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Member postings for Roderick Jenkins

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

Thread: Ba head sizes
14/07/2011 12:20:08
According to Wikipedia the hex bolt head is 1.75x the thread diameter. Thus, for 0BA which is 6mm diam. the head is 10.5mm AF. Be aware that many vendors supply bolts with a one size smaller head e.g. 4BA bolt (thread O.D. 3.6mm) witha 5BA head (5.6mm instead of 6.3mm AF). These smaller headed bolts have a better scale approximation to the full size, mostly whitworth bolts, on full size engines.
Thread: mystery thread sizes
08/07/2011 12:30:13
Brass thread was a constant pitch series for use on brass tubes used originally, I believe, by instrument makers who could use the same chaser on any diameter. The relatively fine pitch allows a thread to be cut on thin walls. Since bicyles were made from tubing then I guess that the manufacturers adopted a similarly useful size.
Thread: I need a mill ? Manual or CNC??
01/07/2011 12:23:47
Can I just emphasize the need for headroom in a milling machine. This is hardly an ambitious setup yet the table is at the bottom of its travel (it's a rising knee - no quill) and the 5.6mm jobber drill is hard against the inside of the 1/4" chuck. There is no chance with a 1/2" chuck. If I had to choose between a small CNC mill and a larger manual one I would go for the manual one (but I'd like the small CNC as well please).
Thread: Machining cam lobes
01/07/2011 08:48:50
As Jason B says, visit Ron's site from the link -use the mk2 version. I wrote the original SIC article and program, first in GW-basic and then in Q-basic both of which are DOS programs. The Qbasic version still runs under Windows XP, though in a rather small window. I have converted the programs into an Excel spreadsheet. PM me if you would like a copy.
If you have CAD then you can draw the cam and then draw a series of lines radially from the base circle centre at 3 degree intervals (using a polar array). You can then read off the co-ordinates of the intersection of the radial lines with the cam profile. This will give you the Z setting for the mill against the rotation from a dividing head
Thread: Case Hardening - can't even harden a washer !
28/06/2011 22:11:10
So, Clive's leather worked OK and they used to use powdered horn. Start saving your toenails chaps
28/06/2011 19:31:55
I'd have a go at laying the washer on a thin bed (1mm) of hardening powder and then put a similar layer on top. Get it carroty hot for a couple of minutes and then pick it up and quench it. If necessary you can re-heat after you take it out of its powder bed, the carbon should already have diffused into the steel surface.
How disappointing that we can't seem to get the good stuff anymore. Fortunately (for me!) I have a little stash of Kasenit that should see me out.
Good luck
28/06/2011 08:40:37
C W,
Are you sure the washer isn't plated or passivated, this could act as a barrier to carbon diffusion? My method is to heat the the piece up to red and then to dunk it in the hardening compound. The powder sticks to the hot piece in a thick cake. Then heat up the piece to bright red (cherry is the usual description but cooked carrots is a better description IMHO). The carbon from the compound then diffuses into the surface of the steel - this takesa bit of time so give it a minute or two at this temperature. Now dunk into cold water. Only the surface will be hard, to the depth that the carbon has diffused. A file should glide off but the washer will still bend because the core is soft.
The straw colour describes the appearance of the metal when it is at the correct temperature for tempering. You shouldn't need to do this for case hardening, which is the major advantage of this method: the piece remains tough but the surface is dead hard and therefore wear resistant.
It is a straightforward process. I would try it on small piece of steel rod with a turned surface.
Thread: Boiler Leaks
26/06/2011 18:40:41
Alec Farmer in "Model Locomotive Boiler Making" recommended the use of high temperature soft solder for sealing small leaks. From what I remember though, you cannot then go back and do any silver soldering, the soft solder will alloy with the hard solder and form a brittle compound. My "Jenny Lind" project stalled at this point many years ago with a pin hole in the firebox crown. I never could decide how to fix this - it's still waiting.
Thread: New Drummond Project
26/06/2011 11:02:30
When I had a Zyto I used a continuous thin flat belt that came off a scrapped diamond wheel cutter. This was very good, much, much better than the leather belt with clicking joiner the lathe came with. You have to remove the spindle to get it on the pulleys but that was not much of a task - you'll probably be doing that anyway. to sell the sort of thing I had.

