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Member postings for Paul Relf-Davies

Here is a list of all the postings Paul Relf-Davies has made in our forums. Click on a thread name to jump to the thread.

Thread: Small Volume PCB printers?
13/08/2019 08:17:55

Hi all,

Can anyone reccomend a service that will produce a small number of PCBs from an image of the tracks?

I have an existing PCB that has been damaged (missing the copper track*) and is no longer available.

I figure I can scan the PCB, 'fix' the defects, then if I need to find someone who can use the image as mask to produce a replacement PCB.

I'm sure there are companies out there who do this...looking for recommendations.



*the damaged track is part of a spiral contact, so it needs to be both electrically conductive and smooth/accurately placed, so simply bridging the gap won't do.

Thread: Is this a bad idea...?
12/03/2019 21:44:10

Indeed. The plan will be for the gears to be in mesh all the time, but as has been mentioned, engaged/disengaged using a single-tooth dog clutch.

The extra 1/8" I found still allows for the gears to be meshed all the time.

I'll post some pictures when I'm done.


12/03/2019 20:00:16

So..I've gone back and re-measured things.

The shaft the tumbler output 'twin' runs on (and the spindle holding the 2 gears) is an 1 1/8" long. I think there is enough clearance beyond that (before there is a risk of interfering with any change gear combo) to increase the spacing between the 2 gears from 3/8" to 1/2". I think that would give enough space to fit a clutch of some sort.

It certainly should prove an interesting little project.


12/03/2019 14:36:19

Thanks Hopper.. I have read all I can fin on Graham Meek's (elegant) design.... but I think it would need a complete re-design to have ant hope of fitting to my lathe....

It was actually Martin Cleeve's March '77 article in ME that got me thinking on this in the first place...

What I'd really like to see are more detailed pictures of the dog clutch on the ML7 that were in that article. They seem to show the clutch operating mechanism on the outside of the gear pair, rather than between the gears, which would seem to give more 'wiggle' to, but I imagine could limit the change gear combination that you could use...?


12/03/2019 12:55:07

Hi all,

I have a small screw-cutting lathe (similar-ish to an ML7, without the gearbox) and I've been pondering how to improve accuracy, repeat-ability & simplify screw-cutting. Specifically, how to automatically stop the saddle at a the same point on multiple passes.

I did a fair bit of research and have found numerous references & articles talking about de-coupling the change gears from the tumbler with a single-tooth dog clutch.

As far as I can see, the primary facility that these dog-clutch assemblies offer is the ability to dis-engage & re-engage the timing gears while the motor is running. The point (assume) being that the dog tooth in the clutch assembly is stronger than the gear teeth, and is therefore un/less likely to break when the gears are re-engage, while the motor is running...? my (potentially bad?) idea...

My requirement is to be able to automatically dis-engage the timing gears only. I'm happy to re-set things manually. As such, I wonder if I even need to add a dog-clutch assembly?....Could I just add a trigger to disengage the existing tumbler mechanism at a repeatable point each pass?

I guess the bottom line is this: while I appreciate that there is a risk of damage if the tumbler gears are engaged with the lathe spindle turning, is there a similar risk when dis-engaging them with the lathe spindle turning?

BTW - the reason for this train of thought is that the shaft that the tumbler output gear & the timing gear chain input gear is 1) very short - about 3 gear-widths long and 2) acts as the pivot for the tumbler-actuation lever.

I'm sure I could design something to fit, but it occurred to me that there might be a simpler approach

Any thoughts, much appreciated!



Thread: combining dis-similar DC power supplies
07/03/2019 19:15:46

Thanks for your replies...

I had a sneaking suspicion that would be the outcome. Fortunately, for the project at hand, I only need +5v and +12v, for which my donor PSU can supply far more current than i actually need.

I was simply curious as I can envisage that the additional potential combinations to be gained could have been useful for future projects.

