Mill power feed speed modification

Hi folks

Hey folks...

I have a Chester 626 mill with power feed, however the useful working speed is somewhere between 0-1 on the pot and I'm noticing that the feed/torque at that low area is irregular at best.

Wondering if anyone has done any mods to improve this, without having to make a whole custom PF? I'm hoping it's as simple as replacing the horrible plastic gears in the unit? I'm assuming they're such crappy plastic items to provide a point of failure redundancy?

Cheers

J

Nothing to do with that; it's all down to cost of manufacture. Once a mold is produced it costs pence to churn out plastic gears.

Andrew

Could be a driver issue. DC motors are inherently pretty good a delivering steady torque down to low rpm but it does need a decent driver to realise that capability. Simple voltage control schemes without proper feed forward and feedback balancing tend to have problems at low rpm.

Clive

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.

It works perfectly well across its range, it's just the thing is too fast at all the speeds the pot can notice. Dialling it right down to where it's useful it seems like the power fluctuates and the speed is very hard to control with any satisfactory accuracy.

I wonder if it just needs a heftier pot?

J

It may be worth trying a resistor between the high speed side wire and the pot terminal of equal resistance to the pot. If it is only suppling a voltage for speed control this will halve the voltage on the wiper terminal so give slower speeds over the range of the pot. If the useful speed is at position 1 then this would move it to position 2.

Martin C

Just an afterthought, what is the speed that you consider useful? How long does it take the table to move 250mm for example?

Edited By Martin Connelly on 24/07/2023 00:49:21

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.

Looking in the OP's albums it is an Align type which seem to give adequate speed ranges on many machines that no doubt have a similar leadscrew pitch so either something is amiss or the OP is expecting different speeds in which case an answer to Martin's question would be useful.

There are certainly different speed requirements from flycutting with HSS and machining a similar surface with a 6 insert facemill. where you could feed 18 times faster.

As I understand things universal motors supplied for AC use can easily be re-configured to run off DC.

If, as Malcom says, it is connected directly across the 110 V supply using only the positive half cycles its never going to do well at low speeds.

Proper electronic controls for "90 V" nominal DC motors are relatively inexpensive these days. Starter price maybe £150 at the affordable end of full on industrial rated ones with rather more output than you need. Probably £50 or less in the lower end, but still decent, import shops but I've no familiarity with what can be found there.

A proper control with feedback and feedforward onboard is enormously better than a simple half wave supply at low speeds.

Due to the characteristics of universal motor half wave works pretty well on a faster, well loaded, motor such as a hand drill. Not so good on lighter loads intending to deliver constant speed.

Clive

When Quinn Dunki ( Blondihacks ) had trouble with the power feed on her mill the problem was with the track on the potentiometer .

Thanks folks. I'll take some measurements re feed speed and come back. I suspect I'll get a buddy to come take a look, who's much better with electronics than I. No doubt he'll understand thes terminology that's gone right over my noggin...

J

There are a number of the plastic gears for about five different makes of these power feeds available on ebay, one might match yours. A replacement for the pot may well be the answer. Shame Maplins no longer exist.

Edited By old mart on 24/07/2023 16:06:13

There are still plenty of other electronic component sellers since Maplin went, try Rapide, RS components , Farnell and CPC, just google components or even Ebay. john

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.

Heftier very unlikely to be the answer, if a pot is too small it gets hot. An oversized pot just runs cooler, buying bigger than necessary wastes money.

Has anyone been 'at it'? The symptoms suggest someone has fitted a log pot rather than a linear type.

Almost for sure a pot fitted for motor control should marked 'LIN'. If yours is marked 'LOG', changing the pot for the correct LIN type should fix it.

(Linear tracks range evenly, whilst logarithmic tracks change progressively more rapidly. Log pots are very common, because the human ear is logarithmic - volume control - but they're inconvenient on motors.)

Dave

Thanks Dave. I'll whip the feed off tomorrow and see what's what.

J