Setting up the timing on a Polly V locomotive

TLC for the Polly V 


Polly V Live Steam 5″ Locomotive

I have been the owner of a Polly V 5″ gauge locomotive for some time now.  It has run OK but I have always had some unexplained happenings with it.  Things like being reluctant to start and sounding ‘out of balance’ to use a clock making term. I also admit that while I know the overview of how it works I have never got to grips with Stephenson Valve Gear.   I decided that some investigation was needed.

New Piston Rings

A bought a new pair of O rings for the cylinders a few weeks ago and decided it was time to fit these as a first step in an engine health check.   This was easier to do than I expected.   The end face of each cylinder is held in place with eight hex head screws and when these are removed the end plate can be taken off.   The cylinder rod can be disconnected and the piston pushed out of the end sufficiently to swap out the O rings.   A liberal coating of silicon grease on the new O ring allows the piston to be gently squeezed back into the cylinder and job done.   Having completed this exercise I was feeling a bit more confident.

The Stephenson Link

Out of curiosity I decided to have a look inside the valve chests on top of the cylinders.   These were a bit more fiddly to get at as I had to remove some of the chassis work but not a major problem.   On the bottom face of each valve chamber are two slots.  These slots allow steam in and out / to and from the piston cylinder.   The timing block moves back and forth across these openings to alternately allow steam in and out of the piston chamber.   The movement of the block is controlled by mechanical linkage and each cylinder is out of phase with the other to balance the drive to the wheels.

Having opened the two chests I noticed that both looked to be set up in a different way and as I rolled the engine up and down on the bench the openings in the bottom face of the chest were not revealed in a synchronised manner.  Some investigation was needed.

When I bought the engine I also got a full set of the construction notes supplied with it from which the previous owner assembled it from new.   I found these notes difficult to follow which was due to the fact that I was not conversant with the names of the individual parts mentioned.  I stuck with it and the first thing that struck me as wrong was the gap at the end of the Stephenson Link was not the same in ‘Forward’ and ‘Reverse’ settings on the quadrant reverser.    I had no idea why this was important and no idea of this was an absolute gap size that was needed (an actual measured distance) or just a relative similar size.   Clearly it was not either of these and I needed to adjust it. 

Much gloom followed as I realised the adjustment for this was behind the side water tank.   This would have to come off.   This looked fairly straightforward except the injector water feed is coupled into this tank in the cab area.   The gland nut holding this is inside the water tank and a real pain to unfasten.  The amount of arc that is possible to apply to the nut is very restricted.   Eventually I managed to free this and the tank came away to reveal the quadrant rod adjuster.   It was then a two minute job to adjust the quadrant rod length to balance up the end gaps on the Stephenson link in Forward and Reverse.

View showing side tank removed and the quadrant rod adjuster

Timing the Valve Gear

Having balanced the Stephenson Link, the next step was to adjust the valve blocks.    I put the quadrant reverser at its Centre position and rolled the engine up and down on the bench watching each valve block move.   The movement of each block was not symmetrical .   In the Centre position the block should only just move a small amount back and forth to just reveal the two port openings in the base of the chamber.

This is easy to adjust by removing the valve actuating link arm screw.   After some back and form adjusting I got the blocks to achieve the minimal movement needed about the centre position.

If I now rolled the engine back and forth on the bench in ‘Forward’ or ‘Reverse’ the movement of the blocks increased to reveal the full area of the two ports and with each side of the engine leading in turn.   I now had a nice action of the valve blocks alternately opening and closing over the front and back ports with no excessive movement.   Things looked much better.   I cleaned up the valve chamber lid and fitted a new gasket to each chamber. I put the engine on the rolling road and using compressed air ran the engine up.  Wonderful !   It now sounded more balanced and was much happier to start in Forward or Backwards.  Here is a pictorial representation of the valve action.

Without the balancing in the Stephenson Link this would not have been possible to achieve.

The Eccentrics

While delving around on the underside I noticed that one of the eccentrics had movement on it and the eccentric arms were sloppy.   On the Polly kit the eccentrics are positioned with a grub screw into pre-drilled holes on the axle. 

Another major gloom session ensued as it looked at first sight that the boiler would have to come off to work on the eccentric arms.   However after playing with positioning of the eccentrics I found I could get the clamping screws out, drop the arms off and so allow tightening of the grub screws.   In the process of doing this I found that there were tiny packing pieces between the faces of the eccentric arm halves.   Clearly the original owner must have believed the eccentric arms were too tight and had packed them out.    I did a trial fit without the packing and the eccentrics ran freely.  Perhaps this had been done as a running in adjustment.  On completion I did a second run on compressed air with no obvious issues. 

