SX2P Gas Strut Modification Issue?

[Dr_GMJNParticipant @dr_gmjn]

OK so I thought I’d update this one. In order to address the reverse backlash issue, I assumed a lower rated strut would be the answer, ie allowing the head to ‘fall’ onto the rack, but retaining enough vertical load to make moving the head upwards easy. Since I didn’t know what load the strut would have to be to achieve this, I bought an adjustable strut. Unfortunately, it was faulty on testing it – hissing noise and leaking fluid during the first few mm of travel. I believe they are sending a replacement. However, since I would release all pressure from it before returning, I thought I’d at least try it out. I did this by gradually releasing pressure until the head couldn’t be cracked downwards after the fine feed was released – ie eliminating the free play shown in the video I posted.

Basically this doesn’t work. Firstly, in order to allow the head to drop onto the rack, the residual friction in the slide is such that it requires the strut to be bled to give virtually no upload from the strut. Therefore the head drops under its own weight. There is no ‘sweet spot’ compromise – at least on my machine. Also, in this scenario, when fine feed is engaged, and the head moved downwards, there is a massive amount of ‘stick-slip’, which makes accurate work impossible. Eliminating this would mean pretty much having no strut assistance.

On initially bleeding the strut, I kept checking the feel. There was a point where the head could be moved downwards fairly easily, but the residual upload was such that it wasn’t possible for me to crack the head down (like in the video). When I get the replacement strut, I’m therefore going to de-rate it so that it once again pushes the head up, but much more than it did with the 150N strut. This makes pull-in to the work less likely.

In many ways I wish Id Just left the spring arm on!

I will report back with a strut rated to 150N +

[Ron LadenParticipant @ronladen17547]

I assume you just added the gas strut mod as it is what most SX2 owners do me included. I am also guessing that you didn't come across this problem with the spring arm as you had not done that much milling before changing to the strut..?

Assuming the above is correct why not put the spring arm back on and test to see if the spring gives the same problem, it would be interesting to know.

[Dr_GMJNParticipant @dr_gmjn]

Ron – As an inexperienced user, I fitted the strut really because it’s touted as an ‘essential’ mod by many/most owners who comment online. Believe it or not, as I was fitting it I actually thought the reverse backlash effect could be a problem, but since nobody had ever mentioned it, I thought I’d made a wrong assumption about this being an issue. After all “everyone” seemed to do it as a no-brainer mod (and 150N rated struts seemed to go-to spec for the SX2P).
 

I never used it with the spring apart from playing about when I first got it. All the spring components were shoved in a box and I think I might have used the axle for something else now. Of course they’re only a few £ to get another, but even so, I can’t see it having any effect other than having a rougher and less consistent action than a strut. As I remember it, the pin in slot gives a varying lever action which would change the level of assistance throughout the height. Either way, any form of counterweight will either give less or more load than needed to balance, or be spot on. From what I can see, less load is bad, since you end up needing a very smooth drop under gravity (doesn’t seem to have a smooth enough action for that with the gib adjusted for no play), or what I’m now going for, which is excess load which I hope will be more than enough to resist any tool grab movement.

The real fix I suppose is to have a zero backlash rack and pinion, or leadscrew, but I really can’t be bothered with that. I was wondering if I could fit some stiff Belville (sp?) washers behind the rack, so as to be able to adjust the rack to be preloaded against the pinion? I assume the down load from a tool grab potentially wouldn’t be enough to push the rack back against spring load, especially since – intuitively at least – the wedge angle of the teeth is working in your favour when the forces are resolved.

Edited By Dr_GMJN on 12/12/2020 07:56:33

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dr_GMJN on 12/12/2020 07:55:47:

[…]

After all “everyone” seemed to do it as a no-brainer mod

.

That may be the problem ^^^

[ interpretation of the term no-brainer ]

”everyone” is sometimes sadly deficient in the cerebral area

MichaelG.

