Member postings for Andrew Johnston

Time spent learning is never wasted, even if you don't know when you might make use of the new knowledge.

Another potential disadvantage of using the maths function of the 'scope to measure a high side sense resistor is to do with common-mode rejection ratios. Have a look at the maths of instrumentation amplifiers as an excuse to further ignore the domestic duties. There is a classic design for a nifty three opamp configuration that provides very good common-mode rejection while not requiring precision resistors.

Andrew

Not sure it'll make much difference? The 40/60 solder starts to go 'pasty' at about 180°C, at which point I suspect it will have very little strength. The 60/40 eutectic melts at 183°C. A better soft solder choice might be CupSol that melts at 305°C.

My traction engines will run at 170psi and if I recall correctly that's a wet steam temperature of about 190°C. So operating at lower pressures, and remembering that the cylinder will be at lower temperature than the steam, ordinary soft solders of whatever constitution should be fine.

Andrew

It's a bit more complex than that. The 60/40, or to be precise 63/37, mix used for electronic work is eutectic, ie, the freezing and melting points are the same. So the solder goes from liquid to solid almost instantaneously, which is good in preventing dry joints. In contrast some mixes used in plumbing had different freezing and melting points, so there was a range of temperatures where the solder was 'pasty' allowing for the joint to be wiped to leave a neat fillet.

Andrew

Darn right!

Lathes don't get jealous if you've got another one, or more, on the side

Lathes don't moan if you ignore them for a period

Lathes don't complain if they don't get serviced as often as they should

Lathes don't get headaches

Lathes are ready to perform immediately after power on

Lathes don't go on shopping sprees, but love it when you buy accessories

Lathes don't get pregnant

Andrew

That's a very important lesson, and one which isn't always understood.

Andrew

Hmmm, looks like the sort of finish you can get on hot rolled steel if the parameters aren't correct. To add to the parameters from Douglas I'd add feedrate. You need the cutter to cut not rub. I'd run the full depth in one go with a chip load of 0.1mm per edge. To summarise:

1. Check the tram of the mill

2. Make sure cutter isn't blunt

3. Run at a sensible spindle speed and feedrate

4. Brush the swarf away - if you're running fast 'n' furious the swarf tends to get thrown away, which helps

Andrew

The problem is that one still needs to account for phase angles, even with averages. So ideally one needs to use complex numbers, or as I prefer to call them two dimensional numbers.

If you're going to do some measurements, and I'd be interested to see the results, you'd be better off (and safer) using a current transformer, like these:

**LINK**

The datasheets are worse than useless and the frequency range is a bit limited. But they'll give an indication of the current waveforms, and it's a lot safer than playing with high side current sense resistors. You may want to check the maximum input voltage wrt earth before you go connecting to the mains.

Andrew

I couldn't possibly comment on the last statement.

The first statement is correct but for the wrong reason. Agreed that when starting a motor direct on line there is a brief time when the motor currents are much higher than the motor rated current. This is while the motor speeds up and starts producing a back emf that opposes the applied voltage. When running via a VFD the start up currents will not be large. The VFD is programmed to increase the applied voltage over time, keeping the currents to a sensible value during start up. I would expect the basic efficiency of the output stage of a VFD to be on the order of 85-95%. So why the large input currents? Two basic reasons, one the appalling power factor as illustrated by Ian Parkin. Two, most small, and cheaper, VFDs do not have a power factor corrector as a first stage, but simply a rectifier. Once the DC link capacitors are charged the input current is anything but sinsoidal. Current only flows near the peak of the voltage waveform when the input voltage exceeds that of the DC link. So the current is taken in short bursts twice a cycle. During each burst the input current is much higher than one might calculate from simple V x I requirements. The bursts of current are also bad as they create harmonics that are reflected back into the power network. To prevent these most VFD manuals recommend using an external power line filter.

Andrew.

