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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: Milling for beginners
26/06/2019 14:00:02

There's no definitive definition. In this picture I would regard all the slittling saws as coarse apart from the one bottom right:

slitting saws.jpg

The size of gullet is the important factor. A fine tooth slitting saw is ok for slotting a screw head. But if you want to take a 1/2" or more depth of cut with a slitting saw you need a large gullet for the swarf, and hence fewer teeth for a given diameter.


Edited By Andrew Johnston on 26/06/2019 14:00:36

Thread: Multi faceted drill bits - really necessary?
26/06/2019 11:11:50

All my standard metric drills are Dormer 4-facet. Probably 90+% of my drilling using them. As Jason says you don't need to faff about with centre popping or drilling with a 4-facet drill. Provided of course that the surface is flat and reasonably smooth.

In 12+ years I've only renewed a handful of the Dormer drills, mostly small ones becaise I've broken, or lost, them. Some have worn out, although they're good for many hundreds of holes before needing replacement. The smaller Dormer drills are so cheap it's utterly pointless wasting time sharpening them; I just buy new ones from Greenwood Tools.

Way back when I was a small kid I used to sharpen drills freehand. Although I've now got a Clarkson and a drill and tap grinding attachment I've never used it for drills so far; taps yes but not drills.


Thread: Solid carbide end mills
26/06/2019 08:55:41

It's rare day when I use small HSS milling cutters. Here's a 4mm carbide cutter machining a cast iron bevel gear:

After Final Cut

There will be no problem using a carbide cutter to form ports. They have a high Young's modulus so don't tend to bend but will snap if mistreated.

The term solid carbide is incorrect. The tools are formed by sintering tungsten carbide in the form of a fine powder in a matrix of cobalt.


Thread: Cross Slide Lock
24/06/2019 14:16:17

Don't need to make anything as my lathe comes with saddle, cross slide and top slide locks as standard. The top slide is locked at all times, except when I need to use it. The saddle is locked during parting off, but never otherwise. I never use the cross slide lock when general turning. It may get used when I'm using the hydraulic copying unit, but it's not essential.


Thread: hepworth copy attachment missing handle
23/06/2019 18:55:33

PM = private message

See the inbox along the top menu bar when logged in.


Thread: Electronic Noise and ARDUINO
22/06/2019 20:22:03

I'd disagree, a complete shield, including cable outers should not be connected to 0V. If you do so you simply introduce noise into the 0V line. You might connect the shield to earth, but that's not the same thing as 0V.

A lot of EMC issues are a matter of sucking and seeing. Spent many a "happy" hour in EMC chambers doing experiments. The most common cause of problems for radiated and susceptibility isues are cables.


Thread: hepworth copy attachment missing handle
22/06/2019 20:13:35

The stylus should replicate the shape of the tool being used. Otherwise you will not get faithful copy. In this example the tool is round with a radius of 1.5mm, so the stylus is simply a 3mm diameter spigot:

hydraulic copy unit.jpg


Thread: Electronic Noise and ARDUINO
22/06/2019 16:01:26

The shield can be perforated. A rule of thumb is that if the apertures are less than a tenth of the wavelength then the shield will act as if it is solid. As the frequency rises the apertures start to act like slot antennas and finally just as a hole.


Thread: Is CAD for Me?
22/06/2019 10:12:26
Posted by Nigel Graham 2 on 21/06/2019 23:23:50:

Even an equal pair of bevel wheels isn't easy to measure and draw though!

Not difficult, but you do need a good understanding of gear design, like this:

governor bevel gear assembly 16 tooth.jpg

And after machining on a CNC mill:

governor bevel gears.jpg


Thread: DTI identification
20/06/2019 20:07:02

I'd be inclined to say metric +/- 10mm with each small division being 0.01mm. However, look at how much the nib is sticking out; which will equate to the total travel, that'll tell you if it's +/- 0.01" or +/- 10mm.

Is it an indicator or a test indicator? The former have a shaft that moves in the vertical direction, the latter normally have a moveable lever that can move in any position it is set.


Thread: Lathe Speed - What am I missing out on?
19/06/2019 11:34:53
Posted by Chris Bradbury on 19/06/2019 11:14:38:

I got a mirror finish once saw a old man looking back at me..........

That's why I haven't got round to fitting a mirror in the newly refurbished main bathroom yet!


19/06/2019 10:47:26

One of the problems with descriptions like "mirror" or "good" finish is that they are subjective and determined by a standard deemed acceptable by the user, which may be different to somebody else. To me a mirror finish means you should get a perfect reflection from the work surface, just like looking into the bathroom mirror.

For a mirror to act as a mirror, ie, mostly specular reflection, the surface finish must be significantly smaller than the wavelength of light. That implies a surface roughness of around 0.1 to 0.2µm. I get about ten times that when turning, which is roughly in line with the range of surface roughness expected, according to Machinery's Handbook. I don't even get a mirror finish on my cylindrical grinder. Better than turning, but not mirror according to my definition.

A look at this thread may be instructive, certainly for carbide inserts the surface speed can be critical depending upon the material ("sticky" steels are among the worst):

While surface speed may, or may not, be important, there are also considerations based on work size, as mentioned. The smallest drill I've used on my lathe is 1/32", can remember what spindle speed but probably 1700rpm. On the other hand turning a 16" flywheel would be pretty exciting at, say, 420 rpm:


Even at 85rpm it's pushing carbide inserts.

As and when I get my standard Pultra instrument lathe running the top speed will be 6000 rpm; the special high speed version went to 10000 rpm.

Opening up the question in a wider sense would anybody only run their mill at a single speed irrespective of cutter size or material? Across my three mills I've run cutters from 30 rpm to 24000rpm. smile o

That's enough pot stirring for the moment; last but one day at work so the countdown has commenced!