Thanks again


07/03/2019 17:19:35

Hi all,

I'm building a bench PSU to support a project in relation to my mill (power for a 3-axis DRO and a powered X-Axis (using an old car wiper motor) & my rusty O-level electronics has failed me blush.

I'm making use of an old but usable PC PSU, which will give various voltages: 3.3v as well as +/-5 &12v.

Given these, I should be able to 'make up' other voltages, for instance +12v & -12v should give a 24v supply.

However, my question is this: Were I to combine 2 supply lines with differing max current outputs, how do I calculate the resultant max current?

In the example above, the +12v supply can deliver up to 15A, but the -12v is only rated up to 0.5A. Were I to combine these to give 24v, what would be the max current I could expect to draw?

I'm sure there is a simple just escapes me!



Thread: Welding helmet
25/02/2019 12:50:07

I bought a 'Tacklife PAH03D' (LINK) from Amazon a few months back. it's currently about £50. It seems extremely good. It is very comfortable & the view you get of work is excellent.


Thread: Sources of 1" bore involute gear cutters..?
14/02/2019 13:30:20
Posted by Brian H on 14/02/2019 13:14:41:

Clock gears are normally a different profile from the involute ones.


Indeed.. I seem to recall reading about cycloidal gears...?

TBH,this is purely experimental.. I'm starting from a set of plans for a wooden clock (ie just a cutting list - no explicit specification of the tooth profile on the wheels) and trying to tweak it to meet the limits of the tools & have (the largest wheel on the plan is 12" is diameter - there is no way I can make that!!.

This is more a learning exercise (in terms of machining techniques) than a requirement to build a particular's be nice to have a 'thing' at the end of the process!

I'm working on the theory that the this point in time, my limitations as a machinist will far out weigh the 'incorrectness' of cutting involute teeth, rather than some other variety (cycloidal?)



14/02/2019 12:44:09

Thanks all, for your input!

I think a new 22mm arbour & a set of 'eBay specials' is the way simplest forward

They will initially be used to cut the wheels for a simple clock, though I can see change-gear cutting duty i the future too.



14/02/2019 10:30:42

Hi All,

I'm looking for a selection of involute gear cutters. I have a suitable arbour with a 1" bore (originally bought to use with slitting saws). Unfortunately I'm struggling to find a source of suitable & reasonably priced cutters to fit this arbour.

Ebay seems awash with cutters with 22mm bores in the profiles I need, at around £9 each

RDG sells then with a 1" bore at about £25 each

That seems to be about it in the ME world, as far as I can find - at least in the cutter profiles I'm after (M2.0)

The bottom line is it would be cheaper for me to buy the cutters from eBay, plus a new 22mm arbour, than to buy from RDG (and and use my existing arbour)


  1. Are RDG's cutters worth the premium price - given I don't imagine they's see that much use...
  2. Does anyone know of any other suppliers of involute cutters with a 1" bore at a more eBay-like price?

I don't object to buying the smaller cutters & a new arbour. or paying extra for the larger cutters, if they represent a significant quality improvement..I'm just interested in exploring as many options as possible



Thread: One off castings
08/02/2019 16:33:59

If you're handy with a computer, you could design a 'virtual' casting plug and then get it 3D printed in metal...?


Thread: Dore westbury mk1
30/01/2019 17:18:12

I'd concur. Its a quiet group, but extremely knowledgeable & willing to lend good advice.


Thread: Hi from North Yorkshire
29/01/2019 14:45:52



Thread: Uncomplicated Steam Boiler for first wobbler please
29/01/2019 12:56:39

I'm not sure where you are based, but you could do a lot worse than to sign up for the Society of Model & Experimental Engineers' 'Polly' course, starting in May (in South London). 

You will end up marvellous little vertical boiler & wobbler engine, plus get a great grounding in a wide range of model engineering techniques.

The ability to ask questions & get feedback on on you progress - plus getting your boiler properly pressure tested - is invaluable. You can't get that from books & videos  


Full disclosure: I built this on the course last year...


