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Member postings for Robin Graham

Here is a list of all the postings Robin Graham has made in our forums. Click on a thread name to jump to the thread.

Thread: A question about reamers and hole tolerances.
16/05/2022 23:25:39

Thanks for replies. I'm actually happy that nobody has been able to explain the table in my opening post. Not having an engineering background I've had many 'Oh gawd, here's another mysterious thing I have to understand' episodes and I thought this might be another. Seems not - the table is just wrong, even allowing for sign changes.

Mick - thanks for pointing out the 40HRC /Glass hard thing, I hadn't noticed that because (apart from difficulties with the numbers) I couldn't really understand the layout of the table. I can cope with 2-D ISO tables!

I have ordered a reamer form ARC, which whilst not 'industrial' will doubtless be more trustworthy than the Amazon offerering.

Comments about the importance of technique and ancillary kit (floating holder) made me realise that even if were to spring £40 or so for a Guhring or Dormer I still wouldn't be guaranteed H7 tolerance. Doh! Well I'm here (primarily) to learn and the comments made me think, so thanks.

The job is one of those 'help someone out' things which crop up from time to time. The 'client' has now agreed to send me the broken bushing so I'll be able to make an accurate copy without worrying about standards. It's been an educational thread for me though.


15/05/2022 21:50:31

I want to make a bushing with a 7mm hole specifically to H7 tolerance. I had a look for a suitable reamer and came across offerings from sourcingmap on Amazon which have the merits of being cheap and being on next day delivery. However I can't understand the table of technical info:


I can't reconcile this with info from other sources which all tell me that 7mm H7 should be between 7.000 and 7.015mm. For example, the entry for 6.5 - 13mm above suggests that for a 7mm mm reamer D will be between 7.000 and 7.005 mm and the H7 column gives a tolerance of +0/-0.027mm. Which seems to suggest the reamer might actually cut under size. Am I misunderstanding the table?

Perhaps I should steer clear and buy elsewhere ( think I probably will actually, ARC's offerings are only a few quid more and they deliver pretty quickly) but I'd like to know what the table above means. If anyone can elucidate I'd be grateful.


Thread: A tyro desoldering question.
13/05/2022 22:08:51

Thanks Duncan, and apologies for delay replying - reply notification got swamped in my inbox. That looks like it should do what I want. A rotary switch would be actually be even better than a toggle for this application.

I'm confident that the existing switch is break-before-make just from visual inspection of the innards.


Edited By Robin Graham on 13/05/2022 22:09:09

10/05/2022 02:20:06

Should have said that if it goes to stage I'll insist that ME gets a credit in the programme!


10/05/2022 02:12:58
Posted by duncan webster on 10/05/2022 00:27:21:

I think you could do it with a rotary wafer switch, not sure how many poles. If you're interested I'll think some more. Something like this. Certainly easier to mount

Thanks Duncan, I had wondered about a rotary switch but I'm obviously out of my depth with this - I had no idea how many different types of switch there are. A rotary switch would be better for this application than a slider from the operator's point of view, so any further input would be welcome.

I got involved with this project because someone else in my town gave up on it and suggested to the 'creatives' that I was the man for the job. The 'creatives' of course see only the effect and are oblivious to the complexities of the mechanisms which underpin their performances . I'm in a grumpy mood!


09/05/2022 23:38:16

Well, I found some 8-pin DP3T slide switches on Amazon (for the princely sum of £6.49 for 20!) and have ordered them. They'll probably be pretty cr@p at that price, but hopefully at least one will be for OK enough for this job.

In the course of my researches I came across a lot of 6-pin slide switches described as DP3T so obviously the designation doesn't specify the switch uniquely.

I also came across a 12-pin DP3T On-On-On toggle switch. A toggle switch would actually be preferable, but I can't see how this one could be made to work in my application. Here is the schematic:


I can't satisfy myself that it's impossible though! If anyone can see a way that it could work or reassure me that it's impossible that would be educational.

