Using mm for inches: can I just substitute 900mm for 3 feet or...

[flockandroll]

Hi, I hope you can put up with this rambling question.

if I wasn't on the autistic spectrum maybe this wouldn't bother me, but...

When planning a layout, can I just substitute 300mm for every foot and 25mm for every inch, or does it need an exact conversion? 

So can a minimum 3 foot radius become 900mm rather than 914.4mm? And instead of asking a timber company to cut the plywood into 2 foot wide strips, should I ask for 600mm or 609.6mm? (Should I expect the timber people to cut to the nearest mm? Or less or more precise? I am assuming that it is as quick for them to set their saw to 609mm as it is to 600mm (I am aware of the difference between accuracy and precision))

And as plywood comes in nominal 8 x 4 foot sheets ( 2440mm x 1220mm ), do I ask for them to be cut to 1200mm long as opposed to just using the width of the sheet?

You need to allow for the kerf, so you can't actually get four 2 x 4 foot sheets out of a piece of plywood. So does mean 600mm x 1200mm would be the way to go?

In OO, if it will go round 3 foot curves and the train will propel over that radius, is it safe to say the slightly tightly 900mm will be OK?

 

I think I have answered my own questions, but I would like some confirmation? Thank you in advance

 

 

 

 

 

 

[Harlequin]

The timber suppliers long ago standardised on 305mm to the foot as the nearest simple approximation. Thus, what were 8ft by 4ft boards are now 1220*2440mm - as you pointed out.

 

305mm is a nice number to work with mentally and things like track radii become very easy, 610mm, 915mm, etc., etc... - and you don't need to ask anyone to cut down your 1220 wide boards - they are already the right size! 🙂

 

When cutting boards you do have to take the wastage into account so, no, you can't get four 1220*610 boards out of a standard sheet of material but the same would be true in Imperial. You've just got to accept that things won't be exact multiples of feet or 305mm.

 

The difference between true 3ft radius and 900mm won't make any difference to OO running.

 

[flockandroll]

Thank you very much!

[ITG]

Harlequin is spot on, but of course if you are planning your track layout, the length of such as points, or set track pieces, are a defined length - be that both imperial and/or metric.  If you round up/down, there is a risk that where you thought a point would sit, between adjoining pieces of track, may not be correct. The result being you could potentially end up with an unwanted overlap, or a small gap. Not such an issue when working with flexitrack, but point still have a defined length.

[Poor Old Bruce]

Some suppliers like Wickes and B&Q (there must be others) offer pre-cut plywood sheets. It might be worth you having a wander round the various outlets (shops to most people) to see what is available as one may be different from another. Always assuming you have any shops nearby and that you  can get to them of course. They may offer a delivery service (for a fee, no doubt).

[flockandroll]

Thank you for helpful replies.

One sheet of  good hardwood ply obtained from http://www.empson-sons.co.uk/ of North Lincolnshire, very helpfully cut up for me. I am now commencing assembly of a small "practice"baseboard which will have a rolling road and curved track so I can check clearances and  practice track laying and ballasting.

[DCB]

Sounds good.  Get yourself some awkward models for checking clearances, In 00  there are some oddballs like Hornby Dublo Royal Mail Coach, Breakdown crane etc which the usual track spacings seem to be designed around, but I have a Hornby Tender drive King with huge overhang at the front and Mk 3 coaches with huge overhang in the middle for checking clearance on curves, and some other oddballs Airfix centenary and Airfix Autocoach for checking platform ramp clearances, and of course the obese 10ft 40mm wide 9F.  The King is also a scale 15ft high which is great for overhead clearance checks. Its ever more important to check clearances these days when people glue track down and drill holes for droppers and ballast it so tweaks become very difficult.  Its actually its surprisingly difficult to lay flexi track straight.   Test layout is the way to go.   I'm not a great fan of taking measurements.  I prefer go/no go gauges, for buffers and , coupling heights  and platform above track level measurement , I made a radius bar to draw accurate circles and part circles even though the pivot had to be a temporary strut across an operating well.  A millimeter variation in height or width is 10 times more noticeable than one in length, like if you have to measure it to see the discrepancy its not enough to worry about.  (and if a loco is 0.5 mm too high and the tender 0.5mm too low you can barely measure it but it looks very very wrong(  ( See Hornby Hogwarts Castle) 

 

[DK123GWR]

If you have a large modern diesel, check that as well. They tend to have underframe detailing right on the ends which could be taken off if you have a tight curve next to a platform. Older diesels (with less underframe detailing) are less likely to suffer in this regard. Also remember that the underframe detailing on some coaches (e.g. Hornby Mk2As) and the overhang on steam locomotives (especially 4-6-0s) will be assymmetric - so check them as they run in both directions.