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.
I'd concur, for steel in particular the supplier matters. I bought this lump of 4" diameter steel on Ebay as it was cheap:
It was to be used as a former for bending sheet metal so quality didn't matter. Just as well, as the finish was poor:
For smaller diameter (less than 1" diameter) I buy a standard 3m lengths from metals4u. For larger diameters, and specialist alloys, I buy from m-machine in Darlington.
|Thread: What is "Mathematics"|
I suspect that is largely the case. I don't suppose that the mathematicians developing the theory of finite fields in the 19th century had any inkling that they would be central to coding theory in the later 20th century. Coding theory underpins mobile phones among other comms systems.
I obviously didn't explain the idea very well. What I meant was related to the statement by PatJ that mathematics is used to describe the universe. Of course it is, but the point I was trying to make is that mathematics does not exist for that reason. It exists in it's own right and is independent of any practical application, at least to pure mathematicians. Mathematics is a useful tool, but is not determined by physical applications.
Calum has explained why that is incorrect. I would add the following to the explanation. Mathematics is not a rule based system, it is an axiomatic system. That is one starts with a list of axioms and then one can create rules that can be proved by reduction to the basic axioms. An axiom is a statement that is taken to be true but is unprovable.
It is possible to create a mathematical system that is complete and consistent. What Godel demonstrated is that a mathematical system that has axioms that allow the existence of natural numbers (in other words is useful) to be derived is incomplete and inconsistent.
The same is true of computer programs; they are not deterministic. Much effort has been expended on developing axiomatic bases and formal verification for computer languages to try and ensure that they will do what is intended. I believe it was Tony Hoare who stated that within any large program is a small program trying to get out. As a starter he invented Hoare logic in an attempt to prove program correctness.
One is highly unlikely to come across inconsistencies in mathematics that excite pure mathematicians. The engineer can safely assume that the mathematics we use will work.
The question of what is mathematics has stumped mathematicians for centuries, but particularly in the 19th and early 20th centuries. Questions like what makes a mathematical system useful, and what are it's characteristics? Or how many infinities are there and what are their properties. In the 19th century it became clear that there are limits to mathematics, but that there are also alternatives to what most people considered to be mathematics. For instance in this thread many people have referred to trig and geometry. What they actually mean is Euclidian geometry, ie, based on planes. There are a myriad of other geometries constructed on other surfaces. Another useful geometry is spherical geometry (a special case of a Riemannian geometry) which, being based on a spherw, has uses in navigation. Each geometry has different characteristids. In Euclidian geometry parallel lines never meet, in spherical geometry they always meet at a point.
In the 1920s Hilbert proposed a study to show that all mathematics follows from a correctly chosen set of axioms, and that said set can be shown to be consistent. This turned out to be impossible on both counts. In the late 1920s Godel showed that within any mathematical system sufficiently powerful enough to contain natural numbers there are true propositions about natural numbers that cannot be proved, or disproved, from the axoims. In simple terms any useful mathematical system is incomplete and inconsistent as a consequence of it's existence. I'm afraid that those posters who think that mathematics is always right, and consistent, are wrong.
There is nothing in mathematics that bears any relationship to the the real world, so I think PatJ is also wrong in his definition. Of course mathematics is very useful to model the world, but it is only a model, which may, or may not, be a good fit. But there is nothing inherent in mathematics that says it reflects the world. Newtonian mechanics provided a good model for describing the physical world up until the late 19th century, when new experiments and measurements seemed to be inconsistent with the Newtonian model. In the end that led to the development of relativity which extends the validity of the model, but has it's own inconsistencies.
|Thread: Making HSS form tools|
Commercially the shapes would be cut on a wire EDM. It is possible to mill HSS with carbide cutters, although some filing would be needed for the sharp internal corner on a curve. Diamond files work well, but they are finishing tools, not for bulk material removal. Heating treating O1 is a darn sight less faff than filing shapes in HSS.
|Thread: chips from cast iron abrasive ?|
I've turned and milled a lot of it, all the bevel gears, pinions and the liners for both my traction engines. It is lovely to machine, soft and consistent with no hard spots or inclusions.
Having said that most of the iron castings for my traction engines have also been soft with no hard spots. I think my supplier uses a small, traditional, foundry.
|Thread: aluminium sticking to end mill|
I know the feeling:
Can we assume that aluminium means an alloy? Pure aluminium is horrid to machine, bit like warm fudge. Summarising a few pointers:
* Use 2 or 3 flute cutters
* ideally use flood coolant or an occasional squirt of WD40
* If no coolant or WD40 can be used then a finish cut climb milling gives a much better finish
* Highly polished carbide cutters are available, specifically for aluminium
* Maximise depth of cut and minimise stepover, tends to give long thin swarf that is less likely to clog the flutes - Edit: for slotting one is stuck with full width cutting, flood coolant is the proper answer. Unless the swarf is continuously cleared not only will it tend to clog but will be recut which doesn't do the cutter any good
* On aluminium alloys I run fast, several thousand rpm and high feedrates so one gets thicker swarf rather than fine which is more likely to clog the flutes. For example slotting 22mm deep on 6082 with a 3-flute 6mm carbide cutter parameters were 2500rpm, feedrate 400mm/min and stepdown 5.5mm per pass. Of course I was using flood coolant, primarily to clear the swarf rather than provide cooling.
|Thread: Myford ID grinder arbors|
This is my internal grinding setup (I don't have the belt guard casting):
I got one internal spindle arbor with my grinder. Judging by the patina and style I judge it to probably be original Myford manufacture:
My spindle thread is the same, 5/8" 20 tpi Whitworth RH. The key dimensions of my arbor are:
Taper small end: ~0.235"
Taper large end: 0.547"
Length of taper: 1-1/16"
Diameter of plain cylindrical section: 0.471"
Diameter of grinding wheel cylindrical section: Nominally 3/8", but is tapered by ~12 thou along the length
The internal thread for the screw that holds the grinding wheel is 1/4" 26tpi RH, ie, BSF.