Thanks again for help, Robin.

09/05/2022 15:03:27

Thanks all, DP3T On-On-On it is. Now I know that I can go ahead with this.


09/05/2022 13:28:38

-Thanks for further advice. Looking more closely at the switch:


it seems likely that if I bend the sides outwards the whole thing will fall apart and reveal pads onto which I can solder.

I'm having trouble finding a replacement though - searching on DPCO as suggested by Mike brings up switches with two rows of three pins rather than the type I need (see earlier sketch). Does anyone know what the configuration I want is called?


08/05/2022 23:48:02

Thanks for advice. I had hoped that I might be able to get away with leaving the switch in position, but after reading exchanges between Nick, Martin and Noel:

Posted by Nicholas Farr on 08/05/2022 21:01:51:
Posted by Martin Kyte on 08/05/2022 13:07:23:
Posted by noel shelley on 08/05/2022 11:57:23:

The switch may be ON in all positions, to different functions so leaving in place may not be an option. Good Luck. Noel.

The info on the silk layer says 3 positions are ON OFF and DEMO with the centre marked off so it's pretty certain that the switch osolates all pads in the centre position. Easily checked with a meter.

regards Martin

Hi, from my (non extensive but over many years) experience, you can't be certain about anything that looks like a switch is configured in such a way that it must be what you would expect. The switch in the photo is a three way, but it has eight pins, which could mean that in the off position, it may be holding up to four different aspects of the circuit off and just adding a new switch without removing the old one or it's contactor components, the circuit might not work or it may even short other parts out, so the circuit needs to be checked out before just adding another switch. More or less what Noel has said.

Regards Nick.


Edited By Nicholas Farr on 08/05/2022 21:03:10

I took a multimeter to the switch and found:


The hatched things are pcb tracks - I can see the L - shaped track connecting 2,6,7 on the component side and infer the existence of one between pins 5 and 8 because they are connected at all switch positions. Presumably on the other side, obscured by the switch body. I have also inferred the switch connection between 6 and 7 in the 'off' position - symmetry seems to demand it!

The problem with leaving the existing switch in the 'off' position and wiring up another one seems to be that pins 2 and 3 will remain connected in 'on and 'demo' modes. Maybe that wouldn't matter but I can't be sure.

I'll have a go with the wick, but if it doesn't look like it'll work I'll resort to Nick's suggestion of destroying the switch - drastic as it seems it might actually be the least risky option.

If anyone's interested the board came from a (now obsolete) Hasbro FurReal toy pony. You can see it here. They cost over 1000 USD when launched I'm told!





Edited By Robin Graham on 09/05/2022 00:38:01

08/05/2022 00:38:02

A while back I took on a community project to repurpose a robotic toy for use by a 'performance artist'. I've got the mechanics sorted out, but I now realise that I need to move the toy's control switch from the 'brainboard' to a remote location. This is the switch from the top:


and from the rear:


My first question is: What are the odds on me removing the switch from the board using solder wick without knackering anything, given that I've zero experience with this technique?

Second Q is whether I even need to remove the switch - perhaps if I left it in the 'off' position I could just take wires from appropriate pins on the back of the board and replicate the 'demo' and 'on' states with a new switch.

Any advice would be welcome!


Thread: MIG welding - beginners question.
02/05/2022 22:05:17

Thanks for replies - especially for Joseph's and Nicholas' detailed explanations of what's going on. I think I now understand things better.

Hopper - encouraging to know that you are having good results with gasless. I've looked at some YT stuff about these low-end machines, but after 10 mins of watching a guy opening a cardboard box, ( I think I'm OK with that) followed by shock that that it doesn't have a plug &c &c I begin to lose the will to live.

The main thing I was after was confirmation that the machine is meant to work in short circuit mode. I had incorrectly assumed that MIG machines were designed to maintain a constant arc length.