[HOWARDTParticipant @howardt]

When I decided to convert to gas strut I weighed the head at around 15kg. My assembly uses two 100N struts with a 2:1 increase in upward force, so 400N. Theoretically this should counter balance about 40kg. I mainly did this to get a better feel than the spring gave and am pleased with the result.

When drilling the force is mostly up so mostly no problem, however when milling the complex cutting forces are caused by the cut material acting on the helix angle of the flute. The blunter the tool the greater the force trying to drag the tool down, obviously there are other factors such as diameter etc.

When milling and drilling the head will normally be pushed up against the tooth of the rack, but there is enough free play in the both feed systems to allow some downward movement if the tool digs in unless the head is locked in position or the counterbalance load is big enough to be greater than the cutting force. On production machines constantly moving vertical slides will have a counterbalance system, probably hydraulic, to reduce the load and wear on the ball screw not to prevent adverse movement, that is what the ball screw and nut tolerance does. If you have too much free play between the ball screw and nut you will still get adverse movement.

So to sum up, my opinion, the strut assembly gives better feel to the movement of the head and reduces the feel of the backlash in the system as the head is always held upwards.. To reduce the tendency for a tool to grab during a downward feed both the coarse and fine feed assemblies need to be attended to, something i don’t think any one has done yet.

[Dr_GMJNParticipant @dr_gmjn]

Posted by HOWARDT on 12/12/2020 12:55:00:

When I decided to convert to gas strut I weighed the head at around 15kg. My assembly uses two 100N struts with a 2:1 increase in upward force, so 400N. Theoretically this should counter balance about 40kg. I mainly did this to get a better feel than the spring gave and am pleased with the result.

When drilling the force is mostly up so mostly no problem, however when milling the complex cutting forces are caused by the cut material acting on the helix angle of the flute. The blunter the tool the greater the force trying to drag the tool down, obviously there are other factors such as diameter etc.

When milling and drilling the head will normally be pushed up against the tooth of the rack, but there is enough free play in the both feed systems to allow some downward movement if the tool digs in unless the head is locked in position or the counterbalance load is big enough to be greater than the cutting force. On production machines constantly moving vertical slides will have a counterbalance system, probably hydraulic, to reduce the load and wear on the ball screw not to prevent adverse movement, that is what the ball screw and nut tolerance does. If you have too much free play between the ball screw and nut you will still get adverse movement.

So to sum up, my opinion, the strut assembly gives better feel to the movement of the head and reduces the feel of the backlash in the system as the head is always held upwards.. To reduce the tendency for a tool to grab during a downward feed both the coarse and fine feed assemblies need to be attended to, something i don’t think any one has done yet.

The adjustable gas strut I have is pressurised to give 400N before any adjustment.

Having tried it in this state last night, I think it might work as supplied. Yes, it obviously needs more downward load on the levers to overcome the residual upload, but it didn't seem too bad.

I'm using cutters no greater than 12mm diameter for the work I'm doing. Do you think that a tool grab would overcome that residual c.25kg (plus friction) ?

I'd still be interested in opinions on the idea of using Belleville washers to pre-load the rack against the pinion.

Thanks.

[Dr_GMJNParticipant @dr_gmjn]

Or…since the issue is only really relevant when plunge milling, could I:

Temporarily hang a weight on the head to overcome the 'up-lash'

or

Temporarily disconnect the strut?

When using the mill for these operations.

I don't want to get into extensive modifications to the machine – I'm not up for that at the moment.

[Dr_GMJNParticipant @dr_gmjn]

So after much head scratching, I came up with what I thought was the simplest and cheapest of several admittedly heavily compromised solutions:

When I need to increment downwards while cutting (counterboring, drilling, spot-facing), I decided to pre-load the head downwards using a spring. I used a suitcase scale to get the load required to always counteract the residual upward load from the gas-strut, plus an excess to prevent the stick-slip effect of the dovetails etc.

I found the perfect tension spring online for a few quid, which unbelievably turned into £40 taking into account postage, and a "handling charge". Somewhat stumped, I then realised that the answer was staring me in the face, in the form of the £5 suitcase scale. So I set to work drilling the travel limit block and the head to take each end of the balance:

It's got the added advantage of the scale which enables me to confirm that I've got the minimum residual load applied through out the depth of whatever I'm doing.