Putting it back together

Re-assembling the chassis work was straightforward except replacing the side tank water feed to the injector.   The original nut had been mangled by the previous owner so needed replacing.   Rather than just fitting another hex nut I made a brass knurled nut.   In the confines of the side tank this gave me better grip with my finger ends to tighten the fitting.   Once it was finger tight I could use the end of a screwdriver to increment it tighter using the knurling surface to give me purchase.

Replacement knurled nut to fasten the injector water feed into the side tank

Other work

While having grease embedded under my finger nails I did a few other jobs on the engine.   

The whistle valve had always leaked steam and I had bought in a new version of this with a stronger spring action.   This was fitted along with a new steam valve feeding the injector.   

I re-positioned the pressure gauge to be central in the cab rather than off to one side as originally fitted.

I fitted a new set of drain cocks on the cylinders.

Finally I adjusted the cylinder lubricator which had been a bit too liberal with its feed delivering a snotty chimney edge and a fine mist of oil to my face while driving.   The adjustment screw was unlocked and moved back a small amount.

All back together and a good days work.   A lot learned which is always a bonus and less fear of the black art of steam engines.

The next day I took the engine down to the club track and made quite a few circuits on the raised level track.   The engine sounded and ran quite differently.   It ran much better on lower levels of steam pressure.  I also did not get a bath of lubricating oil but judging by the edge of the chimney there was still sufficient being passed into the cylinders.

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Stretching FlatCam and PCB milling on Tormach PCNC440

Tormach M Code Expansion Card

I have got a project underway to use the Tormach USB M Code expansion board in association with an A axis rotary table.   Details of this will follow in due course.   The expansion card when added to a Tormach mill allows the operator to embed M Codes in their CNC program which will operate up to 4 dry contact SPCO relays or accept 4 inputs as handshaking acks.

My Cabling Masterplan

As part of this project I need to have cables from the USB expansion board to various devices and in a rush of blood to the head decided to use standard readily available Micro USB cables for this. Now the cable connectors are pretty small and the PCB mating socket is even smaller with its 5 connections.   Glibly overlooking this I asked a colleague to produce a PCB layout for the connector to a breakout connector strip.   Dave duly produced a layout and a scratching of head resulted.   How was I going to produce the PCB and how was I going to solder to the connections assuming I could see them ….

FlatCam to the Rescue

Elsewhere in my blog there is mention of the use of FlatCam to create a CNC GCode listing from PCB Gerber and Excellon files.   This program works really well and many successful PCBs have been produced but I have never attempted to mill such fine PCB tracks. A number of problems needed to be addressed to make this successful.   The PCB sheet needed to be held very flat on the PCNC440 tooling table and the correct milling tool with its associated feeds and speeds needed to be chosen. In the past I have used strips of aluminium to fasten the PCB blank down on the tooling table.   This is never perfect and leads to variations in the pressure around the edges of the board.   With single sided PCB there is a natural curvature of the board material as a result of the surface tension of the laminating process.  A single sided blank has a concave surface on the copper side.  I needed to create something more repeatable.

Milling Window Restrictions

Before I bought the Tormach PCNC440 I had a discussion with John Saunders at NYC CNC and he recommended going for the biggest machine I could fit in my workshop.   I could have squeezed the 770 in at a push but I would have had to sell off my Myford VMB which I was reluctant to do.   My order therefore went through as a 440.   With hindsight this decision has been justified on two counts.   I rarely need a larger working area than the 440 offers and the VMB gets used very regularly for quick jobs that don’t justify CNC.   This project was an exception. I wanted to make a frame that would clamp the PCB blank down onto the tooling table.   In order to get the maximum working area for the PCB blank the clamping frame would have to sit outside the machining area.   How was I going to manufacture it ? Fortunately my tooling plate was designed to have a mix of M8 clamping holes and 3.7mm tooling holes and I was going to use this to advantage.   The clamping frame would be symmetrical.   By adding some matching tooling holes in the frame I could cut just over half of the frame and then flip it round 180 degrees and cut the second half. Here is a picture of the CAD showing half of the machining on what will be the underside of the plate when in use. The outer holes are for the M8 clamping to the table and the four smaller holes are the tooling holes.   Being tight with my materials I did not want to just mill out the centre of the plate and have a mountain of swarf (chips).  Instead I designed it with two slots as shown,  one for the clamping surface and one that almost cut through the stock.   The partial cut was to ensure the central piece did not flip out once cut free and damage my cutter. First one half was drilled and cut and then the plate was rotated 180 degrees and the second half cut.   This left the central island just held in place by less than 0.5mm of material.   This was easy to hand cut through to liberate the central area.   The plate was then turned over and the cut edge cleaned using the same tooling position and doing the same 180 degree rotation. To my surprise the rotation process on the tooling pins worked very well with only a minor step transition at the overlap point on all cuts.  This was probably more down to my 3.7mm tooling pins being not quite concentrically turned from 4mm silver steel. With this finished I now had a much more robust clamp for the PCB material.   I had made the clamping step 4mm deep so I could put sacrificial backing boards behind the PCB being run.   This would allow drilling through as needed.   Checking the flatness of a clamped PCB blank with my Haimer showed variation of a few thou in the top surface of the PCB Z position. The worst case variation in Z was at dead centre where the PCB’s natural bow was most dominant.

