Wednesday, 23 August 2017

Bilge pumps

Good god. The stuff coming out of the sump is horrible. Not surprisingly there is a bit of fine swarf that's got through the gauze strainers and of course a bit of lube and coolant oil. Plus a fair bit of water - the vast majority in fact.

I made a bit of a start yesterday but after adding then removing about a gallon and a half of hot water and washing up liquid, there was clearly a lot of work still to do and already I'd made a bit of a mess. This wasn't going to be quick - or end well. 

A better solution(!) would be to flush it out with a hose and electric pump. Who knows, if I got the inflow and outflow roughly balanced, I could exchange the contents several times without significant drama. Conversely, if I cocked it up I could make a fine old mess. 

So far, almost everything I've flushed out has been aqueous and most of the oil remains floating on the surface in clumps like a turd-coloured slick. It's going to be pretty much impossible to pump that out and it doesn't look as if it will mix with any kind of detergent and then be washed away. 

I dug out the old hot water circulating pump that came from the house when we removed the Megaflow cylinder. It's not a Grundfos but I hoped it would pump reasonably well. However, no matter how I tried to prime it (even using a cold water hose), I couldn't get any flow going. 

So I swapped over to the Grundfos pump that I was previously using as a coolant pump on the Blidgeport. That actually worked very nicely. 

After a couple of fill / empty cycles, the bottom of the sump is visible finally. Still some turd-like floating excrescences are visible but it's a vast improvement.

I'm now pondering the option of removing the inspection cover on the end of the sump. It would release the residual contents, ideally into a shallow catch tray below. I could then clean the remaining crap out using wodges of paper towels. I don't think I'll manage it any other way.

Tuesday, 22 August 2017

Plumbing the depths

Yuk. The coolant sump is full of shitty slop. When the machine mover guy tipped the machine on its side during the unloading, it dumped a load of water and some oil. Not sure where the water came from, as it was apparently filled exclusively with neat cutting oil. However, I know that there was quite a bit of water around, judging by the rust damage in the head and quill. I may never know the specifics but at some time a lot of water got in. Now the sump is full of water, oil, scum and god know what. 

The plan is to suck it out using a drill-mounted sump pump. I got one from Homebase of all places for about a tenner. Along with a couple of lengths of hose and some empty 4 pint plastic milk bottles, I'm ready to go.

Big bucket in case of leaks:

But don't allow the hose to come off the outlet!
Bollocks. I'll come back tomorrow and have another go. I think this may take a few sessions.

Running the 3-phase coolant pump from single phase

As mentioned last time, I can't just run the coolant pump off the same VFD as the spindle due to the wide spindle speed range and I don't plan on buying a VFD just to run the coolant pump. However, it's quite common practice to power 3-phase motors from single phase by providing a sort-of third phase using a motor run capacitor. This seems to require something like 70uF per kW as a rule of thumb. That number comes up when you search for examples, although it's not clear where it comes from, other than perhaps empirical knowledge. I can't be arsed to attempt any form of analysis - I'm just trying to get it to run and not much more and it's far from rocket science.

It seems that once the motor is up and running around base speed you could actually run it perfectly well with just 2 phases. Obviously if you ran it near rated load (100W / 1/8 hp here) on 2 phases it would run hotter but for a 0.1kW motor it seems pretty big, so I suspect it's quite conservatively rated. In this scheme, the capacitor is required to provide a phase angle on the third phase to get the thing moving. Depending which phase you connect this capacitor to determines the direction of rotation. If you could be arsed, the capacitor could then be disconnected and the motor would run on. In this state, it operates just like a rotary phase converter and you could even connect a (3-phase) load across all 3 phases, sort of like an autotransformer. But I can't be arsed, as I've said.

So I found that Maplin keep a range of 250V and 440V motor run capacitors by LCR (seems LCR are still in business) in quite a range of values from 1uF to 20uF. The prices aren't amazing but when you are buying a single example, what do you expect. And it's nice to be able to simply call in and pick one up. Maplin actually have a lot of stock hidden out the back of the shop, so even though it may not be visible on the shelves, they may have stock of what you see on the website - you can check online that they have what you want.

Seemed rude not to go for the 8uF 440V example (for £6), so I bought their single stock item on the way home. Despite being illustrated with flying leads, it actually comes with four 1/4" spade terminals, as described in the text. No problem. And it's nicely insulated, being overmoulded in some form of plastic.