Technique is to lower the limit block to an appropriate level, lock it, then lower the head and fit the scale, raise the head until the dial reads enough, then engage fine feed.

Lowering using fine feed then gives a smooth, backlash-free movement until the load decays to about 20kg on the dial, at which point I know I'm approaching potential trouble.

Ok it's a bit clunky, but it's cheap, but you can't polish a turd.

BTW, In future, I'm considering lining the z-axis dovetails with Trelleborg slideway strips, to try and reduce the sticking issue.; there might be enough gib strip adjustment to fit the thinner version.
 

Edited By Dr_GMJN on 01/03/2021 23:04:24

[James Hall 3Participant @jameshall3]

Not a problem with the Honda Accord which the insurance co. paid for after the old one was written off in a crump – and which, when it arrived, looked for some reason remarkably like an SX3!

No gas strut, of course, and you have to do all the work raising it yourself, but consequently no backlash problems – limited to 0.02mm, presumably in the worm gears. I'm only a newby so am perhaps wrong, but if milling requiring Z-axis precision I lock the column and quill in any case; drilling would have to exert a lot of force to push that heavy head upwards.

[Dr_GMJNParticipant @dr_gmjn]

Posted by James Hall 3 on 01/03/2021 23:53:14:

Not a problem with the Honda Accord which the insurance co. paid for after the old one was written off in a crump – and which, when it arrived, looked for some reason remarkably like an SX3!

No gas strut, of course, and you have to do all the work raising it yourself, but consequently no backlash problems – limited to 0.02mm, presumably in the worm gears. I'm only a newby so am perhaps wrong, but if milling requiring Z-axis precision I lock the column and quill in any case; drilling would have to exert a lot of force to push that heavy head upwards.

How does locking the z-axis help me to drill, spot-face or counterbore something?

Why would the drill pushing the head up be an issue for pull-in?

[James Hall 3Participant @jameshall3]

Posted by Dr_GMJN on 02/03/2021 00:03:11:

Posted by James Hall 3 on 01/03/2021 23:53:14:

Not a problem with the Honda Accord which the insurance co. paid for after the old one was written off in a crump – and which, when it arrived, looked for some reason remarkably like an SX3!

No gas strut, of course, and you have to do all the work raising it yourself, but consequently no backlash problems – limited to 0.02mm, presumably in the worm gears. I'm only a newby so am perhaps wrong, but if milling requiring Z-axis precision I lock the column and quill in any case; drilling would have to exert a lot of force to push that heavy head upwards.

How does locking the z-axis help me to drill, spot-face or counterbore something?

Why would the drill pushing the head up be an issue for pull-in?

I suggest you re-read my post, more carefully this time.

[Dr_GMJNParticipant @dr_gmjn]

Posted by James Hall 3 on 02/03/2021 00:32:20:

Posted by Dr_GMJN on 02/03/2021 00:03:11:

Posted by James Hall 3 on 01/03/2021 23:53:14:

Not a problem with the Honda Accord which the insurance co. paid for after the old one was written off in a crump – and which, when it arrived, looked for some reason remarkably like an SX3!

No gas strut, of course, and you have to do all the work raising it yourself, but consequently no backlash problems – limited to 0.02mm, presumably in the worm gears. I'm only a newby so am perhaps wrong, but if milling requiring Z-axis precision I lock the column and quill in any case; drilling would have to exert a lot of force to push that heavy head upwards.

How does locking the z-axis help me to drill, spot-face or counterbore something?

Why would the drill pushing the head up be an issue for pull-in?

I suggest you re-read my post, more carefully this time.

All you’ve said is that you have a different machine to me, that you have no issues milling with the z-axis locked, and that drilling would need a lot of force to push the head up.

Not sure what point you’re trying to make?

