[itko]

Hi, first post, very new to this but learning bit by bit. Perhaps a silly question, but on a CNC milling machine, I noticed designs where the table is moving and the spindle is stationary and I noticed the opposite (Gantry), where the spindle is moving but the table is stationary. Is there a difference?

Also, I've been trying to find a solid source that describes the different types of designs and their practical use. Any help?

IK

[fizzissist]

Simple answer, really. one design is more advantageous in one application, and a different design is better in another, for reasons of cost, rigidity, etc., etc.,.....

See how easy that is?

[itko]

Could you specify what application that would be? Which is better for rigidity?

[Bob La Londe]

Typically a moving spindle uses less total space for the size of work envelope, and a moving table tends to have a more rigid spindle.

[fizzissist]

Oh, c'mon.

Look at pictures of machines.

Would you use a knee mill to machine the hull of a 12meter sloop?

Let's move on to Question #2.

[itko]

What about when dealing with 4 and 5 axes. I saw machines where the spindle rotates and ones with a rotary table. I assume the rotary table is a cheaper, less rigid option?

IK

[Bob La Londe]

What about when dealing with 4 and 5 axes. I saw machines where the spindle rotates and ones with a rotary table. I assume the rotary table is a cheaper, less rigid option?

IK

It depends on what you are doing which is better....

WHAT DO YOU WANT TO DO?

HOW ARE YOU GOING TO BEST GET A CUTTER TO MEET WHERE IT NEEDS TO CUT?

There is no best!

[wizard]

I realize this is your first post and you are learning but you need to get past the idea that there is a perfect or best solution for machine design. All machines are compromises in one way or the other.

Hi, first post, very new to this but learning bit by bit. Perhaps a silly question, but on a CNC milling machine, I noticed designs where the table is moving and the spindle is stationary and I noticed the opposite (Gantry), where the spindle is moving but the table is stationary. Is there a difference?

You just described the differences. Beyond the obvious you then have to fit the operation to the mill or lathe required. You don't throw a shaft for a steam turbine on a 9x20 lathe, like wise it is pretty hard to put an amoured personnel carrier on the bed of a Bridgeport.

Also, I've been trying to find a solid source that describes the different types of designs and their practical use. Any help?

IK

In general a moving gantry is less rigid than a fixed gantry machine. Beyond that the discussion gets silly real fast unless you want to discuss specifics.

[DareBee]

What about when dealing with 4 and 5 axes. I saw machines where the spindle rotates and ones with a rotary table. I assume the rotary table is a cheaper, less rigid option?

IK

Wrong

Cheaper and MORE rigid.
The more joints or moving parts in each item the weaker it is and this amplifies by multiples into the next downstream component.

[Mstcnc]

As stated before, even rigidy depends how and what kind of part for the machine is made! And of course how much money can be used...
Exaple for large and heavy parts its just not practical make machine with moving/rotating table. Near here is 5 axis machine for machining huge turbines. They take 5mm (0,2") deep cuts 150mm (6") diameter cutter and yes its full 5 axis simultaneus movements, no brakes in milling head!

-M

[wizard]

As stated before, even rigidy depends how and what kind of part for the machine is made! And of course how much money can be used...

For many industries it isn't uncommon to have a machine, be it a mill or lathe or whatever be design for a specific use. It is a completely different discussion from general purpose machines.

As to money to be used sometimes you have to work with what you have.

Exaple for large and heavy parts its just not practical make machine with moving/rotating table. Near here is 5 axis machine for machining huge turbines. They take 5mm (0,2") deep cuts 150mm (6") diameter cutter and yes its full 5 axis simultaneus movements, no brakes in milling head!

-M

I've seen some massive machines with rotary tables and even bigger vertical lathes. Such machines might not be seen as economical when you can often accomplish as much with CNC controlled machines. It ends up being a question of economics.

[handlewanker]

Hiiiiyyyy nooby, the very real answer to all the above is how much dough do you have to spend......when you supply that answer you will soon be aware that a moving gantry router only capable of cutting wood, plastics and maybe alluminium and costing just about a grand on Ebay can be the starting point to your adventure in CNC milling.

If you don't know the reason routers/mills have moving or fixed gantries and the mob on here are at a loss to put it into the simplest explanation, then spend a grand and enjoy the hobby as a green raw beginner, thirsting for knowledge and itching to make something, anything......just as long as you don't expect to become a millionaire overnight, or ever, and just like the idea of messing about with machinery.

Oh yeah, have an extrension available to that grand you will spend out for all the cutters and other junk you need to make the thing sing.......like replacing the controller it comes with, with a gecko G540 controller.

Why?...........you'll find out, if you read ALL the posts on this forum that deal with routers and also CNC converted table mills etc that come from China and sell like hot cakes for around a grand......like the 3020 3040 and other routers on Ebay, that the insides mechanically are quite good but ALL use a crap controller and NO ONE has managed to succesfully "modify" one with any success so far.

BTW, if you go on Utube and browse the aforementioned routers by number you'll get a close encounter with folks who buy those machines and are willing to list the advantages and drawbacks each have......like the Gecko G540 controller cure for the crap Chinese supplied thing.