Made up a cable that is long enough to eventually reach from the coolant sump to the control cabinet where I will control it with a SSR via the CNC controller. Found a cable gland to fit the junction box and connected it all up. 

The cable is cable tied to the motor, along with the cap - that's good enough for the likes of me. The spare terminals are protected with a couple of spare insulated crimps. 

There's a 50% chance that when you try it, the motor will rotate the correct direction. Seems that the coolant gods smiled on me and I got lucky first time round. No need to swap one of the cap wires to the other phase.

The bearings are clearly in good shape. It spins easily and silently, so no need for any work on the mechanics. I haven't bothered to test it with any load, eg bucket of water but as the initial starting load will be fairly low (it's a centrifugal pump), I don't expect any problem on that front.

Job done - awaiting refitting. But first I will need to clean out the coolant sump. Hmm...

Saturday, 19 August 2017

X axis gib, quill lube tube and coolant pump

The X axis gib is easy to adjust. There's a 13mm bolt on the end of the ballscrew, accessible from the LH side of the machine. So it's easy enough to use a crank handle and long extension from a socket set to turn the ballscrew back and forth, then gradually tighten the gib until it starts to bite. Interestingly, at that point, if you move the table with the RH gib screw removed, the gib can self-tighten and make the servo bomb out. So it's necessary to loosen off the RH screw 1/8 of a turn at a time, then tighten the LH (tightening) screw. You can feel the gib nip up to the RH screw - and finally, after enough tightening, the ballscrew starts to tighten.

So I consider it done for now. The backlash is certainly better now and it is what it is. I'll measure it again in a few days and live with what I find.

Yet another job that's been stuck on the list is repairing the autolube pipe in the head. There are various pipes that come from the lube pump to the knee and head - and from there to the individual lube points. Somehow it got torn off, presumably when I was crudely ripping out some of the old cables. The bummer is that access to the broken section is almost impossible. Almost but not completely. I can just see the end of the torn pipe and just reach it with my fingers. It's peeking around the corner at the top left, just above the compression fitting.
 It's pretty tight, so I ended up removing the bottom microswitch and the lube pipe feed that feeds the RH side of the quill.

When John Gunn was kind enough to give me some olives and nuts for the power drawbar air lines, he also enclosed a quick fit coupling for 3mm / eighth inch pipe. If I can chop off the end of the torn pipe cleanly and get my fist of ham into the confined space, I should be able to make a successful connection with the quick fit coupling. I then only have to feed the repaired pipe back through the cavity in the head and ram and connect it back to the lube pump, then the job is done.

Managed to get some Lindstrom cutters in and develop enough leverage ("leeverage", not "levverage", as we are in the UK) to cut the pipe cleanly. Then push the connector onto the pipe end. The end of the pipe was nicely stretched. God knows what I was thinking when I ripped it out. I'm lucky it failed as it did, rather than at the compression joint further out of grasp.

Taped the pipe to a length of brass rod and managed to push it through the ram cavity. Luckily the pipe is still long enough to reach the compression fitting next to the pump. Job done!

Final task for the day is to take a look at the coolant pump. It's a 220V 3-phase motor and I don't want to cough up for an inverter just for this function. And obviously, I can't simply connect it across the spindle motor due to the very wide speed range. I'd really like to be able to operate it from single phase by using a capacitor to generate the 3rd phase. It's unlikely to be able to perform as well as it would if it were operating from 3-phase but it seems to be a common technique and is worth looking in to.

The pump looks to be in good condition and spins easily. It's a simple centrifugal pump, which is typical for this application. 

Need to do some internetting to work out what is required....

Back together - and improved vise mounting

Back in there this evening, aiming to put it all back together with belts, gib(s) etc adjusted, hoping to reduce the backlash somewhat. 

The belt adjustment was simple enough and seems to have reduced the backlash a bit. Didn't write down any actual measurement but we figured out last night that it was one of the contributors.

The gib was easy enough to adjust, tightening it up gradually until it started to bind, then backing off to find the sweet spot. With the cover off the Y axis belt drive, it's possible to move the table back and forth. This allows you to tell when the gibs are biting.