The welder I bought is of the 'idiot proof' type - just choose gas or gasless and wire size, twiddle the single knob to give the 'power' you want and the machine figures out volts, amps and feed. It works, at least on thin (1.2mm) steel which is all I've tried so far, I just need to get used it.

I'll have a look at the MIG site, thanks for the link Anthony.



Edited By Robin Graham on 02/05/2022 22:05:53

01/05/2022 02:29:55

A few weeks ago I posted a question about cheap gasless MIG welders as I was having trouble welding 1.2mm steel box section with my MMA machine. I have now bought a cheap (£150) machine and it's night and day - half an hour's practice and I'm making satisfactory welds using flux cored wire.

The hardest adaptation I had to make was trusting the machine to get the wire feed right as I'd (sort of) trained myself to keep a constant arc length with MMA. This resulted in me moving the MIG torch away from the work when it looked like the wire was going to touch the weld pool. The machine responded by squirting out more wire of course, with bad results.

I've now discovered that a 'standard' MIG process is short circuit/arc/short circuit etc, and maybe that's what the machine is trying to do. Would this be normal behaviour for a low-end MIG welder? I'm happy that it works of course, but would like to know what's going on. The manual that came with the machine is pretty much unintelligible but does refer to an 'inductance' setting which isn't explained but might be something to do with with limiting current in the short circuit phase. I'm in that foggy area where you don't even know what the right question to ask  is with this, Any elucidation welcome.


Edited By Robin Graham on 01/05/2022 02:42:43

Thread: Ambiguous words
24/04/2022 01:50:50

I'm not sure how serious you are in this list-making endeavour Michael. If you were inviting pet hates and puns (of which there have been a few), well it's made for an entertaining read. But entertaining the possibility that you are actually trying to enumerate the finite denumerable set of words which have non-unique mappings to a (possibly!) infinite non-denumerable set of 'meanings' I think you may have a task which would have defeated even Heracles. I'm not very sure it can be done. You might be better off trying to populate the set of words which have unique mappings - it would be the null set I suspect, It's a wicked project though, I have to say.

Fowler has views of course - his take is that ambiguities arise not so much from words but from the ways of putting them together. Which seems sensible.




Edited By Robin Graham on 24/04/2022 01:52:33

Edited By Robin Graham on 24/04/2022 01:53:39

Thread: Gasless MIG welding
17/04/2022 01:28:04

welder - many thanks for your kind offer. I have sent you a PM.

John Doe 2 - I don't think the suggestion of hiring has been made earlier. It looks like I could hire a MIG for about £60 a week, but I'd have to buy or hire gas on top of that. Might as well go the whole hog and buy a welder - bound to come in handy in the future!

Dell - unfortunately I am about 100 miles away from you, but thank you for your generous offer of help. Had you been nearer I'd have been round like a shot. Aluminium would actually better for this, but I couldn't find anyone local with the skill/kit to do it for free - it's a "community project".

It might seem nuts that I'm considering buying a new welder for an unpaid one off, but that's how it works for me. My workshop activities are more about journeys than destinations I suspect.


16/04/2022 01:58:11
Posted by phil holmes on 15/04/2022 20:24:52:

hope above made sense..... just re read it, not so sure. ho hum

Yes, I think that makes sense Phil - thanks for your input. What I've been doing is making a series of tacks as in A in this sketch:


Sometimes (~70%) it works, sometimes it doesn't. When it doesn't it's because once I've blown through the cut edge the arc just runs away and makes it worse. Would you do something more like B? That is traverse across the 'non cut' bit without any weaving and let the pool melt the cut edge?

I've pretty much given up using stick welding for this project because I need 100% success when I make the frame and I'm not going to get there in in time.

I have now tried silver soldering, which I can do confidently, but I have never tried a joint like this before - I was worried that it would be weak because it's (sort of) a butt joint. I was surprised how strong the joins are though - after application of a FBH the tube bent and the joint just gave me a smug look. More expensive and slower than welding of course, but it'll get the job done.