I've turned and milled a lot of cast iron, both castings and continuously extruded bar. Can't say I've ever worried about health issues; after all we are a carbon based lifeform. All the castings I've used have a very soft skin so no issues with hard spots. I also wire brush and grind the castings to get rid of any sand, grit and dirt before machining. I machine dr, and, like Rik Shaw, wipe down first to get rid of oil. Cleaning up is just a simple brush and dump with a final wipe with a paper towel. I turn and mill at speeds appropriate to insert tooling and with DOC and feeds to use the motor power available. I mostly get fine (pin head) size swarf plus some dust. Like this flywheel, 16.5" diameter, 85rpm, 40 thou DOC and 8 thou feed per rev

Andrew

I looked up some CAM I wrote a few years back using a 2mm cutter in aluminium to form heatsink fins. I was running at 4000rpm, 0.02mm chip load, full width and a step down of 1mm. With more experience I'd keep to the same chip load and DOC but I'd be running at a much higher spindle speed.

Translating to a 3000rpm spindle the chip load equates to a feed rate of 120mm/min. Is the finished part clamped down? If not then you may need to restrict the total depth of cut to say, 1.9mm, or program some bridges to stop the part breaking away from the waste material.

Andrew

I've got a number of Clarkson style chucks for threaded cutters so never needed to use ER collets with them. However, I do use ER collets with Weldon style cutters with no problem.

From a commercial stance threaded milling cutters are mostly obsolete and plain shank cutters rule. I've never seen a Clarkson style holder in an autochanger, which could be a problem for industry. My Clarkson holders are infrequently used for medium size cutters, 3/4" and upwards.

Andrew

Ian: Welcome to the forum.

I've got an Adcock and Shipley horizontal, but it's a 2E so I may, or may not, be able to help with the 1ES. Best to ask the question and await the response. It'd be advantageous to give an approximate location as well.

Andrew

It's a production issue. The plastic encapsulation absorbs some moisture. Soldering on commercially produced boards will be done with a reflow oven, where the board with placed components is heated up to soldering temperature and back down again over a few tens to hundreds of seconds using infrared lamps. If the encapsulation has absorbed water it may well boil, cracking the case. It's not an issue for hand soldering, nor after the part is soldered down.

Andrew

Use a larger diameter pipe to the engine and a smaller diameter pipe to the pressure gauge.

Andrew

Looking good!

The poor finish on the top is more likely to be due to slow cutting speed. I assume that the hemisphere was finished using a series of concentric paths, each one moving out and down slightly. At the top the cutter is only working on the centre, so the cutting speed is small (zero at the centre of the cutter). For a given feedrate the chip load is then large and the tool may be more rubbing than cutting. As one moves out and down the cutter starts cutting more on the side so the cutting speed goes up and the chip load goes down, giving a better finish.

Depending upon the part geometry the issue can be overcome by tilting the part, or using a 5-axis CNC mill that adaptively tilts the tool so it's always cutting on the outer part of the ball.

Andrew

Correct, unlike normal bevel gears the parallel depth method uses the small end of the gear to set the design parameters.

Andrew

Reading the blurb the stuff is a lead free solder paste (including flux) intended for surface mount electronic components. The flux will be non-corrosive and/or water soluble. In other words not very effective!

I doubt the paste will work on cast iron, and any resulting joint will be weak.

Andrew

I don't recall being forced to use my right hand in the 1960s, although I started in a private school. In contrast in the 1930s my father was hit on his left hand with a ruler until he used his right hand. That was until my grandmother when down to the school and gave the teacher what for.

I've got a pair of left handed scissors, but I use them right handed.

I fly right handed, except for the Auster where there was only one throttle in the middle, so in the lefthand seat you had to take off and land left handed with the throttle operated by the right hand.

Andrew

Oh dear, the cart seems to be in fromt of the horse.

Ideally one would have specified an acceptable backlash before building hardware and writing software; then one can select the hardware to achieve that accuracy, or at least get close to it. What positional accuracy do you want to achieve?

If we assume you need to move from point X to point Y and then back to point X with a given accuracy then the question arises; can you overshoot position X on the way back and then move forward to it? If so then all you need to do is step past point X by more steps than the backlash and then move forwards again to point X. It's a standard technique when machining for taking into account backlash in items like a lathe cross slide.

Andrew

An industrial DRO should withstand coolant and swarf without loss of function and be reliable over a long period. You also get guaranteed accuracy with an industrial DRO. I got calibration certificates for the sensors with my DRO. Personally I wouldn't believe the accuracy specifications in the link. For a start accuracy and resolution are the same, which is not generally the case when taking measurements.

A cheap DRO may well be adequate, it depends upon the real accuracy that the user actually needs.

Andrew