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Member postings for John Haine

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

Thread: Varibale Speed for Drill Press (Induction motor?)
18/01/2011 15:36:24
Almost certainly the fan uses a "torque motor" which is made having a higher resistance rotor than a standard induction motor.  These will slow down according to the stator current (less = lower) but correspondingly are rather inefficient and get rather warm.  All induction motors run asynchronously*, the rotor going slower than the stator fields, but torque motors are very asynchronous.  Standard induction motors just stall if you try to operate them in this mode, and then get very hot. 
 
Torque motors are used where you need very simple speed control but efficiency isn't important; or when you just want to exert a constant torque at variable speed.  I used to have an audio tape deck with such motors driving the reels - normally the takeup reel exerted just enough torque to roll up the tape; whilst the feed reel was actually driven backwards by the unrolling tape and the torque kept it tensioned.  Fast forward or rewind involved increasing the appropriate motor current to drive the reels fast.
 
*unless they are "synchronous induction motors" in which case they start like induction motors but when close to synchronous speed the rotor locks into synch with the stator field operating as a reluctance motor.  Ordinary induction motors with "squirrel cage" rotors can't run synchronously.
Thread: Parting off on Myford lathes
16/01/2011 16:18:11
I use a rear toolpost of the type Kirjeng sell.  Blade is ground with a small hollow in the top surface made with a rat-tail diamond file.  I follow the advice of GH Thomas, run the lathe reasonably fast so the cut per rev is under better control.  I always use power feed (now the lathe is CNC'd there's no alternative).  Never have a problem.  Also have Q-cut tool which is good but now seldom used, but was used in the Dickson toolholder on topslide.  Again used power feed but less good 'cos being a carbide tip the edge is lee keen so needs more cutting pressure.
Thread: steam turbine and generator
12/01/2011 22:16:28
Another issue is that generators are intrinsically AC machines, even though they may have a commutator, so you have to consider core losses.  At 60,000 rpm a dynamo would be working at 1000 Hz where ordinary laminated motor cores would be rather lossy.  Best would be either ferrite or even (assuming neodymium magnets) no cores at all?  Also use a diode rectifier rather than attempting a commutator. 
 
You could also think about a Tesla turbine which uses plain discs rather than blades - much easier to make.  Very exaggerated claims are made for them (as for everything to do with Tesla [oh dear, I can feel the flames already]) but worth looking at.  Goole will lead you to more sites about Tesla than you want.  Wikipedia has a reasonable page -
 
 
Thread: Hints and tips
12/01/2011 20:57:07
So, I just measured the resistance of my 10 metre extension lead.  Each core has about 0.5 ohms as far as I can tell (quite hard to measure low resistances as the probe contact resistance is significant).  So let's assume each one was 1 ohm, total 2.  At 10 amps (this lead is rated 13A) the loss is "IsquaredR" which is 200 watts, two old fashioned light bulbs, so it will get quite hot especially since cooling inside the drum is minimal.  But typically you might be running a 100 watt inspection lamp (0.4 amps) and an electric drill intermittently, so if the average current was 1 amp (which it won't be) the power lost will be 2 watts which would hardly warm it.  The lamp on its own will result in 320 milliwatts loss which you can forget about.
 
Message - be sensible, if you're just doing some work in the loft with a light and a drill don't worry, you're more likely to break your neck tripping over the excess cable than setting the house on fire.  But if you're running a lead to the shed an running your lathe and a convection heater then rollout all the cable.
12/01/2011 16:19:15
If the field cancellation wasn't perfect (or if for example it wasn't twin cable but a single strand), any series inductance would decrease rather than increase the current as it (sort of) adds to the series resistance.  The only way there could possibly be an increase in current is if there is also a significant distributed capacitance that resonated with the inductance at the frequency of operation (even then it isn't apparent how it would actually increase the crrent in this configuration).  At mains frequency and with the dimensions of ordinary cable this is very unlikely.
Thread: For those little fiddly bits perhaps
11/01/2011 14:59:30
Thread: Machining a metric external thread
06/01/2011 13:20:38
  http://en.wikipedia.org/wiki/ISO_metric_screw_thread is a good start
Thread: Diamond tool holder.
02/01/2011 16:56:22
Thanks for the suggestions Chris.  I took the same approach to rounding, but maybe it needs a few more strokes.  My holder has the clamp screw from the opposite side with a clamping piece in the tool slot (which is twice as wide as in the simple design - seemed a good idea at the time but was fiddly to make).  Clearance problem was at the front, but easily sorted.  I was using a finish cut of 0.1mm (4 thou) under CNC, but using a turning wizard which only cut towards the h/stock ... I need to play around a bit with speeds and feeds I think, and maybe rewrite the wizard to allow a finish cut in the opposite direction.  Good idea to try finishing with the tool a bit high, I'll give it a whirl.
 
