Member postings for Andrew Johnston

The speed of a brushed DC motor is proportional to the applied voltage. Rather than alter the DC voltage, speed control on the these machines is done by pulse width modulation (PWM) of the motor voltage. So to run a 180V motor on a nominal 325V bus you simply limit the PWM to less than 100% so that the motor never sees more than an average180V. That'll be why there are different value resistors in the control circuit. At least that's the theory, anything goes, right or wrong, with these cheap boards.

Andrew

The trouble with experts is that one needs to already be an expert to work out who is an expert and who is merely an ex-spurt.

Andrew

How about:

Why do model engineers keep saying they don't need to use industrial speeds and feeds but are quite happy taking lots of small cuts; and then get obsessed with quick change toolposts to save a few seconds?

Andrew

To answer the specific question, I doubt it makes any difference.

Andrew

The most common cause of broken taps (at least in small sizes) is accidental bending rather than too much torque. I normally drill for 65-70% engagement. A bit more for fine pitch threads and less for materials like stainless steel.

About half my tapping is by machine and half by hand. Machine tapping is done at low to high hundreds of rpm using professional tapping heads. For hand tapping I don't use tapping guides; a turn or two and look at the tap relative to the work in two perpendicular directions and tweak if needed. Probably a quarter of my hand tapping is with taper taps followed by plug if needed. But for most common thread sizes I use spiral flute taps for both machine and hand tapping. Haven't broken one yet hand tapping. Of course all the above is for small taps, say 1/2" or less. For larger taps it becomes more difficult to start by hand and keep alignment. In which case I use a centre in a drill chuck in the vertical mill to locate the end of the tap.

Andrew

In theory a 3-phase motor is smoother than a single phase one as the rotating magnetic field in a 3-phase motor is of constant amplitude whereas in a single phase motor it pulsates. They also provide easy speed changing and useful features such as controlled ramp up and down and quick stopping if a braking resistor is used, They can also be used to jog the spindle which is helpful for tapping or using a die to cut threads. It also depends on how big a PITA belt changing is on a particular machine. That's not an issue for me as all my manual machines are gear driven, except the Bridgeport which is a varispeed belt, so just twiddle the handle.

On the downside at slow speeds the power is reduced as the torque stays constant rather than being multiplied up as with belts or gears.

From a manufacturers viewpoint they allow one to get rid of belts and/or gearboxes which are expensive to provide; an important point when customer purchases are made on the basis of cost rather than performance.

There is an argument that belts and gears can cause vibration that marks the work. Some toolroom lathe manufacturers went to great lengths to isolate the belt/gear speed changing from the spindle itself.

In principle VFDs are pretty straightforward, but you do need a fair degree of knowledge to set one up and to understand the manuals. Whereas belts and gears are dead simple.

Andrew

I rather doubt that every manufacturer writes their own core control code. It's more likely they beg, borrow or steal it. If the latter then it's quite possible the manufacturer doesn't understand it, so they're not going to be able to explain it to the user.

Andrew

It should be built-in, but almost certainly won't be due to cost. I assume that by surge you mean high voltage, fast and short spikes, as opposed to longer term over-voltage. If I remember correctly the standard surge tests have repetitive patterns of both polarities with peak voltages of 500V, 1kV and 2kV.

If the lathe runs off a single phase 13A socket then one can buy plug in surge protectors intended for computers. Provided they are rated for the current then they should be fine. Or one can buy more involved EMC filters that may also contain surge protection, but they're likely to be more involved to wire in safely. That's a caveat bearing in mind the brouhaha going on in the VFD thread.

Andrew

In an induction motor the poles create a rotating magnetic field. This rotating field induces a current in the rotor than in turn produces a magnetic field in the rotor that tries to catch up with the rotating field from the poles. But the rotating magnetic field in the rotor also induces a current (and voltage) in the pole windings. This induced voltage (back emf) in the poles opposes the applied voltage, reducing the current. The rotor never quite catches up with the rotating magnetic field from the poles. The rotor moves at a speed where the back emf and the torqure required (basically set by phase current) balance. The difference in speed is called the slip and it increases as the required torque increases. As the rotor slows the generated back emf falls slightly allowing more current to flow in the poles.

