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Sizeing questions...

thequintessentialman

Swarf
Registered Member
#1
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.
 

Dave Paine

Active Member
Active Member
#3
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

Active User
H-M Supporter-Premium
#4
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

Active User
H-M Supporter-Premium
#5
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

Swarf
Registered Member
#6
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

Active User
H-M Supporter-Premium
#7
.....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

Active Member
Active Member
#8
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

Active User
Active Member
#9
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

H-M Supporter - Premium Content
H-M Supporter-Premium
#10
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
 

epanzella

Active User
Active Member
#11
I think the biggest inconvenience of a lathe too small is when turning long parts that won't go thru the spindle bore. You can work around other issues with speed & feed changes but introducing a steady rest into the mix is a time killer.
 

DaveD

Active User
Active Member
#12
I've been in this hobby about 5 years. Was a woodworker for at least 40 years prior.
I've found that mass and rigidity is important. Can't have too much of either.
Same for horsepower. I have a 2hp rf45 clone mill. It's 'adequate' for my needs. Weighs about 700#. No where enough to take 'heavy' cuts without chatter.
It's a dovetail column. Much better than a round column where you lose your positioning when you raise/lower the head.

My lathe is a 2500# 14x40 Takisawa with a D1-6 camlock spindle. Very nice for my novice abilities. It's ridgid and has a 5 hp, 3 phase motor. Don't let 3 phase bother you. I have a 10hp rotary phase converter setup I built. If you are handy it's not a problem to build one. Mine has a 220v/50a input. Need to be comfortable doing electric work though.

Cam lock spindles are nice. D1-4 is your typical smaller one. I love my D1-6.
eBay is a good place to find used tooling. Stay away from D1-5 camlock spindles. There is a lot less on eBay for D1-5 stuff.

Aloris type wedge lock tool posts are typically the tool post of choice once you get out of the lantern tool post style typically on the 'smaller' machines.

Lastly, expect to spend at least as much money for tooling as you do for the basic machines. That could be a subject unto itself.

Just realize, you can do small stuff in a big machine. Typically can't do the reverse.
 

Silverbullet

Active Member
Active Member
#13
Most big jobs need bigger machines. I'd suggest if your lathe says 12 x. 36 then the work should be no bigger then 10" x 24" if it's bigger it will be extremely hard to do. At the chuck it may swing larger but I doubt it will over the cross slide. Mills are another item entirely Bridgeport can do amazing work on some items that are way to big for them. But don't try it if your not use to handling big heavy items. If your a hobby machinist it's just that for sizing too. Leave the big stuff to the shops set up for it. With big heavy parts things can go bad real quickly.
Most small machines are underpowered for many jobs . There just not made for real machine shop durability. So the old addage go big or go home works for machines.