Of course a machine with box ways will have 2 more gibs to hold the table down, so once I'd removed the small wipers, I was able to check they weren't flapping about. They won't affect the backlash but it seemed the right time to do it.

Front left:
 Front right:
 Back left:
 The left gib comes out easily enough. It also has the anti-friction coating.

The bottom line? Some improvement. With the 10um Mitutoyo DTI I measured the backlash by moving in one direction, zeroing the controller, continuing on past, then coming back and stopping at the zero. The value on the display shows the backlash. I'm now consistently measuring between 50 and 70um (~2-3 thou) on the Y axis. I suppose that's an improvement. Realistically that looks like about the best I'll manage.

I can't help feeling that the design of the saddle, with the ballscrew on the right of the machine, causes the table to rotate slightly due to the slop that must exist if there is any finite clearance on the slideways. I doubt I've managed to adjust it to the best possible condition but for now I guess it's reasonably good. 

Due to the height of the Perspex machine guards and the need to minimise the extension of the quill (it has only limited movement as it is), I ended up mounting the machine vise on top of some steel bar stock. Recently I managed to acquire a pair of 4" high precision parallels from a guy on the ME forum. This will raise the vise slightly further and hopefully also improve the rigidity of the vise assembly. It currently relies on the pulldown studs and I'm not convinced it's adequate. So it's time to remove it, clean everything down carefully and remount it with the new parallels.

Here we go:

Before cleaning. Black steel bars just about visible under the vise. Fitted diagonally in vain attempt to maximise stability in both X and Y directions.

These are the parallels. Cost something like £50, which seems fair.
 The old setup:

Friday, 18 August 2017

Y axis backlash story continues...

Made up a circular spacer from a piece of stainless TIG wire. In fact, I used 1.2mm wire in the end so that it was easier to locate in the space available. Then jacked up the collar again. 

The backlash was better now - 50um instead of around 130um ie 2 thou instead of 5 thou. But that's still crap enough to require more attention. 

Playing about with the DTI, it seems that the backlash measured directly at the ballnut mounting relative to the knee bearing is quite a bit less. And it varies if you lock the saddle and measure at different distances from the ballscrew. So there are clearly several things going on, including some significant contribution resulting from loose gib adjustment.

With the saddle (ie Y axis) firmly locked, the drive belt tightens and loosens as load is applied, so clearly some lost movement is happening there, despite what I thought. Clearly I hadn't tightened it up as much as I should have when reassembling it.

I then started to look more closely at the Y axis gib adjuster. This has a large slotted screw at each end, so you can drive it in or out by tightening the appropriate screw, taking care to loosen off the opposite screw first.

I was quite surprised how little movement of the gib was necessary to completely lock the saddle. I managed to achieve a complete lockup after only driving the gib in by a matter of several mm and had to knock it back out with a hammer and brass drift in the end. During that state of solidity, the backlash measured at the ballscrew pulley was negligible - any movement dominated by springiness rather than hysteresis (lost movement). You'd have to say that's a good sign.

I completely removed the gib strip to see why it was so reluctant to move. In fact, once the preload was removed, it slipped out very easily. Quite simply, the ways are cast iron on chrome and the wear seems to be almost non existent. And despite the very modest taper angle, when the gib bites, it locks the saddle quite suddenly. I suppose that's a good thing to find. I'll measure the taper and the movement range to get an estimate of how much it actually tightens against the ways. That should indicate how little the ways have worn over the years, given that the gib appears to be original and shows no sign of merging from the end of the adjustment slot, which is what would eventually happen. It can't have been adjusted more than a few mm from its original position.

It seems that the taper is 3.7mm over ~280mm. So 1mm of movement of the adjuster will take up a mere 13um of play. 

On the Blidgeport, I had to remove a short length (perhaps a couple of cm) from the end of the X axis gib to stop it poking out of the end of the table due to the excessive wear on the ways. I don't seem to be anywhere near that territory here.

The gib itself is beautifully made, like the rest of this machine. It's been hand scraped in and there is no sign of any wear, damage or gunk buildup. The oilway (through hole) was clear and the oil channel didn't require much cleaning.

I'm not an expert on these matters but I assume the green coating is some form of low friction finish. 

I had to remove the wipers from the front of the saddle to allow removal of the adjuster screw so I could withdraw the gib. They are composed of a Nylon carrier and moulded elastomer seal.