Thread: Domestic Chemistry
15/04/2022 00:23:56
Posted by SillyOldDuffer on 14/04/2022 09:44:50:
Posted by Robin Graham on 14/04/2022 01:01:26:
Posted by Michael Gilligan on 13/04/2022 19:33:38:

Thanks to Dave and Thor … I am reminded of a previous thread on this forum, about micro-bubbles generated in canine toothpaste.

I am happy to believe that the underlying process is generally similar.



Ref. **LINK**

Edited By Michael Gilligan on 13/04/2022 19:36:14


On the chemistry front I'm pretty skeptical about claims that vinegar 'attacks' grease in a chemical way. It's certainly true that grease can be broken down by dilute acid (acid hydrolysis of esters), but I doubt that the reaction is significant in cleaning an oven. Possibly it has some mild solvent action?


That's what I was hinting at when I said the answer was as much to do with physics as chemistry. Dilute Acetic Acid isn't like Caustic Soda, which attacks fats aggressively; Vinegar only has a mild dissolving effect. Simply splashing vinegar on a dirty oven won't get it clean. It's important that the Bicarbonate / Acid mix be a paste, in which the Bicarbonate and Acid are slowly reacting to produce micro-bubbles, AND there are plenty of sharp Bicarbonate crystals to act as an Abrasive, AND there's enough fluid to penetrate tiny cracks and crevices, AND the paste is rubbed in enthusiastically by the operator. Works in much the same way as soapy Brillo-pad, except the Brillo pad abrades with steel-wool, soap breaks up the fat and there are no micro-bubbles.

Cleaning an oven with a solution of Sodium Acetate made by carefully neutralising Sodium Bicarbonate with Vinegar is a waste of time. It's not reactive or abrasive and there are no micro-bubbles.

Caustic Soda paste, made with water, can be splashed on and left. The alkali breaks fats up, so the operator can leave the oven to soak and wash the whole lot off with water a few hours later. Sodium Bicarbonate would do the same but take a lot longer.

Anyone tried Isopropyl alcohol? Apart from the cost, fumes, and fire hazard I think it might work well. Ditto petrol...



Apologies for not referring to your post directly Dave - I think you nailed it when you said it is as much to do with physics as chemistry, Perhaps even 'more' rather than 'as much'? As Michael had asked for an explanation of the chemistry I was attempting to give some background as to why a chemical explanation doesn't work by referring to the chemical basis (ester hydrolysis) for fat breakdown by the individual components of the mixture.

IPA is good for many things, but being a polar solvent isn't great for grease, as Grindstone Cowboy found. Ideally one would use dichloromethane or chloroform for pure solvent action, or a solution of sodium dichromate in

concentrated sulphuric acid for pure chemical action. wink

I'm not sure how one would make a paste of caustic soda in water? Sounds a bit dangerous...


14/04/2022 01:01:26
Posted by Michael Gilligan on 13/04/2022 19:33:38:

Thanks to Dave and Thor … I am reminded of a previous thread on this forum, about micro-bubbles generated in canine toothpaste.

I am happy to believe that the underlying process is generally similar.



Ref. **LINK**

Edited By Michael Gilligan on 13/04/2022 19:36:14

I think you're right to be happy in that belief Michael. I use a 15% solution of sodium hydroxide with a bit of surfactant myself. It works well, but it's messy and even at that concentration it takes several hours for the chemistry to work. I shall certainly try the vinegar/bicarb method!

On the chemistry front I'm pretty skeptical about claims that vinegar 'attacks' grease in a chemical way. It's certainly true that grease can be broken down by dilute acid (acid hydrolysis of esters), but I doubt that the reaction is significant in cleaning an oven. Possibly it has some mild solvent action?