[Ketan SwaliParticipant @ketanswali79440]

Posted by Dr_GMJN on 01/03/2021 22:54:32:

BTW, In future, I'm considering lining the z-axis dovetails with Trelleborg slideway strips, to try and reduce the sticking issue.; there might be enough gib strip adjustment to fit the thinner version.

Do you mean the use of Turcite?… We have used it in the past on a very small number/selected few X3, SX3, KX3 and SX4, during a re-build process.

If that is what you are thinking of doing, depending on how you intend to approach the subject, the process is a little complicated and time consuming, and unsure how useful it would be on an SX2, as never used it on any re-builds of this machine.

You are likely to consider machining of the gib – to which turcite is applied, along with possible machining of the head casting dovetail (to which turcite is applied), using a 55 deg. dovetail cutter (which is not an easily available cutter).

The Turcite strips are not very thin, and they need to be bonded using a good two-part epoxy. Once bonded, there will be high spots, which will need to be scraped, or milled flat. So, to some extent you will need the turcite to be thinker than where you intend to end up. How easy will it be to scrape or mill the turcite in the casting dovetail is something you have to consider/experiment with on sacrificial material before working on the main dovetails.

A chap called Tom Chapman introduced the late JS and I to the idea back in 2009 after carrying out the process on his KX3. It took us a while to experiment with it, and we tried it out after being shown how to do it properly by Birmingham Machine Tool Services – who used to use Turcite in the re-build process of Bridgeports. We used to purchase the Turcite from them. We re-built very few machines using Turcite, and it is a very good product, with great end results. However, as I said before, I am unsure about how useful it will be for your use.

If you want a pdf of Toms approach for his KX3, send me a pm with your email address and I will email it to you.

Ketan at ARC

[Martin ConnellyParticipant @martinconnelly55370]

Milling machines are usually set up to rest their spindle or head weight on the down feed gearing to prevent further downwards movement under cutting forces. I think most gas struts are also non-linear, the force they exert depends on the amount they are compressed. Gas struts may be of some use on machines where the head is raised and lowered for general positioning then locked and the spindle is then moved to do the drilling/milling/spot facing/counterboring. If the machine has a fixed spindle and the only adjustment is by moving the head then I would suggest they are not a good idea. It sounds like you are now doing a modification to correct the result of a modification you did.

Martin C

[Dr_GMJNParticipant @dr_gmjn]

Posted by Ketan Swali on 02/03/2021 16:28:31:

Posted by Dr_GMJN on 01/03/2021 22:54:32:

BTW, In future, I'm considering lining the z-axis dovetails with Trelleborg slideway strips, to try and reduce the sticking issue.; there might be enough gib strip adjustment to fit the thinner version.

Do you mean the use of Turcite?… We have used it in the past on a very small number/selected few X3, SX3, KX3 and SX4, during a re-build process.

If that is what you are thinking of doing, depending on how you intend to approach the subject, the process is a little complicated and time consuming, and unsure how useful it would be on an SX2, as never used it on any re-builds of this machine.

You are likely to consider machining of the gib – to which turcite is applied, along with possible machining of the head casting dovetail (to which turcite is applied), using a 55 deg. dovetail cutter (which is not an easily available cutter).

The Turcite strips are not very thin, and they need to be bonded using a good two-part epoxy. Once bonded, there will be high spots, which will need to be scraped, or milled flat. So, to some extent you will need the turcite to be thinker than where you intend to end up. How easy will it be to scrape or mill the turcite in the casting dovetail is something you have to consider/experiment with on sacrificial material before working on the main dovetails.

A chap called Tom Chapman introduced the late JS and I to the idea back in 2009 after carrying out the process on his KX3. It took us a while to experiment with it, and we tried it out after being shown how to do it properly by Birmingham Machine Tool Services – who used to use Turcite in the re-build process of Bridgeports. We used to purchase the Turcite from them. We re-built very few machines using Turcite, and it is a very good product, with great end results. However, as I said before, I am unsure about how useful it will be for your use.

If you want a pdf of Toms approach for his KX3, send me a pm with your email address and I will email it to you.

