Ballscrew Thickness!

**Ross77** · 17-08-2009

Originally Posted by Gary

I have seen this done before, but this normally involves more money and because the motor/gearing that is associated to this type is moving with the ball nut, the moving mass is higher.

Didnt think of that

. Ist the main problem in this case the actual torque needed to spin the screw? Although the extra weight wouldnt be ideal surley it is a lesser evil ? wouldnt it be possilble to also use a smaller screw which would make it cheaper?

Sorry to hijack your tread :naughty:

options options options, think im going be scratching my head till ive build my machine and ready for the motion! and thats not to far away!

Isnt that all part of the fun.....:heehee:

**AdCNC** · 17-08-2009

haha yeh but its not fun anymore my wallets burnt out!

**Ross77** · 17-08-2009

Tell me about it, this definetly isnt a cheap hobby....:nope:

I would love to throw 5K at a scratch built machine, with everything designed for its purpose....as opposed to buying S/H parts and making the machine fit them :whistling:

Thank god for Ebay tho.

**irving2008** · 17-08-2009

a 16mm dia screw with support both ends would work fine over that distance (~1.8m) up to 700rpm approx, you don't need a 25mm screw...

**tribbles** · 17-08-2009

Well, according to the calculator:

- 14mm root diameter (subtract 2mm due to thread size)
- 1800mm length between bearings
- B-style end fixing (one fixed, one floating)

Maximum speed is 522RPM.

Even with a 16mm root diameter, it's 597RPM.

(But I know engineers like to put some extra for safety, so it probably would work up to 700RPM).

Or do you have another way of calculating it?

**irving2008** · 17-08-2009

Originally Posted by tribbles

Well, according to the calculator:

- 14mm root diameter (subtract 2mm due to thread size)
- 1800mm length between bearings
- B-style end fixing (one fixed, one floating)

Maximum speed is 522RPM.

Even with a 16mm root diameter, it's 597RPM.

(But I know engineers like to put some extra for safety, so it probably would work up to 700RPM).

Or do you have another way of calculating it?

14mm root, C style fixing, 1850 long, 727rpm

**AdCNC** · 18-08-2009

ok. goin for a thiner screw would drastically reduce starting and stopping inertia and rotational mass! meaning that i dont have to go overboard with the motors! do you think a small 430oz-in NEMA23 gear say to 5:1 would move a Y axis of a 6ft (5ft effective) long table with say 16mm OD screw? this got me wondering because having studied joe's 4x4 CNC videos and pics, it look to me like he's using NEMA23 motors with a long screw, more friction than ball screw and his machine has a inch lead, and its surprising to see thats it flys!

**AdCNC** · 18-08-2009

oh btw i would be using 2 motors and 2 ball screws one either side of the x axis

**Gary** · 18-08-2009

If you are set on a smaller screw, and by the looks of it, you could get away with it, then it may work, but you also need to take into account the mass you are moving.
the heavier the Y axis the more mass the X has to move, and if you want it to move quick, that is a lot of inertia it has to overcome and that requires power.
You allready have the motor, so you can allway try them, but i dont think you will get the speed you want, especially with the current drivers.
The voltage of the drivers is the killer, so i suspect you will need to change them to a higher voltage driver.

What are you making the frame out of? if it is quite large it needs to be as ridgid as possible, or any small amount of flex will be seen.

We are starting to develope a Y and Z axis combined but it is made from steel, so is quite heavy.
The attached pictures shows a large Z axis (400mm stroke and a 34 frame motor) and this weighs about 15 KG, so i suspect our Y axis would be about 50KG.

**tribbles** · 18-08-2009

Originally Posted by irving2008

14mm root, C style fixing, 1850 long, 727rpm

Yeah, having two fixed ends does improve things - although on my machine it's one fixed (BK), and one floating (BF), and I would guess that's what the standard machining on Gary's ballscrews would provide.

I suppose you could use BK on both ends, but you'd need to make the ends with a lot of precision (or have a number of different size shims to pad out the end with the screw-thread).