Your ultimate Spindle and axis motors

[Swiss]

Hi all,

I would like to make a 4-axis CNC milling machine for machining small aluminium parts, and I'm trying to decide which spindle and axis motors to use.

The budget is less then £10,000 ($20,000). For that money I would like to achieve:

200mm of travel in the XYZ axes, 2m/s speed.
Spindle speed of 20,000rpm.
Auto tool changing
(plus the usual coolant flow, PC, drive electronics etc, enclosure)

Can anyone suggest a suitable spindle and axis motors to use? So far, the best looking ones are just outside my budget. Spindle FS45 from Hofer Precision: http://www.hoferprecision.com/0311/hfmotorized.htm and linear stepper motors from H2W: http://www.h2wtech.com/stepmottor.htm

Can anyone suggest something as good for less money?

Hugo

[Stepper Monkey]

You are looking at things well outside the scope of milling aluminum there. The spindle really doesn't need to do 120,000 rpm, and you are going to be paying orders of magnitude more for one that does than a more practical 20,000 RPM spindle. NSK has some nice ones for under US$5000, there are good options all the way down to under $1000.
The linear steppers are a bit impractical as well. I would look at any number of pre-built machines, there are good ones out there already, even if you only use the basic chassis to build up something semi-custom from.
Reinventing that wheel completely from first principles is not only likely very unnecessary, its a hell of a lot more technical and difficult than you might think.

[Swiss]

Thanks Stepper, that's very helpful. I have been Googling for spindles for ages, and could only find people offering them in the range of $10,000. I was sure there had to be something less expensive around somewhere.

Yes, 20,000 rpm is exactly the speed I want, not 120,000. Where did you get that number from?

Why are linear steppers impractical? As far as the control is concerned, they are surely identical to steppers on leadscrews, but they require more current.

Normally I'd think about buying a maching, we have three already, but a) they cost a packet, b) they're much larger than they deserve to be considering the size of the parts they can make, and c) they're never quite what we want. It would be great to have a machine which is designed specifically for manufacturing the parts we need. Also, I am very comfortable re-inventing the wheel in this case. I am a professional robotic engineer, so I'm right at home here.

Thanks
Hugo

[LeeWay]

I will state the obvious then. Since you have experience and machines already, then you know milling creates a dirty environment. It is extremely difficult to seal ways off from just swarf even if you don't intend to use coolant. The motors you linked specifically say for light loads, so you are looking for some semi high speed machining? Very light repeat cuts?

These motors certainly appear fast enough. If you take very light cuts, they may do, but at a premium price. I don't have experience with linear's but I am curious how well these might work on the Z, which would carry some weight on it with the spindle.

Here are some relatively cheap spindles.
http://www.homeshopcnc.com/page7.html
I imagine they are for light duty cutting like what you intend. As opposed to hogging off metal.

I was finally able to design and built a mill myself. Just a standard benchtop size mill. The entire machine was likely around $5000.
I did manage to get 6600 RPM on my spindle. Just a standard X2 Chinese made mill head and tread mill motor with timing gear and belt drive. The entire spindle setup was well under $500.

I normally run the machine @ 200 IPM rapids, but set it to 300 yesterday. It would do 350 for certain, but this is pretty dang fast for such a small machine.
I have 14" x 9.5" x 9" travels. You want to move yours much faster. I am using preloaded precision ground ball screws and nice beefy THK linear ways.
My motors are 495 oz running off 72 VDC @ 12 amp PS and Gecko 203V's set for just 3 amps.
Bigger motors and more amps might push this machine up to a speed you could live with. Not sure though.

At any rate, I look forward to seeing what you come up with. The wheel has been reinvented a lot on this forum and most still roll well.

[Swiss]

milling creates a dirty environment

Yes. I have been thinking a lot about how to seal off the slide ways from swarf and coolant. I think I've thought of a really nice way to do it. I'll post some CAD images later.

I am curious how well these might work on the Z, which would carry some weight on it with the spindle.