Ketan at ARC

Thanks Ketan, in that case I’ll leave it . It does what I need it to do, albeit not very smoothly.

[Dr_GMJNParticipant @dr_gmjn]

Posted by Martin Connelly on 02/03/2021 16:50:16:

Milling machines are usually set up to rest their spindle or head weight on the down feed gearing to prevent further downwards movement under cutting forces. I think most gas struts are also non-linear, the force they exert depends on the amount they are compressed. Gas struts may be of some use on machines where the head is raised and lowered for general positioning then locked and the spindle is then moved to do the drilling/milling/spot facing/counterboring. If the machine has a fixed spindle and the only adjustment is by moving the head then I would suggest they are not a good idea. It sounds like you are now doing a modification to correct the result of a modification you did.

Martin C

Not really, because as I said, if the gas strut is removed, the head drops unless the fine feed is engaged, and if fine feed is engaged with no gas strut I get chronic stick-slip when incrementing down.

Retaining the gas strut gives a good feel to moving the head up and down, and the additional compression spring download for fine feed (from the scale) gets rid of the stick-slip. I can also confirm with reference to the scale dial that the load is consistently reducing as I’m fine feeding down. If the needle stops while I’m turning the knob, I know something is sticking.

I’ve also wasted £20 on an adjustable gas strut, and bled it to try and get a slight residual down load, but not enough for the head to stop suddenly. This was do-able, but then the stick-slip issue immediately returned.

So I’m not modifying a modification, because I’ve tried removing the initial modification and that just throws up another fundamental problem.

It is what it is.

[ZebethyalParticipant @zebethyal]

I watched your video and then went out and checked my SX2 as I was concerned that my mill may be suffering the same as yours and I simply hadn't noticed it.

I have a 150N (15Kg) gas spring and wherever I position the head, it will sit there with no pressure on the gib lock, head weight (15Kg) and spring pressure equal each other out, upwards or downwards movement, from any location, require equal degrees of effort.

If I engage the fine feed, then there is about half a turn of worm backlash, but once that has been taken up I cannot move the head either up or down.

I perform milling operations with the Z axis locked, boring operations with the fine feed and some drag on the gib lock and drilling operations with the gib lock loose. Any machining issues I may have had have always been down to incorrect feeds and speeds rather than any movement in the mill.

I believe based on your newer motor that your head weighs the same as mine and therefore a 150N gas spring is the correct size, also gas springs offer a constant level of resistance, across their entire range of travel, in either direction (unlike the original spring).

The issue on your mill appears to be with the relationship between the rack and the worm.

[Ron LadenParticipant @ronladen17547]

Posted by Zebethyal on 03/03/2021 10:45:44:

I watched your video and then went out and checked my SX2 as I was concerned that my mill may be suffering the same as yours and I simply hadn't noticed it.

I have a 150N (15Kg) gas spring and wherever I position the head, it will sit there with no pressure on the gib lock, head weight (15Kg) and spring pressure equal each other out, upwards or downwards movement, from any location, require equal degrees of effort.

If I engage the fine feed, then there is about half a turn of worm backlash, but once that has been taken up I cannot move the head either up or down.

I perform milling operations with the Z axis locked, boring operations with the fine feed and some drag on the gib lock and drilling operations with the gib lock loose. Any machining issues I may have had have always been down to incorrect feeds and speeds rather than any movement in the mill.

I believe based on your newer motor that your head weighs the same as mine and therefore a 150N gas spring is the correct size, also gas springs offer a constant level of resistance, across their entire range of travel, in either direction (unlike the original spring).

The issue on your mill appears to be with the relationship between the rack and the worm.

"therefore 150N gas spring is the correct size"

Is it..? I fitted my SX2 with a 120N it is equally balanced and I have never had any machining issues with it.

Edited By Ron Laden on 03/03/2021 11:18:06

[Dr_GMJNParticipant @dr_gmjn]

All the worm does is step down the gear ratio between your hand and the rack pinion.

