SX2P Gas Strut Modification Issue?

[Dr_GMJNParticipant @dr_gmjn]

Something doesn’t seem right with the stick-slip issue. The bearing length cant be allowing it to jam. Am I missing something as simple as the gib strip moving? Looking at it, I don’t think it was made by a master craftsman…

[Dr_GMJNParticipant @dr_gmjn]

OK so how about this:

1) Cut the rack in half straight down the middle.

2) Drill and tap 4 holes at each end of the column where the ends of the rack are.

3) on the left side the rack would sit on a cap head bolt at the bottom tapped hole, and there would be an eccentric boss at the top. This would fix that half of the rack vertically.

4) on the right side, same thing, but with eccentric bosses top and bottom to enable control of the offset of that half relative to the other.

5) set the right hand rack for zero backlash when meshed with the pinion, lock the eccentrics.

The rack could be held in place by replacing its existing mounting screws (Currently Countersunk into the teeth) with Flat headed screws bearing in ovalised counterbores replacing the countersunk areas in the rack. It’s not like the rack halves could fall out because they are already Located in a slot, and the pinion stops them falling forward.

could also add a Thin central brass Divider between the rack halves (same witdth as the saw cut), which would require the pinion to have a groove cut in it. Or just make it lower than the teeth. This would stop any lateral play.

FWIW a new rack is £13, and the experiment would be reversible if it it didn’t work.

Ive tried the spring scale again tonight and actually am still intermittently getting sticking movement. It’s not consistent at all, and Sod’s law says when I need it to work, it won’t. I’m getting very pissed off with it, and am willing to try something more drastic now. Going from my fettled ML7 to the SX2P is not a great experience, and I’m a believer that the feel of a machine tool can actually influence your attitude to what your making on it.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dr_GMJN on 03/03/2021 21:54:57:

[…]

FWIW a new rack is £13

.

Dare I suggest ? … That very fact probably explains a lot.

Budget your time at ‘minimum wage’ and consider how much it will cost to make your improvements.

MichaelG.

[Dr_GMJNParticipant @dr_gmjn]

Posted by Michael Gilligan on 03/03/2021 22:12:24:

Posted by Dr_GMJN on 03/03/2021 21:54:57:

[…]

FWIW a new rack is £13

.

Dare I suggest ? … That very fact probably explains a lot.

Budget your time at ‘minimum wage’ and consider how much it will cost to make your improvements.

MichaelG.

I know – you get what you pay for.

I just want to improve it. It’s got to the stage where even though modding machines isn’t what Im in this for, I need to do something about it.

If I were to get rid of it, there’s nothing else out there of a similar price and size that wouldn’t need work one way or the other. It’s actually the perfect size for what I need, just that one aspect that I can’t sort out.

[Michael GilliganParticipant @michaelgilligan61133]

I do understand your position, and you have my sympathy.

I haven’t really thought this through, but …

If it was mine, I would probably try to kill the problem at source: Scrap the entire coarse and fine Z-axis adjustment, and the gas strut: and fit a [preferably motorised] ball-screw instead.

MichaelG.

[Dr_GMJNParticipant @dr_gmjn]

Posted by Michael Gilligan on 03/03/2021 22:40:01:

I do understand your position, and you have my sympathy.

I haven’t really thought this through, but …

If it was mine, I would probably try to kill the problem at source: Scrap the entire coarse and fine Z-axis adjustment, and the gas strut: and fit a [preferably motorised] ball-screw instead.

MichaelG.

That’s a major project, and would still retain the dovetails and gib strip. An entire linear slide with ball screw would be easier – just Machine the entire slide system off and bolt the head to the carriage and the guides to the column. No way I’m doing that though.

