Gib Strip Fettling

[JasonB]

On JasonB Said:

Michael, were you hoping for some words about the gibs in that CNC post of John’s? I don’t think he got much further but would probably have milled slots into the gibs across their width.

Yes I was, Jason …

My thinking [such as it was] was that any machine worth the effort of adapting to CNC *must* be capable of being fettled to a reasonable standard … and I was presuming that JS might have addressed any major issues.

The fact that he did not leaves a lot of things open to question: primarily the Quality Control standards of their production <sigh>

Glad you found it anyway … Thanks

MichaelG.

[Dr_GMJNParticipant @dr_gmjn]

Thanks Jason. I think the issue isn’t necessarily accuracy, rather the feel of the machine while achieving that accuracy. Going from the ML7 to the mill highlights this. If I adjust the mill to what I consider an acceptable amount of play/backlash in the axes, the handwheels become sticky. The z-axis is a bit of a joke, but that was the subject of an entire different thread in the past. The outcome was there’s no way to overcome the play/backlash in the rack/pinion/‘fine’ adjustment without major surgery. I seem to periodically go around in circles looking at tooling or technique to overcome machining issues (poor surface finish in y for example), which I believe could be at least partly down to the machine. Looking at the design, the machining and finish on the parts, and how it’s assembled, I could imagine that there’s quite a variation in how each individual machine feels. A ‘good’ one might be very, very good, but a poor one…?
Yes, the ML7 had a few issues too, but the difference there is that I was able to correct most of them to a satisfactory degree fairly easily. This was up to and including measuring, filing and flatting the ways by hand to eliminate binding caused by 50-odd years of wear. It feels like silk compared with the mill, and makes me actually want to machine things, the prospect of using the mill often makes me want to go and build my plastic kits.

[JasonBModerator @jasonb]

Probably why a new Myford is starts at 4 to 5 times the cost of a similar sized imported lathe.

I only have a Myford Vertical slide to compare and that does move nicely but is not a million miles from my X3 which is quite free moving, probably more so than many would set their machine at. My X3 has also seen a lot more use than your machine, I probably make the equivelent to 3 or 4 twin Victorias in a year and have been like that for 17yrs.

Not done much plastic or figure modeling since the mill arrived and I got more into Model Engineering.

[JasonBModerator @jasonb]

Here you go.

X3 Mill with table approx mid travel in X & Y

Measuring approx 300mm away from spindle

Freely rotating handles particularly in X-axis as that is the one I tend to use the most

Good hard push and pull on the end of the table shows about 0.003″ per foot

Lock X and that movement halves, also lock Y and no movement. So generally when cutting and the non moving axis is locked that is 0.0015″ / ft

Backlash approx 15thou in X and 20thou in Y but as I have a DRO I’m not bothered.

You have all seen the models that come off it so fit for purpose as far as I’m conserned

 

[Michael GilliganParticipant @michaelgilligan61133]

Helpful demo. as usual, Jason

I do have to wonder whether “17 years ago” the quality control on such machines was better than it has been in recent years.

My previous comment about Cottage Industry is pertinent, I think … In the old days, the Journeyman [if that’s the right term] would collect and deliver part-built stuff from one cottage to the next, where different craft skills were applied.

It would be his responsibility to accept or reject work.

Such practices worked very well in the Horological trade, and many others … but constant vigilance is essential, or standards can plummet.

MichaelG.

[Dr_GMJNParticipant @dr_gmjn]

Don’t forget also that Jason’s mill is different from mine (larger among other things). I believe the z-axis operation is completely different?

[Dr_GMJN]

On Dr_GMJN Said:

Thanks Jason. I think the issue isn’t necessarily accuracy, rather the feel of the machine while achieving that accuracy. Going from the ML7 to the mill highlights this. If I adjust the mill to what I consider an acceptable amount of play/backlash in the axes, the handwheels become sticky. The z-axis is a bit of a joke, but that was the subject of an entire different thread in the past. The outcome was there’s no way to overcome the play/backlash in the rack/pinion/‘fine’ adjustment without major surgery. I seem to periodically go around in circles looking at tooling or technique to overcome machining issues (poor surface finish in y for example), which I believe could be at least partly down to the machine. …

I wonder if Dr_GMJN is expecting too much of his hobby machine?   The evidence is contradictory: poor finish in Y suggests a fault, but ‘the feel of the machine‘ is subjective.   The ‘feel’ of my WM18 mill is acceptable rather than good, but maybe I have unduly low expectations.  All I can say is that the machine produces the results I expect of it.

