# Thread: cutting forces

1. ## cutting forces

Hi There,

I'm in the preliminary phase of designing a small cnc machine designed for working with aluminum, of approximately 2'x3' working area.

Trying to do some preliminary calculations on what kinds of stress the machine will undergo while cutting, and need some rough estimates of the forces the cutting tool will undergo.

Obviously this is highly dependent on the tool, the cutting speed, and so on. Right now, the only data I have to go on is a paper that assumes 1000N force in each of the X,Y, and Z directions - but this was for a large industrial machine.

Can anyone help me come up with a cutting force estimate?

2. hi dmorton,
I am finding the forces on a lathe headstock by mounting strain gages on headstock while turning operation. once i find the cutting forces on headstock i sure will knew the forces on different parts of the lathe.. but its nit a cnc lathe. i dont know how it will help u?

3. Thanks for the reply.

In another conversation, someone rightly pointed out that the forces at the cutting head are limited by the torque of the motors moving the cutting head and that this might be a better place to start.

4. ## hi,

Its k.. i just wanna know how u will solve this problem.. at the end i suppose to get some idea on designing the lathe.. thanks.

• I've been wondering about how to calculate this, too...your comment about maximum motor torque was the missing piece of the puzzle, so I took the ball and ran with it.

Thrust = (2 x screw efficiency x Torque)/screw lead

I'm using 1/2-10 5-start screws, which means my lead is .5. My steppers max out at around about 1.5 lb feet (288oz-inches). For screw efficiency, I'll use 50% for now, since I don't know the PD of my ACME screw to use THIS calculator. This equation doesn't take other losses into account, by the way.

The equation will look like Thrust = (2 x 0.5 x 288)/.5

So at the ACME nut, my machine will generate 576 oz, or 36 lbs of thrust per direction, max.

In the case of my Z-axis, I'm assuming a worst case scenario of a 12" lever arm between my router mount and the linear rail bearing position.

Since moment of inertia is force x distance, I calculate the maximum torque on my Z bearings as 36 lb/feet.

Does this make sense? Does it seem right?

ck

• First I should say that doing any kind of engineering work in the imperial system is sheer ****ing insanity, and its a wonder that the American civilisation hasn't collapsed under the weight of accumulated conversion errors.

That said, I think you are on the right track. I cant validate the formulae you are using, but the principles are correct.

• The equation for force was not quite right.

Here's a link that will help:

http://www.cnczone.com/forums/attach...achmentid=4954

• ## How about stiffness.

When cutting once the tool/machine deflects more then the chip thickness the cutting becomes unstable and you get chatter as the machine/tool deflects and makes the chip thicker, and the load higher until it bounces back.

So once you have considered the force, you need to know how much the machine will deflect under load.
If it is too much you either must make the machine stiffer or reduce the cutting load.
A machine made from aluminum will deflect about 3 times as much as steel for a given structure and a wooden machine....??? Well lots more. eh.?
http://en.wikipedia.org/wiki/Young%27s_modulus
The above link should be of assistance.