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Member postings for Andrew Johnston

Here is a list of all the postings Andrew Johnston has made in our forums. Click on a thread name to jump to the thread.

Thread: Cutting a worm?
21/05/2019 19:08:39
Posted by Graham Rounce on 21/05/2019 17:42:55:

Yes, that's why I was thinking of square-profile threads, which should cause very little up/down bending of the screws?

For a small worm wheel you'll have significant undercut of the teeth to mate with a square thread so the teeth are likely to break in bending instead.

Square threads are almost obsolete for good reason. They have three interference possibilities all of which are independent.

Andrew

Thread: HSS or CS taps and dies
21/05/2019 10:27:53

Or like this:

grinding tap relief.jpg

Although strictly the picture shows the relief being ground from scratch on a home made tap.

Andrew

20/05/2019 23:38:38

Excellent video, the swarf behaves exactly as I would expect with a spiral flute tap.

Andrew

20/05/2019 21:41:52
Posted by Nealeb on 20/05/2019 16:18:52:

Although nominally "machine taps", they work well for hand tapping as well, and generally are faster as you do not need anything like the same to-and-fro motion to break and clear chips or need swapping taper/second/plug. A while back,

Quite so. thumbs up

With spiral flute taps I find that in ductile materials the "swarf" comes out as a continuous ribbon, one ribbon per flute. Very little, if any, swarf remains in the hole. This is true wih hand tapping as well as machine tapping.

Andrew

20/05/2019 21:38:10
Posted by Bill Phinn on 20/05/2019 16:11:43:

I would be interested to know what taps (i.e. brand/range) Andrew prefers and who the "professional tool suppliers" are he buys them from.

For my most used taps (metric, mostly for work projects) I go to my local tool supplier (CIS) - brands are Guhring or Dormer. For other taps I use Drill-Service, brands vary but OSG and FEW are common. On the very rare occasions I need a 32tpi ME tap I haven't got I use Tap & Die.

Andrew

Thread: Cutting a worm?
20/05/2019 20:01:00
Posted by Graham Rounce on 20/05/2019 16:50:08:

Luddite! Lol

Obviously my mind is rather naughtier than yours. teeth 2

Andrew

Thread: Larger VFD/Motors
20/05/2019 19:56:25

Bleep, bleep, bleep I just lost what I typed. Mumble, mumble &*@# forum software. crying 2

Let's try again, but I'll miss out some of the detailed explanations in case I lose the **** lot again. I can always expand on a given topic at a later date if needed.

--------------------------------------

First I think we owe a vote of thanks to SoD for making these measurements. Darn it, should we ever meet I'd even buy him a beer. thumbs up

The current measurements are pretty much what I would expect. The initial short spike is probably some EMC/filter capacitors charging. Looking at the current waveform after that it is not sinusoidal as stated by SoD. If one looks closely the first cycle or so is sine like, but after that each half cycle pulse becomes slightly narrower with a longer period of no current flow between peaks. Exactly what one would expect for a rectifier charging a bank of capacitors starting from full discharge.

With the motor running the current waveform is as expected. Even on no load the motor consumes some power, so the DC link voltage will decease as current is drawn by the motor. The rectifier can only charge the DC link capacitors when the input voltage is greater than the DC link voltage. This only happens near the top of the input voltage waveform. Hence the short current pulses. If an input voltage waveform was superimposed on the current waveform they would nearly coincide, with the current pulse beginning somewhat before peak voltage and ending at, or slightly after, peak voltage.

While it would be interesting to make measurements of the input current with the motor under load we can predict what the current waveform will be. More current will be drawn by the motor so the DC link voltage will drop further on each cycle. So the input current will start flowing earlier in the input voltage cycle. The current pulse will get wider, with the rising edge starting earlier and the trailing edge staying pretty much where it is.

From an electricity generators point of view the input current spikes are bad. That's why most VFD manuals recommend the use of an input filter. As well as preventing high frequency noise from entering the mains they also smooth out the current pulses so the VFD looks more like a resistive load, thus reducing the current harmonics on the mains.