[Neil AParticipant @neila]

I have had an SX2P mill for some time now, still with its original torsion spring and lever arrangement. I also have had the stick/slip problem, I feel that it is connected with the bedding in of the gibs as I have been able to adjust it out on my machine, maybe I'm lucky. I have also experienced the head run down the column by itself because the torsion spring does not provide enough upward force when the head is at the top of the column, very disconcerting when it happens. I now always fit the limit block so the tool will not hit the work until I am ready to cut metal, just in case.

As regards the backlash in the rack and pinion system I now push on the torsion lever with my left hand to provide a bit of resistance to the downward force, not really the way it should be, but it has worked for me so far.

I did wonder if it would be possible to fit some sort of short stroke hydraulic damper between the head and the limit block to push against and so limit the speed of any downward travel. I have not really thought that one through at the moment, just an idea that might work.

Fitting a ball-screw would be nice if it were not for the amount of machining necessary to do it. There is a thread on converting an SX2P to CNC if you want to see what is involved.

I'm just waiting to see if anyone comes up with a relatively simple solution, at the moment I just live with it. I find the machine is fine in every other aspect.

Neil

[Pete.Participant @pete-2]

Someone on this forum mentioned ballscrews can allow the cutter to be pulled into the work as they have little friction, so it may well not solve your issue.

A traditional lead screw could be fitted to the left hand side of the machine with no machining other than a few holes drilled and tapped, if attached on the outside of the column with a manually operated handle at the top.

Or maybe shimming behind the rack until it meshes with the pinion with acceptable backlash, don't know if this has been mentioned, I haven't read all the comments.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Pete. on 04/03/2021 00:10:26:

Someone on this forum mentioned ballscrews can allow the cutter to be pulled into the work as they have little friction, so it may well not solve your issue.

.

That did cross my mind …

which is one of the reasons I wrote “[preferably motorised]”

I did say I haven’t really thought this through, but …

So please accept it as mere ponderings.

MichaelG.

[Pete.Participant @pete-2]

Absolutely Michael, I wasn't questioning anything you said, I'd seen a few other posts recently that made me think some people might not realise using ballscrews manually can cause issues, I was just throwing a few ideas out there.

[David George 1Participant @davidgeorge1]

I fitted a ballscrew to my Chester champion 16VS and have had no problems but that may be because I have moved the handwheel from the back to the side, with a pair of bevel gears, so I can reach the handwheel.

David

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Pete. on 04/03/2021 01:10:58:

.

… some people might not realise using ballscrews manually can cause issues, I was just throwing a few ideas out there.

.

An excellent reaction, Pete

it demonstrates the merit of of a forum.

I don’t recall seeing any authoritative documentation about the ‘reverse running problem’ of ballscrews but I feel sure it must be out there. … If anyone happens to find anything, it would probably merit a new forum thread for convenient reference.

MichaelG.

[Martin ConnellyParticipant @martinconnelly55370]

It seems to me that the issue is that the head is acting like one of those sprung woodpecker on a pole toys. I'm not sure of the simplest of fixes for this but the toys do rely on friction and a force rotating the sleeve relative to the axis of the pole to work. Once there is some upward force from the tool on the workpiece to balance this downwards force the friction at the column reduces and the head drops.

The use of gas struts and springs cause issues because they do not give a linear response of force over distance compressed or wound up. There are two ways to have a more linear response, one is to use a pneumatic piston with a constant supply of air at a fixed pressure from a regulator, the other is with a counterweight and pulley system.

I think I would try rigging up a temporary weight and pulley system with the upward force taken around the middle of the head to see what effect that had without the springs or gas struts being used. It is easy to adjust the counterweight and gib strip to experiment.

Martin C

[Dr_GMJNParticipant @dr_gmjn]

Posted by Martin Connelly on 04/03/2021 15:57:45:

It seems to me that the issue is that the head is acting like one of those sprung woodpecker on a pole toys. I'm not sure of the simplest of fixes for this but the toys do rely on friction and a force rotating the sleeve relative to the axis of the pole to work. Once there is some upward force from the tool on the workpiece to balance this downwards force the friction at the column reduces and the head drops.

