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Member postings for Andrew Johnston

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

Thread: Electronic Indexers - How Is Cumulative Error Avoided?
30/05/2019 16:50:04
Posted by SillyOldDuffer on 30/05/2019 15:59:07:
......there's not much advantage in using floating point arithmetic which uses more memory and is relatively slow.

Oh dear Dave, you're a bit behind, although at least in the 20th rather than the 19th. smile

By chance I'm looking at a processor for a design update on a CO2 gas sensor board at the moment. The preferred processor has an ARM Cortex M4 core with a full implementation of the IEEE754 single precision floating point specification. Floating point addition, subtraction and multiplication are all single cycle operations. At the maximum clock rate of 72MHz that's about 14ns. One off price of the processor from Farnell is under £4. What's not to like?


30/05/2019 14:24:17

I wrote up a similar method to that elucidated by Mike, and then lost it before posting due to finger trouble. If I were doing something similar to the OP I wouldn't muck about with integer arithmetic. I'd choose a processor with a floating point unit; makes life so much easier.


Thread: Should I have 3 phase supplied to my house?
30/05/2019 11:59:00

I stand by what I wrote in the thread quoted above. The installation was expensive, but partly justified by updating an inadequate single phase supply.

I have recently had to change the single phase consumer unit that was installed as part of the 3-phase installation. The old one was plastic (as recommended by the regulations of the time) and replaced a metal one. The new one is metal again (as now recommended), as the plastic ones were found to have a tendency to catch fire in the event of a fault. So much for the ex-spurts.


Thread: My Lathe Dog
30/05/2019 11:44:50

You need a new dogsbody, the current one is sitting down on the job.


Thread: Taper pins
30/05/2019 11:37:38

Drill Service stock them as well, but not cheap. I assume they must cut on the flutes pretty much the same as a reamer?


Thread: help with gear calculations
30/05/2019 11:27:25

A picture of the existing part would help. The numbers for the gear parameters don't stack up. The OD of a standard spur gear is the number of teeth plus 2 all divided by the diametral pitch. So we have (15+2)/14 = 1.214", or 30.84mm. Not sure how that fits in the with shaft measurements?

Unless you can find a source of proper spares I think it is unlikely you'll be able to buy the part off the shelf. You may be able to buy a suitable gear and fit it to a shaft. It's not difficult to cut a 15 tooth gear, it's 24° per tooth. No need for a dividing head, you could get away with disc marked out and drilled mounted on the end of the shaft. It would help to know what facilities you have available. A pressure angle of 14.5° is most likely, it's an older standard.

Can you measure the depth of the tooth on the rack? It may help to resolve the number issues.


Thread: Taper pins
29/05/2019 21:55:45
Posted by old mart on 29/05/2019 21:26:58:

Small TP reamers have a pentagonal section.

That's odd, my 1/16" taper pin reamer has four cutting edges, with machined flutes like a normal parallel reamer. If the reamer was pentagonal that would give a large negative rake to the cutting edge. Clockmakers broaches are pentagonal in section.


Thread: Feeds and Speeds! 0.4mm milling cutters...
29/05/2019 19:29:24

Not enough information: HSS or carbide, how many flutes and centre cutting or not? What's mind boggling is how they make and grind the cutter in the first place.

For "plunging" I use a third to a half of the normal feedrate, on the grounds that only one edge will be cutting all the way to the centre. In reality I don't plunge straight down; I program either a ramp or helix that means the centre has a chance to cut rather than rub.


Thread: Do you wear a mask grinding HSS tool bits?
29/05/2019 09:58:28
Posted by Hopper on 29/05/2019 08:46:49:

Does ordinary carbide tooling cut HSS without problems?

Yes, although the cutter isn't going to last for ever. Basic cutting parameters are similar to turning hardened steel, ie, high speed, shallow depth of cut and high feedrate. I don't rough by mill that often as I have a large selection of pre-used HSS blanks, so I can usually find something close to what I need. Here's a HSS blank machined on a CNC mill with a 6mm uncoated carbide endmill:

embryo cutter.jpg

The cutter was for shaping an internal gear so I wanted an exact profile, as drawn in CAD. Milling is especially useful for grooving tools where you need to take a substantial amount off the width of the blank - it would take for ever with a grinder.


Thread: Small Trepanning Tool !
28/05/2019 16:41:37

On a full size engine the cores required to create all the steam passages and ports within the cylinder block casting are incredibly complex. In the smaller model sizes it would be very expensive to replicate, and the scrap rate would be high due to the complexity. In addition how do you get the cured core sand out of the internal steam passages?

