Member postings for Andrew Johnston

The bevels aren't critical, provided they have clearance top and bottom. Using files to produce the bevels would be a quick and simple exercise.

Andrew

It'd take some pretty hefty machinery to drive a 1-1/8" BSW tap under power. Similarly you'll need a large workshop gorilla to drive the tap by hand, as well as the problem of keeping it perpendicular. It's simpler to rough out by screwcutting and then clean up with a tap. Of course it may well be that the OP doesn't have 1-1/8" BSW taps, in which case screwcutting is the way to go.

I have a set of 1-1/8" BSW taps; I'd go the screwcut and then tap route if I needed to use them.

Andrew

PM sent to the OP.

Andrew

An involute cutter has the correct tooth form for the lowest number in its range.

Andrew

Correct, annoyingly module cutters are numbered in reverse.

Andrew

Somthing of an oxymoron?

To take big cuts you need a big mill with plenty of power and rigidity. Here's my experience. Last year I bought 50mm, 63mm and 80mm insert milling cutters from Arc. Overall I'm impressed with them. On the Bridgeport I only use the 50mm cutter. The Bridgeport doesn't have the power or rigidity to utilise the bigger cutters; if overloaded it tends to lose tram. I have recently been trimming up a lot of hot rolled steel on the Brdigeport. With the 50mm cutter parameters were as follows - 800rpm, width of cut 25mm, 1mm maximum DOC and 350mm/min feed rate. The mill seemed happy with little cutting noise and the chips coming off blue. Note that the Bridgeport has a mechanical varispeed head so even at 800rpm I get full motor power. If that isn't the case the OP might be better running a 10mm 3-flute carbide endmill at around 2500rpm, the full 8mm DOC, 1-2mm WOC and 500mm/min.

The Bridgeport cannot fully utilise the insert cutters, although they are convenient for milling large surfaces and the inserts seem to be lasting well. When I first got the insert cutters I used the 63mm one on the horizontal mill on some hot rolled steel. Parameters were 450rpm, 50mm width of cut and 450mm/min with a depth of cut of (wait for it) 4.5mm - the maximum recommended. The cutter went through the steel like a knife through butter, throwing chip everywhere, and left an excellent finish. But the mill has a 5hp motor, geared spindle and weighs nearly 2 tonnes.

By all means get an insert cutter but accept that it probably won't be possible to run it to the limits.

Andrew

Correct, at least in the short term. As a first approximation torque is proportional to current. At slow speeds (below the base speed of the motor) the currents can be boosted beyond normal as there is more voltage available than needed to reach the nominal maximum currents. However, heat dissipation is I2R, so the increase in current causes a disproportionate increase in heating. Ok in the short term, but not good in the longer term. On my CNC mill the VFD drops the speed slightly and increases current if it detects a short term overload.

As mentioned by Mike vector control can also help. At slow speeds it can compensate for the reduction in currents caused by losses due to rotor resistance.

Vector control is essentially a 3 to 2 dimensional transform which eliminates the frequency component of the measured currents. So the calculations are reduced to a DC problem with two currents to be controlled D (in-phase) and Q (quadrature phase).

Andrew

NB: dunno why the maths uses D and Q whereas the rest of the signal processing world uses the more logical I (in-phase) and Q (quadrature).

For setting work in the 4-jaw chuck I use aluminium extrusions/gauge plate/HSS toolbits as packing if the work needs to be set away from the chuck body. I never leave the packing in place when turning. Friction is not sufficient to keep it in place with any certainty even if the work is tapped into contact.

For spacing work in the machine vice I use piles of gauge plate or accurate (better than I can measure) parallels made by the late father of a friend, who was a toolmaker. If I don't need to remove the packing I will tap the work down. If the parallels need to be removed (for thru drilling for instance) I don't tap down. The lift on the vice is small anyway.

Andrew

I use STEP and IGES formats for transferring data to my CAM program for CNC milling and for interchange with other engineers using different CAD programs. But I use the STL format for transferring data to my 3D printer slicing program.

Andrew

It depends upon the quality of the drawings. The drawings for my traction engines are riddled with errors and omissions. So I am re-designing in 3D CAD from whence I can create 2D engineering drawings. I also create assemblies in CAD so I can check fit and function before generating drawings, like this assembly of the valve gear:

Apart from the errors and lack of information on the drawings other reasons for CAD modelling are to account for metric, rather than imperial, material and to add features that are on the full size engines but not on the drawings.

I'm idle and I expect to make parts once, and also expect them to fit each other even when made months apart. I aim to do my faffing and fitting on the CAD system, not in the workshop.

Andrew

Given that power is torque times angular velocity (aka speed) then half torque at half speed is a quarter of the power. In reality the VFD will have the same torque at half speed, giving half power.

Andrew

With CCMT inserts I use up to 5mm DOC for roughing and anything up to 1mm for finishing. There are three parameters that can be varied, depth of cut, feed per rev and surface speed. Depth of cut has the least influence on tool life. Doubling the DOC simply uses more of the edge, it doesn't increase the wear per unit length.

Andrew

I don't know about the Warco set but be aware that some suppliers use non-standard, or uncommon, insert sizes so you can't buy elsewhere.

Andrew

For Coventry diehead spares contact Wiseman Threading:

Wiseman Threading

I found them helpful when buying spares for rebuilding my Coventry dieheads.

Andrew

A helix and a spiral are not the same. In a spiral the radius changes as the angle increases. A helix is a special case where the radius is a constant.

Andrew

It's common for the drive shaft from the mill table leadscrew to maintain a ratio of 1:1 until it reaches the worm drive on the dividing head spindle. Bearing that in mind, and looking at the existing gear and witness marks, I think the bevel gears are a special case, ie, mitre gears. So we just need a copy of the existing gear in terms of tooth count and angles.

When spiral milling the division plates are still used, unless the helix is single start. While the drive from the mill table creates the helix:

the division plates are still needed for indexing multiple starts:

The only time I've machined a single start helix I used a 4-axis CNC mill:

Andrew

There isn't a magnetic field in the centre hole, at least in theory. The whole point of a toroid is that the magnetic field is contained purely in the core material.

However, it wouldn't be good idea to put the capacitor in the centre for the reasons mentioned above; heat, danger of creating a shorted turn and problems in clamping the toroid.

Andrew

I wondered when someone would fall for that one.

As Mark says, on my lathe setting 20 thou on the cross slide means 20 thou off the diameter, not the radius.

Andrew

Dunno where you got that from, but it's wrong, at least as a universal statement. For sure, in some materials, especially low carbon steels, the finish can be poor with small depths of cut. But in other materials, such as brass and austenitic stainless steel, the finish is much less dependent on depth of cut.

Being lazy If I need to take 20 thou off the diameter I dial in 20 thou on the cross slide, whether I'm using carbide or HSS. No point in taking multiple cuts when one will do.

Andrew

Correct - Andrew