New Machine Build- Is Thrust in wood cutting a factor for motor size?

[arthernan]

I am serioulsy considering building my own cnc router. And for some reson my first consideration is what kind of forces are going to be excerted while operation the table.

For example as the motor rotates it has to accelerate, I know this requires the most torque. And it needs to accelerate the screw. OK. There is also the screw/nut efficiency. Then the wheigth of the spindle, rpm and screw pitch. But what I am missing is the thrust required to cut througth wood.

In escence I want like anybody else, a CNC machine that is inexpensive but will go as fast as possible. EASY!! I am mainly going to be cutting wood and if the price does not go much higher, I would ocassionaly cut aluminum.

One thing that would help me is to know what is the speed upper limit when cutting wood. Can it really be high???

A possibility for me is to get something reasonable and make the frame so it can withstand higher stress, so if it all works then I jusst upgrade the motors.

I have to say that I have been looking for a while for this info. I happen to have a copy of the machinists handbook, and it does explain how to calculate thrust for cutting metal. The little example they give is for around 6000 lb!!!!

[jsheerin]

Here's some information on how to calculate some of the forces involved in a cnc router:
http://www.cnczone.com/forums/showthread.php?t=8129

How much force is required to accelerate the various parts of your machine will depend on the mass of those parts and how fast you want to accelerate them. How much torque is required from the motors to produce that force depends on your drive mechanism (screws, rack and pinion, gearing, etc.). For example, my machine uses about 2% of the available force to accelerate at low speeds and 10% at high speeds, but I can stall it by taking too big a cut, so I'd say my machine definitely requires more force to cut the material than to accelerate the machine parts.

You can cut wood very fast. 600ipm or higher is used on some commercial machines. There's an optimum chip load for your cutter, so how fast you cut would ideally depend on your cutter, the rpm it spins at, and the material you're cutting. I think Columbo has some info on this in a pdf about their spindles. If you search in this forum you might find a link for it.

You also would want to be able to rapid quickly to position your cutter. You can shoot for over 1000ipm for this.

However a lot of machines that are built on this forum (including my current one) move much slower. Mine tops out in the 250ipm range using acme screws and I usually cut at around 100ipm. For these types of machines, people typically use cutting force targets of around 50 lbf, but that is just a rough estimate based on thinking about how much force you exert pushing a router by hand through a piece of wood.

For going fast, I'd recommend looking at Ahren's (cncrouterparts.com) rack and pinion parts. People have been getting those to move much faster than acme screw setups.

[ger21]

One thing that would help me is to know what is the speed upper limit when cutting wood. Can it really be high???

Depends on what you mean by "wood". You can cut particle board at 2500ipm. But you'll need a $10K-15K spindle.

The larger the chipload (and harder the material), the higher the cutting forces.

As the above post mentioned, if you're using a handheld router for the spindle, figure about 50lbs max. The spindle will probably stall if pushed much harder.

[arthernan]

First off, thank you for taking the time.

I actually thougth about the same thing how much strength do I use to cut with my router. And I thougth it was about 20 lb. Maybe I underestimate myself . Altougth I am a skinny guy. And meditated about what it would take to make a small setup with puleys and guides to test how much cutting force was required.

I did checkout the XL spreadsheet, and I think get it. Put in values price them and repeat that until really happy.

I got a kick out of the other link. The guy was asking about wood and maybe aluminum, where have I heard that??????

The short question is for jsheerin. What size of motor do you use?

Before I commit money, I still have the luxury of dreaming about servos. It seems like several steppers I have seen lose abot 30% at 1000 rpm. If servos on ebay were 30% cheaper than new steppers. It may seem like a good tradeoff with some risks. Does anyone have more than one objection with that rationale?

Thank you guys and gals.

[ger21]

It seems like several steppers I have seen lose abot 30% at 1000 rpm.

Most probably lose much more than 30%, but it depends on the motor, and the supplied voltage.

[jsheerin]

Yeah, I have been planning to do some similar cutting force experiments for quite a while in preparation for redoing my router. Maybe that will happen this winter... Maybe. I was going to use an accelerometer and a hanging weight attached to the wood to be cut (which would move while the router stayed still). From that you can calculate cutting force.

I use 425oz*in motors from Keling, but these are not what I would recommend. They get you a lot of low speed torque but fall off rapidly at higher speeds. To use your example, they've lost 90% of their torque at 1000rpm. There are some newer lower stall torque motors that have higher high speed torque that would work better. These have lost 60-75% of their stall torque by 1k rpm.

[arthernan]

I'm happy not to be the only one thinking about experimenting. If I found something online I would love to use the data. But it may take just as long and I may be able to help others.

My problem is that I just moved and my garage is still a mess.

This is good info. I am going to keep looking at servos for some time now that you guys helped me out with these pointers. I'm not in a hurry. But I sure want to have a CNC machine.

I am thinking of building a small one, just so I can use it to make a bigger one.