They wouldn't have enough strength to lift the weight of the spindle quickly, so I'm thinking of using a spring to support the weight.

Here are some relatively cheap spindles. http://www.homeshopcnc.com/page7.html I imagine they are for light duty cutting like what you intend. As opposed to hogging off metal.

Thanks! These look pretty nice, and not madly expensive. But what I'd really like is one with a pneumatic collet, so the machine can change its own tools. Currently we have a 10-tool changer machine, which is nice, but we do still have to swap out tools all the time. What I really want is at least 20, if not 100. This is feasible for a machine what can just let go of one and pick up the next. (but not for our Emco 105 which has to have the 10 tools installed in tool holders).

I normally run the machine @ 200 IPM rapids, but set it to 300 yesterday....You want to move yours much faster.

Whoops! My bad. I wrote 2m/s! Jeez that would be very fast indeed. It would probably shake itself to pieces. I meant to write 2m/min.

I was finally able to design and built a mill myself.

I'd love to see a pic. Is there one on this forum somewhere?

Hugo

[ahren]

As someone who used to build linear motor (servo) based robotic platforms, I can tell you that 2 m/s is freaking fast. Furthermore, getting up to 2 m/s in a 200 mm work envelope will means you have only 100 mm for ramp up, for an acceleration of more than 2 g's. We did make a pick and place robot that was just capable of this, but starting cost was $80,000. Is there a reason you need to move that fast? At that speed, you'd be able to cover your entire work envelope in 0.2 seconds (assuming a triangular velocity ramp). Seems like these sorts of speeds would be more important for a larger work envelope.

Also, I would counsel against a linear motor for the Z -- it's nice to have this axis hold still when you hit an e-stop. My $0.02.

Ahren
www.cncrouterparts.com

[ahren]

Oops -- saw your reply right after I sent mine. I see 2m/sec isn't required anymore. A factor of 60 less should be completely do-able .

And a linear stepper machine would be really cool to see!

Ahren
www.cncrouterparts.com

[Swiss]

it's nice to have this axis hold still when you hit an e-stop.

That's a very good point. Hmm. How about if the e-stop just cut off the step/direction signals? Or does an e-stop have to cut power too for health and safety reasons?

Hugo

[audioandy1762]

Hi Hugo

Here is the link to Lee's build.
Cutty Shark I

Regards

Andy

[LeeWay]

Originally Posted by Swiss Whoops! My bad. I wrote 2m/s! Jeez that would be very fast indeed. It would probably shake itself to pieces. I meant to write 2m/min. I'd love to see a pic. Is there one on this forum somewhere? Hugo

I thought that was a little fast for most things. That would be a good speed for a printer.

Here is a link to my build thread here at the Zone.
80/20 mill build

Thanks, Andy. That's my router.


As Ahren warns, you may have issues with this on the Z. I use a gas spring on my Z to keep it from spinning down when power is cut. This keeps the head in place on the screw when power is cut. It holds position really well.
My Z axis probably weighs around 70 pounds or so and the gas spring is rated @ 30 pounds. This doesn't over work the axis on the down path and assists in raising some.

How would a linear motor hold position if power were cut? I think you would have to have a precisely weighted spring in order to pull that off. Percise anything usually costs.

[ahren]

In practice you really want the e-stop to cut power to everything. And then there's always the problem of machine startup and shutdown -- I guarantee you'll forget to apply the brake or park the z at its lowest setting. From experience, (we had a high-lead ball screw that would backdrive under the weight of the Z), I would make this axis screw driven. You can still get high speeds, but without the worry.

Ahren
www.cncrouterparts.com

[Swiss]

Leeway, that's a very nice build. Properly stable looking, and nicely machined.

I couldn't get the video to play more than a couple of seconds before freezing and the video from the router seems to have been removed.

there's always the problem of machine startup and shutdown

That's a good point. OK, how about the springs are over powered, and when the power cuts, the spindle automatically lifts up? I don't mind about loosing axis position because I'm going to get the machine to auto-zero the tool height automatically anyway.

Hugo