SX2P Gas Strut Modification Issue?

[Dr_GMJNParticipant @dr_gmjn]

Posted by JasonB on 05/03/2021 08:33:37:

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

Understood Jason, I also assumed that, but as I said previously, even without the gas strut fitted, the sticking is still there. That's why I tried an adjustable strut and de-rated it to force the pinion down into contact under the weight of the head – still the same stick-slip. I then removed the strut altogether, assuming it would cure it, but it was still there.

I then went to spring loading the head down, as near to the column as I could get it, and it still sticks intermittently unless you put about 35kg on the dial, and even then you only get a limited amount of downward travel before the spring load runs out.

[Dr_GMJNParticipant @dr_gmjn]

Posted by Pete Cordell on 05/03/2021 08:41:36:

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

 

 

 

 

Thanks Pete.

Looks basically like another way of increasing engagement with a spring.

I tried incrementally shimming the rack out already, but even when it got the the stage of unacceptable binding and rough operation, there was still backlash. So there didn't seem much point in continuing down that path.

Edited By Dr_GMJN on 05/03/2021 08:54:02

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dr_GMJN on 05/03/2021 08:48:35:

Posted by JasonB on 05/03/2021 08:33:37:

[…]

Understood Jason, I also assumed that, but as I said previously, even without the gas strut fitted, the sticking is still there. That's why I tried […]

.

I would posit that everything else you are trying is futile [perhaps even counter-productive] unless and until you have established smooth sliding of the head on the column.

MichaelG.

[Dr_GMJNParticipant @dr_gmjn]

Posted by Michael Gilligan on 05/03/2021 09:01:20:

Posted by Dr_GMJN on 05/03/2021 08:48:35:

Posted by JasonB on 05/03/2021 08:33:37:

[…]

Understood Jason, I also assumed that, but as I said previously, even without the gas strut fitted, the sticking is still there. That's why I tried […]

.

I would posit that everything else you are trying is futile [perhaps even counter-productive] unless and until you have established smooth sliding of the head on the column.

MichaelG.

So, when it sticks when moved down with the fine feed, it immediately un-sticks once the pinion contacts a rack tooth (ie once the lash is taken up). So I'm assuming that if there's no lash, or a very small amount, the movement will be smoother. So instead of a 5mm drop, I'll have perhaps a 0.1mm drop. I can live with that.

[HowiParticipant @howi]

Posted by Michael Gilligan on 05/03/2021 09:01:20:

Posted by Dr_GMJN on 05/03/2021 08:48:35:

Posted by JasonB on 05/03/2021 08:33:37:

[…]

Understood Jason, I also assumed that, but as I said previously, even without the gas strut fitted, the sticking is still there. That's why I tried […]

.

I would posit that everything else you are trying is futile [perhaps even counter-productive] unless and until you have established smooth sliding of the head on the column.

MichaelG.

exactly!

[ZanParticipant @zan]

If your fine adjustment does nothing when adding cut, then the head drops 5 mm then the rack /pinion interface is strange,. Perhaps the pinion is moving in its bore The sx2 is a cheap light weight machine. Built to keep the costing down. When I converted mine to cnc I found that the z head dovetail casting was warped, probably from machining it when the casting was a bit green. It took a lot of work to get it so that with the column off the machine the head could slide up and down without any twisting. The test was done with a 3/4” silver steel bar in an R8 collet and the head was pushed along the column by hand. I can see that this could be a source of your problems as well.

I think the twisted casting was causing it to rock and this would be a course of the sticking.  I found it by putting the head on the surface plate on the working face of the dovetail.  From memory it I think needed about 15 thou packing under one corner to stop the rocking.

the idea that a ball screw nut will allow the head to fall is bunkum. The nut has to rotate for it to move! Fitting any z feed screw would solve the problem as you will have total feed control and sticking issues will be resolved unless there is serious clearance issues within the z slide arrangements as I had . Fitting a motor to any feed screw would make positioning impossible without the associated electronics and/or computer control. A hard wheel would do the trick nicely, so you end up with an arrangement as in one of the photos above and like the down feed arrangement on the Emco FB2 mill

I could never understand the original spring arrangement as the lever arm resulted in a change of upward force as the head is moved up and down all this and the z feed system was scrapped In my conversion which works very very well without any form of upward thrust. When jogging up and down with Mach 3 I need 1-4 x 0.01 increments in order to obtain a reversal of the head movement . This has not manifested itself as any problem while in use.

fit a full feed screw and dump all the z feed arrangements which I always considered to have too much lag and interface clearances to be effective. Your problems seem to confirm this

Edited By Zan on 05/03/2021 10:06:29

[Ron LadenParticipant @ronladen17547]

Is it not as simple as needing a new rack and pinion, if you have shimmed the rack and it still had backlash surely it must point to pinion being a bad fit to the rack. Is the pinion secure to its shaft, I have never had mine apart so don't know how it's engineered.

I went with 120N on my strut only because at the time of fitting I read somewhere that 150N is a bit too strong (don't know what that was based on) so with my 120N my head is heavy re balance.

Never had the sticking issue fortunately it's as smooth as you want it though a bit heavier upwards but not arm aching so. I agree with Jason to my simple brain I would have thought the head needs to sit heavy not equally balanced especially with the amount of backlash you have in the rack.

