A Very Newbie Question

[KnotsAboutWood]

Hi,

I'm new to CNC but I've tried to read as much as I can and I have the book by Patrick Hood-Daniel and James Floyd Kelly coming soon. B&N says in 2 - 6 days.

So with that said, my main question is fairly simple, why does everyone size their machines as small as possible. I guess a better way to put it is what are the factors that force the hobbiest to build smaller machines?

I'm looking to build a machine that will have a cutting range of approximately 48" X 36". From what I'm reading this is fairly large for the home built machine, why?

Is it weight of the gantry, compared to motor torque? Is it deflection of the drive screw at that extended range? In looking at a few designs, I can see where there could be issues, but I'm really looking for someone that has experience with building a machine of this size.

I'm looking to do signs with this type of carving



Any help, advice, contributions, warnings, etc. would be appreciated.

Thanks

[ger21]

A lot of people have space limitations and don't have room for larger machines. Mine has a 30x45 cutting area, and it takes up a lot of space, about 48x68.

It's also easier to make a smaller machine rigid.

And, smaller machines are cheaper.

Actually, in the 7 years or so I've been here, I've never really noticed anyone building "as small as possible". I see far more people wanting to build as big as possible. Those are the ones that rarely finish machines.

[KnotsAboutWood]

Gerry,

I guess the main reason I say that people are building smaller is because that's about all of the plans I can find. Did you build yours from a plan or from your own design?

Thanks

[ger21]

My own design. You might want to look at joe's 4x4 hybrid. It can be built to a 3x4 size if you don't need the 4x4. www.joescnc.com

[CarveOne]

Many people size their machine's working area to fit a standard portion of a sheet of plywood or MDF. 1/4 and 1/2 sheet is common. Full sheet is less common, but is a standard size. To cut a full sheet of plywood edge to edge requires an ~5"x10" machine due to the width of the carriages and router's offset. Then there are protrusions (like motors) that also take up floor space if you have to walk around them. So, an imaginary rectangular box that includes all of this is going to take up most of a one or two car garage.

How powerful the machine needs to be depends on what you expect to cut and how deep in one pass it must routinely handle.

Simple layman's formulas to build by:

More power = more money

More speed = more money

More mechanical drive systems length = more money

More precision = more money

Lower dust emissions = more money

Better hold down systems= more money

More bling = more money

More brag factor = more money

Sure seems that way to me.

CarveOne

[CarbonKevin]

Just echoing on some previously made comments.

My first machine has a cuttable area of 21.5 x 28.0. Why?

Because that's the size heavy duty drawer slides come in!

I'm designing my new machine around a 2' x 3' spoilboard - I couldn't get cheap enough rails for a 4' x axis, and at 2' x 3', I can still get 5 spoilboards out of a single 4x8 of MDF

[stevespo]

My machine (25x33" cutting area, 40x48" footprint) was sized to fit my shop space *and* the parts I was able to find on eBay and through classifieds. There are times I'd like more capacity, but I honestly don't have the room.

Also, remember that beam stiffness varies based on the cube of the height of the supporting member x the width, divided by the cube of the length. So, the longer you make your frame or gantry you then have to make it significantly larger/heavier to achieve good stiffness.

As CarveOne pointed out more weight = larger motors = bigger power supplies = more expensive electronics = more money....

Steve

[louieatienza]

Originally Posted by KnotsAboutWood Hi, I'm new to CNC but I've tried to read as much as I can and I have the book by Patrick Hood-Daniel and James Floyd Kelly coming soon. B&N says in 2 - 6 days. So with that said, my main question is fairly simple, why does everyone size their machines as small as possible. I guess a better way to put it is what are the factors that force the hobbiest to build smaller machines? I'm looking to build a machine that will have a cutting range of approximately 48" X 36". From what I'm reading this is fairly large for the home built machine, why? Is it weight of the gantry, compared to motor torque? Is it deflection of the drive screw at that extended range? In looking at a few designs, I can see where there could be issues, but I'm really looking for someone that has experience with building a machine of this size. I'm looking to do signs with this type of carving Any help, advice, contributions, warnings, etc. would be appreciated. Thanks

I think a factor of DIY CNC machines is building the most robust machine pissible using readily-available components, such as allthread, skate bearings, EMT or gas pipe, 2x wood, and a lot of those materials have limits on size and strength. Some heavy stuff is moved with small steppers! But as you go bigger, you need bigger more ridgid components, and they almost always cost more!

[KnotsAboutWood]

OK,

So I guess what I'm hearing is, build to your budget. Size really isn't a factor in my shop. I have room for as large a machine as I'd like. My budget is what is limiting the size.

After looking at Joes 4X4 Hybrid and some of the other kits out there, does anyone think it's possible to build a machine with a cutting area of 48" X 48", doesn't need to be all that fast, and have fairly decent tolerances, for $2000.00? I'm going to using it for signs, so I'm in the .1 or larger tolerance range.

If so, what type of design do you suggest? As I said before, my first book on the subject should be here Monday, (Not soon enough if you ask me, but that's another story). So I'm really just looking for ideas right now.

Thanks for all of your help, I really appreciate it.

[ger21]

doesn't need to be all that fast

Everyone who's never built or ran a CNC says that, and yet everyone who builds one, wants it to go faster. Going slow leads to burning wood, shorter tool life, and painful waiting as the machine crawls from one end to the other. Trust me, it'll get old fast.

I'm going to using it for signs, so I'm in the .1 or larger tolerance range.

So it's ok, if, for example, you're carving a letter "O", and when the end missed the start by 1/8", it's OK?

.1" accuracy is unacceptable. Fortunately, it's not that difficult to build a machine accurate to .01 or better, and repeatable to even more precise tolerances.

The $2000 is difficult, though. You'll need to spend about $600 on electronics. If you want to run Windows, then $175 for Mach3. So your at $800. If you go with acme screws, and good nuts, you're looking at around $300.

So your at $1100 for electronics, motors, and screws. That leaves $900 to build the machine. Can it be done? Yes. If you design it and build it all yourself. You may be limited in your linear motion options in the price range, though.

But you won't find any kits or plans that'll come in at that price range.

[louieatienza]

Precision Acme screws are not cheap; though you could order 1/2"-10 at ENCO really cheap. Anti-backlash nuts aren't cheap either though I've seem people make their own.

I ran into the speed dilemna starting out too. The plans I used called for 5/16"-18 allthread, but after assembling my Y axis and testing it with a cordless drill I saw that I wasn't going to be happy with the speeds.

But if you need to get your feet wet, I don't see why you could not build your machine using inexpensive components just to get it up and running, and upgrade each as you go along. Once it's functional, you can even make better parts for your machine USING your machine! (which I plan to do soon...)

[ger21]

Here's the build for my 30"x45" router, that was under $1000 complete. It's not perfect, but works better than I expected when I started it. Some of the components I used were 1/2 the price they are today. I paid $26 for the screws, they are about $50 or more now.
http://www.cnczone.com/forums/showthread.php?t=1356

You can always build a cheaper, slower machine. But you will end up building another, or doing significant upgrades, which in the long run will cost more money than doing it right the first time.