View Full Version : leadscrew conversion thoughts

06-12-2012, 12:17 AM
Hi, this is my first post on this site (other than new members intro ) and am not quite sure where to post!!!

I have a very solid cnc router converted from a pcb drill router. the Z axis is very robust, using a vertical roller bearing slide on each side.

As the machine was not intended for the input of various Z depths it does not have a Z leadscrew, but the axis servo drives a horizontal shaft with a crank and conrod arrangement to drive the Z slide (Ihope that makes sense).

I intend to try to convert the Z axis to ballscrew. At present there is a 24mm hole from top to bottom of the Z slide which houses a spring to help lift the slide on the up stroke. I would like to fit a ball screw through the spring hole, but there is only about 2mm from the back ot the hole to the edge of the slide.

The ideal solution would be to find a ballnut that would fit into the 24mm hole. I think it would be possible to to bore out the top 50mm of the slide to accomodate a nut up to 28mm diameter if the nut flanges did not exceed 28mm. I think it would also be possible to mount the ballscrew offcentre (within the 24mm hole) to permit a larger diameter screw if neccessary.

As the Z stroke will only be about 50mm (total slide length is 205mm) would it be practical to use a fixed bearing at the bottom end (of the static section of the Z axis assembly), the ball nut fixed to the top of the slide, and do away with the top bearing (ie using the ball nut as the top bearing)? Would the Z axis with spindle be too heavy or would I need to incorporate a "helper" spring to lift the axis. Any thoughts would be much appreciated.
Regards, Geoff.

06-12-2012, 11:46 AM
Welcome to the site Geoff.

Think it would be worth your while editing your post into paragraphs, as many folk (well me at least) may find it that difficult to follow that they the wouldn't be bothered to read it.


06-12-2012, 01:16 PM
Hi Andy,

Thanks for your comments.

I have just edited my post into paragraphs, Hope it helps.

Regards, Geoff.

06-12-2012, 04:52 PM
That makes a world of difference Geoff! Sure someone will pop along to help as I'm afraid I don't have the correct know-how ... But can now follow your query.

Sorry, I should probably have sent a PM instead of advising formatting by post.

Maybe the mods can remove our few "formatting" posts from the thread to give better focus to your query?

06-12-2012, 08:01 PM
Geoff, anythings possible with a bit of imagination. I must admit I'm struggling to visualise exactly what you're describing...as they say a picture paints a thousand words, can you take some photos of your machine? Then I'm sure you'll get some of the old hands (figuratively speaking in some cases) chipping in :)

06-12-2012, 09:23 PM
Hi Neil, thanks for your reply. Trouble is I have not yet worked out how to post pictures!!! I'll get the wife to have a go.

Regards, Geoff.

06-12-2012, 09:56 PM
this forum is acctually quite easy to post pictures to, there is a button above the quick reply box, it's the 3rd button after the smiley face, when you hover over it, is says insert picture, click it then you will see 2 tabs, one says from computer the other from URL, if you picture is stored on your computer click from computer, then select files, then search for the picture as you would, t's likely the picture is in my pictures, on on your camera if thats plugged in to your computer.

then when you have found said picture, double click on the file name, and hen in next window click upload files and it will automatically place the picture at the end of your last piece of text

Robin Hewitt
06-12-2012, 10:15 PM
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.

06-12-2012, 10:26 PM
Thanks Wilfy, willtake some pics tomorrow and ive it a try.Regards,Geoff.

06-12-2012, 10:30 PM
Thankyou Robin, I see what you mean, I don't suppose I need more than a fat mans leg.

07-12-2012, 07:37 PM
Hi Wilfy
Will try to upload some pictures now, but don't really know where to post them!!7524752575267527752875297530

07-12-2012, 07:48 PM
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.

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

07-12-2012, 11:38 PM
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

Robin Hewitt
08-12-2012, 10:32 AM
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 :fat:

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


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

Robin Hewitt
08-12-2012, 12:25 PM
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 :thumsup:

08-12-2012, 12:43 PM
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.

09-12-2012, 10:45 AM
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


09-12-2012, 06:09 PM
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.



16-12-2012, 09:36 AM
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

16-12-2012, 11:36 AM
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.!!

16-12-2012, 11:51 AM
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.

17-12-2012, 11:56 PM
Jazz, I've read most of your threads, and don't understand how anyone could think you a pain!!!

Seriously tho' I think that the ballnut at the bottom is the best solution, with or without a second bearing.

The top of the Z slide is only 39mm wide (I thought it was wider) with the 24mm hole just 3mmin from the back. The bottom is about 60mm Behind the spindle).

The back of the Z slide fits back (minimum clearance) tight to the X axis carriage.

The 1605 ballnut is 28mm dia so I can offset the leadscrew in the 24mm hole and bore out the bottom of the slide to accomodate the offset and ballnut.

I was worried about the ballnut beig exposed to dust etc.,but the leadscrew will only be exposed when the slide is near the top of its travel.

I will try to assemble the spare Z slide tomorrow and post some pics. Your comments are much appreciated as my original idea w0uld have meant using a

smaller, more expensive ballscrew.

Thanks again, G.