Member postings for Andrew Johnston

Slowing a motor is essentially a problem of getting rid of the energy in the motor, both mechanical and electrical. If the VFD goes to zero frequency, ie, no drive, the motor will slow as energy is lost to friction. However, the motor also acts a generator which may lead to the DC bus over-volting. So a braking resistor is added to dissipate the regenerative energy and prevent over-volting. This also has the effect of slowing the motor down more quickly. On larger and/or more expensive VFDs the braking resistor can be dispensed with and DC injection braking used instead. In this case the inverter applies a DC voltage to the motor in a sense that causes a DC current to flow in the windings that opposes the current flowing due to the initial magnetic fields in the motor. While this can stop the motor very quickly it cannot be used for long. As the motor slows the DC current will rise, eventually being limited solely by the winding resistance, which will lead to overheating.

Andrew

EN24T isn't particularly hard. It's around 250-300 Brinell, which is mid 20s to 30 on the Rockwell C scale. While that's harder than soft steels it's well below cutting tools and things like springs.

A lot of the early amateur cutting data was intended for lightweight lathes and basic tooling. Carbide inserts weren't even a dream. Unfortunately starting slow doesn't always work, some materials just need to be run fast to get a good finish.

Andrew

The manufacturer of my industrial lathe recommends an ISO68 hydraulic oil for the geared headstock, an ISO68 oil for the apron and slideways (I use a slideway specific oil) and an ISO220 oil for the feed gearbox.

I expect that the maker of my lathe did know what they were doing with regards to lubrication, based on years of making machine tools.

Andrew

Never had a problem with BUE when turning aluminium alloy with aluminium specific inserts. Birdsnesting of the swarf yes, but not BUE. Two things are odd about the pictures. One, the insert doesn't look quite as polished as I'd expect for a CCGT insert. Two, the built up edge seems to run down the side of the insert, not just on the top where you'd expect it to be.

So, as per previous posts, summat isn't right. Cutting speed is correct according to Machinery's Handbook. But we need more details on the material, source of the insert, depth of cut and most importantly, feedrate.

Andrew

For the US equivalent material 4340 with a coated carbide tool and tempered for a Brinell hardness of 275 to 325 Machinery's Handbook gives a turning speed of 400 feet per minute. For 1" diameter work that equates to 1528rpm. Experience indicates that the medium carbon alloy steels need high surface speeds when using carbide tooling to prevent tearing of the surface.

Andrew

I doubt they use algorithms in the sense of reaching a definitive answer without a full search, although probably in the sense that the sequence is gauranteed to halt. I suspect the answers are more likely based on heuristics.

If Google has an algorithm that solves the travelling salesman problem then I expect they're in line for the Fields Medal.

Andrew

It depends upon what is available but as a starting point 70rpm and 2.8"/min feed is good. I'd be cautious and go to the next lower available spindle speed and feed.

I might have been over-ambitious suggesting turning 4" diameter at 1200rpm. I've just turned some 1" EN24T to make some bolts. At 1200rpm the finish was ok, but not perfectly smooth, so i could have gone a bit faster. May be 600rpm would be better for 4" diameter.

Andrew

Indeed; in the majority of cases I'd expect nice round numbers in polar co-ordinates to translate to irrational numbers in Cartesian co-ordinates due to the circular trigonomic functions involved. I expect the reverse is also true, nice round numbers in Cartesian co-ordinates could translate to irrational numbers in polar co-ordinates.

Andrew

In order to use my insert face milling tool on the horizontal mill I need to make a new drawbar with an M16 thread on the end instead of 5/8" BSW. This evening I hacksawed the appropriate length of 16mm diameter EN16T and cut the M16 thread. While I've screwcut many dozens of threads on my imperial lathe this is only the second metric thread I've cut, and the first one must have been 15 years ago:

Since this was a metric thread on an imperial lathe I couldn't use the Ainjest high speed threading unit, so I did it as per the manual leaving the half nuts engaged at all times. I started at 58rpm and dropped to 40rpm as the thread got deeper and I needed to be quicker on the tool retract. The insert tooling left a reasonable finish. It was a bit slow going but at the current rate I'll have kicked the bucket before I need to cut another metric thread.

