Member postings for Andrew Johnston

I'm working on it, with the agreement of our esteemed editor. The introductory article is written; just need to do the trials for the first material. The only downer is that the experiments will be done on a Harrison M300, but that's what I've got. I'm not going to buy a fourth lathe just to do the experiments, and there would be nowhere to put it except the sitting room. And I've already got a lathe in the sitting room.

Andrew

Seems a bit odd. A circle can be uniquely defined by three planar, and not co-linear, points. Anything more is over-constrained. So three measurement points are guaranteed to sit exactly in a circle. A fourth point cannot be so, unless the circle is perfect. That's the reason most bore micrometers have three points. In all cases the measuring instrument has be aligned with the bore axis to get an accurate reading. With four points, and assuming the circle is near perfect, then it may be easier to judge when the instrument is aligned?

Andrew

I use 0.2mm, 0.4mm and 0.8mm radius inserts. The 0.4mm inserts get used most for general turning and the radius is a good compromise when a really sharp internal corner is not needed. I use the 0.2mm inserts for smaller diameter work and where a sharp internal corner is needed. The 0.8mm inserts are used for heavy roughing, especially in steel and cast iron, and where I want a nice fillet.

To some extent the smaller the insert radius the smaller the final depth of cut can be and still maintain a decent finish, although many other factors also need to be considered. Similarly the larger the radius the higher the feedrate can be for a given finish, which may help with chatter problems. Speaking of which I've never had a chatter problem due to the 0.8mm radius inserts, but I do drive them fairly hard.

While I find it useful to have all three options available the 0.4mm insert is a good compromise.

Andrew

I'd disagree with that. Sure if one runs a tool into a rotating chuck for instance then something is mostly likely going to break. But if I overload a machine tool simply by being over-ambitious with the cut then I expect the machine to survive without damage. By experiment I've proved that most of my machine tools will happily survive an overload without damage. The only exception is the horizontal mill; as I've never got close to overloading it.

Andrew

Hang on while I go and get my wind up handle. Does the syllabus cover non-Euclidian geometry then? You can't calculate pi, only approximate it, as it's an irrational number.

Wouldn't want to sit my school exams today, although I didn't exactly cover myself in glory at the time either. My secondary school assessed me in the stupid category.

Andrew

In Euclidian geometry the circle is defined by the distance known as the radius. So it's logical that properties of a circle, like circumference and area, are defined in terms of the radius.

The equation given by Nigel is only true if there is no pilot hole, which would seem unlikely, although it hasn't been explicitly stated.

Andrew

Ah well, that's the Bridgeport in the flower bed then. When I stalled it recently playing with a face mill it's knocked the head slightly out of tram. Not surprising it stalled, I was pushing my luck in terms of metal removal, about 2.5 cubic inches per minute. That's pushing it for 1.5hp. But at least I now know where the limits are.

There's no reason why a DC motor drive should be less reliable than a VFD. There's a saying in the world of power electronics that any fool can design a circuit that works, but designing one that doesn't let the magic smoke out under fault conditions takes skill and costs money.

Andrew

Thanks for that; furnace number looks to be 9/82/1050, so just into the 80s.

Andrew

Welcome to the forum; it turns out I've got one of your products:

Very useful it's been too.

Andrew

No! For external threads I normally turn to the nominal diameter. Although to be fair for most of my external threads I screwcut or use a Coventry diehead rather than split dies.

Andrew

Thanks, I'm quite pleased with the finish, which is straight off the mill.

Andrew

It's been an omnishambles of a weekend involving making scrap and terminal harm being done to cutters in pursuit of making parts. However I have produced two spectacle plate rod bushes, or at least that's what I call them. They sit on the spectacle plate on my traction engines and provide support for the rods running to the regulator and 'starting' valves on the cylinder. Here are the parts, based on castings used on the full size engines:

And in situ on the spectacle plate with home made bolts and nuts:

And the rear view, also showing the front plate complete with cutouts for the water pump rod and the steam distribution block:

Tomorrow's job is to make another part; one of those shown in the first picture is scrap.

