[Tommy B]

There are a lot of designs out there. I just saw a listing for a small CNC frame that is sold on ebay with all metal construction. It touts itself as being able to cut wood and non-ferrous metals, and so just isn't rigid enough to cut steel.

I've been fiddling around with my own design, using all steel. I'm not a mechanical engineer, so I have to make-do with what little knowledge I have (use lotsa triangles and make sure the components are arranged so that the steel is strongest where it gets the forces).

I've been using this handy-dandy little applet to do my calcs for me. I'm using 1" hardened rod for the ways, and if the rest of my setup weighs 40 lbs, then I'm only going to get .001 deflection. That's good enough for cutting steel when I'm done, right?

bending applet

Anyway, what's the design threshold between being able to cut steel or not able?

[UUU]

That must be the lightest steel-cutting machine of all time. So either your design is fantastic, or you need more bulk in it.

If that 0.001 is millimeters, yes it's good enough. If it's inches, no it's not good enough. You might be cutting at 0.001 tooth-loading, so your deflection may double the load. Or more, when you add the flexing of the tool itself

[M250cnc]

If that 0.001 is millimeters, yes it's good enough. If it's inches, no it's not good enough.

Ha ha .001 of a millimeter is

0.0000394" so it won't be good enough.

Phil

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[Tommy B]

That must be the lightest steel-cutting machine of all time. So either your design is fantastic, or you need more bulk in it.

If that 0.001 is millimeters, yes it's good enough. If it's inches, no it's not good enough. You might be cutting at 0.001 tooth-loading, so your deflection may double the load. Or more, when you add the flexing of the tool itself

Well, you just told me what I need to know right there. I have to modify my design until I get .001 millimeters, or is there some other number that is a cut off?

Phil said that I need to get it down to 0.0000394", but what's the amount of deflection that is generally considered stiff enough to cut steel?

[jsheerin]

I can cut steel on my cnc router. I cut a hole in a stainless exhaust pipe the other night. It took 20 minutes and the cut quality was not that good. Similarly, I cut aluminum occasionally - once again, it takes a long time and the cut quality isn't that good.

What influences how well a machine will cut various materials is how stiff it is. What I've found from researching is that you want a stiffness in the 60k-230k lbf/in range to cut steel. Mori Seiki advertises their high precision machines are around 680k, iirc. For softer materials, I've read that the reduction in stiffness required correlates to the modulus of elasticity, so for aluminum you can be about 0.35x the stiffness as needed for steel, or around 20k-80k. My 4'x4'x6" wood router made from 80/20 extrusions (3060,3030 and 1530) and cncrouterparts.com components measures and models to be around 2k lbf/in which explains why it doesn't do that great cutting aluminum or steel.

[Tommy B]

I can cut steel on my cnc router. I cut a hole in a stainless exhaust pipe the other night. It took 20 minutes and the cut quality was not that good. Similarly, I cut aluminum occasionally - once again, it takes a long time and the cut quality isn't that good.

What influences how well a machine will cut various materials is how stiff it is. What I've found from researching is that you want a stiffness in the 60k-230k lbf/in range to cut steel. Mori Seiki advertises their high precision machines are around 680k, iirc. For softer materials, I've read that the reduction in stiffness required correlates to the modulus of elasticity, so for aluminum you can be about 0.35x the stiffness as needed for steel, or around 20k-80k. My 4'x4'x6" wood router made from 80/20 extrusions (3060,3030 and 1530) and cncrouterparts.com components measures and models to be around 2k lbf/in which explains why it doesn't do that great cutting aluminum or steel.

I didn't know that. I didn't know that a CNC machine can get a rating for how stiff it is, and thereby let the user know its suitability for cutting various materials. My machine is 12" by 12" by 5" work envelope using steel tubing. Like I said, I was just going by that deflection applet to see what errors I would get from sagging of the ways, even before I made a cut.

From what else I've gathered here on the forum, you have to calculate in the kind of drill bits you are using as well. I probably won't bother with this. I'll just figure that a stronger frame is better.

[jsheerin]

Sagging under gravity doesn't really matter as such - you could compensate for that when you build the machine if you wanted to.

What matters is how the machine flexes under the loads of cutting and accelerating the various parts of the machine. You can calculate this directly as a deflection for a given load, but it's easier to specify it as a stiffness. When I run FEA on my machine frames, I specify some random load - say 100 lbf. Then I see what the deflection is for that force. But I don't know what the actual force from cutting will be, so I don't know that the deflection I calculate is actually how much it will deflect, but I do know how stiff I want it to be so I can take those two numbers and calculate the stiffness. Similarly when I measure the stiffness, I pull or push on the machine and measure the force while I also measure the deflection.

The bits you use will deflect, but the numbers I gave are just for the machine frame.