Member postings for Andrew Johnston

Rim; one can see the aluminium blocks, bolted to the faceplate, in this picture:

Note the four steel spigots bolted to the faceplate. These provide the drive to the flywheel, and radial location. The clamps on the spokes do not need to be more than nipped up. They are there to stop the flywheel falling off the faceplate.

Andrew

Edited By Andrew Johnston on 05/02/2022 10:09:14

Plus one, did it once, never again. More chatter than an Islington luvvie.

When I machine a flywheel I set it up on a faceplate so I can machine one side of the rim and boss, the bore and the rim at one setting. That only leaves one side to get the rim and boss to width. The flywheel doesn't need to run true to do that, which simplifies mounting on the faceplate.

Andrew

Correct!

Andrew

Not really; two of my mills are floor standing. For the third (CNC) I made my own stand and coolant tank:

Andrew

The cart is before the horse. Before we can advise on what to buy we need to understand what size holes, on what size work, how are the holes placed and to what accuracy?

Everything moves to some extent, so it is unrealistic to demand no movement. A cheap XY table plus swivel vice is inviting the wobbles with associated inaccuracies.

Andrew

Looks like a brazed carbide tool? Wrong tool for the job. You need HSS with a large top rake, or a CCGT polished insert designed for aluminium. Plastics tend to need to be cut rather than sheared.

Andrew

Why not? Phase to ground is no different to single phase 240V. Phase to phase will be a larger voltage, which could cause a larger current. Although the above states that 120V has a bigger current than 240V: people are negative resistance?

Andrew

Depends what you want to do. I rarely run the Bridgeport beyond 3000rpm, as it gets quite noisy. But on the CNC mill I regularly run at the maximum of 5000rpm. For small cutters I have a separate high speed spindle. With a 1mm cutter I run at 24000rpm. High spindle speeds allow a higher feedrate and reduced machining time. But they also have the effect of minimising tool breakage due to small variations in the feedrate.

Andrew

Correct, the range of teeth listed on a given cutter refers to the full circumference, ie, a complete gear. Any given cutter is correct for the lower tooth count, the errors as the tooth count increases get larger, until the next cutter in the sequence is needed. As the number of teeth increases the involute curve becomes more like a line, until, with an infinite count, you get a rack with straight sides. A rack is just a gear of infinite diameter.

Andrew

Don't even think about using a die. The thread should be screwcut for the reasons given by Jason. I use full form insert tooling to get the correct profile. The 1" diameter 20tpi Whitworth thread on this embryo worm wheel was screwcut to fit a large size Clarkson chuck:

Andrew

The pullout plate should have the coolant pump, hydraulic pump, air compressor and a 1/3hp motor all mounted together.

The headstock has two co-axial lever switches, one for forward/reverse and the other pole switches for fast and low speed ranges. They can be operated seperately, or together, irrespective of whether the motor is running or stationary. I think the switches were proprietary, as Britan list silver replacement contacts in the manual. The the direction control switch needs to be break before make. This may be of some help:

Andrew

Unfortunately I no longer have any 3/4" collets. The style of dead length collet used by Britan comes under the DIN6343 standard. The 1-1/4" collet does not appear to a standard size, and I expect the 3/4" collet is the same. The following company claims to cover Britan collets, although I have no personal experience:

Collets

Changing to an ER collet would cripple the machine in terms of use. If I needed a special size collet I'd make one to match the existing design. I have the means to grind collets, but wouldn't bother for home use.

In general capstan lathes are not much better than a boat anchor without tooling. Before I did anything else I'd make darn sure I had all the parts to make a functional machine. All the switch gear on the headstock is missing. As is the lever for high and low speed, if the machine is so equipped. It looks like the machine has an autolock on the cross slide. The toolrest seems to be missing. More importantly in the righthand end of the machine is there a pull out plate with an auxiliary motor that runs coolant, hydraulic and pneumatic pumps?

A picture of all the parts spread out would help identification.

Andrew

I think you're probably on a hiding to nothing trying to use the Myford. I agree with Baz that slitting saws always run slightly eccentric. Another issue is that they can go walkies if not properly supported. Even on my Bridgeport slitting saws often don't cut true when deep slotting. On the horizontal mill they are fine, but it's a heavy old beast.

When I make clevises it's easier to use a slotdrill or endmill end on rather than use a slitting saw to make the slot. A problem with stainless steel is that some alloys work harden if you let the tool rub. An issue may well be that the Myford isn't rigid enough to take a decent cut per tooth to prevent work hardening. If I was going to use a slitting saw I'd use a coarse tooth one, as below, apart from the one bottom right:

The coarse teeth ones have a big enough gullet for the swarf to escape without jamming. Fine tooth slitting saws are intended for shallow slots only.

To summarise, forget slitting saws and use a slotdrill or endmill. Or drill out the waste and finish by filing. When making clevises I cut the slot first on a longer length of material and then turn the remainder using a split bush.

Andrew

The item with square fins, below the transformer, is a selenium rectifier, mostly likely to provide DC for the electromagnetic clutch.So you will need the panel to use the clutch.

Andrew

Paint codes are not standardised across suppliers.

Andrew

The bottom cutting edges of slotdrills and endmills are hollow ground, usually by around 1 degree or so, to avoid rubbing when slotting. A corollary is that when plunge cutting they do not leave a flat bottomed hole. The tools look to be usable without regrinding.

Andrew

The data implies the motor will only run at 1400rpm (100Hz) intermittently, ie, not continuously. I can't see anywhere the duty cycle for intermittent operation. It will only run continuously at 1000rpm (70Hz). Otherwise it seems fairly standard and does what one would expect below 50Hz. The only caveat being that at very low speeds the power output is negligible.

The VFD is basic but seems to be fine. The easy guide gives clear diagrams for fitting external speed and direction control switches. In standard form it is not possible to fit a potentiometer for controlling the speed. One would need to buy the optional I/O expander.

Andrew

At least my machines are ex-industrial so they don't issue red cards when you stall them.

Andrew

VFDs are a convenient way of using an existing 3-phase motor, and make changing speed simple. They also have other useful features such as soft start and current limiting. But consider the following:

1. Some people don't understand how VFDs work, and the tradeoffs involved

2. Some modellers don't run their machines to the limit, so a VFD is often fine

3. Some people don't know the following equation: Power (W) = torque (Nm) x angular velocity (rad/sec)

Andrew

The graph shows what one would expect for a motor driven by a VFD with base speed at 60Hz. Below 60Hz the torque is constant, and power decreases linearly, Above base speed power is constant, and torque decreases in proportion to the increase in speed.

Metal removal rate is determined by available power, not by torque. Looking at the graph if the motor is running at 30Hz then power is down by 50% and you will only be able to remove 50% of the metal per unit time. A belt, or geared, drive system is constant power, which is what you need. As the speed decreases the torque increases to keep power constant.

Reducing speed by using a VFD, without changing the belts, is simply going to cripple the mill in terms of metal removal rates.

Andrew