Thread: Horizontal mill cutter
18/06/2019 20:49:27
Posted by JasonB on 18/06/2019 16:24:25:

OK so you have big tools but would you not agree that on the lighter machine that the OP has a smaller diameter cutter such as this would give it more mechanical advantage and that the larger the dia the more likely it is to stall.

I wouldn't regard a 4" cutter as big. smile For a given spindle speed then I'd agree that a larger diameter cutter will have less force at the tooth and is therefore more likely to stall. But if the spindle speed is reduced in proportion to the increase in diameter then the cutter is no more likely to stall.

The point I was trying to make is that 1hp can take what looks like a big cut. So a half horsepower should be able to do a whole lot more than the OP intimates. I suspect the drive system. A 1" pulley seems a bit small for what is presumably a V-belt. It's not clear if the motor is being driven by a static, or rotary, converter or by a VFD. And if the latter at what frequency. It would also be useful to know how many poles the motor has. If it's 4 and the motor is running at 50Hz that gives about 210rpm at the spindle, which is a little fast for a 3" cutter in steel.


Thread: Lathe Speed - What am I missing out on?
18/06/2019 20:35:54
Posted by Blue Heeler on 17/06/2019 23:24:38:

I use HSS and carbide insert tooling, turn steel, brass, copper, cast iron, aluminium, plastic and get a mirror finish on all materials.

Good grief, I've obviously still got a lot to learn then. embarrassed First thing is to stop fiddling with the spindle speed knobs. smile


Thread: Horizontal mill cutter
18/06/2019 15:55:55
Posted by JasonB on 18/06/2019 14:50:08:

...........than a 2" or so dia slab mill like Andrew shows

Errr, as stated the slab mill shown is 4" long and 4" diameter.


18/06/2019 14:43:58
Posted by George Jervis on 18/06/2019 03:34:32:

........what I'm finding is either the cutters won't cut or if I take a cut deeper than 3 to 5 tho it trips the 3 phase converter.....

Half a horsepower ought to better than that. Here's a 4"x4" slab mill at work:


Spindle speed is 88rpm, width of cut 40mm, depth of cut 2.5mm and feedrate 160mm/min and the material is hot rolled steel. The slab mill is not blunt, but neither is it sharp; bought secondhand on Ebay some years ago. No doubt somebody will whine that it's an industrial horizontal mill and not applicable to the amateur, aw diddums! smile Sure the mill has a 4hp motor on slow range with belt drive to a countershaft and then a gearbox. But if you do the calculations the mertal removal rate is about 1 cubic inch per minute. A rule of thumb says that requires about 1hp. So half a horsepower ought to be capable of far more than a few thou DOC.

First I'd suspect the drive arrangement; is it definite that there is a half horsepower available at the cutter? Second I'd look at the feedrate. Too slow a feedrate simply means that the cutter rubs rather than cuts, especially with low DOC, which takes more power than cutting. I normally start with a chip load of about 0.1mm per tooth.

I don't know what power motor the mill originally took, but half horsepower seems a little low. I'd be inclined to go up to 1hp.


Thread: Making High Speed Steel Injector D Bits
18/06/2019 09:50:20
Posted by Steamer1915 on 17/06/2019 19:03:40:

If you are grinding between centres, the centres should not rotate. This removes any inaccuracy created by the spindle bearings.

Well, I never knew that. Having looked at the sectional drawing of the workhead spindle on my grinder it does indeed look as if the centre does not need to rotate. However, on the real workhead, where the nose fitting is different, the centre does rotate. The spindle has a indent lock, but it locks the whole spindle, not just the centre. I can't work out how to get the nose fitting off, and if I use too much BF&BI something is going to get broken.

For the time being I'll stick with what I've got. I can grind to size and parallel to tenths, which is the minimum I can measure, and fine for my needs.


Thread: Improving runout in a slitting saw
17/06/2019 21:45:05
Posted by old mart on 17/06/2019 21:19:42:

All of mine are eccentric too, probably not being British made is the reason.

Yep, that'd do it!

With a quality arbor and slitting saw I'd expect the eccentricity to be better than 0.05mm, give or take. A value of 0.3mm is awful. Having said that even with a perfect arbor and quality saw there will still be some eccentricity, and the saw will still go ching ching. If you're taking a reasonable depth of cut and using a good feedrate a small eccentricity doesn't matter.

I've never seen an adjustable slitting saw arbor, or needed to think about it. I guess it comes down to cheap saws probably needing adjustment and better ones not.


Thread: Making High Speed Steel Injector D Bits
17/06/2019 15:03:28

Posted by Nigel Graham 2 on 17/06/2019 13:09:52:

Now, the injector wants the steam to condense almost entirely in the combining-cone, so it can convert as much of the heat as possible into mechanical energy.

Errr, that's not how the combining cone works. The steam is condensed by the water and the latent heat of evaporation released as the steam condenses goes to heat the water, but it doesn't add to the kinetic energy. The key to understanding the operation of the combining cone is conservation of momentum.

The release of the heat in the steam determines the maximum inlet water temperature at which the injector will work, and is dependent upon the inlet steam pressure.


Thread: Limitation of a milling machine with low spindle RPM.
17/06/2019 14:43:32

While in theory it is possible to use small cutters at low rpm by reducing feedrate experience says it leads to broken cutters. The problem with manually feeding at a low rate is keeping the feed constant. It's only too easy to go a bit slow or a bit fast over a fraction of a turn. Too slow isn't serious but too fast can easily exceed the chip load for the cutter. Equals ping and a broken cutter. Been there done that.


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