Thread: leadscrew conversion thoughts
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.
Geoff I do of course mean ballscrew nut mounted on the top of my Z slide. G
Last edited by GEOFFREY; 07-12-2012 at 08:53 PM. Reason: clarification
sorry i wasnt offering any help to your situation as i'm in no way qualified to do so... i was merely explaining how you get pictures here so you may grab the attention of the clever sods to provide you with some help :D
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.
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
Thinking about this...
If you sprung the motor sideways to pull on the belt and depended on the pulley flanges to keep the belt on the pulley, all your problems go away and you might actually get zero backlash from a cheap ball nut up to a useful vertical loading.
You can usually feel a bit of wobble on a cheap ball nut, not sideways wobble, rotational wobble 90 degrees out from the screw axis. The dust shields constrain it but the balls are slightly loose so you expect 20-50 microns of backlash after it beds in.
If you heaved the screw sideways using a belt tensioner, you would trap the balls between the screw and one side of the nut removing that backlash.
If the screw bends a bit, or the nut isn't concentric to the bottom bearing, the pulley will still turn concentrically. Meaning the motor never has to move appreciably sideways against it's spring during any single revolution and the load will not vary as it goes round.
This no top bearing idea could be a bonus
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.
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
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.
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
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