As this is drifting a bit off topic, the answer to your original question is (in my very humble opinion)..... the moving gantry type, unless made humungeously massive is not capable of cutting anything that is harder than wood or plastics, whereas the fixed gantry and moving table ones, can be made extremely rigid and capable of milling steel and other hard metals, but do have the footprint extended due to the table having to move it's full length out beyond the cutter both front and rear, so if you have a table a metre long the machine is about 2 metres long....width is the same for both types of machine......all other factors are also about the same, pro rata.

You could build your own, if you have the mechanical apptitude and also the machinery etc to do the job.....some have wood frames and do very creditable work......horses for courses.

One of the main reasons the cheaper moving gantry type are susceptible to problems is that you have a carriage that spans the width of the machine with a very short bearing guide on each side of the Y axis rails, and as the gantry is driven by a ballscrew up the middle of the frame (it must be a ballsrew and if it ain't.... it ain't even worth having for free) when the cutter is to one side of the table you get a slewing effect on the gantry due to lack of guidance from the short side bearing blocks that the gantry rides on.

If you have the side bearings made longer to compensate for slewing you also extend the frame and that puts it in the same league as the fixed gantry model.

Attached is a pic of a fixed gantry machine I found while browsing the 'Net, and you can see the advantages of the more rigid structure that can be achieved by having the gantry attached firmly to the base frame.....but this one does cost a bit.

Correction, a moving gantry mill CAN be made to cut harder metals etc, but like someone who cracks walnuts with his dentures the dentures have to be made that much stronger to resist the cracking forces....so does the moving gantry.
Ian.

[hanermo]

There are LOTS of industrial moving gantry machines .. in the 200.000$ and up size, and starting from about 20 metric tons in mass.

Milling machines cut metal .. and routers cut wood and plastics (sometimes alu).

Mass and rigidity are the defining factors.

Bascially, there is a very clear relationship betwen mass, rigidity and power.
You can look up the specs on any industrial size machine, the smallest being the well known Bridgeports, and go from there.

Ie Bridgeports starts at 1000 kg for 1 Hp, for milling, R8 collet.
BP M head, 800 kg, .750 Hp, MT2 collet.

Modern VMCs start at 4000 kg or so, with upto 40 Hp (HAAS DT1).
For about 60x30 cm table.

[DareBee]

+1
Mass = rigidity and strength

mass combined with gravity and atmospheric pressure wreaks havoc on the requirements of motion

[underthetire]

I was going to ignore this thread..

It's not mass of the machine that makes a bed mill, for argument sake, more rigid. What a bed mill has, is more dampening. A traveling column mill can be just as or more rigid than a bed mill, but bed mills tend to dampen vibrations better. What hasn't been discussed yet is positional accuracy and speed. A column mill can be faster than a bed mill, and position better than a bed mill. The main reason for this is tuning. With a bed mill, the tuned weight can vary by thousands of pounds ( workpiece, workholding etc). With a column mill, the only change in tunable mass would be the tool holder and tool. This generally doesn't change by more than 100 LBS.

I see a lot of "more mass=better more rigid machine". This is far from true. With today's Finite analysts, higher end builders can put mass and support where its needed, keeping some structures light for accuracy and speed. While this approach is not for your average DIY builder, it does pay to try and copy what the builders do.

[itko]

Thank you all for taking the time to respond. Learned quite a bit. Will take everything into consideration.

[handlewanker]

Hi, I tend to agree with you....mass does not automatically guarantee a successfull outcome...it's design, and mass can occur.

If you only ever want to machine with a 6mm end mill and the material will never exceed plastics or MDF, then those flimsy moving gantry routers made from alluminium extrusions sold on Ebay for a grand or so are ideal, but most financially and technologically challenged newcomers participating in the CNC field will still attempt to mill steel and wonder why they can't.

Mass can overcome some shortcomings, but can also add to the problem with cantilevered forces that resonate at the drop of a hat.

What has been made in industry using FEA techniques cannot be made by DIY fabrication if the materials are on one hand aged castings and on the other fully welded and non stress relieved composite structures.....both weighing the same.

It's difficult to apply the rule of copying others build strategy, because everyone has a different idea of the ideal machine for all seasons.

The best approach is to adopt the "adjustability" factor when simple DIY structures are on the go......lots of adjustability occurs during the settling process in a DIY machine's lifetime.......that's what you pay not to have in a shop bought machine, especially if you intend from day one to modify it for CNC use.

In most cases of modification the only lengths you can go to is to replace Acme drive screws with ballscrews, and add VFD drives to the spindle motor, whereas the actual slideways being dovetailed have so much friction the lost steps become just another glitch to live with.

The ideal modification to an existing bench type mill would be to machine off all the dovetail slideways and replace them with linear rails, and this takes some real retro design work to become a success instead of weakening the structure by losing some iron.

I dislike alluminium as a build material and as a material period, even though it's the most preferred material of all DIY'ers.

I have to smile when I hear a designer's requirement state that they want to get .02mm tolerances, and alluminium is the main structure material.....even more so when the tolerances relate to the longest distance the slideways can move, and the environment is not temperature controlled....LOL.

It's amazing how much force a cutter cutter can exert on a framework, and if the force is in the same direction as the inbuilt locked up stress and the inertia of the frame resonates accordingly..... .02mm is a wish you'll never achieve.
Ian.