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SX2P Gas Strut Modification Issue?

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Dr_GMJN05/10/2020 09:37:10
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1602 forum posts
Posted by oldvelo on 05/10/2020 02:50:29:

Hi

"As it stands, it’s just wrong to have the head forced up by default. It must be detrimental to accurate work - if you have a hand feed lever, you intuitively assume it’s limiting the tool position downwards. Currently, it’s basically moving the tool to its highest limit - all bets are off in terms of where it could end up in the workpiece".

"intuitively assume" are not terms consistent with precission engineering. To hold the head from being pulled into the job and elliminate the effect of the backlash is with a higher spring pressure to have the head held firmly UP at all times.

1 ) Backlash in an X2 is the clearance in the Rack and Pinion

2 ) The clearance in the Worm and Worm Wheel

3 ) End float on the worm to housing

4 ) Clearance on the Teeth on the Dog Clutch

5 ) Clearance on the Key and Keyway on the Pinion Shaft in The Dog Clutch

6 ) Two Universal Joints on the Worm Shaft Drive

All adds up to an excessive ammount of backlash. Around 3/4 of a full turn on my reasonably adjusted machine.

Eric

Thanks Eric.

What I meant by "intuitively assume" is that if I move a lever of handwheel in a certain direction, I assume that I'm moving the tool in a corresponding direction and setting a hard stop wherever I leave it. I'm assuming that it's not an arbitary position that can then move further under load. It has nothing to do with how I approach precision engineering (not that I pretend to be much good at it at present anyway), but more how I expect a tool to work. I agree that it's not possible to do consistent precision work like this, which is why I'm trying to solve the problem.

I don't think adding yet more upward load is the best solution, for a couple of reasons:

1) The magnitude of the extra load required to resist all likely downward loads generated by cutting is difficult to determine, and would effectively have to be a guess, erring on the high side - which is definitely not a phrase I'd associated with precision engineering!

2) It adds unecessary load on all the mechanisms resisting the weight of the head, and makes the hand lever and fine feed wheel stiffer and more difficult to use - Instead of using gravity to advantge, I'd be fighting against artificial load.

The backlash is annoying in term of feel, but I have a DRO, and use that for getting positions. So if the z-position was to a hard stop downwards, backlash would be irrelevant. It's also not relevant for any operation where I can lock the head.

A further assumption in all this, albeit one that seems valid from the simple checks I've done, is that the fine feed worm wheel drive is not back-driveable (further external down-load won't turn the worm gear). Any further downward movement of the head once set would be due to deflections (bending) within the machine.

Dr_GMJN05/10/2020 09:44:32
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1602 forum posts
Posted by Michael Gilligan on 04/10/2020 23:07:54:
Posted by not done it yet on 04/10/2020 22:01:37:

Adding weight to the head seems like the wrong way to solve the issue.

Cabinet Enforcer was not offering that option as a solution to your problem - only as a means of testing your theory. Little point in changing the strut if it is not the solution.

.

Agreed yes

MichaelG.

Apologies - I realised that, I was just thinking (or writing) out loud in terms of possible solutions.

Dr_GMJN05/10/2020 09:44:45
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1602 forum posts

Here's a video, which will hopefully work if you click on the image:



There must be about 5mm -10mm of movement after setting the depth.

oldvelo05/10/2020 18:39:05
297 forum posts
56 photos

Hi

Lets put all contradicting assumptions to one side they are the basis for misunderstanding and cause for many major disasters.

The plan in mind is to deal in facts only and avert such happenings.

The 5 mm 10 mm backlash that you demonstrate in the video is explained in my posting 1) to 5) Which is easily moved by light finger pressure this also allows a cutter that has not had the excessive lead backed off to drag the head down when machining Cast Iron, Bronze, or Brass.

The Worm Wheel does actually turn slightly as the clearances are taken up in this order 1) 5) 4) 3) 2) with a single start worm it is normaly self sustaining.

Perhaps the answer "using gravity to advantge" would be to add a 5kg-8kg weight to the cutting head to take out all the backlash when the head is being lowered and remove the weight when you need to raise the head.

Eric

Dr_GMJN11/12/2020 19:35:54
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1602 forum posts

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 Laden12/12/2020 06:25:35
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2320 forum posts
452 photos

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_GMJN12/12/2020 07:55:47
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1602 forum posts

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 Gilligan12/12/2020 09:24:05
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23121 forum posts
1360 photos
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.

HOWARDT12/12/2020 12:55:00
1081 forum posts
39 photos

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_GMJN12/12/2020 13:39:19
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1602 forum posts
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_GMJN12/12/2020 14:04:26
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1602 forum posts

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_GMJN01/03/2021 22:54:32
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1602 forum posts

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 301/03/2021 23:53:14
92 forum posts
12 photos

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_GMJN02/03/2021 00:03:11
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1602 forum posts
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 302/03/2021 00:32:20
92 forum posts
12 photos
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_GMJN02/03/2021 06:44:07
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1602 forum posts
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 Swali02/03/2021 16:28:31
1481 forum posts
149 photos
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 Connelly02/03/2021 16:50:16
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2549 forum posts
235 photos

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_GMJN02/03/2021 17:32:22
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1602 forum posts
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_GMJN02/03/2021 17:43:39
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1602 forum posts
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.


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