Member postings for Macolm

On an old item of unknown condition, it is possible that the capacitor was replaced with the wrong value at some point. If it runs without becoming unduly hot, it should be OK with a value of capacitor that works. However, it may not have a centrifugal switch and may be designed with a permanent run winding and capacitor. If so, be more wary of overheating, and the capacitor must be continuously rated.

Unfortunately, the other possibility is shorted turns in one or other winding, and this would likely cause overheating. When testing, be sure to have a senible low value breaker or fuse in the supply.

There is also a type of start relay with a coil in series with the main winding (in fact only a few turns) that pulls in a contact supplying the start winding. This is activated by the initial surge, then drops out at the run current level. There is adjustment for the transition point that has been set up during manufacture.

Obviously a loss of calibration or sticking can give rise to the start winding remaining energised.

I am not clear as to the exact configuration, but I have managed to extract bearing outer tracks by using a suitable size rawl bolt. A split ring can be made up if necessary to fit between the rawl bolt outer and the bearing inside diameter, and aluminium is good for this as it will indent to help with grip. Discard the actual bolt, and substitute with the right size of screwed rod that fits the expander thread. Fit a nut to the other end to actuate the expansion and tighten this well. The tail of the screwed rod then goes through a suitable size sleeve (often one or more pieces from a socket set can be used), then a washer and nut to do the actual extraction. Clearly it depends on sizes and what you have available.

Presumably distilling involves a fair range of temperature. PTFE has quite a high thermal expansion, roughly 1% over 0C to 100C, with a step change near room temperature due to a change of state. It would be better to redesign to avoid threading the PTFE, and to clamp it across a thin section to reduce the effect of the size change.

Edited By Macolm on 09/08/2023 21:05:28

My understanding of how both a screw mounted drill chuck and for that matter a threaded lathe spindle register should work is as follows. Axial truth (parallel-ness to axis) is controlled by contact of flat faces, both parts needing to have these exactly true running. The threads control the centring, with each requiring to be an accurate constant pitch spiral. When tightened, the threads seat all round on the angled faces, determining an exact concentric point (even if there is slack) in combination with the axial alignment of the face contact. Thus the combination of thread and face contact defines an exact repeatable mounting.

There is therefore no point in a register as well. At best it will define the same concentricity as the threads, but any error will inevitable fight with the position defined by the threads. In the same way, a taper interface could have a concentricity error similar to a register. It also is likely to have lower stiffness than the face to face configuration, but if carefully executed it will, of course, work.

I have had good results making up drill chuck mountings on morse blanks by carefully screw cutting and facing, and making sure there is not a”register”.

Presumably you now have access to the back of the damaged screws. If you can lay hands on small self tapping screws about 3.5mm to 4mm outside diameter, you can drill 3mm holes right through the remains of the headless screws. Try to keep the holes centred, but this does not need to be very good. Then screw in the self tapper from the back, and with a bit of luck this will fetch out the remains, leaving the original thread undamaged. If locktite is suspected, enter the self tappers, then heat the part to 150C and unscrew while hot (hold in a cloth).

In this sort of operation, the danger is that the self tapper (or easyout, or whatever) expends the part being removed, thus wedging it tighter. Therefore if it does not turn at reasonable pressure, quickly resort to heat.

Here is a circuit diagram of an Align power feed. There are several incorrect diagrams on the web, but this one seems to be correct for my unit (though it is a different model).

It is certainly worthwhile checking thoroughly for any defects. Near minimum speed, any mechanical unevenness will easily cause speed fluctuations. The best of luck working inside the unit, it is not well designed for component access, though the circuit board isn’t too bad.

According to my ancient GE SCR Manual, circuits like this mostly use motor back EMF to help with speed stability. Unfortunately this must rely on residual magnetism to work, the field not being energised at the point of triggering the SCR (thyristor). In some cases the motor field yoke is made from a material with high magnetic remnance. Anyway, as already said, my unit works surprisingly well, with the available speed range pretty well matched to needs.

To use the motor with a different control method would probably need the field to have its own power supply (parallel field), otherwise you would be stuck with a series universal motor configuration. The voltage for the separate field supply might be able to be increased relatively, resulting in slower motor speeds. Also note that the armature will probably need only 50V DC for full speed, and only a few volts for the wanted speed range.

If the power feed is indeed the Align type as shown on the Chester website, I would suggest there is little chance of improving things by interfering with the control circuit. This connects directly across the 110V supply, though the motor is only energised during positive half cycles. The speed control potentiometer is floating with respect to the supply lines, and such circuits normally use a clever fudge to provide speed stabilisation feed back.

If the setup is just too fast for what is needed, it would seem to need a further mechanical gear reduction somehow. Alternatively, completely redesigned electronics might extend the speed range lower, but that would be a lot of work.

