Gallery, Projects and General > How do I??
How do I measure approx 60" ACCURATELY ?
Joules:
Or as previously demonstrated, use DRO as giant caliper. Using a razor blade against the toopost, mark stock and place in 3 jaw (assuming lathe) align razor to mark then advance saddle 15-30" depending on machine, new mark, repeat, this will be about as accurate as you can get. At worst you are going to be within the scratch mark at that "temperature" you did support the material used to eliminate sag, at both ends. You could also do this to check how accurate your tape measure is. No really good way to do this in the home shop unless you have 60" worth of gauge blocks, and a true flat surface to place them on.
https://www.madmodder.net/index.php/topic,12110.msg143732.html#msg143732
Joules:
Another thought occurred, the laser measures, once you eliminate the initial error, how consistent are they. Is the 1mm error always 1mm, ± how much ? I will try doing some measurement experiments and see if the laser exhibits any unusual results over 60 or so inches using the lathe carriage to move the laser. Lots of assumptions on the internals of the laser pointing true, but look for phase shift errors in the electronics of the laser, i.e each mm differs by up to ± 0.2 or the error is consistent. You may also be more accurate reading your 60" from a few meters away.
Joules:
Far from the most rigorous lab setup for metrology :lol:
Not accurate enough for Andrew, but some interesting results just the same. I am using a very cheap ALDI laser measure, if I remember they quote something like ±2mm. During the test I noted very good reading to reading results and was suspicious, damn right, if the reading doesn't vary by the error, they take the last reading. So take a different distance and remeasure. If the change is only a few mm's the error can be quite large, but point at another distance target and come back, I could consistently get to within 0.5mm. Now you may note the Heath Robinson test setup and I haven't got space to try over a larger distance unless I clear some room round the mill. You can certainly test how consistent a laser measure is over a set distance and use gauge, 1-2-3 blocks to offset the laser and check consistency. I suspect the 1mm precision lasers should be able to do around 0.25mm accuracy. A linear rail and bearing block with stops could improve this test a great deal.
Sorry for taking the thread off on a tangent (I believe thats what you are after) Andrew.
I should add, this accuracy was achieved only over this distance, it may vary better or worse at different distances, will try some targets laid out on the floor at some point, over say 3.5m 60" being 1524mm. Probably far better using two metre steel rules end to end.
awemawson:
Joules no problem with tangents !
I've been taking in all everyone has been saying and am very grateful for the contributions, but not actually 'doing' a lot recently - this whole business, which is taking me into diversions off detours via interruptions, and is itself a side shoot off the main picture is frankly a bit depressing. It supposed to be a quick 'let's make a gauge' one day job but there's a 'gotcha' at every corner it seems !
Never mind I won a HUGE (estimated 100 kg) crate of soft jaws for the Beaver TC20 lathe on ebay yesterday and my son (bless him) has just picked it up in Cirencester and is bringing it over at the weekend. Chap was asking a fortune, but in the end accepted a pittance (barely over the price of one set of blank jaws!) And I get two pallet sleeves that the wife uses as raised beds for veg in the polytunnel :clap:
I'll try and get back to the grinder tomorrow !
philf:
Andrew - What's wrong with turning a gauge on your CNC? If it's not going to be hardened, grinding seems a bit over the top - particularly when you consider how awkward it is to set up the grinder. I was going to attempt to be amusing by suggesting you use an Oxymoron gauge to measure "approximately 60" accurately" but thought better of it!
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