[BrentonSpear]

Hi,

I am currently designing my cnc machine using Sketchup and it is coming along nicely.

My ideal machine is 1800mm in the Y axis and 1200mm in the X axis.

I am looking to use two balls crews on the Y axis, one either side and a single ballscrew on the X axis.

The ball screws will be either 16mm or 20mm with 5mm pitch and I would drive them with NEMA23 steppers from a gecko driver.

Before I get too carried away with my sketching, is 1800mm on the Y axis too long for a ballscrew to handle?

Thanks.



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[ger21]

5mm pitch is a very poor choice for a router, as it will result in slow speeds, or require high rpm's, which will cause the screw to whip.

The minimum size screw that I would use for an 1800mm axis would be a 2510.

[rowbare]

A lot depends on how fast you have to turn the screw and how the screw is mounted. Nook Industries Engineering Tools has a critical speed calculator. Look for the heading: "CRITICAL SPEED – The input speed which excites the resonant frequency of a system."

bob

[BrentonSpear]

Thanks.

I will look for 10mm pitch ballscrews.

So the length of 1800mm is not an issue?


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[louieatienza]

Thanks.

I will look for 10mm pitch ballscrews.

So the length of 1800mm is not an issue?


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There are machines that use longer screws... But they also have larger diameter... And will require larger bearings, motor...

[BrentonSpear]

I am looking to use NEMA23 stepper motors on the ballscrews.

I am currently looking at 2010 & 2510 ballscrews.

[hanermo]

I disagree strongly on 10 mm rise screws.
This needs servos, at the very least.

Also, for 1.8 m length, something like 32-40 mm is the right size.

I use 32 mm D in 1200 mm length, for a mill and a lathe.
Going from 1.2 m to 1.8 m is about 3x less rigid.
Free length cubed.

Also, you sould use a rigid screw, and a rotating nut, for best results.

[beernuts]

As ger21 stated, 2510 will be fine over 1800mm. A 2010 screw will do "just" but limit the speed potential of your machine.

A 2510 screw with a length of 1800mm has quite some weight, the Nema23 motor will turn it but your acceleration will be very low. Not a good combination.

Consider HTD5mm belt. The Nema23 motor will like that. Use 20mm wide HTD belt with stainless steel cable inserts, you will be surprised how good that works.

[thegolfer]

Hi mate,

According to the critical screw speed formula taken from the Hiwin ball screw catalogue, if you were to use a 20 mm ball screw over 1800 mm you would be limited to around 772 RPM screw rotation speed. This would give you a maximum rapid travel speed of 7.72 m/min with a 10 mm pitch ball screw. At this rate it would take 13.99 seconds for your machine to travel the 1800 mm.

If you were to use a 25 mm ball screw over 1800 mm you would be limited to 974 RPM. This would give you a maximum rapid speed of 9.74 m/min and it would take 11.09 seconds to travel the 1800 mm.

Both of these examples are pretty slow. I'd want a machine that big to be moving around at more like 15 m/min rapid speeds.

For a machine of the size you're looking at, I'd go for a rack and pinion system for both the 1800 mm and 1200 mm axes. As beernuts points out though, belts would work as well.

Toby

[pippin88]

I'm using a 16mm screw with 16mm lead. It rapids very comfortably at 20m/min. On brief testing it works fine at 25m/min. Starts stalling around 30m/min - scary watching it move that fast.

[ger21]

How long is it?

[louieatienza]

How long is it?

Well at 20m/min that's about 1250rpm? Sounds reasonable for a ballscrew, though I don't think most stepper setups I've seen here spin that fast under load...

[pippin88]

Sorry, meant to include that. 1300mm travel, approx 1400mm overall length.

[UA_Iron]

I disagree strongly on 10 mm rise screws.
This needs servos, at the very least.

Also, for 1.8 m length, something like 32-40 mm is the right size.

I use 32 mm D in 1200 mm length, for a mill and a lathe.
Going from 1.2 m to 1.8 m is about 3x less rigid.
Free length cubed.

Also, you sould use a rigid screw, and a rotating nut, for best results.

Servos require much higher RPM than Steppers, unless you use a gear reducer - but now we're talking big $$$.

Find a screw with a 20mm lead, you'll be in a favorable low RPM range for stepper motors (when their torque is the highest). With a table that big I am assuming that you're not expecting an accuracy tighter than 0.010" anyways... You'll get good repeatability though.

[BrentonSpear]

I have decided to redesign my CNC machine and use rack and pinion for the X & Y axis and a small ballscrew for the Z axis.

You can see my design here: http://www.cnczone.com/forums/diy-cn...65814-cnc.html