Wednesday, 24 August 2016

X-axis drive

The previous post (some considerable time ago) ended on a slightly downbeat note as I had found significant bed wear. Well, the good news is that it seems to have largely gone away. I don't know what happened, maybe things were a bit out of place and have now settled down, but the wear is now just enough to make the carriage a bit tight at the tailstock end, rather than enough to make it loose at the chuck end, so I decided to press on with the rest of the project.  First I tightened the nut retaining the Z-screw, which turned out to be a slightly interesting task, so here is a picture. I used a large ring spanner on the nut, and then used an ER32 collet chuck on the bearing surface of the other end to apply the counter-torque. Luckily this lathe is 20" between centres and not 60".


I had previously ordered a casting to contain the X axis drive chain, and the first thing to do was to machine it and prepare the housing for the angular-contact bearings. 



 The bearing preload is supplied by a screwed-in cap, with hole spacing to suit the pin-spanner of my angle grinder:


At this point I made the first irreversible change to the lathe. I cut off the extension that the taper-turning attachment connects to. My lathe came without the taper-turner anyway, but it still felt like a step too far. However my workshop is small (very small), and with the lathe close to the wall, the extension hits the wall before the tool gets to the centre-line, so it had to go. 


I also needed to slightly increase the size of pocket in the slide to make room for the ballnut. I had tried all sorts of ways to squeeze it in without doing this, but it was just too difficult. Only 1mm in width and 2mm in depth needed. 


And here is the screw and nut in place. The block clamps round the ballnut thread, and then the threaded hole is how the cross-slide is connected. 


The new screw does not sit in exactly the same place as the old screw. I thought long and hard how to bore the new housing for the end-bearing in the correct location, and finally came up with the idea pictured below. I pushed the slide all the way back so that the nut was right against the back face, snugged up the gib and tightened the slide-to-nut screw. Then I squodged epoxy putty into the gap around the ball screw end (which had been previously machined for a bearing). When the epoxy was set I was able to unbolt the cross-slide and slide it off, then centre my coaxial indicator on the centre-hole in the end of the screw to exactly align the horizontal milling spindle with the screw. Then the screw was tapped/prised out and the bearing housing was bored with my Wohlhaupter boring head. 


The next three pics show how I made a path for a proximity sensor cable (the X-axis home and limit switch) back into the apron casting where the electronics is. I used a CAD package to work out the compound angle required to get from the proximity sensor bore up and over the V-way and into the leadscrew tunnel. 




All blanked off by a little phosphor bronze plate. No reason for the material, except I had some in the right thickness. 

The proximity sensor uses two shallow holes milled in the underside of the cross-slide as targets for homing and limit. In normal use it is never exposed, the photo below was taken with the connecting screw removed and the slide pushed back. 


 


Also visible in the photo above are the oil grooves for the oiling system, and the fact that I made a new, shorter, gib-adjusting screw out of a shoulder socket screw because I had a clearance problem. 
The oiling system is fed from an oil sump in the apron, and feeds oil to the cross-slide and saddle ways. I don't think it lubricated the compound or the screw directly. I thought about an electronically controlled oiler based on a solenoid, but decided not to bother. 




This is the new pump piston, which simply fits in a bore in the apron casting. I had made provision for this in the casting shape, though I had perhaps not given quite as much thought as I should to the oil routing from the pump to the saddle. I ended up with a very wiggly copper pipe and a drilled grub-screw at each end to swage it and hold it in a counter-sunk hole. 


Around this time (looking at the photo dates) I decided to move the VFD nearer to the motor and further from the PC to make a bit more space. This was partly necessitated by the purchase of a rather bigger VFD, as the first one seemed to struggle a lot. I am not sure why, but the 3hp VFD over-currented at 12A whereas the new 4hp VFD runs the motor nicely at a max of 6A. 
Anyway, whatever the reason, I made a bracket:


And mounted the VFD in the new position. 


