[awerby]

Machines designed to cut steel are usually made from cast iron. If you have an iron foundry in your shop, perhaps you'll be able to build a machine that does a good job on it. They are based on a big casting (or several castings bolted together) in the shape of the letter "C". A 3'x4' working area is rather small for a router, but it's large for a mill, especially on the Y axis. A knee mill with a 3' X axis is normal, but it won't usually have nearly a 4' travel in Y; half that would be large.

If you look for an older mill with a broken or obsolete control, you can often find them for about their value as scrap metal. Machine-shop owners usually want them hauled away to make room for new equipment. I don't know how true that is in Mexico, but if you're willing to wait for a good deal and travel north, then you can find them in the USA. Ball screws are generally included, often the motors and spindles will work; all you have to do is a "brain transplant". That's a lot easier than building a mill from scratch.

[Victt7]

The problem with that i think would be the shipping cost and desinging one will cost me around 450 ~ 600 bucks and 4 axis insten of 3

[diepchess]

Aloha!

Old thread - month ago you posted question - yet let me drop a few coins.

It's possible to buid a reasonable gantry that works on steel. The whole thing is about how much time you want it to cut such sheet.
The trick is to use a very lightweight spindel motor.

For example my spindel motor is very heavy. About 9 kilo. That means if i put it in a steel box, that box is soon 40 kilo if not more. If that box then hangs onto a long tube on a gantry, that means i need some very strong stiff tube.
Now next is to limit the Y-travel.

Deflection of your gantry bridge that moves over the machine is Newtons force times length of the Y axis to the power 3.

For example simplistic point deflection:

Say we have my 9 kilo spindel that in total has a weight of 60 kilo with motor and box and so on and then we add 50 kilo of Z-axis force it is using to hammer onto the workpiece.
Far over 1000 newton. Let's use 2000 newton. Now let's use a cheap tube as our moving gantries bridge that rolls over X axis. Let's use a tube of 100 x 100 millimeter thickness 5 millimeter.

First we calculate how strong that tube is: (100 ^ 4 -90 ^ 4) / 12 = 2,865,833
We then take length of our spanning of the gantry bridge say 3.5 feet => lets take 1000 mm.

I would then want to calculate with deflection (millimeters) = (2000 newton * 1000 ^ 3) / (48 * 210000 (steel) * 2865833) = 0.069 mm deflection
And we didn't even add the weight of the tube itself yet, though we did calculate as if all force is concentrated in the middle of the tube whereas it probably
is not entirely in the middle located. So in reality probably we measure 0.05 mm deflection.

That is a big problem in short if we get nearby 0.05mm as human eye will see that easily.

Now we do the same calculation for a cheapskate spindel of 1.5 kilo that we hang at a light structure of 4 kilo and because we mill with a smaller mill we also generate less force onto workpiece. Say maximum 10 kilo.
That's a weight of something over 15 kilo. Let's take double that in force. 300 newton.

So now if we do exactly same calculation with 300 newton, we suddenly are having a deflection that is nearly 7x less, or in short less than 0.01 mm.

Easy as a pie.

Yet of course milling time suddenly is very long as you can take away very little steel from the steel plate each second.

[aarggh]

Diepchess,

If your talking the el cheapo spindles on Ebay, good luck with that!

At 1.5kg weight for the spindle however, I'm not sure what you think you'd find as even a weak little 800w spindle weighs around 2-3kg. Unless you mean a die grinder?

In short, machining steel requires brute force, and a LOT of mass and rigidity. Without the horsepower to cut, and the mass and rigidity to dampen the vibrations and keep the cutter on target, you'll find the cutter will be shrieking like a screaming banshee, and will be skating all over the material. You'll also destroy cutters very, very fast this way.

For practical hobbyist purposes in most user designs, rigidity, and mass to a degree, is generally inversely proportional to cutting area. The bigger you make the work area, trade-offs are usually made to lighten the gantry and X axis in order to still be able to move it adequately.

A bigger machine could be made to cut aluminium to varying degrees of success depending on mass, rigidity, size, and quality of machine components, but to machine steel is a very different ball game.

cheers, Ian

[diepchess]

Yeah we fully agree upon the ideal situation, which is why i got something that's 380 volt and a bunch of kilowatts.

