You could always fit a dro to your lathe(s)? Metric threads are only complicated in that the threading dial cannot be used and you may need a conversion gear. MOD gear cutters seem more easily available and while 20 degree pressure angle for them is clearly wrong – whether imperial or metric – does it really matter? If it does that may decide your choice.

OS says ‘most people’. I would say ‘virtually everyone’. I’m quite sure that metal thicknesses are not all exactly scaled to the originals, so does it really matter, as long as it is at the correct(ish) scale and it works as expected?

The important thing is not to mix the two systems without taking extra care to sort out the ramifications of doing that.

Drawings and castings for my Ruston proctor SD came from Live Steam Models and were in inperial units but I built the engine in metric, mainly because of the availability of tools and materials.

The boiler drawings that were supplied used 10" OD (254mm) pipe which is not available. Commercially available 10" nominal bore pipe is 273mm OD. Therefore many parts had to be modified to suit the larger boiler barrel diameter. Fortunately, I worked as a mechanical design engineer/draftsman, so re-drawing everything to suit available materials in metric was not a problem and the complete engine was modelled in 3D using Solidworks. You would not have these problems if you used the modified drawings that use available materials in metric. But it is worthwhile double checking everything before you start machining because the drawings may contain some mistakes.

Paul.

Of course I'm biased, as I'm building from the LSM imperial drawings. My castings are from elsewhere and follow the original Filby design in some areas, so the LSM drawings are not appropriate. There are a number of show stopping errors, and a lot of missing detail, in the LSM imperial drawings. I'd be surprised if they had been corrected in the metric versions.

As I mentioned to Karl yesterday at the Forncett exhibition day one area that has gone metric is sheet and plate. So 1/4" plate for the hornplates is simply not available. I used 6mm plate which necessitated changing the dimensions of some other parts. However in other areas the situation is more complicated. I've just looked at the rear axle material, specified as EN8 and 1-5/8" diameter. Looking at my local steel stockholder 1-5/8" is no longer available for order. Both 1-1/2" and 1-3/4" are available and in stock. The metric equivalent is 41.275mm. The nearest listed is 42mm, but this is not available for order. The only size that can be ordered is 45mm, which is on 14 days.

An interesting question is what have LSM done with the gears. The DP values do not correspond exactly with standard module sizes. So changing to module would require some significant changes to centre to centre distances, which has a number of knock on effects. Similarly have they made new patterns for castings, or are the castings still based on the imperial design?

When I started I planned to change all threads to metric. Internally I used a number of metric, mainly M6, SHCS and will continue to do so. But externally they just don't look right, so I've reverted to BSF and BA. Since I'm making most of my own bolts, studs and nuts, availability of off the shelf items isn't important. Nor is the availablility of the correct hex AF stock, as I use sizes that correspond with the hex collets I have available rather than what is nominally correct.

My lathe is imperial and both manual mills are metric. Generally this is not a problem either way. However, as NDIY says the main issue is metric screwcutting on an imperial lathe. I tend to screwcut a lot of my threads, a number of which are specials. The imperial drawings call out a number of BSP and 32 & 40tpi ME threads for which there are no direct metric equivalents. Before anyone points it out I know that there are ISO pipe thread standards, but they're based on the BSP system. So for 3/8" BSP you'll still need to be able to cut a 19 tpi thread.

Andrew