Member postings for Andrew Johnston

I'd agree with that. I have an AVO (bought from a friend mostly for old times sake) but it does sometimes get used instead of the DVM. Digital 'scopes are great in many ways, but I've seen a number of professionals get confused due to aliasing giving a misleading picture of the waveform.

Andrew

Err, with 1% resistors I think the midpoint voltage will also vary by 1%, ie, 4.455V to 4.545V.

Andrew

The IRF3205 doesn't look like a logic level MOSFET according to the datasheet I downloaded? A simple NPN transistor, such as the FMMT491, will work fine. For bipolars it's good practice to have a resistor from base to emitter, say 10k. Same for a MOSFET, say 100k from gate to source. When processors power up most GPIO pins come up as inputs by default. The purpose of the resistors is to ensure that the transistor is off when the base/gate is essentially floating.

I'd agree with John Haine that an Avo is not the best choice of meter; a 'scope would be ideal. It's not true to say that MOSFETs need no drive current. They can require substantial current, albeit for a short time. On the datasheet gate charge is the key parameter. For digital ICs the majority of the current consumption is due to charging and discharging the gates during switching.

Andrew

I double checked, just to make sure. My reference was Machinery's Handbook, so I don't think it's a case of just far eastern cutters being different.

Each cutter is correct for the lower tooth count in its range. I don't know what tolerance is applied to get each range and I'm not bored enough to do the maths.

Hmmm, RDG seem to have the same numbering for both DP and module; so as always it's buyer beware - check and double check, don't assume.

Andrew

That's true for DP gears, but not for module gears. The numbering of cutters for module gears is reversed. So a #1 cutter for DP gears will cut the same range as a #8 for module gears.

The depth of cut should be marked on the cutter. This is important as it is not necessarily fixed across a given value for DP or module. For DP gears the addendum is 1/P as is the theoretical value of the dedendum. However, mating gears need clearance at the bottom of the space so the dedendum is made larger than theory. A value of 1.157/P is often used, making the total tooth depth 2.157/P. For large numbers of teeth, where less clearance is needed, a total value of 2.125/P may be used whereas for small numbers of teeth, where more clearance is needed, a total value of 2.25/P might be used.

Andrew

You've lost me already! Although I do know a little bit about scanning electron microscopes. When I was doing my Ph.D, some research groups met for coffee each morning on the 3rd floor of the engineering laboratories rather than use the main cafeteria. The two main groups were signal processing and electron microscopes. Although retired one of the regular attendees was Sir Charles Oatley who had been instrumental in turning the scanning electron microscope from a laboratory curiosity to a commercial instrument in the 1950s and 60s.

At which institution are you studying?

Andrew

+1

A dividing head isn't the best tool for the job as the work ideally needs to be horizontal. Talk of involute tooth forms and cutters is irrelevant. From the drawings/pictures of the part the tooth forms are nothing like involute. The only important parameter for the tooth form is that it matches the mating rubber coupling. The parts should be pretty simple to make; don't get bogged down thinking it's a gear, it isn't.

Andrew

Same in the UK:

Apart from cost the main issue with Kurt vices is that they are too big for most hobby mills.

Andrew

Actually I wouldn't be surprised. When I was in college I had a long talk over lunch with one of the Fellows. Turns out he was a biochemist and a pretty good experimentalist. You may have heard of him - Frederick Sanger.

My neighbours are biochemists, he works for Astra Zeneca and has been helping to organise the vaccine trials. So I'm picking up some knowledge from the horse's mouth so to speak.

School is primarily about imparting basic knowledge but university is, or should be, more about how to learn on one's own. For a higher degree it's more about learning to question and how to put facts and ideas together in new ways, or developing completely new ideas.

Andrew

Welcome to the forum.

Swing is a convenient number for marketing, but not necessarily that useful. As it's measured to the lathe bed unless the work is a tube you might not be able to hold work of that diameter as the chuck jaws will hit the bed. Also be aware that the swing over the cross slide will be smaller, so you will not be able to machine a part of diameter equal to the swing and also of the length between centres. I'd add at least an inch (~25mm) to whatever work diameter you might need to get a swing. Same goes for centre to centre, unless you really do want to machine between centres.

Andrew

Welcome to the forum.