Dr_GMJN’s comments about backlash worry me slightly.  Far Eastern and many pricier Western machine tools only implement the most basic anti-backlash mechanism.   They allow backlash to be reduced rather than eliminated, expecting the operator to compensate for what remains.   A cheap simple and mostly effective compromise.  A booby trap is that an operator who constantly tightens the adjuster to minimise backlash causes rapid mechanical wear.    The law of reducing returns applies,  and the owner either accepts limitation, or upgrades the entire mechanism!   And here’s the bite:  effective anti-backlash mechanisms ain’t cheap or easy, which is why they tend to be found only on big money machines.

Fortunately, there’s an excellent alternative!  The best way to do manage backlash is with a DRO, a device that eliminates many other dial-related operator errors.  Fitting the cheapest model available means I don’t worry about backlash at all – it’s irrelevant!

Other compromises can be seen in other aspects of machine tool design.   Gibs are a simple method of adjusting sliding ways, their chief disadvantage being they are fiddly to adjust, sometimes devilishly perverse!   Fitting Brass Gibs to mini-lathes was a popular upgrade,   that I suspect resulted in much bad language and not much improvement!    Even the best lead-screw is inferior to a ball-screw, and on an older machine lead-screws are unlikely to be as accurate as their proud owners imagine!  Most lathe gearboxes are of the simplest possible construction, and even expensive lathes may be fitted with rather ordinary headstock bearings.   The bed may be hardened, or not, by a variety of more-or-less effective techniques.   Ways are sometimes hand-scraped to improve oil-retention or flatness, but even in the good old days this was often an outright con, designed to create the illusion that a master-craftsman had finished the machine to the highest possible standard.    The list is endless:  fudged castings, Mazak fittings, under-rated motors, manual lubrication etc etc etc.

No compromise machines are available, but seriously expensive.   Prices start at 6x hobby machine prices, and 30x or more is not unusual.

My view of machine tools is they are what they are!   My Far Eastern tools all worked out of the box.   The mini-lathe benefited from some mild fettling, and I made a couple of small improvements to the WM280.   The WM18 is untouched, apart from adding a DRO.

None of my machines are idiot proof in the sense they do exactly what I want simply by twirling the dials!  They all demand a certain level of skill.  When things go wrong, I check my three main suspects.   They are:

• Me.   Even when this chap knows what he’s doing, he makes a lot of mistakes.   As success depends enormously on me getting it right, I find it pays to double check measurements and assumptions.   Ego and over-confidence being common human failings make it all too tempting to blame the machine, when the truth is that remarkably good work is done by skilled machinists on badly worn old kit and wonky hobby gear!   This requires understanding tools, cutting speeds, materials, machine limitations, measuring, workholding, and recognising issues etc.  There is an enormous amount to learn, and mechanical skills require a lot of practice.
• Material.   Not all metals machine well, and many are downright evil.  After a very bad experience with a totally unsuitable junkbox, I’ve found buying in known free-cutting metal saves a lot of grief.
• Machine.    Yes, these can be worn, mis-assembled, broken, badly installed, corroded, rough as old boots, abused,  poorly  maintained, tarted up by Coco The Clown,  or repaired by Billy Bodger!    But in my experience, the machine is the least likely suspect when things go wrong.    Don’t dismiss the possibility though, especially if the machine was dismantled to improve it by a newcomer.     Although not rocket science, they don’t come apart and go back together in an entirely straightforward way.   I recommend using the machine to cut metal for some months before messing with it, and then only making one tweak at a time.  If cutting metal with a new machine suggests an obvious fault, contact the supplier first!

Hobby machines are fine if you just want to cut metal in an ordinary unhurried way, accurate to ±0.02mm (thou), and are prepared to work within their limitations.   Not a good choice for those need to work accurately in a hurry, where it’s worth paying for a well-made machine and keeping it in tip-top condition.   Or maybe buying a hobby mill and upgrading it thoroughly, expense no object…

Dave

 

[JasonBModerator @jasonb]

Yes it’s a different machine and why I did not show Z play. But posted as an example of what sort of play can be tolerated and still get good results.

Anyone want to wobble the end of the table of their VMC, Senior or Bridgeport and report back?