On canine tooth cleaning - I don't think I reported back on my plan to use the ultrasonic toothbrush for rust removal. Pure funk I'm afraid. The risk of domestic disharmony if it went wrong was too great. I expect it would work though. Might be good for cleaning ovens (slowly) as well.


Edited By Robin Graham on 14/04/2022 01:03:16

Thread: Why is electricity so expensive?
07/04/2022 00:17:04

Thanks for replies. I'm not surprised that that the title of the thread has invited wide-ranging discussion, which is welcome. We humans are indeed in a bit of a pickle. I liked SoD's inversion of the question - why is electricity so cheap? I am a 'baby boomer' on a final salary index linked pension myself and can see that nice as that might be for me, it's a sort of fluke. Not sustainable for future generations I fear.

MichaelG - that Pathe news clip took me back a bit! But doesn't answer my my question about the relative prices of electricity and gas which I don't think anyone else has addressed either.

To put it another way.

Wholesale forward gas and electricity prices in GB (prices in GB pence per kWh, ratios electric/gas, source Ofgem)

March 2021 - Electricity 5.0 , gas 1.7 - ratio 2.9

Dec 2021 - Electricity 24, gas 9.0 - ratio 2.7

Jan 2022 - Electricity 20, gas 7 - ratio 2.9

So it's a pretty consistent - per Joule, electrons at the plug are getting on for three times as pricey as gas molecules at the nozzle. OK, derate that as electricity is 100% efficient for heating, gas maybe 70-80% . But still a big difference.Why would that be?

In my case, the ratio from my supplier ( elec 32p, gas 7p) is more like 4.6. But I'm with Green Energy so I get special electrons of course - I had a close-up look at the blighters through my welding helmet tonight and they did indeed have a greenish tint. You have to pay for that!



Edited By Robin Graham on 07/04/2022 00:19:57

Edited By Robin Graham on 07/04/2022 00:20:34

Edited By Robin Graham on 07/04/2022 00:23:28

Thread: Polishing Brass
06/04/2022 23:31:58

Ian. I have spent many hours polishing brass trying various methods including belt sanding. I agree with others that belt sanding is not the way to go. There are various problems with this approach. One is that it as you go through the grades, it is necessary to turn the work at angle of at least 45 degrees (preferably 90) between grades so you can see that all the scratches from the previous grade have been eliminated. In your case (unless you have have a linisher with a very wide belt!) this will involve running the work across the belt at right angles. It is pretty much impossible to do that without getting 'dig ins' when the work crosses the edges of the belt. Even if you have a massive belt you won't be able to hold those slender strips down on the belt with consistent pressure. You may also find that even at low belt speed the the work runs away from you - there's a lot of friction and with such thin material you don't have much to hold on to. By all means give it a go, and if you have success please report back - I have never got this method to work despite numerous attempts.

What I do now is old school wet 'n' dry up to 3000 grit then polishing compounds/mops. - I use mops and the Menzerna compounds from The Polishing Shop where you will find information about the various types of mops and compounds available.

Autosol is OK from maybe 1200 or 2000 grit if you're not too fussy. It won't give a true mirror finish on brass though - you will still see scratches if you look hard enough. If you want a specular finish (which I aim for) it's polishing compound, elbow grease and meticulous hygiene - you need to be sure that not a single particle of grit or compound from the previous stage is anywhere near. A flea can jump about 160 times it's body length, a grit particle would laugh scornfully at that.

Obviously I'm a bit obsessive about this, you may be less so!


Edited By Robin Graham on 06/04/2022 23:33:41

Thread: Why is electricity so expensive?
05/04/2022 01:21:44

Like everyone else I guess have had unwelcome news from my energy supplier - £0.32 per kWh for electricity. Gas has gone up from about £0.04 to ~£0.07 per kWh too. It made me think - given that most UK electricity is still generated by domestic gas fired power stations, which presumably operate at better than 25% efficiency, why are the electrons so pricey?


Edited By Robin Graham on 05/04/2022 01:22:17

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