leadscrew conversion thoughts

[Robin Hewitt]

Here's a rule of thumb:- A 1Nm motor on to a 5mm pitch ballscrew will lift one fat bloke off the floor.

Assuming 1 fat bloke = 282 lbs

Substitute a 0.5Nm motor or a 10mm pitch screw and you can only lift half a fat bloke. The maths is easy.

Wide ball screws are used where you need long travel or massive power.

You don't need either so a skinny ballscrew and dinky motor will be fine and dandy.

[GEOFFREY]

Thankyou Robin, I see what you mean, I don't suppose I need more than a fat mans leg.

[GEOFFREY]

Hi Robin,
I have just posted some pictures and did them as a reply to Wilfy.

One of the reasons that I wanted to use a "beefy" leadscrew is because I am hoping to be able to use a ball screw mounted on the top of my Z slide as the top bearing. I don't know if this will work, but intend to drive the screw via a servo/encoder fixed parallel to the spindle and using toothed belt and pulleys.

I would be very interested to hear any comments about this.

Regards
Geoff I do of course mean ballscrew nut mounted on the top of my Z slide. G

[Robin Hewitt]

One of the reasons that I wanted to use a "beefy" leadscrew is because I am hoping to be able to use a ball screw mounted on the top of my Z slide as the top bearing. I don't know if this will work, but intend to drive the screw via a servo/encoder fixed parallel to the spindle and using toothed belt and pulleys.

Hi Geoffrey

Ah! Didn't realise you had a "real" machine. It looks gorgeous

There are three considerations. How far does the pulley have to move sideways before the teeth can slip, what force is trying to bend it and how concentric is the nut to the bottom bearing.

Let me grab some numbers out of the air and try it.

Assuming half a fat bloke up down force to move it, 50mm diameter pulley, XL belt.

A 4.5 lb side loading has to bend the screw 2mm before it slips. Doesn't sound like it will be a problem.

But the shorter the distance from nut to bottom bearing, the more concentric the nut has to be. Without the top constraint the shaft will tilt as the nut moves up and down. Concentricity decides how much it tilts.

Good luck

Robin


Here's the boring bit in case someone wants to check it...

To slip the belt has to fit around a circle pulley radius + belt tooth height
Assume a 25mm radius pulley and a 1.27mm XL tooth.
157mm circumference has to become 165mm, a difference of 8mm
But we lose half on the semicircular contact and another half because the slack happens on both sides.
2mm of bend to slip.

Next the bending force. Assume half a fat bloke on a 5mm screw so 0.5Nm motor.
Our pulley is .025m radius so the side load on the shaft is 0.5 * 1/0.025 = 20 Newtons = 4.5 lbf

[JAZZCNC]

Geoffrey would there be any chance you could post pictures with the Cam/Motor assembly off the machine with a top down view.

From what I'm seeing then it doesn't look like it will be a massive Job to achieve thou you will need the top of the Ballscrew supported if using Timing pulley setup other wise the tension will obviously try to bend it causing sticking.! You can easily get away with the bottom hanging unsupported but not the top.

[GEOFFREY]

Hi,
Thanks for your comments, I am posting some pics of my spare carriage so you can see how it travels on the X axis and carries the Z axis slide.

The heavy duty top plate will need to be retained to allow fixing of the X axis ball nut and linear scale. Everything else above that level will be removed.

I like the idea of mounting the Z ball screw nut at the bottom of the slide and fixed bearing to the heavy duty plate, but am a bit concerned that this would leave the ball nut and the end 50/60mm of ball screw exposed to sawdust.

Regards
Geoff

[GEOFFREY]

Jazz, thanks for the suggestion, you are right it makes far more sense to do that. I am getting the bottom bored out for the ballnut and will worry about sawdust etc later. G

[JAZZCNC]

Jazz, thanks for the suggestion, you are right it makes far more sense to do that. I am getting the bottom bored out for the ballnut and will worry about sawdust etc later. G

Geoff Am I missing something.?

Can't see how your going to do it with ballnut at the bottom.? The screw would need to hang down below the Z axis to give available travel.!!

[JAZZCNC]

Sorry to be a pain but can you give some top down shots of the spare assembly with Z axis close to in place has possible.

Also of the Z axis like pic 1538 in post #11 but tuther way round.!

Don't think this would be a major job to mount in traditional way but without clear pics of clearances etc then it's difficult to tell.
Thou less than the ideal I'm sure with the width of the Y axis carriage that some form of offset screw could be adapted if needed and eliminate any ugly screw sticking out the bottom and risk of damage to screw.

[GEOFFREY]

Hi Robin, thanks for the nice comments about my machne. Whilst a bit old and scruffy it really is georgeous(bit like me really).

I actually have a second identical, but unconverted machine which I bought for spares, but am finding it difficult to bring myself to break it up!!!

The "spare Z slide is the one pictured in the last couple of pics(you can also see my now completed vacuum table I referred to in my NM introduction).

The 24mm hole i referred to is shown clearly in pic 6 and there is in fact plenty of room to bore this out much bigger(I'm getting braver by the day).

The servo/encoder would be mounted parallel to the spindle almost directly under its existing position(you can see in pic 1 that there is space for it.

The idea of using a very heavy duty leadscrew was to try to virtually eliminate any deflection of the screw as at max Z travel there should never be more than about 60mm of screw poking its head above the slide.


I have bean called to breakfast(3rd time) so have to go now

Regards,Geoff.