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: Drilling cast iron - where did I go wrong?|
This is how it's ended up.
Following advice I tried drilling CI at the lowest speed (475 rpm) on my drill press without a pilot - just spotting with a 90 degree bit (wot I had) then going through with the 135 degree 10.5mm split point. No chatter and a neat hole. So learnt something there. Thanks.
I'd assumed that the misalignment was the result of my inexperience, but I wasn't too happy with working round the problem without understanding where I'd fouled up on my first attempt.
I put the pieces together again (they are prevented from moving against each other radially by a register), rotated them, and found that the holes actually line up well in one of the four possible orientations. Surely that can only happen if the holes in the bought backplate aren't at 90 degrees to each other. So maybe I didn't foul up, except by taking the accuracy of the commercial product as an article of faith.
Edited By Robin Graham on 21/05/2019 23:52:52
Edited By Robin Graham on 21/05/2019 23:57:25
Thanks for your suggestion Graham. It was helpful in a way you probably didn't anticipate - I think the phrase 'if you don't mind an extra couple of holes' sparked a realisation that I'd gone about this project arse about face. For alignment it would have been easier to clamp or glue the pieces together and drill new holes through into the backplate. I have an irrational fear of modifying bought parts - in this case a D1-4 backplate- it creates a blind spot in my vision of how to make a thing. I need to get over that!.
Thanks for replies and apologies for delay in responding. Having a bad back I volunteered to be a subject for student training at an osteopathy school. I'm not sure if that was a good idea - it certainly took my mind away from workshop activities though*.
I think the mistake I made was changing too many things at the same time. I have little experience with CI, less with split-point drills and none at all with using pilots for a hole of this size. Normally I'd just spot or centre drill and go straight in with 10.5 mm, but I'd read that one should creep up to the final diameter and that split-point drills were the bee's knees. I thought it might be a good test ground as I didn't need to be very accurate.
All I want to do is make some matching holes For M10 screws:
They don't match even after drilling the clearance holes to 11mm. I really didn't think this would be a problem but I have to accept that I fouled up somewhere (can't blame the machine or tools). I'll think on it and make a new set of holes at 45 degrees to the first go in the hope that it'll look like it was meant to be like that...
*Rib relocated - felt great at the time, agony afterwards.
Edited By Robin Graham on 18/05/2019 23:06:51
Edited By Robin Graham on 18/05/2019 23:18:57
I have almost no previous experience of working with CI, so this is very much a beginner's question.
I want to make some counterbored M10 clearance holes in a slab of CI to take cap head screws. I bought a counterbore tool and a 10.5mm 135 degree split point drill to suit the pilot. Holes were marked out and centre punched as usual, then started with a BS3 centre drill and opened up to 7mm with a standard 118 degree drill. No problems so far, everything spot on. But when I went in with the 10.5 drill the work chattered horribly and I ended up with a hole with a ragged entrance and centred over 10 thou from where it should have been.
I've done the same sort of thing in mild steel many times (albeit with standard 118 degree drills) and ended up with clean holes within 2 or 4 thou (depending on the weather) of where they're meant to be.
I was going at around 1100 rpm for all the ops.
|Thread: Silvering brass|
A long time ago (1964ish?) I worked in the QC lab of a non-ferrous metals foundry. One of my duties was to polish brass and bronze samples for microscopic examination of grain structure. After polishing we painted the surface with an acidic solution of mercury(II) nitrate which we prepared ourselves by dissolving metallic mercury in concentrated nitric acid. Mercury deposited on the polished surface and formed a silvery amalgam which revealed the grain structure. I always assumed that residual nitric acid in the brew preferentially etched the grain boundaries and the silvery amalgam gave contrast, but maybe the deposition/amalgamation process was also sensitive to grain boundaries. Same process I guess.
No need to apologise for the egg sucking tutorial Dave - it happens that I have a background in chemistry/physics, but there will be people who light on this topic from internet searching and benefit from your explanation.
Dr Ure's opus looks rather wonderful! Do you have the original (C19) volumes? They seem to be quite rare - I had a look round (I'd not heard of the work before) and it seems I can get a 1999 Routledge reprint from Amazon for a bit under $2000!
Returning to silver chloride, I reckon you're right that it's about controlled deposition. Tartrate ions will complex the silver from the chloride and, after further research, it seems that excess chloride will do the same job.
I'm intrigued by the inferior buttons. Corrosive sublimate is mercury(II) chloride I think, so mixing that up with the silver and zinc sulphate is going to make some sort of amalgam possibly.