Edited By Paul Davies on 29/01/2019 12:59:50

Thread: failed Dial Caliper
25/01/2019 10:44:53

ah.. it appears my reply from yesterday didn't stick.. so..

Thanks for the You Tube links!

Unfortunately, opening it up isn't an option. The dial/bearing assembly is bonded to the caliper body, so disassembly would result in a 0% chance of re-assembly.

Peering at what I can see of the mechanism, the dial's drive pinion seems well supported & fully, its not 'stuck' away from the rack.

I think the problem is that the way the caliper has been assembled, the rack simply does not engage (more than a though or so...some times...) with the pinion. The rack is both screwed & glued down. were it just screwed, I'd attempt to shim it a few thou...but as it stands I would guess that any attempt to un-glue it would result in a very bent rack.

I have a feeling that this is a hopeless quest...but I'll hold onto it, in case I get a burst of inspiration at some point in the future!!

Thanks all for your input!


23/01/2019 16:59:36
Posted by Michael Gilligan on 23/01/2019 11:32:12:
Posted by Paul Davies on 23/01/2019 11:18:43:

Hmm.. interesting...peering at the mechanism, the pinion gear seems to be slightly above the rack (when the jaws are (or close to) 'closed' ). so it may be stick 'up'. Is this what you meant by 'string-loaded'?

I wonder if a squirt of air from my compressor might persuade it back in to proper engagement..I shall try?


You are the first to mention 'string-loaded' Paul

... I wrote 'spring loaded'



P.S. ... careful dismantling, inspection, cleaning, and re-assembly is usually a better idea than 'a squirt of air'

hehe.. it may be that the problem here is that spring is in-fact made of string.,...!!


23/01/2019 11:18:43
Posted by Clive Foster on 22/01/2019 17:57:00:

I think Micheal has the answer.

Sticky mechanism on the spring loaded pinion preventing it from swinging back into proper engagement with the rack. As its dead probably nothing to looks from giving it a blast up the innards with a miracle aerosol hoping to free things off. Not WD40 tho'. Needs to be the type that doesn't leave a sticky deposit behind. Just a trace of long lasting, instrument quality oil.

Had similar issues due to build up of stuff in the rack bought to a head by a large lump. After extracting the lump I carefully cleaned the rack a couple of times. After the initial clean of the rack it was clear there was a lot of stuff caught up in the pinion which worked out onto the cleaned rack and subsequently needed removing. In retrospect the action of the caliper had got very bad over time. Should have investigated sooner.

Always annoyed me that you cannot find verniers having the easy to read double length scale and proper knife-edge internal jaws.


Hmm.. interesting...peering at the mechanism, the pinion gear seems to be slightly above the rack (when the jaws are (or close to) 'closed' ). so it may be stick 'up'. Is this what you meant by 'string-loaded'?

I wonder if a squirt of air from my compressor might persuade it back in to proper engagement..I shall try?


23/01/2019 11:10:35
Posted by Chris Trice on 22/01/2019 14:54:38:

Corrosion is the only thing I can think of that would cause the difference between when you last used it and next time you picked it up. Either teeth have corroded away/stripped or the shafts/bearings they turn on have failed.

Not at all.. there is no corrosion at... were it not for that fact that it is not working, one might think it brand new!

Indeed looking at the pinion gear (with suitable magnification) I can see that it is simply not engaging with the rack.


22/01/2019 14:04:48
Posted by Mick B1 on 22/01/2019 13:47:43:

When I was a miller/turner back in the mid 70s, loads of workmates bought the then-new dial calipers and many suffered issues similar to what you describe - even when the caliper was from a 'reputable' maker. For that reason I've never had one, and lived on verniers, mics, and more lately digitals instead.

However, I have to admit that, since those old days, I haven't heard similar current stories about dial calipers. So I don't know whether makers are reaching down for the old standards again(!), or whether dial calipers are endemically prone to such faults and their use has receded in the interim?

Thanks...very interesting...


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