  • Replace the gib, adjusting it carefully to be at the point of biting. This is a bit more difficult to judge on a CNC machine without handles than on a manual machine but needs to be done carefully as it clearly contributes to the backlash.
  • Tighten up the belt to eliminate visible movement under load.
  • Retest.
Ideally I'd get the backlash down around the 25um / 1 thou mark. The original machine spec claimed 10um accuracy, although I've no idea what that actually looked like in practice.

Thursday, 17 August 2017

More slop than a school dinner

I'm a bit pissed off to be honest. Although the Shiz has clearly been looked after very carefully over the years, I know it had about 23 years of use before it was laid up for a further 10 years. 

I have now replaced the X axis bearings and the spindle bearings to address damage due to pitting and / or slight rust. The Y axis bearings looked fine, so were simply cleaned up and regreased before careful reassembly. That seemed to take care of any backlash due to the bearings and the possibility of poorly adjusted belts, loose pulleys etc. 

Furthermore, I investigated and (to some extent) improved the backlash on the Z axis which seemed to be mainly due to movement of the ballnut yoke against the quill, presumably one result of the 30+ years of work. I think I've largely fixed that now.

Yesterday I thought I'd better do some vaguely objective backlash tests to put some numbers to the X and y axis backlash. I have to say that although I was well chuffed at getting the thing up and running and taking a part from CAD to CAM to CNC to metal for the first time (with mostly reasonable looking results), I was less than overwhelmed by the patchy surface finish of some of the curved surfaces formed by the X and Y movements. I'm no trained eye when it comes to CNC machines but even I can tell that it's the result of backlash (lost motion) between the servos and the table. So let's get calibrated here and see what we are dealing with.

Ideally I'd load the table up before doing these backlash tests, perhaps by simply tightening up the table locks a bit. However, I thought I'd start by simply measuring the backlash with the table unloaded and rely initially on just its friction / inertia. After all, that's what most willy wavers seem to do when they are boasting about their machines on Youtube and the various metalworking forums....

Simplest way seemed to me to be:
  • Position the table and quill at a suitable position and then use MDI to select G54.
  • Zero all axes (still in G54) and then set up a DTI aligned to the desired axis, to pick up movement in that direction. I started with X not surprisingly and used the coarsest DTI - the "Kurt" branded Chinesium thing with 0.001" resolution. The Baty 0.01mm and Mitutoyo 0.001mm jobbies might be a bit optimistic at this stage.
  • In MDI:
    • G01 X0.5 (move 0.5mm to the right in X direction)
    • G01 X0 (move back to origin in X direction)
    • Zero the DTI carefully
    • G01 X-0.5 (move 0.5mm to the left in X direction)
    • G01 X0 (move back to origin in X direction)
    • Measure the difference from when the DTI was zeroed.
  • The difference in zero readings is the backlash, measured from the table moving one way and back to zero, then back the other way and returning to zero. Approaching the same alleged zero from each direction will unfortunately result in a different table rest position due to said backlash.

Finally, issuing a G01 X0.13 following by G01 X0 repeatedly would result in the needle just moving each time. A lesser value would usually not move the needle. More would consistently move the needle quite noticeably. That is a quick way of demonstrating if the measured value of backlash is about right.

Oddly (and disappointingly) enough, I found a very similar value for the Y axis. Note that these measurements were made near the middle of travel, where the machine probably spent most of its operating lifetime. If I were being really methodical and anal, I'd have repeated this test at different positions along the travel. 

The bummer is that my measured value of 0.13mm is 0.13/25.4 inches or about 5 thou in old money. Bugger and f*ck - that is just completely shite, hence the despondency. Unless I am happy to put up with this, which I'm not, I will have to do something about it. What to do?

Firstly, I removed the Y axis belt drive cover to check if the ballscrew is actually moving axially (against the bearings) when the table is moved. Even with the table locked, the most I could see was less than half a thou, so clearly not a significant factor. 

Then (with the table still locked) I measured the approximate circumferential movement of the 58mm pulley. I found roughly 7mm at a radius of ~30mm, which is equivalent to about 13 degrees. Working back from the measured backlash, 0.13 movement on a 5mm pitch ballscrew is equivalent to about 10 degrees (without any load). That's pretty close agreement, given the rough and ready test method. So it's clear the backlash is almost entirely within the ballscrew and ballnut.

