Member postings for Andrew Johnston

That's interesting too. When I was in the Bedford model engineering club in the early 1970s one of the members gave me a lot of help, and allowed me loose in his workshop on Saturday mornings. I think he worked at Brookhurst Igranic, which made heavy duty electrical control systems for cranes, mines and the like. As I recall the company was taken over by Cincinnati Milacron, although I'm not sure when?

Andrew

Errr, I don't. If I'm in the workshop it's because I've got something specific to do, find or make. Time is precious.

Andrew

Interesting; my syndicate partner in my big glider (Nimbus) did an apprenticeship at Cincinnati in Biggleswade in the mid to late 1960s.

Andrew

I mostly tap dry. If lubrication is needed I use Rocol RTD; mostly on difficult materials like stainless steel and/or bigger taps, say 3/4" upwards. The RTD goo works well, but is a PITA to remove from tap and hole. On aluminium alloy I sometimes use WD40 just to stop the swarf sticking to the tap.

As for drill sizes i usually aim for a thread depth of 60-70%. In tough materials I'll be nearer 50%. For fine pitch threads I aim for 70-80% engagement.

If an internal thread is screwcut I go for 100% thread depth.

Some years ago I did some experiments on thread depth. Material was 6082 combined with a high tensile (12.9 grade) SHCS. The bolt fractured before the internal thread stripped with 50% engagement; I didn't bother testing the higher engagement percentages.

Andrew

Nice machine; if I ever win the lottery a man sized planer is on my list of machine tools. Double column as well. I've always wondered about the planers with only one support column for the tool slide. There must be one hell of a twisting load in the column? I've always had a soft spot for planers ever since I saw one in operation back in the early 1970s at W.H.Allens in Bedford. It must have been 8x8x30, or thereabouts. On my shop floor tour it was planing fabricated crankcases for medium size diesel engines.

Andrew

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

Or like this:

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

Andrew

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

Andrew

Quite so.

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

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

Obviously my mind is rather naughtier than yours.

Andrew

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

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.

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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.

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

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

Andrew

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

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.

Andrew

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:

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

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

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

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:

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:

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:

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:

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

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