Jason –

Footling about with that milling-machine’s hand-wheels is the least of my worries for it! I have first to build its drive system, then rebuild its lost table-feed – the worm-wheel is still there but all the rest has vanished.

The feed-screws are: Longitudinal and Knee 6tpi; Cross-feed 8tpi; the knee and cross-feeds are graduated in 25s by 5 (0 = 25). No long feed dial.

Of course we work to the nearest 25ths of 6ths of inches (1/150 inch….. 0.006 recurring).

Alibre practice?

Very witty!

Never mind how to draw any such geared scale – I doubt I could even if I knew the design. Nor could I design and cut the wheels. Anyway, a tiny gear-train to convert sixths of inches to conventional units, on a half-inch diameter shaft on an antique machine, is impossible by arithmetic and lack of space. I only wonder why Denbigh used these strange pitches, and on two, not all three, screws.

The important part is the mill-wrighting. That is too hard to draw in CAD, even TurboCAD‘s direct orthographic mode; but anyway the print would be illegibly small.  I still need design it though. So I build such projects from rough sketches on paper, based on measuring the existing parts.

To rotate that horizontal mill’s spindle from a 1350rpm motor will involve 4 hefty big gears I happen to have, to give 2 speeds (roughly 65 and 130rpm) at the spindle, and various pulleys, bearings and belts.

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Michael –

I did indeed re-read that thread and its basic lesson seems to be less which system of units to use (for our purposes inches or mm is by personal choice and tools available: we don’t have to please the ISO!) but calls for consistency and good-quality dimensioning.

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Martin –

I too have an ancient Vernier-caliper I was pretty sure was in binary fractions!

In fact examining it now, shows decimal scales of inches to 1/1000″ and of cm to something now too faint to read. I keep it as a curio, as it is very hard to read even with a powerful magnifying-glass – and is probably out of calibration. Oddly the internal-measuring jaws are not edges as we normally use but very sharp points, and I wonder if they were for measuring between centre-drill or centre-pop marks, or scribed lines. The make is unknown. If it was there, it has long faded away.

I also have simple firm-joint calipers, read against a rule, and I do need use them occasionally.

Well, there isn’t!

I wonder if a reason not often put forwards for so many model-engineering drawings being in binary inch fractions was that scale.

Most of the full-size things we replicate to scale were made to feet and inches, so many model designs are reduced versions with slight adjustments.

Not always very slight though: if we use the track gauge as our guide, the scale for a 5″ model is 11.3 to 1. Yet a truer equivalent (1/12) gives a track gauge of 4.71″. All right, make that 4.75″…. So it’s all very well until you want to work as closely as possible to prototype from works drawings, then the arithmetic starts being awkward. Unless all our miniature locos are right to scale in everything except gauge!

The other gauges?

3.5″ equates very closely to 1/16 (3/4″ to the foot).

7.25″ is 1/7.8 (approx 1.5″ to the foot.) An oddity of scale is that a narrow-gauge engine like the “Wren” seems to dwarf a model Standard Gauge main-line locomotive!

I don’t though, know how and why our gauges were established, but the full-size Standard Gauge is an odd value to scale anyway.

At least we can work to integer scales, Imperial or Metric, for non-railway models. For Metric I’d convert all the original feet-inches-and-bits to all-inches in decimals first, then use something based on N X 1mm = 1 inch (N being an appropriate divider or multiplier constant). A spreadsheet takes a lot of the graft and error risks out of this.

….

I have just placed an order with Hemingway Kits, and it asks you whether you prefer to work in Imperial or Metric units, saying most of its kits are still in inches. I replied Imperial but can work in both.. That is because my lathes, jig-borer and milling-machine are Imperial, but I have fitted the mill with a 3-axis DRO giving the option.

In fact I often use the milling-machine’s inch dials, with the DRO switched off, for simpler or less critical, inch-based, tasks; to maintain my ability with them.

.

A thought occurred to me about that Denbigh horizontal mill’s two 6tpi screws. It’s easier than it seems.

Their dials are divided into 25 X 5, so 1/150 inch per division. This means 3 divisions = 0.020″.

Aha! Immediately much more logical provided you don’t need work much more finely; but a secondary pointer to show mid divisions like a single-point Vernier, will give 0.01″ limits.

So how to use it? Safest to produce a list in planning the work, and count the turns with pencil-and-paper as you go.

Say you need a 0.5″ deep channel:

0.5 / .02 = 25.

So count 25 X 3 divisions = 75, which is a nice neat 3 complete turns, and Denbigh’s drawing-office, bless ’em, specified adjustable dials on this machine!

5/8 deep though? 0.125″ = 6 X (0.020) + 0.005

= 3 X 6 = 18 divisions, + 1 division which being 0.006″, will take the depth a massive 0.001″ over.

So set the dial to 0, raise the knee by 3 complete turns, then go further to 18. (Test before taking that last few “thou”, as normal.)

Ending the channel short, such as a connecting-rod flute with an elegant concave run-out (the prototype was quite possibly machined on a big horizontal mill itself), needs marking out the work or fitting a rule to the machine, as the long feed is not calibrated.

I don’t know this machine’s history. Its deceased last owner had it in a very scruffy workshop for his hobby of restoring old cars. As with any similar milling-machine though, if its knee screw is worn unevenly such as by a lot of production-work, I would expect the raising to read progressively under as the worn thread flanks take charge. Which at least errs on the safe side.