Finally, check that the measured backlash is not just caused by the saddle gibs being loose. If there was sufficient slop, you could imagine the table slewing about and appearing to display backlash.
The backlash is clearly between the ballscrew and the connection with the saddle.

I suppose the most "obvious" approach would be to get the table and saddle off, get the ballscrews out on the bench and take a closer look. But to do that would be quite a task. I've already had the X and Y axis drives and pulleys off and that wasn't a walk in the park. I'd have to do that all over again - and much more besides. Besides, the next step would involve some significant and unwelcome effort.

A quick estimate of the table mass focuses the mind. I estimated the equivalent dimensions of the table at around 4 x 40 x 120cm (those approximations allowing for the negative contribution of the tee slots etc), which is a volume of about 38 litres. As cast iron has a density of about 7, that equates to about 270kg. Call it a quarter of a tonne, give or take.

There is also the question of how to get the ballscrew out. Unlike the "normal" AN-S model, my AN-SB model has the leadscrew stationary (fixed to the saddle) and the ballnut moves along it, fastened somehow to the bottom of the table, taking the table with it. Clearly I'd have to remove the ballscrew with the table, after liberating both ends from their bearings. Then somehow get the table onto a bench without buggering the ballscrew by applying the 270kg weight of the table to it. I'd much rather not take that route - surely there must be a less painful solution - or at least one involving less effort.

And then there would be the saddle itself. Removing it simply to get the ballscrew out doesn't actually look necessary - on these Shizuokas, the leadscrew actually sits outside the machine, so it may be accessible enough to service without having to remove it and the saddle.

So, taking the path of least resistance, I thought it best to tackle the Y axis first.

From what I have seen of the Z axis ballscrew and the parts list exploded views of the (very similar) AN-S machines, the ballnut is almost certainly a double nut affair, with a ground shim between the nuts to remove backlash. On the Y axis, it appears that the ballnuts are held captive within the cast lug on the saddle, by means of 2 collars, each held in by 4 hex socket bolts.

Access to the front 2 bolts isn't so bad but the rear 2 are on the almost-impossible-side-of-difficult. Getting an Allen key in there and undoing the tight fixings looked a bit of a challenge. I tried and gave up on the front collar but managed to shift all 4 of the rear collar bolts. That reveals the end of the ballnut assembly. 

I was hoping that I would be able to lock the table and drive the ballnuts out of their bore by turning the ballscrew (by manually moving the toothed belt), then reshim the gap by adding shimstock (about 5 thou, then), before plopping them back in. Beyond that, whether or not I would be able to get the X axis ballnuts into a suitable position to repeat the trick would remain to be seen, even if I succeeded with the Y axis.

*** LATER***

I managed to push the ballnut out of the saddle lug - almost no effort required. Sure enough, the nuts are located between the 2 collars and prevented from rotating by a large keyway. There are a pair of semicircular shim pieces between the ballnuts. Here's one of them:

The key was a bugger to remove. I have to admit that I ended up using a centre punch to drift the end up and out of the slot. Didn't seem to be any other method on the cards, as it was well tight.

With the key removed and one of the shims back in place, tightening the nuts against each other ensures zero backlash. If the shims are the correct thickness, the key will then simply drop down into both keyways. If not, the keyways won't line up. 

Here the shim is trapped between the nuts. Although the key is not engaged with the right hand nut, you can see that it would drop right down directly into the keyway. In this case, they are spot on. Certainly a lot better than 10-15 degrees...

My expectation was that the shims would be a loose fit, requiring some additional gapping to take up the slop but in fact, they were a good tight fit. That was what I also found when I dismantled the Z axis ballnut assembly, so it looks as if the ballscrew wear on these machines is negligible. Pretty impressive after 33 years.

That's encouraging. And it suggests that the backlash movement is between the ballnut assembly and the 2 collars, so that the ballnuts are actually moving axially within their bore. If so, I could deal with that by simply packing out one of the collars to ensure the ballnut assembly is under compression ie fully constrained. That would be a fine result. Best way to find out - rebuild the assembly with some form of packing and see if that has got rid of the movement. I imagine a 0.8mm dia stainless steel TIG welding rod would form a suitable ring-shaped packing piece. Let's find out....

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