[Dr_GMJNParticipant @dr_gmjn]

Strangely enough we have a medium sized old Bridgeport at work, and a 1950’s Colchester lathe. They were bought as examples of ‘old school’ machines, and how they can be improved by modern sensoring and monitoring systems (they aren’t re-furbished as far as I know). They were probably the most battered examples available from the local used tool supplier. You can imagine how they look next to sate-of-the-art manufacturing systems, in a clinically clean setting.

I had a play with the slides today on both machines and they felt smooth as silk. They must be horribly worn, and I had no means of establishing free play (I couldn’t feel or hear any) but the feel was very nice.

To comment on a previous post:

Ive previously backed-off the backlash plates in order to eliminate excessive stiffness there.

I fitted DROs when I got the machine. Without those, at my skill level I couldn’t use it for anything other than cutting metal to rough shapes.

[Tony Pratt 1Participant @tonypratt1]

The Bridgeport and Colchester lathe would have been made with a lot of care to a high standard of accuracy and checked at every stage of manufacture. The hobby machines we buy these days have none of this shall we say attention to detail so problems occur such as the ‘gib strip issue’ but the machines are affordable!

P.S My Warco 290V is a lovely machine which cuts perfectly parallel but scratch the surface and it’s not pretty.

Tony

[JasonBModerator @jasonb]

Yes I was just looking at the BCA link Michael posted yesterday. one of those in 2000 would have cost £10,000. Looking back at a couple of MEWs from that time there are no adverts for a similar size machine. So in 2007 when I bought my X3 for £849 (would have been cheaper from ARC) an X2 size mill was about £450.

It’s a bit like expecting a Halfords £99 mountain bike to ride like a £10K top spec one from a respected brand or that Dacia to be like a Porsche.

Interestingly they also used the simple split feed nuts and jacking screws that these machines have

Yes they are a bit rough around the edges as Tony says but learn to use them and they can produce good work, In my case I have to judge that by being able to win medals & awards or people wanting me to make models for them as they like what they have seen.

There is also some skill involved that will only come with time, not going into the workshop for months at a time will set you back a bit and it will take a while to get back up to speed.

I do also feel there is too much of a tendancy to work to numbers. It is so easy now to pick up the latest digital  measuring device and just because it reads to X No of decimal places think you need to hit that number. When I started with the Unimat 3 I did not even have a dti after a year or two I made one of these which together withs  100mm tri square that I made at school was all I had to set up work or the machine. Still managed to make a 10V that I can blow into now and it runs.

[Dr_GMJNParticipant @dr_gmjn]

Re. the Bridgeport, I was simply commenting on a question someone asked about free play on that machine. I obviously don’t expect that quality of build at all on a cheap machine.

Re. the figures Jason is getting. I’ve spent a couple of hours this afternoon rebuilding the x/y stage and so far have got nothing like as good as that. For fairly smooth running hand wheels, I’m getting more than 0.010” side-to-side play (rattle), around 250mm from the axis.

I will persevere a bit longer, but really this is the whole point of what I’m saying – surely with a smaller mill of similar origin, I should easily be getting better figures than a larger one, not worse?

[JasonBModerator @jasonb]

If we assume both need a similar amount of clearance in the ways then you WILL get more movement on a smaller mill with shorter dovetails than a larger mill with it’s larger dovetails. That applies to any cost or quality.

Are you setting the gibs with the lead screws engaged or not? if things are tightening up as you adjust the handwheels then the problem is unlikely to be in the gibs

One other thing that I don’t recall seeing mentioned is are the gibs narrower width wise than the space they go into. I have seen pictured where the gib made contact top and bottom before it made contact with the ways due to being too wide.

[Dr_GMJNParticipant @dr_gmjn]

Jason,

I’m assuming smaller tools (eg for watchmaking), would have better precision than larger tools (e.g. for making ship engines or whatever).

I’ll have to check clearance between the gib strips and the moving surfaces.

I tried again, and can get no better than 0.005” 150mm from centre on the x-axis with the x-axis locked. In other words, the y-axis appears to have some kind of issue. I can only reduce this figure by deliberately over-tightening the y-axis gib screws to the extent there’s unacceptable binding of the handwheel. The x-axis seems to be less problematic, despite it being by far the shorter of the two.

The handwheels are free in their bearings, and also the lead screws are free in their castings.

The axes are not smooth enough to setup without the handwheels – far too much stick-slip to judge to movement properly.

