[Dietrichv]

Hello to everyone:
After building my own CNC for wood/aluminium I would want to make a new gantry CNC for steel and have found some troubles with spindles. I would really want to keep on a budget by now (thinking on upgrading later) so I'm trying to figure out what kind of spindle should I use, belt driven one with an electric motor? Direct driven with a stepper motor or with an electric one?
Also, as I'm going to process long pieces (that's why I have chosen gantry) but not too deep removal - just 3.5*mm in the deepest part - I don't need it to be very aggressive in deepness in exchange for saving money. So what kind of torque/power would I need?

Thanks to all and sorry about my bad English

[joeavaerage]

Hi,
if you are cutting wood, aluminum, plastics etc. then a high speed (24000rpm) asynchronous spindle and VFD are the best and cheapest means.

This style of thing:

https://www.ebay.com/itm/15553006229...Bk9SR7LeiduQYg

Just be aware these spindles are high speed but low torque....so just the wrong combination for cutting steel. If you want to cut steel thats a whole different ball game.

Craig

[joeavaerage]

Hi,
if you want to cut steel then you want a much lower speed but way WAY higher torque.

For instance if you want to use an uncoated carbide tool in mild steel the the traget surface speed is 100m/min.

6mm diameter= 0.006 m
RPM =100 /(0.006 x pi)
=5305rpm.

The asynchronous spindles of my previous post CAN go that slow, but you'll blow it up. They absolutely hate going slow. You should never run them at less than half speed unless you don't
care about wrecking it. Second issue is that even a 2.2kW 24000rpm asynchronous spindle will have about 0.9Nm of torque. You'll stall the spindle if you try taking a decent cut in steel.
For steel and a 6mm tool I'd suggest a minimum of 2Nm, and for bigger tools, say 12mm 4Nm or more.

I have an 800W 24000rpm spindle and I use it a lot, but when I'm cutting steel I use another spindle I made which uses a 1.8kW Allen Bradley servo, 6.1Nm (cont) @3500rpm and 18Nm overload.
Cost me nearly $2000NZD ($1200USD) to make, so not cheap, but AC servos do make good and rugged spindle motors.

Craig

[RCaffin]

600 W Baldor DC PM motor, 3,500 RPM max, huge overload capacity. Steel, titanium anything.
I find 600 W quite enough.

Cheers
Roger

[awerby]

It's almost impossible to build a moving gantry machine that's rigid enough to cut steel. Look at fixed bridge/moving table designs instead. As far as spindles are concerned, I agree with Joeaverage. Look for a big motor and fit it with a pulley cluster to give you the speed and torque you'll need. No stepper motor is going to be suitable for this, although some people have used servos to keep track of revolutions so they can do things like rigid tapping. Since the motor is going to be pretty heavy, you'll want to look into splined shafts that will transfer motion without having to move the motor up and down with each motion of Z. In general, it's much more difficult to build a machine capable of cutting steel than a wood router. It's a lot easier to find an old machine that was designed to do this in the first place, and retrofit it with new controls or whatever else it needs.

[Dietrichv]

I already have a 1.5kw motor with 2400 rpm (I'm talking by memory). So pulley will be!!
About making/transforming a fixed milling machine with the movement in the bed... I would need at least a 2800mm bed, which seems enormous to me. Don`t really need to be very accurate so don't know if trying to make it gantry anyway, but you have worry me

[joeavaerage]

Hi,
I think a machine with a 2800mm bed that is rigid enough to cut steel is unrealistic.

I think you woefully underestimate the difference between steel and aluminum even, let alone wood. With steel if the tool is held to engage the steel to cut it the steel is
'stiffer' than the machine and the machine flexes so that the tool has insufficient pressure to cut, it just 'skates' over the surface.

Craig

[joeavaerage]

Hi,
this is typical of the many small mill-drills out there. Its not CNC but none-the-less it is instructive:

https://rongfu.com/mill-drill-machin...-machine-rf45/

This machine is rigid enough to cut steel....'with care', its just rigid enough to do the job. Note the cast iron construction and how limited the travels are to secure the necessary stiffness.
Note also that the spindle speeds are 60 rpm to 1500 rpm.

If you want to cut steel then you'll need to at least be able to match the rigidity of this machine, and a machine with a 2800mm bed will likely weigh many tons to match it.

Craig

[Dietrichv]

Hello to all. Thanks for all your advices, don't really know what I will be doing but have decided to give it a bit more time to think about it.
Now questioning the gantry part, but don't really see possible a fixed one as some of my pieces are 1300mm long so the table will have to be very big.

[joeavaerage]

Hi,
what is it you are trying to cut?

Could do a small section of it, then shift it along a bit and do another small section and so on until its done?.

Typically machines that can cut steel are very large and heavy and eye wateringly expensive.......so what is it about what you want to do that should be so contrary to that norm?

Craig