There are two versions of my model. The original design by Pete Filby and as modified by LSM. The LSM version has the passages cast in and uses thin liners, ~1/8", allowing precise positioning of the steam ports. In contrast the Filby design uses thick liners, 3/8", and has the steam passages machining into the outside of the liners along with some simple drilling of the cylinder block. This results in a simpler and cheaper cylinder block casting. Mind you I think the cylinder block for the Filby design is now over £500, I don't know what the current price for the LSM version is, but I'd sit down before looking.


Thread: Do you wear a mask grinding HSS tool bits?
28/05/2019 15:13:00
Posted by thaiguzzi on 28/05/2019 11:07:57:


What bit don't you understand and I'll try and explain?


28/05/2019 10:22:59

If you've got a lot of metal to remove from a HSS blank it's quicker and easier to mill it first, then use the grinder to touch up and add the relief angles.


Thread: Taper pins
28/05/2019 10:18:23

Probably, but I just measured the small end of the pin roughly where I thought it would exit the shaft and drilled a couple of thou smaller.


Thread: Are these spot drills?
27/05/2019 21:51:36
Posted by Michael Gilligan on 27/05/2019 20:55:05:

but I note that carbide versions are advertised for use on hard steels, etc.

That's what confused me. Drill Service list straight flute drills but all for very hard materials, not brass.

The picture posted by Jason is what I remember, athough not from 1895. The drills would have been in my fathers toolbox when I was a kid. So I expect them to be from the 1940s, and almost certainly carbon steel.

The slow spiral drills are good on brass but what would be really useful on the Britan is left hand, slow spiral, stub drills. But you don't seem to be able to get them either. sad


Edited By Andrew Johnston on 27/05/2019 21:53:07

Thread: Small Trepanning Tool !
27/05/2019 20:59:22

Yep, seen his videos a while back and duly impressed. smile o Man size machines too. I think one of his lathes is a DSG but good to see an old capstan lathe earning it's keep. Horrid materials, pricy as well. I'd expect that Inconel bar to be over £10k.

I'm thinking about doing something similar. For my traction engine cylinder liners I've been supplied with solid cast iron bar. Seems a real shame to drill and bore to end up with a 3/8" thick liner. I could drill on a circular pattern half way from each end, but that would leave a sharp inner part. I fancy having a go at a similar method to the video. I'll have to make one, as I can't find anything available commercially.


Thread: Are these spot drills?
27/05/2019 20:42:39

Bottom is a carbide spot drill, top is a slow spiral drill for brass and bronze:


Years ago I'm sure I remember straight flute drills for brass, but i'm darned if I can find any reference to them now. Did they exist, or have I lost my marbles? On second thoughts don't answer that!


Thread: What Did You Do Today 2019
27/05/2019 08:12:19
Posted by SillyOldDuffer on 26/05/2019 13:00:17:

As all true music fans know even the best amps and speakers are garbage unless connected with Industrial Gauge Oxygen Free Gold Plated Silver Litz Wire Tri-filar wound on a helical PTFE core, and then fitted with bespoke insulating jackets hand-knitted in special-mix Kevlar, Carbon Fibre, and Vucana by nubile maidens. Nothing is too much trouble for proper audio pleasure...

You forgot to mention the gold plated mains


Thread: Early metal lathes
26/05/2019 20:26:59

The obvious answer is shafts, such a crankshafts and the like. However it's likely that other parts were turned bwtween centres and the centres cut off afterwards. A 3-jaw chuck would have been expensive and the average ME may not have had one. So it was a choice of between centres or the 4-jaw chuck. In contrast I've turned between centres only once; to machine the crankshaft castings for my traction engines.

To illustrate how workshops have changed I actually do a lot of work between centres, but that's on the cylindrical grinder. smile

The lathes mentioned were bought because they were cheap, and could be afforded by the working man. In days gone by the lathe would have been pressed into service for almost everything, ie, turning, milling, line boring and so on. Often the only other machine tools in the workshop would have been a pillar drill, and possibly a bench grinder.

A lot of work was done by hand with hacksaws, files and cold chisels rather than machining, think locomotive frames for instance.


Thread: What Did You Do Today 2019
26/05/2019 13:49:22
Posted by Neil Wyatt on 26/05/2019 12:31:45:

Let's steer clear of getting political..

I'm not sure if the matter under discussion is a comedy or a tragedy, but it sure ain't politics. smile


26/05/2019 11:32:41

Sadly May is going to run until the end of July, when the fisticuffs are over and a new incumbent emerges.


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