Tooling and Feeds and Speeds

The next problem was the milling tool and feeds and speeds.   I experimented with various V shaped routers but was not happy with the results.   The 5 thou tip on a 10 degree V tool was incredibly fragile.   Also because the tool was V shaped, any residual bow on the PCB surface lead to a variable width cut.   In the end I opted for Think & Tinkers 15 degree, 2 flute tapered stub (P/N EM2E8-0051-15VC).  This has a 5.1 thou cutting tip which is parallel for the first section so depth variations have no impact on the width of cut.  I ran the program at 10,000 RPM (PCNC 440 maximum) and at 150mm per minute feed rate. The PCB does not look particularly beautiful after milling as there are burrs and shavings present but a gentle rub over with a fine wet and dry removes this and leaves a remarkably clean cut tracking.   The images below show some of the results.   The fine tracking for the USB connector connections is shown on the microscope with a scale for reference.   This shows the five fingers occupying 120 thou with fairly similar track to gap widths of around 15 thou. So now I just have to solder the connectors in place …. I will let you know how it goes.

Overview shot of the clamping plate in position on the my tooling plate on the PCNC440
Finished clamping plate in position on the Tormach PCNC440 holding down a 6″ square piece of single sided PCB.
Tracking on the USB micro connector mounting
Zoom shot on the USB connector tracks with the graticule giving an idea of scale (small divisions are 0.5mm)

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Tormach PCNC440 Bellows Protection

The Problem

It has been one of those things that has been nagging for some time….

I have had a couple of frights while severely destroying metal which were brought about by the rear Y bellows on the Tormach having got filled with swarf (chips).   The machine had tried to do a severe Y movement to the rear of the machine and the bellows began to try to crush the swarf (chips) that had accumulated in its grooves.   While it may not do any fundamental damage it does sound awful and does give rise to a transient expectation of an underwear change.

Some time ago I had ordered in some 1mm Nitrile rubber sheet to solve this problem and it had been sat gathering dust waiting a “non busy” day.   Today was that day. Time to sort this out.

The Nitrile sheet I had ordered in from EBay was 500mm square.  When cut down the middle it would nicely span the bellows.   I also had some asymmetric profile plastic angle section measuring 30mm x 20mm x 1mm which had been brought back from the Brico in France.  (For me the French Bricos are a regular source of material as their range of aluminium, steel and plastic sections far outshines our UK DIY stores).

The Method

I cut two lengths of the plastic angle at 250mm long to match the Nitrile, one for the top of the Z axis bellows and one for the table end of the Y bellows.   On the shorter arm of the each piece of plastic angle I put 5 x 3mm clearance holes and marked these though onto the rubber sheet.   I cut the matching holes in the Nitrile sheet using a rotary punch.   I put a slight countersink on the back of the holes in the plastic to reduce protrusion down into the bellows.

The Z axis mounting consists simply of two M5 clearance holes 120mm apart on the wider arm of the plastic angle.   I made these holes elongated to make adjustment of the mounting easier.

The Y axis was not so straightforward in that the two M5 bellows mounting holes into the table are set below the level of the slideway.   These are spaced at 210mm. The plastic angle had to be hacked out to allow for this but this was simple to do on my Gabro notcher as the 1mm plastic is quite soft.   Once again I made the two M5 clearance mounting holes in the plastic angle elongated for ease of adjustment.

The two pieces of plastic angle were fastened to the Nitrile using M3 x 5 countersink screws with a washer and nut facing out from the bellows.   Rather than tighten these down hard and distort the rubber I soft tightened after adding some Loctite.

Fitting the angle to the machine is a bit fiddly as I was working blind.  With hindsight the mounting holes should have been slots rather than elongated holes. This would allow the plastic angle to be slid into place without taking the bellows screws out.

The 500mm length on the Nitrile seems just about right as it does not bulk up too much at extreme table positions.  A little longer perhaps but no shorter. The 250mm width gives plenty of overlap across the bellows to keep them clear of swarf/chips.

I am pleased with the result and I am sure long term my underwear will also benefit.  Below is a guidance sketch of the plastic angle details and a few finished shots of the Nitrile in place on the Tormach PCNC440.

Bellows protection on Tormach PCNC440 using Nitrile sheet
Plastic angle dimensions for the Z and Y axis bellows Nitrile cover mounting
Full image with Nitrile sheet in place
tormach, bellows protector
Z Axis bellows protector sheet top mounting on Tormach PCNC440
tormach, bellows protector
Y Axis bellows protector sheet table end mounting on Tormach PCNC440

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