It's the gap between the pinion and the rack that's the issue; that gap can be taken up by a tool grab, and the only thing preventing it is the very small residual upward load on the head (as you say, a 15kg strut balances it very nicely).

Reduce that residual upload until its a slight down load – ie taking up all the gap that a tool grab could pull through, and you get laughable stick-slip, which is even worse that the initial problem.

Increase pinion-rack engagement to reduce backlash and you get terrible notchy movement.

Lose-lose.

What it needs is a redesigned z-axis with a zero backlash method of head movement. If it was simple, I'd do it, because the rest of the mill is pretty good considering the price. In fact the whole thing is more than good enough to do decent work considering the price, it's just that things like feel/hassle of operation really started to piss me off.

[ZebethyalParticipant @zebethyal]

Posted by Ron Laden on 03/03/2021 11:16:36:

Posted by Zebethyal on 03/03/2021 10:45:44:

I watched your video and then went out and checked my SX2 as I was concerned that my mill may be suffering the same as yours and I simply hadn't noticed it.

I have a 150N (15Kg) gas spring and wherever I position the head, it will sit there with no pressure on the gib lock, head weight (15Kg) and spring pressure equal each other out, upwards or downwards movement, from any location, require equal degrees of effort.

If I engage the fine feed, then there is about half a turn of worm backlash, but once that has been taken up I cannot move the head either up or down.

I perform milling operations with the Z axis locked, boring operations with the fine feed and some drag on the gib lock and drilling operations with the gib lock loose. Any machining issues I may have had have always been down to incorrect feeds and speeds rather than any movement in the mill.

I believe based on your newer motor that your head weighs the same as mine and therefore a 150N gas spring is the correct size, also gas springs offer a constant level of resistance, across their entire range of travel, in either direction (unlike the original spring).

The issue on your mill appears to be with the relationship between the rack and the worm.

"therefore 150N gas spring is the correct size"

Is it..? I fitted my SX2 with a 120N it is equally balanced and I have never had any machining issues with it.

Edited By Ron Laden on 03/03/2021 11:18:06

'within the acceptable range' then.

There were comments that the OP should use a 120N spring instead of a 150N, someone else (with an older heavier motor) is using 2 x 100N, so there is obviously a fairly wide range that works, in which case multiple individuals are equally amiss claiming/implying that their spring force may be the 'correct' one, potentially yourself included.

Regardless of spring force used, the root cause, as just stated by the OP, is with the rack/pinion interaction, something I do not appear to be experiencing on my mill. Maybe I have slightly better engagement between my rack and pinion – would a new pinion gear possibly reduce/remove the issue?

[Dr_GMJNParticipant @dr_gmjn]

Posted by Zebethyal on 03/03/2021 11:53:34:

Posted by Ron Laden on 03/03/2021 11:16:36:

Posted by Zebethyal on 03/03/2021 10:45:44:

I watched your video and then went out and checked my SX2 as I was concerned that my mill may be suffering the same as yours and I simply hadn't noticed it.

I have a 150N (15Kg) gas spring and wherever I position the head, it will sit there with no pressure on the gib lock, head weight (15Kg) and spring pressure equal each other out, upwards or downwards movement, from any location, require equal degrees of effort.

If I engage the fine feed, then there is about half a turn of worm backlash, but once that has been taken up I cannot move the head either up or down.

I perform milling operations with the Z axis locked, boring operations with the fine feed and some drag on the gib lock and drilling operations with the gib lock loose. Any machining issues I may have had have always been down to incorrect feeds and speeds rather than any movement in the mill.

I believe based on your newer motor that your head weighs the same as mine and therefore a 150N gas spring is the correct size, also gas springs offer a constant level of resistance, across their entire range of travel, in either direction (unlike the original spring).

The issue on your mill appears to be with the relationship between the rack and the worm.

"therefore 150N gas spring is the correct size"

Is it..? I fitted my SX2 with a 120N it is equally balanced and I have never had any machining issues with it.

Edited By Ron Laden on 03/03/2021 11:18:06

'within the acceptable range' then.

