Member postings for Roger Hart

Glad you got that sorted. Now I thought I knew a bit about electrics and your comment about using one plate for 2 or 3 phase motors intrigued me. So I looked up 'Chinese 3 phase colours' and got this site :-

http://www.openelectrical.org/wiki/index.php?title=Cable_Colour_Code

Which confused me a lot! Maybe it is wrong, but the entries for Ireland and Portugal made me wonder....

Perhaps those who know can advise - I am thinking of handing in my Avo.

Very nice machine. Enjoy the restoration, will you be keeping the 'extended' feed handles, useful for those watchmaker jobs.....

FWIW I would advise as simple a psu as possible - transformer + bridge and maybe a cap - say 10,000uF, oh and a fuse is nice too. I have found fancy power supplies don't like motors because motors tend to draw a heavy current at startup and throw a clever psu into overcurrent shutdown. As they say - KISS.

Re 'definitely', a very big question, which is why I said 'definitely' and not definitely. I could be wrong but I thought the context was a simple stepper for a hobby application in 'open-loop' ie no feedback encoders or the like.

As I am sure you know steppers have tolerances and micro-steps vary according to position and load, they are not always monotonic and steps can get lost if you push your luck. When you add in leadscrew variability and backlash some sort of position encoder and feedback loop is needed for the highest accuracy and consistency which makes for a servo system, but it costs. Whether the drive motor is a stepper or a dc motor or an ac motor is not important, what matters is that when the CNC controller says 'x step 0.001mm' the servo controller does its best to deliver 0.001mm, usually by delivering several steps to the drive motor, sometimes a few more and sometimes a few less, whatever it takes to get feedback that means 0.001mm plus or minus tolerance. The ins and outs of fast and accurate servo systems provide many engineers with hours and hours of happy fun and many very thick books. Oh, and even a servo system can only be pushed so far.

Normally you don't bother with 'move-check' with stepper motors. If the pulse drive levels are right and the timeouts correct the stepper will 'definitely' move the right number of steps - no need to check. Usually you make the step drive routine a subroutine with a parameter - a number in a register - that says 'go this number of steps and CW or CCW'. As is said the usual way to check is with a simulator or run it slowly at first. Getting the phasing right is the usual cause of trouble.

The only excuse for checking is if lots of money or lives depend on the stepper doing its thing. For finding a definite position then some sort of sensor and a 'homing routine' is needed because steppers by themselves have no idea where they are.

Without using interrupts the timer routine can tie up the processor twiddling its thumbs, but timer routines can be designed to 'drop through' and the rest of the system designed to return control to the timer. This can work but makes precise timing difficult but probably good enough for steppers, this technique can also result in a horrible tangled program but OK for a simple job.

Start simple and work up.

Good luck with changing the cap. If you can wind it up slowly on a variac in case of nasty surprises.

If you need to replace the whole psu (imho the easiest solution) the modern ones have mostly SATA power connectors and not many of the old molex connectors. Luckily the wire colours are (normally) the same so a simple way out suggests itself.

I agree with sharp HSS tools, a good cutting oil and centre support - and/or go very carefully. Even so I often find silver steel gives a poor finish. My own trick is to finish off with a very fine file and then wet n dry plus oil backed by a ruler or similar. Take care not to catch your knuckles on the chuck jaws and mind the burr on the non-reduced section. Inspector Meticulous might not approve though.

Travelled to rural South Asia recently and visited a blacksmith's shop. I found this tool interesting, a sort of spokeshave for steel, the smith used it after filing the cutting edge to even up the blade and to take off metal such that the cutting edge became more acute. Surprisingly effective. Never seen anything like it, maybe swordsmiths would be familiar with the technique.

FWIW I reckon a refrigeration compressor - by Westinghouse maybe.

So, my apologies, its a capacitor start/induction run motor so my earlier comment was wrong. But the windings do not seem to be on the usual terminals, so going by winding colours (harder to confuse) we have red+black as the running winding so to live and neutral. The starting winding plus switch is blue+yellow and go in series with the capacitor to live and neutral. To reverse direction then change over either red+black or the blue+yellow.

You may be tempted by Les' suggestion which I would also follow - but be careful.

According to my copy of 'Model Engineers Handbook' the red wire and the yellow wire go to live, the blue wire and the black wire go to neutral. To reverse direction then reverse the yellow and blue only. Of course earth the motor frame. The yellow and blue are the start winding in series with the centrifugal start switch.

I have had trouble like this. The problem started as an occasional trip and got worse and worse. If the flashover (to frame) has persisted for some time the mica insulation will track and it may be impossible to clean it up to remove embedded metal particles. Worth a try though. The alternative is the skip.

Thank you everyone. Will have to consider whether the effort is worth it or to stick to old fashioned methods. I like the miniature ball screws - have not yet enquired re price.....

