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View Full Version : A little help in fitting motion to a Minitech Minimill3



nicad
03-18-2004, 05:12 AM
Howdy all-- long time lurker, newbee poster. :)

I am looking to purchase my next cnc mill and am seriously considering the Minitech Minimill 3. They offer a servo upgrade with it, but I think I would rather go with a "barebones" system and fit my own motion to it... it would be to my advantage financially and I would like to learn all I can about the motion controll of servos..
Right now I am thinking of going with 4x Geckos 320's, Mach2, the enclosure and accessories from Camtronics, and I already have 4x Globe 75vdc (3000rpm), 7A(max) ~200 in/oz servos with 500cpt encoders. I do not HAVE to use these motors, but would be nice since they are already here.

since this is my first servo CNC project, I would like some (actually all I can get) advise. :)
Does this seem like a fit for this mill setup? it has 5mm/turn ballscrews on all axis... will these motors be OK to connect directly to the ballscrew?
what power supply should I look at running.. 80Vdc, ??amp.

If these motors do not seem like a good match, any suggestions on ones that would be better (~$80/ea budget)? I would really like to try to stay away from belt-reduction drive to the ballscrews.

Swede
03-18-2004, 11:13 AM
Nicad, I can offer a comment on direct drive, since my own mill uses 5mm pitch ballscrews and THK linear bearings, which is what the mini-mill 3 has, I believe. With this setup, torque requirements are very low compared to dovetail or ACME screw systems. I put servos on mine, NEMA 23 in the 120 continuous oz/in class, which will run all day ice cold. They simply don't heat up at all. Torque to spare, and very high speed. I set the control for .0002" resolution and they'll do 240 ipm. The Z-axis screw will easily pull up 40 pounds of Z carriage and spindle with no counterweight. Despite that I still installed a gas spring to neutralize gravity on the Z.

If the 200 oz/in for your servos is continuous, then I'll go out on a limb and say that those servos will be killer in every way.

I think you are doing a smart thing installing your own hardware. I bought a flashcut servo system WITH the motors, and the motors added hugely to the expense of the package. I found better servo motors later on eBay at 10% of the cost of the stock motors. Surplus servos are plentiful and cheap.

What spindle are you going to use with the minitech?

nicad
03-18-2004, 02:08 PM
Swede-
Thanks for the quick reply! I have actually been following your mill project over on 5bears very closely for a while. Your reference to the Minitech minimill in regards to the linear rails is what got me looking into those systems. I have been lurking around trying to scrounge up components to build a mill very similar to yours, and when I heard that the minimill3 ususes ballscrews now, a barebones system of theirs is exactly what I was after. but now- on to the motion!

The Globe motors I have are not 200in/oz peak, continous. Im sorry I should have been more clear on that. according to http://www.globe-motors.com/21en_in.pdf , they are about 35 in/oz continous. (they dont list my 75v versions of thise-- I guess they were specially wound for some special order back in the day). If your running 200in/oz continous motors on your setup, which is 99% exact same setup as the minimill im looking at, then these Globe motors I have here suddently look pretty small.

I also have some 35V Magnetek motors which I have tested at 625 in/oz @ 25amps (highest my meters will go for now), and seem to get barely warmer than room temp at 6amps putting out 210 in/oz.
With these my main concern would be weight.. they are 4"x6" and weigh 10Lb... hanging out on the end of an 12"-16" Xaxis it seems would eventuall destroy the linear ways or something? maybe 10Lb is not big deal? My experience so far has been with a MaxNC and it runs on aluminum dovetail gibs and regular acme leadscrews.. suspended weight is a BIG deal on the MaxNC! :)
Any suggestions for a good (small, lightweight) decently size servo for the minimill3? Is it OK to mix-n-match servo sizes/types. ie-- larger motor on Z to pull the weight of the spindle motor up faster, smaller motor on the Axis since its moving less, etc?

As for a spindle, I was looking into one of Sherline's Industrial line of spindles for about $185. Will the Sherline spindle motor they offer run the spindle up to 10,000RPM? From my understanding it will only go 3500rpm, or is that the motor's RPM and with a 1:3 pully ratio it will turn it that fast?
I'm needing a 10k spindle setup on this.

Thanks for you time.. if anyone else wanna jump in on this feel free!
Colin

Swede
03-19-2004, 11:24 AM
Colin, I am not familiar with the specs of the Gecko 320 setup as far as voltages and types of servos it can power. My flashcut box uses Logosol digital servo drivers, 12A continuous, and I am nowhere near that with my system, although that's just me guessing based upon how cool the servos run. I think the mill I built replicates the minimill 3 in size. My table is cast iron, 6" X 18", with a stroke of about 11" on the X. I think the minimill uses an anodized aluminum table. With even less weight there than CI, that'll also reduce torque requirements.

I was wrong about what my motors are rated for. Here is the MCG motor I am using:

http://www.mcg-net.com/electric_motors/id23000.php

The motor is the ID23005. It is rated at 57 continuous, 400 stall, 17.2 ou-in/A, 12.7V/Krpm

Dimensionally it is almost identical to the one you propose. The terminal voltage on these is 60V. If yours are wound for 70 volts, my feeling is that they will perform identically to my MCG motors. What it boils down to... manufacturers have no "magic" ability to pack gobs of extra power in a 5" long NEMA 23 motor if both are run at similar voltage levels. Your motors are pretty much identical to mine and I don't think you'll have the slightest problem with them. I'd go ahead and use them with confidence. Servos pack more "effective" power than steppers, as the torque of a stepper drops dramatically with RPM.