Larger and/or more expensive VFDs may have a power factor corrector at the front end. This replaces the simple rectifier and is essentially a rectifier followed by boost converter that controls input current to keep it proportional to input voltage, in which case the input current will be sinusoidal.

To save SoD looking for his notes the output waveforms, from each phase to neutral, will be a PWM waveform swinging between the DC link voltage and neutral. The base PWM frequency is normally in the range 4-16kHz and the width of the PWM pulses changes every cycle so that the fundamental of the PWM waveform is a sine wave of whatever frequency is desired and with the appropriate phase shift.

Andrew

Thread: Cutting a worm?
20/05/2019 15:56:07
Posted by Graham Rounce on 20/05/2019 15:32:32:

It started off when I saw a basque being tied (zig-zag ribbons down the back).

I would have thought that close quarters manual assistance was the only possible solution. teeth 2

Andrew

Thread: HSS or CS taps and dies
20/05/2019 15:53:00
Posted by Roderick Jenkins on 20/05/2019 13:32:33:

Edit: Why am I so crap at proof reading my own work?

I blame quantum effects. When one proof reads, the words have both the correct and incorrect spelling. Since one has a priori knowledge of what was written one sees the "correct" spelling. It's only once the "post" button is pressed that each word has to make a decision, and some choose to use the incorrect spelling.

Andrew

20/05/2019 15:49:01

Personally I don't buy from TT anymore - chipped the teeth on too many taps. My old ME thread sets are undoubtably CS, as are some of my very old secondhand taps. But the vast majority of my taps and dies are HSS. I buy from professional tool suppliers, and I tend to buy spiral point or spiral flute taps as I do a lot of machine tapping, so CS items are simply not available.

I'm not convinced by statements that CS is harder and/or sharper than HSS, at least without qualification. Hang on while I get a beer and let battle commence. smile

Andrew

Thread: indexible internal thread cutting tool
19/05/2019 21:48:19

I've cut a fair number of internal threads in steel, cast iron and brass with a similar tool, 3/8" BSP in this case:

screwcutting_bsp_me.jpg

I've not had a problem with finish. I'd make the following observations:

  • The insert holder came from a professional tool stockist - Cutwel I think?
  • Likewise the inserts came from Cutwel
  • Is the insert full or partial form? If full form it shouldn't be leaving burrs on the crests at full thread depth
  • I recall the rake angle being zero or slightly positive
  • Is the insert clearing the work? If the holder is at a peculiar angle the insert could be rubbing.
  • Have you tried cutting with the same set up in a more forgiving material?

Andrew

Edited By Andrew Johnston on 19/05/2019 21:48:59

Thread: Source of Machined Nuts
19/05/2019 20:54:54
Posted by Paul H 1 on 18/05/2019 12:33:50:

Andrew, have you made any collets for this lathe yourself?

I haven't, although some of the odder size collets I have look they have been locally made rather than purchased. It wouldn't be too difficult to make a collet if required. The main issue would be forming the hole if not round.

Andrew

Edited By Andrew Johnston on 19/05/2019 20:55:34

Thread: Thread Pitch Info.
19/05/2019 20:50:27
Posted by Howard Lewis on 19/05/2019 16:42:47:

If UK, probably 55 degree, Whit form threads. If American, possibly Unified with 60 degree threads. If European, or more recent Far Eastern most likely totbe Metyric, 60 degree thread,

One needs to be careful about making assumptions. The spindle nose thread on my Elliott 10" universal dividing head (presumably made in the UK) is 2"-10 UNS. The thread form is quite a sharp V, so not exactly like UNC.

Andrew

Thread: Source of Machined Nuts
18/05/2019 10:44:53
Posted by Swarf, Mostly! on 17/05/2019 20:58:30:

Do I remember rightly that you have a dedicated repetition lathe? It might interest some members to recap on such a machine's capabilities?