Edited By Robin Graham on 14/05/2019 00:17:45
Edited By Robin Graham on 14/05/2019 00:18:17
Edited By Robin Graham on 14/05/2019 00:36:45
Thanks for replies. As MichaelG cannily perceived I didn't think Renwax would silver brass - it was a subsidiary question born from old man grumpiness about the Repair Shop programme. It's heartwarming to see heirlooms restored, but frustrating when we don't get to see the nitty-gritty. It just annoys me.
Armed with info from here I might just make my own version of Renwax . I have a chunk of micro crystalline wax.
Dave - I understand the basic chemistry of the process, but I wondered about the use of silver chloride, which is pretty much insoluble in water, as the source of silver ions. Why not use a soluble salt such as silver nitrate?
|Thread: Which thread for T nuts|
I've always assumed that the shear forces (ie the forces parallel to the plane of the bed) when milling were carried by friction between the workpiece, or vice, or whatever you clamp to it, and the bed. In which case the shear strength of the fastener is immaterial, except in the sense that Andrew mentions. Is that wrong?
I recall reading of Colin Chapman saying - ' you can hang a double decker bus from a quarter inch bolt'. If that's true, given that the coefficient of friction between steel and steel is about 0.1, I agree with Nick that M8 will be more than sufficient.
Edited By Robin Graham on 12/05/2019 22:47:52
|Thread: Silvering brass|
Thanks - I've heard of it but never tried. I shall get some.
I saw a bit of 'The Repair Shop' on TV recently and there was a guy who silvered a bit of brass (a strangely shaped miniature sword) by wiping some blackish sort of gunk on it. Frustratingly they didn't tell us what it was - Beeb policy I suppose. I also saw one where a 'special wax' was used to finish a woodworking project. The label on the container was (in)conveniently turned away from the camera. Back in the day we all knew what they meant by 'sticky-back plastic'!
I haven't any pressing need to silver brass, but it interested me and I had a look round. It seems that horologists use a paste of silver chloride and potassium hydrogen tartrate to restore clock faces. Presumably the tartrate ions form a complex with the silver which brings it into solution enough to react with the copper in the brass? I haven't found out what the black gunk is though!
Does anyone have experience in this area? Although not pressing I sometimes make things from brass and adding a bit of of silvery bling might be - well - entertaining.
Edited By Robin Graham on 11/05/2019 23:05:35
|Thread: Chinese Lathes|
Cripes. You've never seen how Lola and Huian glare at each other across my workshop Andrew. Lathes, like ships, are somehow female.
More on topic- I've not had any insurmountable problems with Chinese machine tools, I can turn/mill as close as I need - even unto tenths of a thou. When I was working the main guy in the machine shop had 'his' lathe - which was no doubt a good British one, but he wouldn't let anyone else touch it. He had a relationship with the machine, they got on together and understood each other's quirks,..
Edited By Robin Graham on 09/05/2019 00:37:00
|Thread: Oversize ER collets|
Thanks for replies. Clearly I was mistaken about the range of ER40 collets - everywhere I looked said 3-26mm so I thought that anything bigger was 'oversize' and accuracy might be compromised in some way. Not so it seems. As I said I have little doubt that the RDG collet will be OK for what I want to do, I'm not chasing microns, so it was an an acadaemic question I suppose. I was just interested in the limits of the design given the external sizes of the collets in the series. 32mm for ER40 seems pushing it for example - there must come a point where the steel gets so thin that that there is a risk of permanent deformation on compression.
I had thought about alternative ways of holding the screw but they all seemed to have problems of one sort or another. Bruising the threads, faff of moving indicator over threads in the 4-jaw as JasonB said, etc. Truth to tell I've had a hankering after a lathe collet chuck for a while but have been working round it - this project tipped the want/need balance: I should grasp the nettle then (with a stiff upper lip) bite the bullet and make one. It's going OK at the moment and will be more versatile than making a custom collet,
I've no idea how I would get soft jaws for my 3-jaw. It came with the (Far Eastern) lathe. There are no identifying markings on it. Are the scrolls made to a standard so any jaws for a 6" chuck would fit?
Edited By Robin Graham on 06/05/2019 23:57:08
I need to grip a 28 x 5mm trapezoidal leadscrew so I can turn some threads off to make a shaft, as concentric to the screw as I can, and thought that a collet chuck would be a better option than trying to do it in the 3-jaw. It seems that the standard for ER40 goes up to 26mm, but oversize collets are available. Cutwel and others list precision collets up to 30mm, but these are not compressible. Their compressible collets stop at 26mm. RDG list oversize compressible - I have bought a 28mm one and it looks OK.
I expect that the RDG collet will meet my needs, but I'm interested that Cututwel et al , who I think supply to a more demanding industrial market, don't offer the larger sizes as compressible. I'm guessing it's because they can't guarantee compliance, but I'd like to understand the why of it.
|Thread: Machining a chuck backplate|
Ta Jason and Colin - steel it'll be then. I'll ask at the local steel place if they can do me a part bar of EN24T, but I want to crack on with this so shall make a MK1 with the EN1A I have in stock.
