mills,
fly cutter, lathe tool holder, etc...) and work holding (lathe chucks, collets, mill vice) for
the operation you are doing.

Even the center to center distance on a lathe generally doesn't tell the whole story even when
you are using 2 centers. You'll need a way to center drill both ends of the work piece and the
lathe dog you'll need will limit you to how much of that center to center distance you'll actually
be able to machine.

 

[Frank Ford]

I'd say "largest practical" is one of those variables open to interpretation. I think I'd look at the "swing over cross slide" as a guide for the maximum, and use a figure of about half that as the real comfort zone.

For example my lathe is a solid 11" swing Hardinge clone, and exactly right for me, I think. I rarely work on pieces over 1.5" diameter, but I have turned as large as six inch diameter or larger steel occasionally - it gets kinda scary at that size. Six inches is the max diameter that will clear the cross slide. Clearance becomes a big issue there, depending on length, etc. Parting over 2" isn't worth the anxiety, so I don't even try doing that. By comparison, working six inch diameter on a regular 15" lathe seems like a safe walk in the park.

Bear in mind that I'm talking from the perspective of doing a LOT of turning in my particular range, because even though I'm a home shop guy, I frequently make parts in batches of 100-200, in addition to making tools, prototyping, etc.

I would not want a bigger lathe because I'd lose the high speed (4000 RPM).

 

[bfd]

you can always upgrade get what you can come up with and use it that's the most important. when you realize the short commings of what you have then you can answer your own question. you then move up or down. I started with a 6 inch lathe in a one car garage then moved up to a 12" lathe in a 2 car garage then to a 16" lathe in a shop. nothing stays the same including you bill