Correct, I bought it locally from a two man band that were retiring.The lathe is a Britan, made in Cambridge, probably in the mid 1960s. Here's a picture with the key components annotated:

britan lathe - annotated me.jpg

It's quite a small machine, about the same footprint as a Myford ML7, but weighs about 1500lbs. It's similar to a capstan lathe, but is more versatile and is intended for mass production of small parts. Note that the tools are behind the work but the right way up. In normal operation the main spindle rotates clockwise. There are no dials or DROs, everything is done using adjustable stops. In industry the lathe would have been set up by a skilled man and then run by operators on piece rate.

The lathe has a two speed reversing motor; speed and direction changes can be made on the fly, no need to stop and start the motor. There is also an auxiliary motor that drives a coolant pump, a hydraulic pump that provides auto traverse and an air compressor for the bar feeder.

The lathe is collet only, I have the larger 1-1/4" version. The collets are dead length meaning the material doesn't move axially when the collet is closed. The collets are similar to the DIN6343 but are not standard, by coincidence this is a 0.820" hex collet for BSW:

dead length collet.jpg

Like all lathes of this type they are pretty much useless without tooling. Fortunately my example came with a significant number of collets and accessories. And I've managed to buy more tooling via Ebay. More collets are always useful, although I've got nearly a full set of imperial rounds. Here are some accessories:

britan_tooling_2.jpg

I haven't used the lathe to anywhere near it's full potential but I've made most of my own nuts, bolts, washers, studs and threaded "rivets" for my traction engines:

bolts nuts washers.jpg

I'm currently using the Britan to shorten steel rivets before using them on the rear wheel strakes.

Andrew

Edited By Andrew Johnston on 18/05/2019 10:47:44

Thread: Work Holding For CNC Milling
18/05/2019 09:53:16
Posted by John Pace on 16/05/2019 18:20:23:

Should have said pinking shears ,here they are ,one of the first
cnc type jobs that was done back in 2006 .The blades are
gauge plate hardened and only sharpened on the face ,there is
no clearance behind the cutting edge but they worked well enough
to cut the required tapes from paper.

Thanks for the elucidation. Very neat looking job. As I recall full size pinking shears didn't have much, or any, clearance. We used them on Irish linen when recovering the Tiger Moth and on madapolam for gliders. For full size aircraft you could buy narrow rolls of cloth with pinked edges, thus saving a lot of hand cutting!

Andrew

Thread: Source of Machined Nuts
17/05/2019 19:29:05

Plenty of suppliers for machined BA and small metric nuts but not so for BSF. I've been caught by a supplier advertising machined BA nuts sending cold formed BSF nuts. I wanted the old style thicker nuts, so I made my own, 1/4" BSF full and lock and 5/16" BSF full:

nuts_me.jpg

Andrew

Thread: DC Treadmill Motor
17/05/2019 16:20:56

In addition 48V is a standard industrial distribution voltage within automation cabinets and the like, as well as for power over Ethernet units. The problem with 60V is that it is at the SELV voltage, so regulations get rather more onerous.

Andrew

Thread: What Did You Do Today 2019
17/05/2019 14:55:40
Posted by Ron Laden on 15/05/2019 13:31:28:
............from starting how long has it taken you to get to this stage.

I'm embarrassed to say about 10 years. But in my defence I'd note the following:

  • I'm building two engines, so everything in the picture is doubled
  • There are quite a few parts made but not yet installed, like hollow pistons, eccentrics and brake handles
  • All the parts exist in 3D CAD for several reasons; some imperial materials are no longer available, I've made a number of design changes for increased authenticity, I've redesigned a lot of items as the original drawings are poor and riddled with errors
  • I have an out of control garden
  • I've got sailplanes and aeroplanes to fly
  • Until recently I've been working full time

That's probably enough excuses to disguise my indolence. smile

Andrew

Thread: Work Holding For CNC Milling
15/05/2019 20:56:44
Posted by John Pace on 15/05/2019 20:39:26:

In the original article the subject matter was the production of a pair of
shears to make some dummy cover tapes for a model aircraft.

Miniature pinking shears?

Andrew

Thread: Ever have one of those days..?
15/05/2019 10:24:21

It's a rare workshop day when I don't make a mistake. Likewise it's a rare day at the gliding club when I don't get a rollocking for something or other!

Andrew

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