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[foam27]

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[dkowalcz]

Well, here's a quickie calc:

300 oz in input from servo
becomes 2550 oz-in at gearbox output (85% eff)
becomes 15221 oz of force delivered by the screw (95% eff)
which is 951 lb.

You've got two motors, so that's 1902 lb acting on 200 lb - accel is then 9.7 g's. (I'm ignoring a whole bunch of things, like screw and motor inertia, and friction, so when you fire this up I'd WAG more like 7g). Not sure what "4" and "30" mean in terms of units, but 7g is very respectable for machinery. I built a hydraulic system with a 10g accel/100g decel once, it was quite exciting to watch it in action. And quite fun to go around loc-titing all the bolts in place so they wouldn't keep vibrating out.

Be aware that things flex and wiggle a bit when they're being accelerated, so you may need a bit more steel in your gantry to shore it up, or fill the tube with shot. This will be something you'll have to find by experiment, or pay an engie to evaluate it for you. But if you're making 5' square aluminum molds, you can probably afford an actual consultant.

[jimbo]

foam27, I'd like to hear more about your machine if you have the time. I am building something very similar. I have the same ballscrews and a 48"x60" work area. It will be a moving table design.

[foam27]

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[foam27]

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[jimbo]

foam27, Thanks for the pictures. You did a good job on that frame. Its funny, I have many of the same pieces. I am still in the parts acquisition and design stage. But I do have (4) 76" IKO LWHD 25mm rails, and (2) 1.5" 1tpi barnes ballscrews, (2) z slides, z ballscrew, all purchased locally. I have one 750w ac brushless servo for one axis (x, table). I would like to use an 3-5hp elte spindle in the end, but may start with a 2hp hitachi router I have. I don't have a frame yet, nor the drives and remaining servos, plus all the little bits and pieces. I have roughly $800 so far in it. I havent thought it out completely, but probably will have to have a 2:1 or 3:1 reduction with timing belts, to get 1000 ipm rapids.

[dkowalcz]

Originally Posted by foam27 dkowalcz, Thanks for the awesome and enlightening info....so the force in oz-in goes from 2550 to 15221 even though the ball screw has a 1" lead? I 'm guessing that is going from torque to mass force? The "4" and "30" accel numbers are what I have in CNCPRO..I will have to check the units....I think I am going to go to MACH3, but I STILL DON'T TRUST WINDOWS.

Yep. One full turn of the screw means that conceptually, 2550 ounces goes thru 6.28" of distance (a full circumference at 1" radius). At the same time, the gantry moves 1". So you have a factor of 6.28 of leverage. Same as a see-saw or inclined plane, just wrapped around a cylinder.

[ViperTX]

Color me confused......you've designed a CNC Router from a collection of parts, you've spent $5K plus you have a $150 Hitachi Router and now you want to know if this is going to give you what you want.......

Ah....well the Hitachi Router was a good buy, but for continuous duty at high speeds.....hmmmmm.....

[foam27]

Hehe, Viper.....you silly man..(or kid...)

A-It already gave me what I wanted, I just want more.

B-It dosen't run continous duty, or high speed, and it's proven itself for long period durations, with frequent brush and runout inspection .

C- It has already produced beautiful molds.

D- We all want to tweak more, that's why we are here.

E- I didn't pay $5K for ballscrews, or $150 for the router, Sherlock.

F- Leave your bad day( or life) out of the forums.......

My only concern was inertia matching with this motor/ball screw/GB combo....which apparenty isn't a problem...

Guys it's people like this that make people like me want to pull the pix and not share, and I am sure others as well.

It's pretty sad when the only enjoyment some get is criticizing others work.

I've seen his rude posts elsewhere, so I expected this behavior to come my way......hehe...reminds me of that crankorgan guy a ways back...

Hmmm...where's that ignore button..........

[vacpress]

Your 1" Ball screws dont sound too big at all. With nice big servos on them, and your really nice looking router frame, I bet the extra ridgidity and mass of the screws will be satisfying for cutting aluminum.

dkowalcz,
I wish I had a clearer idea how to do these calculations, as i have a similar set of screws comming, and plan to use 990oz/in stepper motors with them. Any chance you could write a really quick explanation on how to calcualte this stuff? (sorry if this is in the forums allready like 100 times!)

Thank you.

[dkowalcz]

Vacpress:

A quick google search found this, which is the usual way to calculate screw force from torque and vice versa:

http://www.roymech.co.uk/Useful_Tabl..._Screws_1.html

For a ballscrew, you can get the same answer with this formula:

Force = (2 x pi x torque x 0.95) / lead

Use oz-in for torque and inches for lead, your force is then in oz.

At a bare minimum, if you're using gibbed slides, you want the force available to be the same as the weight of the slide.

I have an excel spreadsheet for stepper sizing with acme and plain threads (bolts and threaded rods) with the same equations as in the link above. Anybody who wants it just email at "admin@dakeng.com".

[JRoque]

Oops, I wanted to ask a question about your machine, Foam, but your posts are gone. If I remember correctly, your design looks very similar to mine. Do you see flexing in the X axis? I started with an 80/20 #3045 extruded aluminum bar across X and later added a solid steel bar 0.5" thick. If I lower my Z onto the table and feed downward, say, 0.01" I can see X twist a bit. Granted the whole frame is made out of aluminum but I didn't expect that much flexing there especially after adding the steel bar. Comments?

JR