 

 

[Dr_GMJN]

On Dr_GMJN Said:

Re. the Bridgeport, I was simply commenting on a question someone asked about free play on that machine. I obviously don’t expect that quality of build at all on a cheap machine.

Re. the figures Jason is getting. I’ve spent a couple of hours this afternoon rebuilding the x/y stage and so far have got nothing like as good as that. For fairly smooth running hand wheels, I’m getting more than 0.010” side-to-side play (rattle), around 250mm from the axis.

I will persevere a bit longer, but really this is the whole point of what I’m saying – surely with a smaller mill of similar origin, I should easily be getting better figures than a larger one, not worse?

As Jason says above, I would take the hand wheels right off. I would then tighten one axis fully so you can’t move it at all and adjust the other axis until a satisfactory result is achieved then attach the hand wheel and run the axis over it’s full useful travel and see what you get, at the moment you have too many variables in play.

I just read your post re sticking, you still need to move the slides without hand wheel input to start tracking down the problem, I am feeling your pain!

Tony

[HopperParticipant @hopper]

+1 on get the handwheels and feedscrews completely out of the equation when setting gib strips. Dovetails should slide freely when pushed by hand. Get that set up right first. Then if things tighten up when the feedscrews and handwheels are added back into the equation, you know the problem lies with the feedscrew interface somewhere. Possibly the alignment of screw and nut with table etc.

This may be adjustable via plates on the table ends that the handwheels mount on. They should be set with the screws loose and the table wound all the way to that end and then tightened up. Telltale sign of misalignment is the table is free in the middle of its movement but tight at one or both ends, due to flex in the middle of the long thin feedscrew.

It might be time to get out the bearing blue and use a thin layer to find out exactly where the dovetails and gibs are contacting and look for high spots and burrs and as Jason says, binding on the edges of the gib strips. Proper divots in the gib strips may in some cases help locate them and stop them wedging up into the dovetail and jamming. Hard to say without being able to see the machine in the flesh.

[Dr_GMJNParticipant @dr_gmjn]

My last post must have got lost somehow:

When the x/y axes are adjusted for free, smooth movement with no hand wheels fitted, they show excessive play.

However it’s possible to fit the hand wheels (which are free in their bearings and threaded casting blocks), and to further tighten the gibs (slightly) and still get acceptable handwheel drag – at least in the x-axis. However, further tightening the y-axis gibs to give acceptable free play makes the y- handwheel excessively stiff.

I know how to fit the handwheels with the bearing block as close as possible to the axis threaded casting block before tightening.

I’ve been through all this many, many times believe me.

I never expected to get it perfect on a machine like this, but I did want to get to a stage – if possible – where both handwheels are reasonably nice to turn, with minimal axis play. This does not seem to be possible using the usual gib adjustment techniques, and so my assumption is that there’s some kind of issue with my y-axis that I don’t have the ability to identify or overcome without expending significantly more time and effort. Since I find this kind of work very tedious, I will just get it as good as I can and live with the resulting issues. These being an unpleasant to use sticky handwheel action with a possible consequence being poor surface finish/grabbing when side milling in the y direction (a recurring issue with my particular machine for a long time).

I have no doubt that other similar machines have both axes working perfectly well – I’m not suggesting otherwise.

Anyway, thanks to everyone who has contributed to this thread, but I think it’s run it’s course – I’ve had a week of messing about getting pretty much nowhere, so it’s time to move on.

[Dr_GMJN]

On Dr_GMJN Said:

Jason,

I’m assuming smaller tools (eg for watchmaking), would have better precision than larger tools (e.g. for making ship engines or whatever).

… the y-axis appears to have some kind of issue. …

 

I was taught on a course to never assume anything, the point being drummed home with the catch-phrase “ass of u and me”!

The precision to which a tool is made is almost entirely down what it’s for, not size.   Taking ship’s engines as an example, nothing particularly special in the way of accuracy is needed to make triple-expansion marine steam engines; a ‘thou’, is good enough.   The same equipment isn’t good enough to make marine diesel engines or turbines, both of which require much tighter tolerances.

The Y-axis is definitely suspicious.  I wish someone as experienced as Jason could get his hands on the machine, because he could tell instantly if something is amiss compared with other mills.    Could be as simple as the gibs being a right pig to set up correctly, or as complicated as a bed that warped when the dovetail was cut, so no amount of fiddling with the gibs will correct it.   If so, as others have advised, out with the engineer’s blue, and start hunting for the error.   Once the cause is identified, it should be fixable.

Dave

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