Member postings for Andrew Johnston

I don't have a CNC lathe, so I'd have made the pin on a manual lathe, or the repetition lathe, depending on quantity. I chose my words carefully. I am making all of the studs and bolts, and most of the nuts, for my engines rather than buying them. They are mostly made on the repetition lathe - a sort of handraulic CNC. But I don't model the parts in CAD of any sort; just a quick paper sketch with key dimensions calculated.

Andrew

The problem with plunge cutting with a centre cutting endmill is that theoretically the centre isn't cutting, as the surface speed is zero. If absolutely necessary to plunge I reduce feedrates by half to two thirds. On the CNC mill I use a ramp or helix path to depth.

Andrew

They're true bevel gears where the tooth profile changes uniformly along the face width. Neither of the mathods using a dividing head and horizontal mill achieves that. I've never machined parallel depth bevel gears but have machined bevel gears on a horizontal mill using bevel gear involute cutters. The special bevel gear cutters no longer seem to be available commercially.

Andrew

Beaten to it.

Edited By Andrew Johnston on 05/04/2022 17:24:53

Using carbide cutters for full width slotting I use a depth up to 50% of cutter diameter for 6mm and above, Below 6mm I use 25% of cutter diameter. For less than full width I'll use a DOC up to the full length of the flutes. I use these rules for any metal up to and including low carbon steel. For tougher materials I back off a bit.

This morning I needed to tweak the crankshaft bearing holder caps, as for some reason I didn't machine the oil reservoir cutouts to the drawing. Using a 3-flute 6mm carbide, cutter depth of cut was 0.25" and width of cut up to 6mm.

Andrew

Fraid so!

I make onesies and twosies on my CNC mill as well as larger quantities of parts, like the spokes for my engines. I also do a lot of 2D parts, as well as 3D and 4th axis parts. Some parts could be made on manual machines; others like these bevel gears, cannot:

Many of my parts are designed in 3D CAD, irrespective of machining method, so the only extra needed for CNC is the CAM program. The time for that is often offset by simpler workholding and set up.

Andrew

It is fairly simple to calculate leakage currents due to filter capacitors. It would be more difficult to calculate DC leakage as this would normally result from a fault condition. I agree that a type F is ideal for a VFD, but in practice with the relatively small VFDs used in the home workshop it probably doesn't matter. The only surefire way to know how close one is to the trip limit is to actually measure the leakage currents.

I take the pragmatic view that if something is working then leave it alone.

Andrew

A residual current detector (RCD) is basically a current transformer. Normally the output of the secondary is proportional to the current in the single turn primary winding. However, if two wires are used then the output will be proportional to the algebraic sum of the currents in both wires.

For a single phase system both live and neutral wires are used. If there is no leakage the currents in the live and neutral should be the same magnitude but opposite sign. So the magnetic fields will cancel out, and the output of the current transformer will be zero. If there is leakage to earth then the currents will not be equal, the magnetic fields will not completely cancel, and there will be an output from the secondary.

The same principle applies to a 3-phase RCD. There are two cases, with and without a neutral wire. Consider a 3-phase system connected in star but with no neutral. Applying Kirchoff's current law to the star node the algebraic sum of the three currents is zero. Hence the algebraic sum of the three phase currents is also zero. Whether the system is balanced or not is irrelevant. In this case a 3-phase RCD simply has all three phases running through the current transformer. Intuitively the same is true for a system connected in delta, but I don't have the inclination to prove it mathematically.

If we now look at a 3-phase system in star, with a neutral line, the same applies. The algebraic sum of the the currents at the star point, including neutral, is zero. In this case four wires (three phases and neutral) are run through the current transformer.

So a 3-phase RCD may have three or four wires, and they will work irrespective of a balanced or unbalanced system.

Andrew

Ah, that explains quite a lot.