Bit unwise to mention a Ph.D. though; the ignorati will now (wrongly) assume that you're practically incompetent.

Andrew

Ok, so if you've only got one gear on the TDI then you must be limited to a few pitches?

Andrew

Just to be clear I'm talking about gears that fit on the TDI, nothing to do with the gears between spindle and leadscrew. A picture of the chart would be useful.

Andrew

I'm afraid so.

A VFD driving an induction motor effectively creates three sine waves spaced 2pi/3 apart electrically, and mechanically in the motor. These create a rotating magnetic field of, theoretically, constant amplitude.

On a brushless DC motor each set of coils is simply activated in turn and is either on or off. So the magnetic field does rotate and the rotor follows. But it's more like a stepper motor than an induction motor.

In either case there's nothing that says the rate at which the frequency changes needs to be linear or continuous. With regards to the drill it will be much cheaper (and reliable) to switch in steps than having to use a potentiometer. Practically speaking, changing speeds in a series of steps rather than linearly isn't going to make much difference. It's a piecewise linear approximation.

Andrew

On industrial metric lathes the TDI has a choice of gears that mate with the leadscrew as metric thread pitches are not all simply related. May be it's something to do with selection of the appropriate gear, if the lathe has them?

Andrew

Unfortunately there is no absolute number for the value that is good or bad. It all depends upon the value of the item to the buyer. As an example I have no interest in it, so for me the value is zero.

Andrew

Charging lithium based batteries is complex, involving both constant current and constant voltage. Any battery pack should have internal circuitry that prevents over-discharge by disconnecting the battery from the outside world. If a battery is fully discharged when put on charge the charger will operate in constant current mode at a reduced current, often a tenth of normal charge rate. Once the battery reaches a preset voltage the charger goes into constant current charging at the full rate. Once the battery reaches another, higher, voltage setpoint the charger will switch to constant voltage so the charge current decreases as the voltage rises. At a final voltage setpoint the charger will stop the charge cycle.

The voltage setpoints need to be accurately measured, better than 1%. This was a problem with early charger ICs until the IC manufacturers upp'd their game and incorporated accurate reference and measurement functions. The setpoints also vary with battery temperature. As a minimum many battery packs have a means of allowing the charger to measure battery pack temperature.

Andrew

I'll leave you to work that one out.

Did you see the recent news story about someone who bought a cheap battery pack on Ebay and is now claiming it burnt his house down after he put it on charge overnight? Even more dumb he hadn't bothered to renew the house/contents insurance.

Check your insurance before trying to charge the battery!

Andrew

Definitely not. They have specific properties, including those listed. My slideway oil is slightly stringy so it stays where it is put; not all slideways are horizontal.

Andrew

Induction motors and brushless DC motors are not the same.

An induction motor is an asynchronous motor, ie, the motor does not run at a speed equivalent to the applied frequency, but slightly slower. The speed difference increases slightly with increased load, and it is the speed difference that generates the torque. A VFD drives an induction motor with a PWM signal where the PWM width varies from pulse to pulse to simulate a sine wave.

A brushless DC motor is simply a DC motor without the commutator. The mechanical commutator is replaced by electronic switches. Like a brushed DC motor the brushless motor is a synchronous motor, ie, the motor turns at a speed commensurate with the applied frequency - no speed difference. The applied waveform is either on or off for a period each cycle. There is no need for PWM as with an induction motor.

The electronics within a VFD could drive either an induction motor or a brushless DC motor, but the control software is very different, with the induction motor being significantly more involved mathematically.

Things like braking resistors and DC injection braking can be applied to both types of motors, although there are are differences. The quickest way to brake a DC motor is to reverse the applied voltage for a short period. The reverse voltage needs to be removed before the motor starts turning the other way. The same can be done with a induction motor. When I change direction on my repetition lathe I simply flick an electrical switch, the motor grunts, and changes direction all in less than a second.

It's a moot point as to whether the DC motor controller on a hobby level lathe would have these advanced features. In theory turning the speed control down quickly should be fine. Effectively it reduces the frequency of the switching waveforms leaving the motor out of sync. The out of sync magnetic fields have the effect of braking the motor until it's back in sync. But it depends upon ones faith in the designer of the original controller. Reading the various tales of woe on this forum I wouldn't trust the designer further than I could throw him.

Andrew