Member postings for John Haine

You should look at the views expressed by Apple customers about Apple support and posted, and left, on Apple forums. RDG and MyTime Media just look weak, the first for getting so iffy about people using a word that has been common currency in model engineering for 60 or 70 years and the other for giving in to them.

Well, I just cancelled my direct debit having thought the same about the latest issue as well as several previous ones.

Get R8 taper, then buy an R8 - MT2 sleeve to use your drill chuck. R8 vastly superior for holding milling cutters and the collets are fairly cheap.

I bought mine from http://www.colletsdirect.co.uk/

PXF - essential! Apart from smooth and easy facing cuts, I think it is George Thomas who recommends in his articles on a rear parting tool holder that PXF should be used for parting to ensure a slow and steady feed. Since reading that, and having a rear tool post, I have always used it and never had a jam. Also works fine with the inserted-bit tct parting blades in the front tool holder.

Joe, have sent you a PM.

So, for a while I have had a print + digital subscription. I thought I'd download the new App after the letter with my latest issue, and with some difficulty found the screen where you can activate digital access, input my subs number and surname and clicked Activate. Got a message saying "This subscription is not active". That's why I'm trying to activate it, dumbo!

Anyone else had this problem? Is there a way round it. Please help!

What do you want the threads to do? Short metric threads for fasteners can be approximated on an imperial lathe by the right choice of change wheel often to within the accuracy of the lathe. There are details in other threads here. If you want to make exact threads you will need 127 and 50 tooth wheels to give an exact metric lead fro your imperial leadscrew.

I'm not saying it isn't noise, but it is perhaps coupled more through earth loops than through "radiation" from unscreened cable. The fact that the speed of spurious steps gets higher as the motor slows down might be because the in-line filter gets less effective at lower frequency out of the the VFD. Why did you mention a broken wire? But I have had very peculiar things happening on my cnc mill when wires have come adrift, with quite the wrong axes moving and so on.

Zapp Automation sell screened 4 core power cable. I have bought stuff from them and they are a good supplier.

I think the "choke" is an in-line filter - a choke on the 3-phase output of a VFD would not work very well! (Some variable DC supplies for commutator spindle motors, such as are fitted to Denford mills, have humungous chokes in series with their outputs but these are smoothing DC not AC.) By the look of your photo, your spindle uses one of the the high-frequency 3-phase induction motors as are supplied from China - water cooled, right?

Frankly I doubt that noise coupled from the VFD cable into the stepper cables would directly generate spurious pulses, but it's worth trying screened cable. Many of this kind of problem are caused by earth loops - too many earth connections.

In the recent thread about demagnetisers someone mentioned welding cables "twitching" when the arc was struck, supposedly because of the interaction between the current in the cable and the Earth’s magnetic field. This seemed unlikely to me, I thought it was more likely to be due to interaction between the cables, so I decided to do some figuring, with the help of formulas in Wikipedia.

The Earth’s magnetic field (according to Wikipedia) varies between 25 and 65 micro Teslas depending where it is measured. The force on L metres of wire due to a current I Amps in a field B Teslas is given by the formula B x I x L Newtons. So the maximum force on 1 metre of cable carrying 100 Amps in the maximum Earth’s field is (if the cable runs E-W perpendicular to the field) 0.0065 Newtons. If the cable was parallel to the field (running N-S) there would be no force.

The “Ampere” force per metre between two parallel wires 1 metre apart each carrying A Amp is 0.0000002 Newton-metres per Amp-squared. The force is given in Newton-metres since it is inversely proportional to wire spacing. So two parallel cables 1 metre apart and carrying 100 Amps have a force between them of 0.000002 x 10,000 = 0.002 Newtons, about a third of that due to the maximum Earth’s field. However if the current was increased to 200 Amps, the force between the wires would be 4x greater; and if the cables are closer together it would be greater still. Another factor is that the Ampere force is in the same direction whether the welding current is DC or AC, whereas the net force in the Earth’s field would be zero for an AC welder.

So it looks like the force in the Earth’s magnetic field is of the same order as the “Ampere force”, depending on the actual current and configuration of the cables. But what’s puzzling is that neither force is very big! A one-kilogram weight exerts a force of 9.81 Newtons, so a Newton is about a tenth of the force exerted when you hold up a bag of sugar (in fact about the weight of an apple). The electromagnetic forces on a metre of welding cable seem to be measurable in apple pips – so why do the cables twitch? Are the peak welding currents much bigger? Or is there some other mechanism at work?

The way I did it was to set the 8 degree nominal using the topslide scale; hold a bit of silver steel in my 3-jaw and slide on an ER collet of the same nominal size as the steel, which meant that it slightly gripped the steel and hence ran true; and tweaked the topslide for no error on a DTI run along the side of the collet. Had to be careful that the DTI plunger avoided one of the slots. Seemed to work fine.

Search "glass - metal seals" on Wikipedia. Common problem making vacuum tubes.

Be careful with some of the ikea led replacement "bulbs" as the drive electronics are not well smoothed. The one I use in my bedside light gives a pronounced strobe effect which could be dangerous on moving machinery. The Jansjo ones though are fine, they have an integral led and seem well smoothed.

Robbo, worth noting that PDF viewers usually have a 'snapshot' tool which allows you to copy an image of any part of a doc such as the front page. You can then save the photo as a jpg which can be posted here. Or use the Windows clipping tool.

Jason, ouch!

The primary expects mains voltage but only if it has its complete magnetic circuit. With open-circuit secondary the current is limited by the primary inductance but it certainly doesn't saturate the core!! If it did then the transformer wouldn't work at all when you try to take current from the secondary because the core would saturate even more!

In a normal transformer the magnetic circuit has no air gap to maximise the inductance and minimise the no-load current ("Magnetising current". If you create an air gap so as to put your tool in, the inductance greatly decreases (even a small air gap has a dramatic effect) and the current when connected to the mains will be MUCH bigger than it would be in the original transformer. The coil will probably get very hot and smoke, if the fuse doesn't blow.

If you use a transformer winding, ditch the laminations. Drive the coil either by (a) using a variac to limit the voltage (if you have one), measuring the current with an AC ammeter; or (b) connect a 100W incandescent light bulb in series (if you still have one, but they are still available for special purposes).

My demag coil was made by winding a pound of 28 gauge wire on a 2 inch plastic tube former about 2 inches long with a couple of cheeks made of thin ply araldited in place. I have driven it both from a variac and using the light bulb. Made short work of demagnetising the quill of my VMB.

Finally I doubt that welder cables kick in the earth's field. Any loop of wire (such as formed by the two cables from a welder when you touch the rod on the workpiece) will try to expand due to the interaction of the fields from the two wires - so-called Ampere force. This is the basis of the Ampere Balance used to define the amp in terms of other basic SI units.

It would however work better with a smoothed supply. The field winding (2.5 k) voltage can be kept constant, you could consider feeding the armature from a speed controller, such as a KB Electronics one. Some of these also have the rectifier to drive the field winding.