Let's take a standard motor designed to run at rated voltage and power at 50Hz. As the load increases and the torque required goes up the current in the poles increases as the rotor slows slightly. The motor will be designed so that at rated voltage and full torque, at 50Hz, the rated current flows in the poles. This current is driven by the difference between the applied rated voltage and the back emf. The speed at 50Hz is the base speed.

As we increase frequency the rotor turns more quickly and generates a larger back emf that opposes the applied voltage. But we're already at rated voltage, so the difference voltage driving the current through the poles cannot be increased. Instead the current (and hence torque) falls as the greater back emf opposes a fixed voltage. But of course the speed has increased, so although the current (and torque) falls above base speed the power remains constant, at least to a first approximation.

This is in contrast to frequencies below base speed where it is normal to keep the current (and torque) constant and the power decreases in proportion to the speed.

Andrew

If you want to single point screw threads be aware that many small lathes aren't set up for screwcutting and/or only cut a very limited range of pitches. You may need to make a list of threads to be cut and then select a suitable lathe.

Andrew

Seems a bit odd, one would expect the phase currents to go down above the base speed of the motor (probably 50Hz). I'd agree with Steviegtr, reset and start again altering one thing at a time.

Andrew

Welcome to the forum. I was in the north east over the last few days retrieving my glider; could have dropped in for a chat.

Andrew

They look excellent. What method did you use; parallel depth?

Andrew

Interesting, all the square silver steel I've bought has been ground on all four sides. Silver steel is normally made in 2m lengths and sold in 2m, 1m and 330mm lengths. I've not had a problem using the ends of silver steel.

Andrew

Square silver steel used to be available, but it's pretty rare nowadays. Square gauge plate is still available and is ground on all four sides, unlike rectangular gauge plate. The picture in the first post looks more like key steel to me.

Andrew

Beryllium copper still seems to be available. It's not beryllium copper per se that is the problem but when it's in the form of dust or swarf that can get into the lungs. Another useful property of beryllium copper is that it is non-sparking, so tools made from it are mandated at the more stringent end of the ATEX regulations and can be bought, at least in the US,

Beryllium oxide is still used in semiconductors as it has the second highest thermal conductivity after diamond while being an electrical insulator. Again it is toxic when in the form of dust. Back in the 1970s when I started dabbling in electronics the advice was to never cut open the new high power RF transistors that were coming in, as they contained beryllium oxide. Another saying was that a cut due to beryllium metal never healed, although I've never tried it!

Andrew

I've bought a 0.5mm slot drill; good grief it's tiddly widdly. I had trouble seeing the cutting edges even with a magnifying glass. So it's time to make some decisions on feeds and cuts.

Andrew

Normally one would invoke the following equation:

Ebay insert = crap shoot

However the linked to Yamaloy insert (even if they're not the ones being used) are sold by Cromwell, albeit at a slightly lower price. But they're on 2 days which makes me slightly suspicious as Cromwell customers are most likely to be commercial machine shops, not used to waiting.

From the picture the ridges seem quite obvious, but they're stated to be a bit over 2 tenths of the thou deep. At that sort of depth I don't think they'd be so obvious? Not sure it has a bearing on the finish, but 3 thou is a pretty shallow finishing cut.

I've not really had a problem turning bronze or gunmetal with inserts so difficult to make suggestions. But the wide spacing makes one think of a machine related problem rather than the basic cutting process or, possibly, the casting itself?

Andrew

Definitely in series, for the reason given by Les.

I designed an Iridium system for oceanography some years back. Must have been 2G as it took 2A pulses for a few hundred microseconds. Sorted that by using a fast buck converter with several hundred microfarads of low ESR tants on the output. A supercapacitor wouldn't have worked as the PCB needed to fit in an existing 'phone.

Andrew

Wouldn't worry me, but no touching the machine tools without strict supervision!

Oh dear, I've got lots of micrometers, imperial and metric, and I've never bought a new one in my life. Only had one duffer, a 4-5" micrometer bought on Ebay that was about 15 thou under-reading. Sold it back on Ebay and got my money back.

Andrew