In a fit of zeal, I started painting things with Tractol Paint, recommended somewhere on the Internet for painting machine tools. I chose 7031 - Blue Grey as the colour, which I thought was close to the original Holbrook colour, though in practice is a bit more blue. 



If you are wondering why the  sudden switch away from the X-axis drive and onto other things, it was because I moved the lathe to the other side of the workshop to get to the VFD and motor. This also seemed like a good time to fit the monitor post and to paint the back of the lathe and rear covers.


As part of my policy of not having any visible wires (my first retrofit is positively festooned with them) I led the wires up through the bed, up the monitor tube, and out the top. 

Much late-night cogitation was expended in figuring out how to mount the X-axis servo in such a way that the chain tension could be adjusted without taking the saddle off the machine. (A more than slightly tedious process)
In the end I came up with the idea of using a mounting plate with two T-slots, clamped up by the two lower screws that hold the chain cover, and with a second screw up through the bottom to apply a tension adjustment. 


And here it is in place, except with much shorter screws and minus the chain cover. The sprocket drive is taken through another tapered interface between the sprocket carrier and the ballscrew. Morse-like angle but a non-Morse dimension. One nut clamps the bearing inners against a thin spacer, and pulls the taper into the sprocket carrier. Having learned my lesson with the Z-screw there is a little hex milled on the end of the X-screw to apply counter-torque. 


I had decided to have a pair of jogwheels on the actual apron. If I had planned these in time their bosses could have been part of the apron casting, but perhaps that would have been a step too far. I machined some aluminium mounting plates, and wired them to a Mesa 7i73 inside the apron. This also interfaces the proximity sensors and an extra rotary switch which will eventually adjust the jog-increment. It also has a push-action button included, though I haven't yet decided on a function for that. Any ideas? All the IO is interfaced through a single CAT5 cable this way, which seems like a good plan when it all runs in a cable chain. I got special oil-resistant cable-chain rated wire


A stainless box protects the end of the cable chain for the apron. The cable chain contains the CAT5 for the 7i73, the servo motor power cable and some special resolver/encoder cable I found on eBay. 



It was all a bit floppy and awkward at this point as I made the connections to the motor and 7i73. The motor uses a pair of Lemo connectors and the CAT5 is just a normal RJ45 plug. Which feels like two ends of the connector-quality spectrum. 




 The cables were pulled through a conduit. Not after some struggles, and in fact I had to split the bend in half and re-assemble with cable ties to get the cables round the tight corner.





I was then able to assemble the lathe and start making parts!



Which seems like a reasonable place to end this edition of the blog. I hope both my readers are still awake :-)





Sunday, 5 June 2016

Re-keying Euro-cylinder locks.

I recently managed to lose my house keys. I don't know quite what happened, but I came down one morning to find my front door open and my house keys (but nothing else) missing.

Soon after I moved in I had changed all the locks to use the same key, by buying a keyed-alike set of locks from locksonline. So the front and back doors, garage door, shed door and garden gate are all on the same key. That's 6 locks and 12 cylinders.

Knowing that somebody out there had my keys, and knew which house they belonged to wasn't super-comfortable so I made things secure by putting back some of the old locks and leaving a key in the lock on the inside on others. (You can't get a key into one side of a euro-cylinder lock if there is a key already in the other side that it turned slightly. )

I was going to get a whole new set of locks, but that looked like costing at least £140 even for cheap locks, so I decided to see if I could just re-key the existing set of locks. I found lots of information on the internet, but most of those were starting with a picked lock, not a lock to which you have a key. And whereas I can pick locks reasonably well, I didn't want to do 12 of them in an afternoon.

With a conventional cylinder lock you can simply withdraw the unlocked cylinder and slide a plug in from the back to retain the pins. This isn't an option with a Euro-cylinder, you need a special "keying shoe". These are available very cheaply, so I made one.


It is a bit of aluminium bar with a groove milled in it, and a bent bit of piano wire (1.6mm). I had to grind the piano wire to a square-ish section to allow it to fit nicely into the bottom of the lock key-slot. 