Edited By Roderick Jenkins on 26/06/2011 11:02:51

Edited By Roderick Jenkins on 26/06/2011 11:06:03

Thread: Something to ponder 01
24/06/2011 12:32:27
As a man who has absoulutely zero experience of steam engines large or small I feel eminently qualified to join in the discussion
I guess that the 1200HP comes from the rate at which the steam is generated. When the engine is standing in the station and the safety valves are gently hissing then the fire is only just big enough to keep it at that pressure. When the train starts then the pressure will rapidly drop as the steam is consumed in the cyclinders until the fire is built up again in order, at full chat, for that 1200HP to be available continuously. And thereby lies the skill of the fireman, ensuring that the safety valves are sizzling in the station but wailing like banshees at the bottom of a steep bank, feeding the coal and water at the appropriate rates and adjusting the blast to keep the fire just right. Has this got anything to do with question2?

Edited By Roderick Jenkins on 24/06/2011 12:33:45

Thread: flip up toolholder
21/06/2011 09:14:53
My comments referred to the practise of setting the topslide to half the thread angle, I should have made this clearer. No method is universal, the Tubal Cain system falls down when you are cutting a small thread between centres: the topslide fouls the tailstock. There have been several designs for retracting/rising tool holders over the years and this one seems to me to be the simplest and most practical - I shall be making one for myself, 'cause the memory is failing! So, thanks chaps.
20/06/2011 19:58:40
I favour the screwcutting method advocated by Tubal Cain in "Simple workshop Devices". He set up his topslide to be parallel to the job and used the cross slide to set the depth of cut. On larger threads he advances the topslide by a thou or so every cut to ensure that the tool only cuts on the leading edge. He describes a cross slide depth stop but I just set the cross slide dial to zero and can then advance the tool cut by cut until the proper depth for the screw is reached. I only have to remember the setting for the time it takes to return the tool to the start of the thread and even I can remember it for that long (mostly).
Thread: Speed Increaser
19/06/2011 17:14:06
I've created this cross section from the Hemingway drawings, the pink bits are the gears. Clearly some tweaking will be required on manufacture to get the gears meshing properly. I'll make it according to the drawing and see how it goes. I am a bit nervous about locating the ball bearings using only Loctite but we'll see. It can always be dismantled and modified later.

Edited By Roderick Jenkins on 19/06/2011 17:15:05

18/06/2011 14:47:34
Thanks for those comments. I bought the kit and started manufacture without really thinking about it but have now become uncomfortable with the design. Whilst it may be fine for engraving I do not think it is robust enough for a 3mm endmill.  The drawings seem fine but could really do with a cross-section. There are some 3D views but these do not show the critical internal workings. I'm drawing up the cross-section myself and will give serious consideration to your suggested improvements.
many thanks,

Edited By Roderick Jenkins on 18/06/2011 14:49:21

17/06/2011 18:22:46
Thanks, I am aware of that. I wondered, when I put an inverter drive on the mill, what on earth use reverse would be. Now I know
Thread: Gib Adjustment - how tight is too tight?
17/06/2011 15:57:59
I guess it might avoid distortion of the gib by having the screw bite into it but probably it just makes the adjustment smoother.
Thread: Speed Increaser
17/06/2011 14:32:43
Has anybody made Dick Stephen's speed increaser as supplied by Hemmingway? Any comments on construction?
Thread: Gib Adjustment - how tight is too tight?
17/06/2011 14:29:33
Clive wrote: My Myford has the standard gib screws fitted (whatever Myford supply). Now I've not checked the ends of these yet to see the shape, but I'm guessing they'll be the commonly-available conical (pointed) or flat, rather than domed.
Not on my S7, they are beautiful black screws with the end reduced to the core diameter finished in a hemisphere. This thread has shamed me into taking out my locking screw and shoving a BB down the hole. There's quite a bit of inverse snobbery about Myfords on this site but it's little touches like this that make me grateful to have been able to buy a good s/h one.
Thread: Tachometer.
15/06/2011 21:01:27
AS11G purchased from Wilkos in Basingstoke lunch time today. Fitted to my Sharp mill (with inverter drive) in about 10 mins. Thank you Michael.

Edited By Roderick Jenkins on 15/06/2011 21:03:10

Thread: never seen anything like it
15/06/2011 19:58:40
Taper reamers for making the holes for musical instrument pegs have only 3 flutes on half the circumference, the rest being plain. If you try to cut a hole in a soft material like wood using a conventional reamer the reamer tends to pivot around the cutting edges in turn and you end up with a facetted hole. I guess a similar thing can happen in soft metals, it certainly can happen in sheet material. I would expect your reamers to be very accurate, if a little slow cutting. Spiral flute reamers are now available for musical instrument which helps to eliminate the problem but they still have a least one flute missing (or at least that's how it is on my lute peg reamer).
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