SX2P Gas Strut Modification Issue?

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

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.

'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.

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

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.

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

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?

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...

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.

.

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.

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.

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

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.

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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.

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.

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