Andrew

Agreed; pussyfooting around just means that the cutter tends to rub rather than cut, especially with conventional milling. Consider the following:

*** Cutters are consumable items, they wear out and need to be replaced ***

Andrew

I disagree with Blondihacks on a lot of things, including not using carbide cutters on hobby mills. Carbide cutters work just fine on smaller mills. There is one caveat though; there is no point in paying for fancy coatings as they often require high temperatures, meaning high speeds and feedrates, to provide any benefit. I would agree with Blondihacks on not buying cheap cutters. There are two rules for buying cutters:

1. Don't waste money on cheap cutters

2. See rule 1

Cheap cutters are often poorly ground and it is difficult, or impossible, to get a good finish. Also note that carbide is not a homogeneous material; it consists of carbide particles in a binder material, often cobalt. Cheaper cutters tend to have less carbide.

I use K2 cutters from Cutwel for most machining, and their Alu-Power HPC cutters for aluminium. I also use the premium range from Arc and have found them to be excellent.

Using a 4-flute cutter for aluminium is not a good idea. Since aluminium is soft one can use a high feedrate with a large chip load. The problem then becomes one of getting rid of the swarf. With 4 flutes the gullet is small and the swarf can 'weld' to the cutter, like this:

Traditionally 2-flute cutters were used for aluminium, but 3-flute are more common now. Aluminium specific cutters are highly polished as this helps prevent the swarf from clogging the cutter. Flood coolant also helps, but I assume this is not available. Alternatively a squirt of WD40 can help.

Andrew

I was thinking in terms of mechanical output power:

Power = torque x angular velocity

Assuming both voltages are at 50Hz the angular velocity is constant. The voltage reduced by the square root of 3 leads to phase currents reduced by the same. To a first approximation the torque is proportional to the phase current. So the torque, and hence power, is reduced by the same factor, ie, the square root of 3.

Andrew

In a word - no. Running a 415V star connected motor with 240V phase to phase simply reduces currents, and hence torque and power, by the square root of 3.

Andrew

The zero references are arbitrary, not absolute, so MG is correct.

Andrew

I buy my acetone 25 litres at a time from a glassfibre materials supplier, about £2.50p per litre.

Andrew

To avoid confusion on the left is a dial indicator, on the right is a dial test indicator (DTI):

The dial indicator is intended to make precise measurements with the plunger moving axially. Using it to indicate while applying a side load is inaccurate.

If the greater movement of a dial indicator is needed to centre work in a lathe chuck then the technique being used is wrong. One can get well within 0.5mm by aligning the jaws on the chuck with the concentric rings on the chuck body, or by eye against a lathe tool. Or just note where the DTI touches and where it doesn't and adjust according. Then fine tune position using the DTI; I normally aim for less than 0.02mm total runout. I only use a DTI for aligning parts in the lathe chuck. I find it to be fast and accurate, given the correct technique.

Andrew

PS: I acknowledge MikeK beating me to it

Edited By Andrew Johnston on 24/03/2022 16:15:54

I think it does, using Loctite on parts that have been "cleaned" with white spirits doesn't work.

For removing superficial rust on tooling I use wire wool and a quick spray with WD40. For all other cleaning, tools and machined parts, I use acetone.

Andrew

If I have a lot of holes to tap I use a reversing tapping head on the vertical mill, fast and accurate. Takes a few seconds per hole. For smaller numbers of holes I put the tap in the drill chuck, after drilling, and start the thread by rotating the chuck a few turns by hand. The thread is finished by hand on the bench. I normally use spiral flute taps for the majority of my tapping. If using hand taps, for freehand tapping, I start the taper tap a turn or so, and then fine adjust the tap to be perpendicular in two planes by eye. In all cases it's a lot quicker to do the operation than describe it.

Andrew

The picture posted by JohnP is definitely a spiral bevel gear, as the teeth are curved. The gear set probably is driving the spindle, you wouldn't bother with spiral bevel gears just to move an axis. I agree with a previous post that nylon seems an odd material to choose.

Andrew

It's not really a spiral bevel gear, but the teeth do look slightly skew. That seems an odd choice, and makes manufacture more difficult. If possible a picture of the mating gear would be helpful as, presumably, it is less worn.

Andrew

Time to settle down with beer, nibbles and a dictionary, pausing only to don appropriate PPE against flying toys.

Andrew