Andrew

Edited By Andrew Johnston on 11/10/2019 21:49:52

For those whose brains have exploded don't bother reading this!

The problem of working out what change wheels are needed for a given pitch is an example of the travelling salesman problem. In mathematical terms it is NP-complete, in that a solution can be found by a brute force, but finite, search. However, an algorithm to reach the answer quickly doesn't exist.

Andrew

I'm mystified as to why polar co-ordinates should be inherently more accurate than Cartesian co-ordinates. They are both Euclidian geometry and there are precise equations for converting from one to the other, albeit involving trigonometric functions which are inherently irrational. There are functions that are more elegantly expressed in polar terms, but that's not the same as accuracy.

A supplemental question is that if different co-ordinate systems in one geometry have different accuracies how do different geometries, such as spherical, fit into the picture?

Andrew

I don't; both imperial and metric threads are built into the gearbox, although being an imperial lathe the metric threads are an approximation. As standard the manual recommends keeping the half nuts closed when cutting metric threads.

So in theory I can't use the AInjest unit for metric threads. One can get a unit called a Travadial that essentially tracks distance travelled along the bed allowing one to cut metric threads with the Ainjest unit. However, it is not exact and the errors accumulate with number of passes. Should I ever need to cut any significant number of metric threads I'd add an encoder to the spindle, and a bedstop, so I can reset the saddle to the same place and also reverse the spindle such that the spindle to leadscrew alignment is also reset.

Andrew

Incidentally I make the OD of a 36 tooth 10DP gear to be 96.52mm.

Andrew

Simples!

The Equation of a circle has three unknowns. Three unique points give you three indpendent equations. Three equations and three unknowns allows a unique solution for the centre and radius, with a bit of algebraic manipulation.

Andrew

+1 on an Ainjest unit.

I've got an imperial unit, on an imperial lathe, and it makes cutting imperial threads a doddle, even into blind holes like these 3/8" BSP glands:

Once I'd got the DOC sorted it was four passes and about a minute per gland at 260rpm with 4 thou clearance at the back.

Andrew

Even on an industrial lathe some fiddling is required to machine near capacity. This flywheel is over 2" smaller than the listed capacity in the gap, but look at the overhang on the top slide, along with a LH boring bar:

Andrew

That's fine - Andrew

The late, great, JohnS had strong views about the "flexibility" of the Bridgeport mill!

I have great sympathy with your colleague. RS-232 clamp screws are 4-40 UNC and made from non-ductile brass. I've lost count of the number I've broken by "just" nipping up the last little bit. Rotten things, and they're not cheap.

Andrew

Edited By Andrew Johnston on 10/10/2019 12:10:52

There are 36 teeth on the gear being cut.

Just noted that the involute cutter is around 65mm diameter. My spindle speed of 60rpm was based on a 3-1/4" diameter cutter.

Andrew

Must be getting old, I thought I made my 6DP traction engine crankshaft gears from EN8, but looking back I actually made them from EN24T. Gears shown here, before the splines were machined:

If I recall correctly I had to turn at fairly high speeds to get the nice finish with insert tooling; probably 800 or 1200rpm. Depth of cut and feeds were nothing special.

I cut the gears in one pass; anything else is a waste of time and just increases wear on the cutter. Spindle speed would have been around 60rpm. I can't remember what the feedrate was, but I normally aim for around 4 thou per tooth when horizontal milling. Anything much less and the mill shakes. Gear cutting shown here; coolant not essential but it does help cool things down:

The cutter was secondhand, bought from the US via Ebay. A new cutter should cut a complete gear without a problem, although it may depend upon the source of said cutter.

Andrew