The use of gas struts and springs cause issues because they do not give a linear response of force over distance compressed or wound up. There are two ways to have a more linear response, one is to use a pneumatic piston with a constant supply of air at a fixed pressure from a regulator, the other is with a counterweight and pulley system.

I think I would try rigging up a temporary weight and pulley system with the upward force taken around the middle of the head to see what effect that had without the springs or gas struts being used. It is easy to adjust the counterweight and gib strip to experiment.

Martin C

The woodpecker thing is a narrow collar though – it's like the mastic gun principle. The mill head has a much longer area of contact.

I put the sprung load as close to the column as I could, to minimise the lever effect as much as practically possible, but it still sticks.

Sod it. I'm going to saw the rack and offset each half. That appears to be the simplest way of giving positive motion from the feed wheels, with continuous pinion contact both up and down. If it doesn't work I'll get another rack and live with it until something better turns up.

[Pete.Participant @pete-2]

Posted by David George 1 on 04/03/2021 08:14:13:

I fitted a ballscrew to my Chester champion 16VS and have had no problems but that may be because I have moved the handwheel from the back to the side, with a pair of bevel gears, so I can reach the handwheel.

David

That looks like an excellent solution to the problem, very nicely done also.

[Pete.Participant @pete-2]

Posted by Michael Gilligan on 04/03/2021 08:43:56:

Posted by Pete. on 04/03/2021 01:10:58:

.

… some people might not realise using ballscrews manually can cause issues, I was just throwing a few ideas out there.

.

An excellent reaction, Pete

it demonstrates the merit of of a forum.

I don’t recall seeing any authoritative documentation about the ‘reverse running problem’ of ballscrews but I feel sure it must be out there. … If anyone happens to find anything, it would probably merit a new forum thread for convenient reference.

MichaelG.

Just to clarify, I have no experience of this happening, just something I read on here from someone who had tried it, be interesting to her more from anyone who might have experienced this.

[Michael GilliganParticipant @michaelgilligan61133]

Pete,

I don’t think this helps enough to deserve a new forum thread … but it’s a start: **LINK**

Ballscrew Back-Driving Holding Torque Calculator

Big problem seems to be that you need to input % efficiency [which seems like the big unknown]

MichaelG.

.

Does anyone have some ‘typical’ numbers we could try in this calculator ?

.

Edit: __ Just found this : https://www.youtube.com/watch?v=pCD2HXjiWRQ

Edited By Michael Gilligan on 04/03/2021 17:30:45

[oldveloParticipant @oldvelo]

Stop looking for complicated answers to a simple problem 5 months on and the problem not solved.

Time to learn how to learn how to back off the cutter lead angle to 90 deg for machining caste iron, bronzes, and brasses.

Only my biased opinion based on a lifetime of engineering after all it's your time and money.

Eric

 

Edited By oldvelo on 04/03/2021 17:32:19

Edited By oldvelo on 04/03/2021 17:32:33

[ChrisLHParticipant @chrislh]

I had similar problems to the OP with my milling machine. Specifically, the head had a habit of making its way down into or up out of the work depending on how tough the going was. Of course, it was OK if I locked the z axis but this is not an option for CNC operation.

My chosen solution was a heavy counterweight (album photo) of sufficient size to ensure that the leadscrew was always loaded in the upwards direction. The application point of the counterweight load was estimated to minimise the couple, and therefore friction, experienced by the z axis slide. My analysis of the mechanics suggests that this application point is not in line with the leadscrew. To do this properley one would need to know the weight of the counterweight, the weight of the head and the position of its centre of gravity. Rather a tall order in the case of the latter two measurements but perhaps doable on a small machine.

To be clear, the aim would be :

weight of head x distance CG of head to leadscrew CL = weight of counterweight x dist. application point to leadscrew CL

Offered as food for thought.