Next problem is how to measure the PCD of the blind M10 threaded holes in the backplate so I can transfer to my adaptor. ARC do the very thing for the job in the form of threaded transfer punches, but I don't want to wait. Preliminary attempts to do this gave a difference of almost 1mm between measuring from the holes in the 4-jaw to which the plate was attached and from the holes in the backplate itself. Maybe that's why I had to take a hammer to to an Allen key to get the screws out in the first place, or maybe I'm not measuring right.
Thanks for replies. In retrospect JasonB's suggestion of a double register might have been the way to go. However I am of a nervous disposition and have an irrational fear of modifying 'official' parts - daft really, they're just lumps of metal. I was pleased that the adaptor I made fitted the backplate with a wriggle, a push and a satisfying click when it went home, so maybe my skills have advanced enough to cross the line.
Nick - I read the same about 3/4 jaws somewhere and didn't understand it. I still don't. Surely if the bolts are tight enough the frictional forces between the mating surfaces are dominant and the bolts don't see any significant shear forces?
My original plan was to buy a chuck from Arc, but having slept on it I'm going to have a go at making one from scratch. I'm after ER40 and have enough 3" EN1A or CI in stock to make the chuck body. Is their any advantage in using one or the other? I like CI because it machines so sweetly, but maybe steel would be better?
Edited By Robin Graham on 03/05/2019 23:56:29
Edited By Robin Graham on 04/05/2019 00:17:18
I would like to have an ER collet chuck for my lathe. The machine has a D1-4 spindle nose and to save money on buying a new backplate I'm making an adaptor to convert the one from the four jaw (which I too rarely use) to fit the new chuck. Four jaw and backplate:
The register is 125mm diameter and about 4.5mm deep on the backplate - the recess in the chuck is about 6mm deep, so I'm making the adaptor in the same style:
So to get to to my question - given that the 'rim' on the adapter is only 12.5 mm ( I had a bit of 150mm CI to hand) , is this the way to do it? My gut feeling is 'yes, it'll be fine', but I read somewhere on t'internet that I should make the rim short so the faces internal to the rim mate.
Not done this before, so any advice/opinions would be welcome.
Edited By Robin Graham on 02/05/2019 23:52:37
|Thread: A visit to Manchester Sci and Eng Museum|
That's interesting Oily R. I can't remember when I last went to Snibston but I recall lots of interesting and educational exhibits - even a working lens polishing machine complete with bitumen pad and 'random orbital' drive, I wonder if the move to TV documentaries might have been as more to do funding cuts than a change of strategy. Must be cheaper, and no H&S concerns!
The whole thing about sci/tech education is a hard biscuit for sure - it's great that kids have access to so much information so easily, but it's difficult to navigate through all that stuff without guidance. In my own field (physics) I have seen many changes in teaching. When I retired I did some private tutoring - I was surprised to find that the first module on the AQA AS syllabus was concerned with sub-atomic structure. The students were required to remember the number of up and down quarks in a proton, draw 'Feynman diagrams' , understand conservation of strangeness etc. Later on in the syllabus we get to wonder about how things push against things, rub against things and sometimes fall over. Seems arse about face to me.
Sorry, ranting again, Robin
Edited By Robin Graham on 01/05/2019 23:26:39
Edited By Robin Graham on 01/05/2019 23:43:38
|Thread: A question about centre drilling|
Hmm. In the article GHT said his lathe was aligned 'beyond reproach'. But he still thought it worth doing, and found he could get get a centre within 0.00015" of the true middle. That raises questions in my mind, but all I really want to do is make some rolls, I doubt that a few microns one way or the other is going to make any difference,
Edited By Robin Graham on 30/04/2019 01:49:04
Thanks for explanations - I get it now. I think that this is one of those 'real life' techniques which can be hard to come by without a practical background in mechanical engineering. Maybe I should should read more GHT.
I am making a sheet roller following G. H. Thomas' design published in the October 1976 ME. He gave the following advice for drilling the rollers before turning between centres.
"Face the end; put a suitable centre-drill in the tail-stock chuck with its two cutting edges in a horizontal plane, i.e. one towards the operator and one towards the back. Put a piece of plain bar or any blunt instrument in the tool-post; start the centre-drill into the end of the bar and then bring up the blunt ended “tool” and cause it to bear with slight pressure on the centre-drill whilst it is being fed in. Ease off the pressure gradually as you finish feeding the drill."
I'm sure that this must be good technique or GH wouldn't have said it. But can anyone explain why it works better than just whacking the centre drill in?
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