I ground the tool on the Quorn with a cup wheel, this gives a flat face, and if I finish off with passing the tool across the wheel until it sparks out it has virtually a mirror finish.
 
However, first impressions are that this is a great tool.
 
Yes, I should be at Ally Pally, are you demonstrating again?
 
John.
01/01/2011 14:29:22
Well I spent quite a while making my own version last winter, but only yesterday got around to grinding a tool up for it.  After machining a bit more off to obtain enough clearance I found it works a treat, though I think I need to have a slightly greater clearance on the tool. 
 
So, a question to those who have used these a lot: though it cuts very easily, the finish isn't wonderful, I think because of the very sharp tip.  How much rounding do you give the tip, and how do you do it?  I rubbed the appropriate edge of the 3/16 toolbit on a diamond lap and just took the sharp corner off.  Is there any other secret to getting a good finish?
 
All suggestions gratefully received!

Thread: rotary tables
01/01/2011 14:10:15
I would agree that 6 inch is generally fine.  I would suggest that you get one with 4 slots rather than 3 - then a 4-jaw chuck can be mounted very easily without a backplate.  4-jaw chucks bolted to the mill table are a little known but very useful workholding method for smaller parts, as they can clamp against 2 orthogonal surfaces both of which are perpendicular to the table.  Putting the chuck on an RT gives a further improvement in versatility.
Thread: Rulers - my pet peeve
28/12/2010 14:46:12
I have a 15cm and 30cm made by "Products Engineering" in the US.  Very readable satin chrome finish with black engravings, no superfluous clutter.  On one side the top scale is graduated in mm and cm and the bottom in 0.5 mm.  When you turn them over the top
scale is 0.5mm and the bottom in mm.  Best of both worlds.
 
Many years ago the NPL did a lot of research on gauge graduation and for a while you could buy rules where the small graduations were longest in the centre of the unit (cm or inch as appropriate), tapering down to shortest at the "whole unit" lines.  This made it MUCH easier to read, I wonder why no-one still makes them? 
Thread: Mild Steel as material for gears
18/12/2010 17:00:30
Someone, I think Dick Stephen, has described making pinions from mild steel and case-hardening them
Thread: Lignum vitae under the saddle?
09/12/2010 14:28:49
Lignum Vitae was used by John Harrison in his clocks, I think for both arbor bearings and pallets, because it didn't need lubrication.  The Brocklesby clock has been working I think for a couple of centuries without lubrication.
Thread: You are going to the Model Engineer Exhibition aren't you?
07/12/2010 21:04:05
I love the fact that the map on the MEX website shows Esher but doesn't mark the racecourse...
Thread: End mills in a drill chuck
07/12/2010 20:56:49
Is this one still going?
 
Anyway, I read somewhere that one of the big problems doing this is that if you're taking a deep milling cut (as opposed to axial feeding e.g. to spotface), the varying sideways  loading tends to work the chuck off the Jacobs taper.  So OK for axial feeding and very light milling (like Lautard's idea to mark out for cross drilling), but only a last resort for real milling.
 
I guess one solution is to use ER pattern collets so you can hold any size of cutter, but myself having used both ER and pozilock collet chucks I don't think you can beat R8 straight in the mandrel - more rigid, more daylight.
Thread: Myford Super 7 -59 metrification?
01/12/2010 11:16:26
Even a "native metric" S7 (I bought a new one a few years ago, with the large bore mandrel) only has a 1/8" pitch leadscrew, so you do still need metric conversion gears if
you are doing long screws.  However for most applications the approximate metric gearing that Myford give in the handbook is quite good enough at least for fixings.
 
Fitting DROs has been covered in at least one MEW article - problem solved I would think?
 
Or do a CNC conversion and let the PC take care of it....
 
What's gone wrong with the web formatting on this site?
Thread: Another MEW index 1-170
14/11/2010 10:56:44
  Authorname does seem to appear in places - hit ctrl+F to get a search window.
13/11/2010 11:37:11
Brilliant!  Thanks for this, it's very useful.
Thread: Which New Lathe; choices, choices...
12/11/2010 21:17:45
Myford.
Thread: What do we really mean when we say we work in "X" units.
08/11/2010 12:42:13
The only thing to remember is that Myford supply topslide and cross-slide screws to metric, but not the main leadscrew!  I bought my S7 as metris when new and was puzzled when trying to part off to an exact length which needed several turns of the L/S which I assumed was 3 mm pitch...oh no, it's 1/8 inch!  Now driven by CNC so it doesn't matter...
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