Chris

[Neil AParticipant @neila]

In MEW 251 Phil Dawes showed his approach to using a counterweight to balance the head of a Sieg SX2P. Might be worth you having a look at that article to see how he did it.

Neil

[Dr_GMJNParticipant @dr_gmjn]

The way I see it, balancing the head isn’t the fundamental issue, it’s balancing *and* eliminating sticking *and* eliminating backlash. In fact balancing is probably the least important of the three. Addressing one Z-axis issue but not the others seems pointless, since the one(s) remaining will eventually get you.

I understand about the couple on the head and position on the counterbalance link to both reduce sticking and balance the head, but as soon as you put cutting loads on the head, the theoretical link position will change slightly (if I understand correctly).

I think the most relevant improvement in this case would be to eliminate backlash – or reduce it to a sensible level. You can see from the video I posted that it’s quite severe – about 5-10mm. Combined with the stick-slip it’s asking for trouble.

If the head’s not balanced, it just gives a bit more load on the hand wheels (the head won’t drop anyway with fine feed engaged). The gas strut is fine – in fact almost perfect notwithstanding the offset load it gives.

If there’s any sticking, it’ll be overcome by the action of positively moving the head with the hand levers. The worst that can happen I assume is that there would be a momentary increase in load. I doubt this would be felt to any significant degree by the time the worm drive has increased the handwheel torque on the pinion.

So I’m left with swapping the rack for a screw, making a spit/preloaded pinion, or experimenting with splitting the rack. The latter seems the cheapest, simplest and least risk option. Having said that, I can find no examples online, which is concerning, but there we go. The nearest example I’ve been given is a split and offset rack in a mountain railway system, but that’s not to reduce backlash, it’s to ensure consistent pinion engagement. The usual method for precision rack drives us to split the pinion, but this is a far more involved modification. I can understand why this is a preferred option, since it doesn’t mean messing about with adjusting potentially long lengths of rack.

[Ron LadenParticipant @ronladen17547]

Just save me wading back through every post I keep seeing the mention of "sticking" are you saying that the head actually sticks to the column..?

[Dr_GMJNParticipant @dr_gmjn]

Posted by Ron Laden on 05/03/2021 07:19:38:

Just save me wading back through every post I keep seeing the mention of "sticking" are you saying that the head actually sticks to the column..?

It’s a ‘jerky’ column movement: stick-slip. It’s intermittent. If I rapidly turn the hand levers down, there’s often a distinct thud-thud-thud as the head moves down. Do it slower and occasionally it’ll stick, and wont unstick until all the backlash has been taken up and the pinion forces it free.I’ve tried adjusting the gib strip and lubricants, but I think short of scraping the slides (which I’ve never done before), and fettling the gib strip the issue will remain.

It’s strange because I’ve checked that the head isn’t moving around, with a DTI, and I can push the head up and down by hand quite freely with the rack removed. This suggests the gas strut is pretty much spot-on in terms of balance.

probably something to do with how the pinion is applying a lever load to the head, or maybe the gib strip is not contacting all along it’s length, or the screw detents aren’t right, or a billion other things.

[JasonBModerator @jasonb]

All this balancing out the weight of the head stops it sitting firmly down on the screw, as soon as you make the head "weightless" then you get backlash and the sticktion is not overcome by the weight of the head so you have to push/pull with the leadscrew to overcome it. strut on the side also twists the head on the column so you may balance out the weight but the head itself is not balanced centrally.

If the head was "heavy" then that would have a similar effect to you pulling it down with your hand in the video.

I'll stick with my unweighted X3 and a strong hand to wind it up

Edited By JasonB on 05/03/2021 08:38:38

[Pete CordellParticipant @petecordell95786]

Backlash-free rack and pinion

An improved rack and pinion has been developed that eliminates backlash without using split gears. It is amenable to easy retrofitting of existing systems.

Might be worth having a look

[Author]

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