I have the original manual for my MG12, with the correct serial number manually typed in red on the page regarding serial numbers. The section on the internal attachment is unhelpful. Mostly about how to fit it, and how to do the initial fitting and alignment if it was bought at a later date rather than with the base machine.
I have an older MG12. The internal spindle bolts to the front of the wheel spindle, replacing a plate, as opposed to the later pulldown internal spindle. The dimensions given above do not relate to the internal arbor I have. As far as I am aware the internal spindle didn't change between the MG9 and MG12.
|Thread: New To CAD? No, but....|
As per my example.
If a hollow tube, or shape, is inherently part of the design then I tend to implement it using sketches. But for smaller holes, especially if countersunk, counterbored, or threaded, I use the 'hole' function. The 'hole' function also allows one to take account of the drill point; useful when checking whether a blind hole will unintentionally break through or not.
|Thread: Minimum depth of cut possible with carbide tooling on S7|
Best to sit down and put on the seat belt before looking at prices for CBN inserts.
|Thread: 3 phase query - for a charity project in Africa|
When one is dirt poor needs must. The heating elements are a non-starter due to power requirements. Best option is to try running the fan motor with a Steinmetz connection and see what happens.
|Thread: Minimum depth of cut possible with carbide tooling on S7|
How is an "excellent" finish defined? I'm generally happy with an Ra value in the range 1 to 3 um.
I rarely take cuts of less than a few thou, and only use CCGT inserts for aluminium and plastic. The finish one gets with CCMT inserts at low DOC depends on the material, insert nose radius and feedrate. I normally use 0.4mm radius inserts, with 0.8mm radius for roughing, and 0.2mm radius if I need a sharper internal corner. I almost never drop below 4 thou/rev feedrate for finishing.
Some low carbon steels, like EN1A, brass and austenitic stainless steels (303 and 316) seem fine with low DOC. But EN3, for instance, goes to pot and tends to tear with low DOC.
There is no definitive answer and never will be; experimentation is needed. Just because a particular combination works at one point doesn't mean it will work again in the future; there are too many ill defined parameters.
|Thread: Re-boring Grind Wheels|
If the wheels are that bad from new then bin them. It would make me wonder what else was wrong with the wheel. It's a grinding wheel; so if you try drilling the bore it will equally grind the drill rendering it useless.
For an 8" wheel £11 is very cheap. Personally I wouldn't touch them with a bargepole. I buy my grinding wheels from professional suppliers and reputable makes. At recommended speeds the periphery of a grinding wheel is doing about 60mph. Being hit in the face with a rock doing 60mph just to save a few quid doesn't seem like a good deal to me.
|Thread: Milling Table Flatness - What is acceptable|
Exactly. As SOD says making meaningful measurements is much more difficult than it seems. Often what is thought to be measured isn't anything of the sort.
|Thread: New To CAD? No, but....|
Crosshead CAD - Part 2
Sketch for the cutout for the connecting rod on the XY plane, only two dimensions needed:
Extrude cut to depth midplane, so equi-spaced around the XY plane:
Mirror everything in the ZX plane:
Add hole for the little end pin. Dimension of the hole is in the submenu, no dimensions are needed in the associated sketch, just constrain the point to be on the Z-axis:
Sketch on the end face of the part for the piston rod boss. Only diameter dimensions are needed, both circles are constrained to be on the X-axis:
Extrude to length:
For each feature only one dimension is needed, which ripples through the part if changed. Careful placement of the original sketch avoids the need for many dimensions and construction lines.
Edited By Andrew Johnston on 11/05/2022 21:04:46
Crosshead CAD - Part 1
First sketch on the XY plane, with dimensions:
Line at bottom is constrained to be on the X-axis and on the left to be on the Y-axis. Extrude the sketch on the Z-axis:
Sketch on the end face of the model for the slidebar cutout, only two dimensions needed:
Extrude cut through all:
Mirror everything in the XY plane:
Mirror everything in the YZ plane:
|Thread: .dwg and .dxf files|
When I had parts laser cut the DXF I sent was a 2D drawing with border and dimensions. Why not ask them what they want?
|Thread: Fusion 360 CAM Milling|
It's always difficult to interpret CAM options across packages, but of those shown I think the relevant ones are scallop, spiral and radial. I take scallop to mean that the package chooses a variable stepover to maintain a constant scallop height, giving a consistant surface roughness. Spiral creates a spiral path around the object, although it is not clear if the spiral constants are fixed. Radial creates a series of radial paths starting from the centre. To some extent what path is chosen depends upon the purpose of the part and aesthetics.
I've used all three methods but mostly tend to use scallop as it gives the most consistent surface finish. I assume the size of cutter is determined by the internal fillet at the bottom? In general it is not a good idea to use a cutter the same size as the fillet as there will be a large engagement with possible chatter and poor finish. I try and use a cutter that is smaller than the fillet or internal corner. Assuming the cutter is carbide I'd be running at 10000rpm and 400/600mm/min feed as per JasonB. One needs to be cautious on feedrate. Remember that, depending upon the cutter geometry, on near flat surfaces only one tooth may be cutting, even on a multi-tooth cutter.
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