Is the power feed of Align make (or a competent copy)? The genuine Align (Taiwanese) unit uses a universal motor and a traditional SCR controller circuit running from 110V, and the one I have works smoothly over a surprisingly large range of speed. If yours is similar, I suggest you initially check the brushes are free and not contaminated with grease etc.

The main speed reduction is a small gear (5 teeth?) driving a large moulded plastic helical gear, and this is not of a particularly high standard. On my unit this causes a slight cyclical variation in speed, which in turn can occasionally be seen as slight patterning on a fine finish like flycutting. Again, in general the results are satisfactory. Do check out the gear in case you have a bad example. You can remove the unit from the mill to see whether it then runs smoothly or not. This will be without the final gear which fits on the leadscrew.

Assuming mild steel rim on a cast iron wheel, there will be next to no differential expansion between them by heating. The stuck shrink fit parts will also be in sufficiently good thermal contact that they heat up in unison (unless you can attack the rim with a large jet oxyacetylene torch, which might just work if it doesn’t melt first).

There is really no alternative to resorting to press fitting to complete assembly. What this needs is tooling (parallel thickness spacers to directly support the offending parts, and a press or vice that ensures parallel and axial motion. With a sound setup there is a good prospect of successful recovery.

The cheap, nasty, but successful way to do this is to first cut an (oversize) 5mm hole with a slot mill. Then mount the  cutter in the toolpost, one cutting edge aligned just like a boring bar. Now size the hole using it as a boring bar. When you think about it, this is the same as it being offset due to tailstock misalignment, except you can vary the offset accurately.

Edited By Macolm on 19/07/2023 10:18:16

As I understand it, this sleeve is intended to repair a Jacobs chuck with a split outer shell. In turn, the outer shell needs to be sufficiently tight on the internal split threaded actuating ring to prevent slip. I have had Jacobs chucks where these parts slipped when being fully tightened despite not being split! I doubt the repair will be successful unless the sleeve is substantial and a tight press fit.

The convergence of plain lenses is simply a matter of offsetting the centres. They need to go further apart for the L and R images to converge. Unless the focal distance and the convergence roughly match, the brain is unhappy and you get eyestrain.

Proof testing each and every hull they made could have been simply to lower it to the proof depth in a deep enough ocean trench. This does, though, need a long enough length of string!

Also interested in this as it is on the list to try. Surely though, the broach should not “wobble” when rotated, merely rotate on an axis one degree displaced from the rotation axis of the work. Presumably also the broach is advanced along the work axis, not its own rotation axis.

Edited By Macolm on 02/07/2023 15:37:58

Right click and choose "open with - Notepad. This will be text only and harmless, though perhaps of limited use. Generally though, it does help work out what is going on. However, to get this to work on all or most files you may need to enable some options in folder options/view.

If you need to substitute M3.5 screws for 4BA in old electrical boxes, you can buy "rethreaders" from Screwfix and Toolstation amoung others. The resultant fit is not particularly good, and best not to over tighten!

For what it is worth, I did a quick calculation:-

Heating Acetal to 120C:

Thermal expansion coiff of 140ppm x 100C >> 1.4%

for 7mm diameter, hole size increase is roughly 0.1mm

Cooling elastomer to -20C

Thermal expansion coiff of 140ppm x 40C >> 0.64%

for 7mm diameter, insulation size decrease is roughly 0.04mm

So circa 0.14mm increased clearance ( about 6 thou)

Is this enough for feasible assembly? Probably not.

Edited By Macolm on 25/06/2023 10:11:40

Screened cable of itself does not attenuate interference much, interference entering the cable appears little reduced at the other end. Thus both ends should consist of closed metal boxes, and the screen be well connected at each end. This should confine the interference (provided it cannot escape somewhere else). This is why it is sensible to use some form of 360 degree termination of the screen rather than just a single wire to an earth point.

As others have suggested, the mains input should also be filtered to limit escape by that route. Again, the control pendant is another possible escape route, but hopefully the maker of the VFD will have taken some precautions here.

Finally, the motor might not constitute a particularly good screened box, in which case capacitors or other filter components connected at the VFD or within the motor might help. Ensure any such items are rated for the correct mains voltage at least. Be careful! Assuming 240 volts, examples;

https://www.bitsboxuk.com/index.php?main_page=index&cPath=65_81

Unfortunately it can be very difficult to work out what is going on, and many of us have had experience of intractable problems with interference and filtering. Carefully noting what happens when changes are made can build up some insight. Even if something makes matters worse, that information may prove useful.

Screened cable can be useful in managing emissions, but be aware that high frequencies can propagate through it as a coaxial mode. Unlike ventilation holes which work as short sections of cut-off waveguide which have a lower frequency transmission limit, coax has no lower cut off frequency. Thus it should be used to provide distance between the interference and the susceptible circuit.

If things need to be in close proximity, consider feed through capacitors, failing which any filter needs to be connected with as short wires as possible. This should be as near the interference source as possible.