The Sherline industrial ER16 spindle is a pretty nice and cost-effective setup. I really like the ER collet system, and it is possible to create QC tooling based upon the ER taper and a power drawbar. I recently boosted a Sherline spindle to 8,000K with a 1/2HP DC motor. I haven't had a chance to cut with it yet but it is promising. The spindle does get warm at these speeds, but honestly they are so cheap that if you cook one, it is not a huge loss. Sherline recommends reducing the preload a bit which is easy to do. I'm not really confident that the Sherline motor will create decent torque at those speeds, but it is worth a try. If you buy just the spindle, it'll be $180 or so; with the motor and speed control, the package goes up to $400+, but the advantage to the latter is that it is very easy to mount. I did an update at my hobby site showing the mounting of the 1/2hp motor to the Sherline, take a look, maybe it'll give you an idea.

Mix and match servos shouldn't be a problem, I added a NEMA34 to my Z. It all boils down to the Geckos, and I suspect they'll be happy so long as the installed motor is within the Gecko's rating.

Good luck, I looked hard at that mill and was impressed by its specs. It's definitely on the high-end of bench CNC mills.

NeoMoses
03-19-2004, 01:03 PM
While it's true that the minimill/3 appears to be nicely made, with THK bearings and precision ballscrews, I think it is over-priced. Please tell me you're not planning on spending over $2000-$3000 on this. The price they quote of $8500 for the stepper drive system is just too much for the size of mill. You can buy a used Bridgeport Series I with ballscrews and Servo motors for that. I know, a BPort isn't exactly a desktop mill, but still, I've always been a fan of getting the most for my money. Have you looked at any of the large mill/drills? That, too, would seem like a better bang for the buck. But, that's just my $0.02, it's worth exactly what you paid for it. ;)

nicad
03-19-2004, 03:20 PM
Sweed-
OK now I'm feeling a little better about my NEMA 23 motors here. :)
The Geckos limits are 80V max and 20A max. since these motors are rated 75V, Im thinking ill just go 75 or possibly 80v and should be OK. I ran the numbers and the motors I have here look like they only want about 1.3Amps continous to be happy, and really no more than 7 or 8 Amps Max... well within bounds of the Geckos. :)
Looks like Sherline (just recently??) is offering either a larger motor or higher pully setup, but they are getting 10kRPM out of the spindle now (which is the speed im targeting) http://www.sherlinedirect.com/merchant.cfm?pid=165&step=4.
Where did you get your 20kRPM Dayton motor, and if you dont mind, how much was it?
What exactly is the difference between the #1 MT and the ER-16? I have not seen an ER16 setup.. is it just a longer taper inside the spindle? does the spindle still have the 3/4"-16 threads on the outside for collets? can either MT#1 or the ER16 clamp a 3/8" endmill with a collet? I always see the 3/8" tooling clamped in a sleeve screwed to the spindle nose. To me this just cries out "not centered!". :)
I have been looking at that 'ATC for desktop mills' by homeshopaccessories (I think??). interesting drawbar config in the sherline spindle. I want to find out more about the setup he has.

NeoMoses-
I have about $6000 in my Maxnc so far (including origional price).. fixing it up to make it "work" like I need it to.
I have spoken with Jack at Miniteck and he was able to get me some VERY good bare-bonse system pricing. Since I have my own motors, the rest of the motion components that I am getting from 'here and there' will be running me right around $1000. Lets just say after its all said and done (going by the numbers so far), this new system will be very close to what I have invested in my Maxnc so far.
I dont have the room for a BProt, and I have always been shy of the retrofitted mill/drills for some reason.

Thanks for the input yall.. keep it flowing! :)
Colin

Swede
03-23-2004, 12:14 PM
Hello again Colin:)

ER16 - This is a steep-taper collet system designed to hold tooling. A full set of ER16 collets will grip everything up to 3/8" or so shank. The collets have a collapse range which allows a given collet to grip down the the next smaller, meaning you can grip 1/8", 5mm, 3/8", 3.456mm, etc. Very versatile and accurate. Sherline offers ER16-tapered spindles in their "industrial tooling" section on their website.

I checked the runout of my own ER16 Spindle with a good Brown and Sharpe 0.0001" DTI and the runout was below 0.0001", probably 0.00004". In contrast, my R8 Harbor Freight minimill spindle was no better than 0.001", 20X the runout.

A full set of new German-made ER16 collets can usually be snagged from eBay for < $100 U.S. Don't buy them individually or they will cost too much.

#1 Morse is a shallow taper, and Morse taper spindles in a mill are not as versatile as an ER16 setup. I'd do the ER16 if I had the choice long before the Morse.

The 20K RPM Dayton motor was purchased in a surplus store, and there was only one. It was a lucky purchase. I've wondered what it was designed for; my guess is a vacuum cleaner. It's not easy to find high-speed motors in that wattage, maybe someone has a source.

If you do buy a bare bones MM3, work hard on setting up a spindle mount which will allow you to easily mount and register different spindles. There may come a day when you have 3 or 4 which are different or specialized, and it's nice to be able to swap them out without an hour of work aligning them.

Let us know how it goes, I haven't seen any detailed reports on this mill.