What is frustrating is that most of cnc seems doable fairly cheaply right up to the mechanics - but whichever way I twist the problem there seems no cheap or easy way past that problem. So to forget it or JFDI is the question.

Looking at feasibility of building a small miller for watch parts - so about 30mm X and Y and say 20mm Z. I have a microscope XY table as a possible start but the question of feedscrews arises. I reckon I can make a reasonable 0.5mm x 5mm screw and lap it and make an adjustable nut but is this likely to have small enough backlash to allow dial engraving/rose engine work?

Alternatively I have some optical grating strip - so would it be better to go the servo route? Or get a big lump of wood, some CI and brass and make a rose engine and do the numbers by hand.

Just to add re use of handle. I find you cannot just wind back out, it is necessary to move the tool away from the work a bit then wind out then reset the tool position. I clip a terry-clip to the cross slide collar to remind me where I got to and slide it back up to vertical when I have added the extra cut.

Try using a mandrel handle. Then everything happens at a sane speed and is much easier to see (and stop) when things are not right. I do most of my smaller screw cutting by handle. I will even admit to keeping the clasp nut closed and winding out - did a 4 start thread that way - fairly easy.

A tale of woe. Recently given a telescope eyepiece and so rummaged in junk box for a front lens, a few bits of brass and rainwater pipe and turned up a nice brass lens cell, all very elegant. Bought a bit of brass tube for the eyepiece to slide into and all worked very nicely - but the eyepiece tube was unfinished. So cleaned up the ends and got to work with wet-and-dry and Brasso - lovely job - tube now slides nicely. So stood 'scope vertically and of course the eye tube dropped straight down the main tube and smashed the objective lens rear element. Of course I should have fitted a collar but ..... At least I now have a magnifying glass project on the stocks....

Oh, forgot the bit about lead and lag of brushes wrt commutator. Normally the brush set was set to lag behind the crossing of the field magnetic lines by a few degrees. So if you reverse the rotation then in theory you have to shift the brush set (not always easy). Also from limited experience I found the brushes object to reversal at least until they bed back in and get used to the new direction.

I have no practical hands-on experience of 3 brush dynamos so what I say may be rubbish. However if I understand the theory aright the 'third' brush picks up enough volts with respect to the 'ground' brush to drive the field coils and allow some regulation, which way round the volts are should not matter. But there is a snag, for a given direction of rotation there is a 'sustainable' and an 'unsustainable' way of connecting the field coils. If you change the direction of rotation then you must swap over the field coil - armature connections and reflash the field coils afterwards. It is the reflash that decides the polarity and the swapover that decides whether the dynamo will startup and generate. If you want +ve wrt to ground then that is the way you flash the coils - no mysterious reversals needed.

For a given direction of rotation a working shunt dynamo (what this is) can deliver +ve or -ve wrt ground depending on which way it is flashed - which is why jump leads and confusion over +ve and -ve earth cars caused so much trouble back in the old dynamo days.

Sounds like Pete may be the main man here.

Spotted a very beatup Mk22 'Sensitive Altimeter' at a boot fair and thought 'wonder how that works'. Luckily it was cheap enough to take a look. Inside an aneroid capsule levers one end of a 400Hz differential transformer and a 400Hz biphase dragcup motor and very fancy gear system and snail cam seek to balance the other end of the differential transformer.

The servo drives a cyclometer and dial mechanism from 0 to 100,000 feet!!! A rectifier bridge drives a flag solenoid to show the thing is 'on' (I suppose). All seems to work - the servo plate says 8 volts but it works merrily on about 4v rms. What now? it seems to change altitude with the weather(ish) but the dial divisions are 100 feet each and by my calculations 1 millibar equals 27 feet change so maybe not one for the sitting room wall.

A mystery is that a front knob sets an auxiliary display with a minimum setting of 800, possibly local air pressure in millibars at takeoff time - anyone any idea?

I do agree that my workshop is my own and my responsibility and I will do as I jolly well please in it. I have usually found the dangerous jobs go OK, it is the simple things that catch you out. Do what you like but try not to hurt anyone - the coroner or the lawyers may have harsh words to say.

Here is a picture to warm the cockles of your hearts - repumping an old Crookes Xray tube about 8 years back. The copper item is a single stage diffusion pump to a 1920s design, in the background is the homebrew vac gauge and the old Edwards mechanical vane pump lies rigth at the back. The white tube hanging out on the right is a capillary tube controlled leak - Crookes tubes need some air in them - a really good vacuum is no good. The spark coil is the big black object below with a motor interrupter to its right. The heater was a 2KWatt theatre lamp run on a variac and a water cooling pipe can be seen emerging from the diff pump. This was a bit Heath Robinson even for me but it did work and delivered the classic apple green glow. I did stand outside when running the tube.... and I am still kicking. Yes I know it was dangerous and I probably broke a few regs too.