using linear motors to build a small cnc

Has anybody built a small cnc using linear motors? I am toying with the idea of building a small high speed mill/router and am
wondering if using linear motors is a good idea and where to find some. Brand names seem pretty expensive. I would like to have feedrates of at least 1000 ipm so that eliminates stepper motors. Travels would be at least 12" if not 24" on all 3 axes.
Also, what could be used for spindle? Ideally 10,000 rpm
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Paul



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It wouldn't be cost effective and/or they wouldn't have enough torque. For example my CNC knee mill has 880 oz/in servos. They make 10 revolutions to move the table an inch. You'd have to get 8800 oz/in linear motor to do the same. That's 50 lb.ft. I remember buying linear motors that big when I worked at 3M for $5000 or so. My servos are on Ebay every day for $300.
Karl

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in message

...
I don't know if that high a speed would be possible in that short a distance. My servos max at 1700 rpm or 170 ipm on my CNC knee mill. But I limit max speed to 50 ipm. (Chicken s%^&t here) You wouldn't need anywhere near the torque though, if you geared it 3:1 you'd get 560 ipm top. If you want to go faster, you'll need big servos.I've seen calculaters for table weight, speed, acceleration, etc. that give the required servo torque, on a vendor website
I've no experience with routers. Do they make a three phase router head? I think so. Running one of these on a VFD would give you variable speed.
Karl
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wrote in

They have made major advances in linear motors in the last two years. They are now the fastest accelerating motors you can buy. The cost has came way down also. There are several machining centers now being manufactured with linear motors because of the speed.
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Anthony

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wrote:
Linear motors would work great, but they are likely to be too costly to implement on a small machine
Currently one can get linear motors on many high end CNC machines (DMG seems to be the biggest proponent)
1000 ipm does not need linear motors from strictly a speed point of view anyways (backlash etc is another matter). At 1000 ipm go with brushless servo motors. You can buy the controllers from advanced motion controls http://www.a-m-c.com/ They are pricier that stepper controls, but reasonable wrt servo brushless.
You may find that building one machine may be more expensive that just buying a used CNC - or even pretty close to the price of a small newer one.

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My DMG has a linear motor, you should see the size of the drive it uses, size of a old at case computer. Lotsa lotsa power. fast tho
Paul Ragot wrote:

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Paul Ragot writes:

This seems impractical.
CNC drives for small mills should develop literally tons of linear force.
Do linear motors do that?
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No.
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SVL



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well, for hundred grand they do
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snipped-for-privacy@truetex.com says...

Tons sounds high, my SWAG is 1000# would be adequate for a Brideport sized mill. There are linear motors that produce that much force, and more, but they're certainly not cheap.
Ned Simmons
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Ned Simmons writes:

F = ma. The table and saddle weigh together 400 lbs or more. Add the rotational inertia of the motor, pulleys/belts, handles, and screw. You'd like the table to get up to a respectable slewing speed (for a hobbyist) of 100 ipm or better, in some fraction of a second. You must also exceed the side cutting forces for a typical task by some multiple, from the _Handbook_, at cutting speeds.
My smallish 0.1 HP PMDC motors at http://www.truetex.com/mcgpd34002.htm produce about 800 lbs of linear force with 2:1 timing belt pulleys on ballscrews on a freshly rescraped Bridgeport Series I with Teflon ways. It is certainly adequate, but like a speedboat, you can never have too much. Fun to watch at 100 ipm, but you wonder what 200 or 500 would be like.
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snipped-for-privacy@truetex.com says...

But with a linear motor you don't have all that rotating inertia. If you're looking for quick acceleration with a lead screw drive, the torque required to get all the rotating stuff up to speed is usually much greater than the torque required to move the load at a steady velocity.
To get to 100ipm in 1/10 sec is only .04g , or 16lbf for a 400lb table.

So say you're using the full 1 HP at 50 FPM => 660# at the periphery of the cutter.
I backed into it from the other direction, assuming a reasonable force on the table cranks and 50% efficiency for acme screws to get my 1000# guess.

Indicate your vice in 1/2 sec <g>
Ned Simmons
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wrote:

Are the Mazaks with 3.5g acceleration not linear motor driven?

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Richard J Kinch wrote:

The new fancy Trans do - Mag Levs aren't they! Martin
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