Just a thought straight off the top of my head but if the pinion is a bad fit in the rack and you shimmed the rack until it felt rough but still had backlash then I would have thought the tips of the pinion teeth are bottoming out but leaving gaps between the sides/mating faces of the teeth. I can't see where else the backlash could come from. If that is the case I would be tempted to reduce the o/d of the tips so the pinion sits deeper and mates with the rack The rack would need shimming again of course to make up the difference.

I could of course be wrong but with your explanation of the shimming and still having backlash that's what I imagined must be happening.

[ZanParticipant @zan]

rod Interesting! Just checked my junk box there’s a lot of interfaces in the system

1 the rack and pinion are well clear in their respective spaces, but the bottom of the pinion grooves seem to be rounded and could be causing interference.

2 the universal joint has about 0.04mm slack

3 the worm shaft can move in and out due to clearance prob 0.04mm

4 fit between worm and wheel ? Can’t determine

5 the dog clutch between wheel and pinion shaft 0.25mm?

6 clearance between rack and pinion unknown but it must be there.

conclusion A lot of interfaces all of which need clearance to work or clever expensive engineering to eliminate all contributing to the backlash . This makes me even more certain a z feed screw of some type is the way forward and ditch the gas strut and spring system as I did  I never lock the z , not a good idea anyway with cnc.  I have to use light cuts , nothing like Jason in his blogs, but the results are excellent and well worth the effort of converting the machine 

interesting. The rack is 25 wide. Why did they go to the expense of making the pinion gear 75 long? A bit of a waste the end could be reduced to below root dia and a bush inserted to reduce the hole diameter, or use a reduced hole..which to me seems cheaper.  I wonder???

 

edit.  
re sticking n my mods. I scraped and filed the casting flat, then scraped the dovetails to fit the column. This left the head mount planes out of parallel , so the top was milled true on the Bridgeport   The column was straight within 2 thou. Quite good!  I also had problems with the column  seat on the base casting. It had been attacked with a power sander and would not allow tramming  ( one corner was 30 odd thou below the others) also re machined on the big mill   See my write up of the full cnc conversion  in ME  about 3 years ago.  A lot of parts!

Edited By Zan on 05/03/2021 11:29:37

edit. Autotype errors

Edited By Zan on 05/03/2021 11:37:54

[Dr_GMJNParticipant @dr_gmjn]

Glad you posted that Zan, because I'd totally forgotten the backlash in the fine feed drive.

So there's no point doing the split rack, – saved me some hassle, but still no closer to a solution, unless I go for a ballscrew – or what about a leadscrew like on the x-y table? Could that be fitted with less drastic modification than turning the motor?

What's the best method for getting the handwheel horizontal?

What was the concensus about a ballscrew being back driven – I thought they could be?

Thanks.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dr_GMJN on 05/03/2021 12:13:15:

[…]

What was the concensus about a ballscrew being back driven – I thought they could be?

Thanks.

.

I don’t think we have consensus … but I did post a couple of relevant links, yesterday

MichaelG.

[Martin ConnellyParticipant @martinconnelly55370]

Ball screws can be backdriven but that is easy to avoid. Pinch the head dovetails slightly is one option, another is to put a fixed split plastic collar on the ball screw somewhere where it can be adjusted as needed to give enough braking to hold it against unwanted rotation. An energised stepper motor also has enough torque to hold them against being back driven but you may not want to go down that sort of route at the moment.

Martin C

[Dr_GMJNParticipant @dr_gmjn]

This has jogged my memory about ballscrews. Yes, they can easily be back-driven.

About 10 years ago we made a small platform about 600mm square, that could be raised parallel to its base. We used four ball screws, one in each corner. In order to prevent the platform lowering under its own weight, we incorporated a worm/pinion drive on one of the screws. This drove the other three via a timing belt drive. The worm was driven by a hand wheel.

We were amazed to find that despite the worm drive being specified well within its non-back-drive ratio, the platform lowered very gently and smoothly under its own weight. We ended up having a locking screw on the drive shaft.

Is it easier and cheaper to incorporate a conventional leadscrew and nut, similar to those on the x/y table. They can't be back-driven?

[Martin ConnellyParticipant @martinconnelly55370]

You are trying to avoid backlash so that the head is driven up or down predictably. Ball nuts and screws are better for this but standard leadscrews are harder but not impossible to back drive. So if you go the ball screw route you need to prevent back driving. If you go the leadscrew route you may need to be able to adjust out backlash.

Martin C

[Dr_GMJNParticipant @dr_gmjn]

Posted by Martin Connelly on 05/03/2021 15:00:37:

You are trying to avoid backlash so that the head is driven up or down predictably. Ball nuts and screws are better for this but standard leadscrews are harder but not impossible to back drive. So if you go the ball screw route you need to prevent back driving. If you go the leadscrew route you may need to be able to adjust out backlash.

Martin C

And unless I want to feed the head using a wheel stuck horizontally on the top of the machine, I'd need another gear system to re-orient the drive.

Chances are I won't bother with any of it to be honest, way too much hassle to start major modifications to the head. I just wanted a fairly quick fix that would reduce the issue if not eliminate it, but I guess if there were any out there, someone would have already done it.

Anyway, thanks all; it's been emotional. Hopefully I can now start some threads that don't have the word "issue" in the title…

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