The first thing to do is to remove the circlip from one side. There are special tools, but I just used 2 screwdrivers. The photo above isn't really very good, because I was trying to hold two screwdrivers and a phone camera. And ran out of limbs.


The second "special tool" you need to go with the pinning shoe is a key with the back milled off. You could file it off, it would work fine, but I had the mill set up and running so I used that. 
You put in the special key, rotate the cylinder by 180 degrees, then slide in the pinning shoe and slide out the key, pins and cylinder. 


Here you can see the cylinder removed and also see how the bent wire holds all the pins and springs in place. 


This is the new key with the existing pins re-arranged. The key was a blank bought from the local key-cutter. I marked the pin positions by passing a drill down the pin holes, then filed them to height to suit the new pin order. If you were very lucky you might find that you had a key that already fitted the permuted pins. 


It is probably worth describing the re-assembly procedure. It is possible to get the key and the cam 180 degrees out of phase. 

You won't be able to slide in the barrel with the new key fitted, and if you can slide it in with the modified key then you haven't changed anything. 

Do not forget to put the spacer-blocks back in. The way that a Euro-cylinder works is that the key pushes a widget into engagement with the cam. There are spacer blocks to make up for the many possible cylinder lengths. Having put the cylinder back together leaving out those blocks would be annoying, as you would then need to make a cut-back version of the new key. Luckily I noticed in time. 

Push the barrel in far enough to hold the pins down, but not quite all the way. Then withdraw the keying shoe. 

Put the cam somewhere in the rest position, ie pointing towards the retaining screw hole, then rotate the cylinder into the normal orientation. When it is close you should be able to engage it with the cam and also let all the pins drop home. Your new key should now open the lock. But be very careful, as it is easy to pull the whole cylinder out at this point and all the springs will go everywhere. Don't ask me how I know this. It is possible to re-insert the springs and top pins, starting from the front of the lock and working back with tweezers and sliding the pinning shoe in one pin at a time. But it's tedious and fiddly. I told you not to ask how I know. 

Put the circlip back, and do the other half. 

When you have the rhythm it's a pretty quick job, less than 10 minutes per lock. 



Monday, 29 February 2016

Holbrook: Movement at last

In the last instalment I had the Z axis motor mounted and the Z-screw ready to install. But the apron casting still needed a hole through it, and a socket for the ballscrew.
First, though, I needed to make a new gib strip to suit the new apron. I couldn't use the old one, as that had gone missing at some point during the rebuild prematurely curtailed by the demise of a previous owner.
I had a couple of strips of iron cast for the job. I have previously made a gib from a length of round Durabar, and it took an awfully long time. Part of the problem was that the round bar was difficult to hold while machining-away most of it.
For this gib I made a fixture to hold it, a lump of aluminium with a slot milled in it and a bunch of allen screws:


I machined it to as close as possible to the same taper as the groove I had machined in the apron casting (CNC makes this really rather easy) then scraped it to final fit to blue using an arrangment of edge-clamps on the mill table to stop it sliding about.


And here it is in-situ. No adjusting screw yet.


At this point it became evident that the bed is worn near the headstock end, the gib wants to be about 20mm further in at that end than at the tailstock end. I am trying to decide what to do about that. I got a quote for a regrind (£940) but that includes a lot of work that I don't need, such at Turcite under the saddle to retain the apron height and a cross-slide rebuild. Further thought is needed. Maybe another hand-planing setup is called for, it worked well on the Rivett. 

With the apron now fitted and gibbed-down I was faced with drilling a long hole right the way through the saddle, and a socket for the ballscrew that needed to be in an accurately located place. My milling machine is much too short of travel to make the hole. But I had a cunning plan. It isn't a coincidence that the socket for the ballscrew is No3 Morse Taper, I did that on purpose to allow me to do this:


The 1kW servo motor is more than capable of spinning a drill but I had to take something of a break from machining to use this idea, it was necessary to start wiring the system up, including fitting the PC and finding a place for the servo power supply. I finally settled on putting the servo PSU behind the headstock. 