Andrew

That's one reason I use full form inserts; you don't need to faff about cleaning up threads. Why make life difficult? My personal view is that 1.5mm pitch is rather coarse for the application.

Andrew

While most 3-jaw chucks have two sets of jaws, small precision 3-jaw chucks for use on instrument lathes normally only have one set. which are reversible. The 'threads' on the rear of the jaws are like a very flat ellipse.

Andrew

The arbors I wanted are finally in stock so on Thursday I ordered a 50mm and 63mm facemill, a R8 and ISO40 arbor and 10 off of each insert type. The order was delivered on Friday morning.

The facemill and arbors are a nice slide fit and the inserts sit snuggly in the pockets. I haven't been able to try the 50mm face mill as one insert screw is missing, but I have been experimenting with the 63mm facemill this evening.

I used the ISO40 arbor on an Adcock and Shipley 2E universal horizontal mill. This is a solid mill at a bit under 2 tons with (in the high speed range) a 5hp motor driving through a geared spindle. First I tried 6082 aluminium with the ground and polished inserts. Here's the setup:

My mill is limited by feedrate, the maximum being 430mm/min. I wanted a chip load of about 0.1mm per tooth so (wiht 5 inserts on the facemill) I chose a spindle speed of 900rpm. I started cautiously with a cut of about 60mm wide and 1mm deep. The finish was good as shown here:

At one edge the surface roughness measured 1.04µm, in middle 2.07µm and at the other edge 1.05µm, all Ra. One would expect a variation across with width as the teeth cut at different angles across the width relative to the measurement. Next I went for broke with the same parameters except for the depth of cut which was now 4.5mm, as per the maximum recommended for the inserts. Roughness measurements at edge, middle and the other edge were 1.22µm, 1.66µm ad 1.06µm respectively.

Next I tried hot rolled steel with the ordinary pressed inserts. Spindle speed and feedrate were the same, with a total width of cut of 40mm. Again I started with a depth of cut of 1mm. A similar setup was used:

Another good finish resulted, seen here:

Surface roughness measurements, specified as before, were 0.93µm, 1.49µm and 1.40µm. Finally I gave the facemill a run for it's money using the same cutting parameters as above, but with a depth of cut of 5mm. Surface roughness measurements were 1.15µm, 1.11µm and 0.91µm respectively.

Initial impressions of the facemills are excellent. in all cases the surface finish was good and very uniform. Surface roughness measurements are also consistent.

All cuts were very quiet with no discernable cutting noise and not a hint of chatter. Of course the milling machine is in the brick built outhouse category, which helps. But I'm beginning to wonder what  on earth I have to do in terms of cutting parameters to test the mill to anywhere near it's limits.

Andrew

Edited By Andrew Johnston on 09/02/2020 22:40:32

Duralac?

Andrew

Edited By Andrew Johnston on 04/02/2020 19:33:00

Even with a flat bottomed cutter you don't get an exactly cylindrical pin. Assuming a finite number of flutes on the cutter you end up with a series of facets that are on a helix. On a practical note the centre of the cutter will be operating at a very small surface speed, so it is likely that the surface finish will vary across the pin.

Andrew

For riveted assemblies on my traction engines such as strakes to rear wheel and spokes to wheels I spray the mating areas with grey primer before assembly. My guess would be that the joint would be sufficiently tight (all rivets closed cold) that the thickness of paint is immaterial. The rivet expands to fill the hole, so it's a pretty tight fit. Try getting one out when forming the head has gone pear-shaped!

Andrew

I've recently stocked up on 32tpi and 40tpi Whitworth inserts from Cutwel. Errr, a 32tpi full form insert will cut a 32tpi thread, and the tip radius should be as per the Whitworth thread specification. Here's a short 32tpi Whitworth thread being screwcut in cast iron:

Andrew

While I have the wherewithal to accurately grind a HSS toolbit (Clarkson T&C grinder and swivel vice) I'm idle and just buy threading inserts.

Andrew