Yet he could buy a kress spindle which has very little runout and weighs 1.4 - 1.5 kilo and 1050 watt. Costs over here (Netherlands) a 150-160 euro. Of course milling some mild steel with that takes forever and lots of cooleant, yet milling with very small diameter carbide tools is also possible. Of course that thing is bloody noisy and milling at such high RPM requires some good calculations on how to mill

The ebay stuff in general has a very bad runout - that's asking for problems on tool wear isn't it?

No one said If you want to mill cheapskate that you do not need to be a very good machinist to do it

Furthermore if he finds even a couple of hundreds of dollar too much for shipment of something great it's obvious that has to rethink to pay for 500+ kilo of steel and a lot more in concrete which he should to build such gantry

[Yoddha]

Could you recommend some good text books that focus on rigidity of milling machines? Any help would be greatly appreciated. I am about to start my senior year of ME and besides trying to tie together what I’ve learned from statics and Kinematics, I don’t have any examples of problems that could help me learn more about the general design requirements for milling machines.

Aloha!

Old thread - month ago you posted question - yet let me drop a few coins.

It's possible to buid a reasonable gantry that works on steel. The whole thing is about how much time you want it to cut such sheet.
The trick is to use a very lightweight spindel motor.

For example my spindel motor is very heavy. About 9 kilo. That means if i put it in a steel box, that box is soon 40 kilo if not more. If that box then hangs onto a long tube on a gantry, that means i need some very strong stiff tube.
Now next is to limit the Y-travel.

Deflection of your gantry bridge that moves over the machine is Newtons force times length of the Y axis to the power 3.

For example simplistic point deflection:

Say we have my 9 kilo spindel that in total has a weight of 60 kilo with motor and box and so on and then we add 50 kilo of Z-axis force it is using to hammer onto the workpiece.
Far over 1000 newton. Let's use 2000 newton. Now let's use a cheap tube as our moving gantries bridge that rolls over X axis. Let's use a tube of 100 x 100 millimeter thickness 5 millimeter.

First we calculate how strong that tube is: (100 ^ 4 -90 ^ 4) / 12 = 2,865,833
We then take length of our spanning of the gantry bridge say 3.5 feet => lets take 1000 mm.

I would then want to calculate with deflection (millimeters) = (2000 newton * 1000 ^ 3) / (48 * 210000 (steel) * 2865833) = 0.069 mm deflection
And we didn't even add the weight of the tube itself yet, though we did calculate as if all force is concentrated in the middle of the tube whereas it probably
is not entirely in the middle located. So in reality probably we measure 0.05 mm deflection.

That is a big problem in short if we get nearby 0.05mm as human eye will see that easily.

Now we do the same calculation for a cheapskate spindel of 1.5 kilo that we hang at a light structure of 4 kilo and because we mill with a smaller mill we also generate less force onto workpiece. Say maximum 10 kilo.
That's a weight of something over 15 kilo. Let's take double that in force. 300 newton.

So now if we do exactly same calculation with 300 newton, we suddenly are having a deflection that is nearly 7x less, or in short less than 0.01 mm.

Easy as a pie.

Yet of course milling time suddenly is very long as you can take away very little steel from the steel plate each second.

[diepchess]

Machines designed to cut steel are usually made from cast iron. If you have an iron foundry in your shop, perhaps you'll be able to build a machine that does a good job on it. They are based on a big casting (or several castings bolted together) in the shape of the letter "C". A 3'x4' working area is rather small for a router, but it's large for a mill, especially on the Y axis. A knee mill with a 3' X axis is normal, but it won't usually have nearly a 4' travel in Y; half that would be large.

If you look for an older mill with a broken or obsolete control, you can often find them for about their value as scrap metal. Machine-shop owners usually want them hauled away to make room for new equipment. I don't know how true that is in Mexico, but if you're willing to wait for a good deal and travel north, then you can find them in the USA. Ball screws are generally included, often the motors and spindles will work; all you have to do is a "brain transplant". That's a lot easier than building a mill from scratch.

Andrew - we fully agree - the problem when you want to cut sheets is of course that if i take aluminum here in Netherlands. If i buy in small quantity of aluminium plates 1x1 meter the price is 4 euro a kilo. If i order part of it say 30 cm x 30cm, then i pay 10 euro a kilo. The cheaper price starts from 1x1 meter and longer (say 1 x 2 meters etc).

With aluminum so very expensive of course there is no option yet to order 1x1 meter. Cutting that by hand into smaller pieces is a lot of work of course. Easier is to have gantry that cuts out the shapes. Then you clamp the cut out pieces into a more accurate metal milling CNC machine that is doing the more accurate milling