I started with the centre drill as above, then switched to an MT2 long-series drill in an extension adaptor to drill a though-hole:


I used a carver clamp in the saddle to give the tailstock something to push against to supply the feed, and after rather a long time with lots of chatter and squealing, I finally saw the drill emerge from the far side. 


I then fitted a boring tool to the Z-drive to create a register exactly concentric to the drive as a reference. 

The apron casting was then transferred to the mill, where I centred to the reference diameter using a coaxial indicator  and bored the pocket for the ballnut with the boring head. 



A bit of CNC milling made a pocket for the ballnut flange. 


The casting was then reversed on the mill to allow me to come in from the other side. I didn't clock-up to exit hole of the drill as it was evident that the drill had wandered rather a long way. As it was cutting through radiuses and skimming along a few mm inside the top face of the casting cavity this was no real surprise. I kept the same height setting as the ballnut side, and set the lateral position relative to the rear reference face and then corrected the hole position with the boring head. 
Even with the 100mm boring tool bought specially from China for the job it wasn't possible to get right to the middle, so I ended up using a lathe boring bar to "knife-and-fork" out the central 1.5" of the ballscrew path. I had decided to fully-enclose the screw, so I pushed in an aluminium tube with a couple of O-ring seals round it. This keeps the screw separated from the X-motor and its wiring and keeps the oil tank completely sealed. The tube also provides the required register for the spiral spring cover that will eventually be fitted. 

If I was doing it all again I would definitely mount the ballnut on the left side of the apron. It makes assembling everything easier, it means that most cutting forces are pulling against the nut flange, it would put the nut in a convenient place to be oiled, and it would leave more space round the motor. But hindsight is a wonderful thing. 



It was then finally time to thread the preloaded ballnut onto the ballscrew and put the apron back on the lathe. 



Then push the screw home and tighten the retaining nut. Actually I haven't done that yet, It is going to need a deep socket and a special tool. 


But even finger-tight there is enough drive on the MT3 taper to drive the carriage back and forth:





It's nice to see it moving, but I would be happier if I knew what I was going to do about the bed wear. 
Luckily if rectification means that the saddle drops, it just means a bit of a skim off the top of the apron casting to put things right. 

Friday, 15 January 2016

Holbrook Minor Drivetrain Autopsy

The previous instalment of this blog ended with the drivetrain locking solid. This is not a trvial thing to diagnose as the lathe lives hard against the wall, and all access to the drivetrain is through the back.
Luckily, as detailed in another previous installment I anticipated the problem and I have made a frame to allow me to jack the lathe up on to castors to move it about.
It actually took rather longer to make a space to move the lathe to than to push the (700kg) lathe across the garage, and I was left feeling pretty smug about my foresight.

The lathe manual seems proud of the fact that "The entire drivetrain is removable as a unit for easy servicing" which is indeed true. What they don't make such a big deal of is that the drivetrain on its baseplate weighs about 200kg and has about 1/2" of headroom. I eventually managed to extract it with the help of my Crowbar of Doom (which is useful for all sorts of machinery moving jobs and is a superb bit of equipment, all the shapes and angles are perfect).


The Kopp Variator actually freed-up as soon as I took the fan cover off and wiggled the motor fan, but I decided to have a look inside anyway. It also relieved me of any worries that the problem was actually with the 2-speed gearbox. 

Getting the Variator across the workshop onto the bench was a struggle by myself. The manual says that with the motor flange (but not the motor) it weighs 75kg. I was left with a rising sense of panic as I was unsure I would make it to the bench, had nowhere else to put it down, and was wearing carpet slippers[1]. 

The Kopp manual says that you should dismantle from the output end. The Holbrook manual hints that they fitted the Variator back-to-front. I can confirm that in the Holbrook installation you want to take off the end cover nearest the speed adjusting worm shaft, in my case that was the motor end. 

All fasteners on my Variator appeared to be metric. This incuded the tiny locking screws for the end-float adjusters.