There were comments that the OP should use a 120N spring instead of a 150N, someone else (with an older heavier motor) is using 2 x 100N, so there is obviously a fairly wide range that works, in which case multiple individuals are equally amiss claiming/implying that their spring force may be the 'correct' one, potentially yourself included.

Regardless of spring force used, the root cause, as just stated by the OP, is with the rack/pinion interaction, something I do not appear to be experiencing on my mill. Maybe I have slightly better engagement between my rack and pinion – would a new pinion gear possibly reduce/remove the issue?

A decent quality rack and pinion would certainly help. Positive backlash-free control would solve the issue straight away. I think someone even mentioned splitting the rack down the middle, and moving one side up and the other down to get rid of backlash (I may have imagined that though).

I also thought of putting belville washers behind the rack to load it against the pinion, but that would not be very rigid, and would probably only serve to make the movement as rough as when I tried shimming the rack further into engagement.

[Martin ConnellyParticipant @martinconnelly55370]

What slideway lubricant are you using for the z axis movement? A properly formulated product should be used to avoid stiction, I use Rocol Ultraglide. A little bit goes a long way so it may seem expensive but it lasts a long time.

Martin C

[HOWARDTParticipant @howardt]

You could replace the straight rack and pinion with a helical one. Ideally the helix angle should be enough to give at least two, preferably three teeth in contact. This would mean making some parts as the gear s part of the shaft. Also the gear shaft runs in the cast iron housing so some form of bearing would be better fit. There would still be slack in the worm drive to take care of.

I packed the rack out on mine to reduce the clearance and feed down with the gib lock slightly nipped.

[Ron LadenParticipant @ronladen17547]

Posted by Zebethyal on 03/03/2021 11:53:34:

Posted by Ron Laden on 03/03/2021 11:16:36:

Posted by Zebethyal on 03/03/2021 10:45:44:

I watched your video and then went out and checked my SX2 as I was concerned that my mill may be suffering the same as yours and I simply hadn't noticed it.

I have a 150N (15Kg) gas spring and wherever I position the head, it will sit there with no pressure on the gib lock, head weight (15Kg) and spring pressure equal each other out, upwards or downwards movement, from any location, require equal degrees of effort.

If I engage the fine feed, then there is about half a turn of worm backlash, but once that has been taken up I cannot move the head either up or down.

I perform milling operations with the Z axis locked, boring operations with the fine feed and some drag on the gib lock and drilling operations with the gib lock loose. Any machining issues I may have had have always been down to incorrect feeds and speeds rather than any movement in the mill.

I believe based on your newer motor that your head weighs the same as mine and therefore a 150N gas spring is the correct size, also gas springs offer a constant level of resistance, across their entire range of travel, in either direction (unlike the original spring).

The issue on your mill appears to be with the relationship between the rack and the worm.

"therefore 150N gas spring is the correct size"

Is it..? I fitted my SX2 with a 120N it is equally balanced and I have never had any machining issues with it.

Edited By Ron Laden on 03/03/2021 11:18:06

'within the acceptable range' then.

There were comments that the OP should use a 120N spring instead of a 150N, someone else (with an older heavier motor) is using 2 x 100N, so there is obviously a fairly wide range that works, in which case multiple individuals are equally amiss claiming/implying that their spring force may be the 'correct' one, potentially yourself included.

​​​​Just to be clear I didn't claim 120N is the correct size, it is for my machine but I made no suggestion it was the size others should be using. To be fair it was you that said 150N is the "correct" size.

Regardless of spring force used, the root cause, as just stated by the OP, is with the rack/pinion interaction, something I do not appear to be experiencing on my mill. Maybe I have slightly better engagement between my rack and pinion – would a new pinion gear possibly reduce/remove the issue?

 

Edited By Ron Laden on 03/03/2021 12:58:12

[Dr_GMJNParticipant @dr_gmjn]

I’ve been using Slidex lubricant, and have recently got some slideway oil from Pennine Lubricants. Doesn’t make any difference.

Is splitting the rack and offsetting it a daft idea?

[Author]

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