The Variator is rather clever. The speed control rotates an iris plate into which spherical rollers on the axles of 6 balls fit:



The ends of the axles run in tracks in the end housings So as the iris plate rotates the axles of the balls tilt one way or the other. In the horizontal position the gear ratio is 1:1, with the axles tilted all the way one way it is 3:1, and the other way is 1:3. 




The main problem with my Variator had become clear before I removed it from the lathe. There was very much less oil than there should be in there. No more than half, possibly even less. Certainly when I took it apart everything seemed completely dry. Luckily all the friction surfaces seemed pretty much OK and the problem that I had had with it seizing appeared to be due to overheated oil turning to sticky laquer. (Perhaps it was the wrong oil, Allspeeds Ltd are very insistent that only one oil will do, Shell Morlina S2 BL10) 
These are the balls as I found them. A tidy workplace and absolute cleanliness are paramount. 



And here is one ball dismantled from the axle. I have not seen split-cage needle rollers before. Now I know that they exist I can think of all sorts of uses for them. 




 I cleaned-up the balls with white spirit and the very finest of Scotch-Brite pads to remove the brown staining.



The picture above is also a good one to explain some details of how the system works. 
The outer ring matches the ball diameter  on the inside and floats on the balls. (It's an RHP part, I think it is the outer race of a spherical roller bearing). The two drive cones are not keyed in any way to the input or output shafts, instead they have a set of  ramps and a roller-thrust bearing arrangement. There are matching ramps on the input and output shafts. So: Input torque causes the rollers to roll up the ramps, which pushes the cones inwards. The balls slide out a little until restrained by the outer ring, at which point the pressure increases until the output shaft starts to move. So there is only ever the right amount of pressure to drive the system. 

It is important to have the end-float correctly adjusted for this mechanism to work properly. I found it better to back-out the end-float adjusters before final assembly, then adjust them afterwards. You just need to loosen the tiny locking grub-screw then use a pin-spanner to adjust the adjusting plugs until they are both in the same axial position relative to the end cover, there is no backlash/play between the shafts and the unit turns freely by hand. 

I have ordered some special Variator oil to see if it does in fact work better with oil in it, even though I have actually decided that I won't be using the variator in my CNC conversion. 

The 2-speed gearbox

 I will. however, be using the 2-speed gearbox. This is electrically controlled so easy to integrate with CNC. Inside there are two clutches to engage either straight-through 1:1 gearing or the 6:1 ratio. Slightly oddly the reduction gears are 12:144 then 84:84 which violates normal design practice of having a "hunting tooth" so that over time every gear tooth sees every other gear tooth.


In the external cover lives another clutch that serves as a spindle brake. Unlike the other clutches this one is not easily disconnected, as the terminal nut is inside the case, and the wire passes through a gland. 
On my lathe the brake had been disconnected, and I don't know it it was working. It certainly wasn't after I had extracted the drivetrain, as I forgot about the wire. When I looked inside the case the wire had pulled out of the eyelet. This was a minor problem until I tried to fit anew eyelet, and the terminal screw snapped as I tightened the nut.

So, I took the brake off to see what I could see. 



The plates are on the left, these alternately mesh with the central hub (keyed to the gearbox shaft) and the outer dogs of the electromagnet (on the right). The plates are all ferromagnetic, and want to stick to each other when current flows. 
This wasn't going to work very well unless I could make current flow, and the broken off screw had no electrical continuity to the magnet. (when the screw broke off it came with solder and a bit of wire. 

I had some reason for optimism, though, as poking a multimeter probe down the hole did find an electrical connection. 



After toying with a number of elegant solutions I decided to just solder a wire into the hole and keep adding solder until it found something to make contact with. The magnet windings are all encapsulated and mounted in a heavy-duty steel case. 


Luckily this appears to have been a complete success. I will be inserting a knot in the cable inside the case and a bullet connector outside to avoid making the same mistake again in the future. 

I have the parts on order to make a motor bracket to fit in place of the Variator for direct drive via a VFD. Then I will experiment to see if I want the Variator back. 



[1] I exaggerate, I was wearing hiking boots, but only by accident.