Cheap square angle tolerance

[Cheshirechappie]

Words like "accuracy", "tolerance" and "spot on" seem to mean different things to different people, depending on the trade or profession with which they are most familiar.

For someone brought up in a precision engineering environment, "spot on" is a meaningless term, because they are taught that nothing is perfect, but some things work with particular degrees of inaccuracy. For example, a machine tool fitter or inspector would be measuring to microns, and would think a spindle runout on a new machine of a couple of thousandths of an inch would be grossly unacceptable. A bricklayer, however, would be working to something like a millimetre or two for his best work, and would regard that as "spot on", because most buildings would be perfectly fit for purpose built to those limits (or even wider ones!). Neither the bricklayer nor the machine tool fitter is 'wrong' - both are working to appropriate accuracy for their work.

That tends to reflect in the comments on threads such as this - an error you can barely see is "spot on" for the building trades, but the toolmakers would want it expressed in thous or microns, because that's what their minds have been trained to do.

Trying to put numbers to try-square accuracy is perhaps a bit meaningless once you get to thous and microns, because wood isn't that precise. It could change dimension by more than that just with the change in humidity during the day. Consequently, even us engineers have to be a bit pragmatic when working wood, forget our metrology training, and accept that "fit for purpose" is good enough - as i suggested earlier, what's OK for garden decking might not be for cabinetmaking, but no woodworker needs engineering inspection grade metrology equipment!

 

[MikeG.]

.........For someone brought up in a precision engineering environment, "spot on" is a meaningless term, because they are taught that nothing is perfect........

That's just illogical, though. All we are after is something to mark out 90 degrees. Why is every angle down to a millionth of a degree or more possible, but exactly 90 degrees impossible? If nothing were perfect, then absolutely every other angle is possible, but 90 degrees would be impossible. Unobtainable. That isn't a logically sustainable position.

What they really mean by that is that they can't be certain that something is or isn't precisely 90 degrees, but that's a different argument altogether.

 

[Cheshirechappie]

.........For someone brought up in a precision engineering environment, "spot on" is a meaningless term, because they are taught that nothing is perfect........

That's just illogical, though. All we are after is something to mark out 90 degrees. Why is every angle down to a millionth of a degree or more possible, but exactly 90 degrees impossible? If nothing were perfect, then absolutely every other angle is possible, but 90 degrees would be impossible. Unobtainable. That isn't a logically sustainable position.

What they really mean by that is that they can't be certain that something is or isn't precisely 90 degrees, but that's a different argument altogether.

Maybe I should have worded that a bit better - yes, in theory, perfection is possible, but in practice, it doesn't happen. Thus, you quantify the degree of imperfection that is acceptable in a particular situation. A calibration grade square has a smaller degree of acceptable imperfection than an inspection grade square - and so on. It's all rather high falutin' for the practical woodworker, though - which is why these threads so often end up disappearing up their own fundaments, as we end up (rather pointlessly) trying to apply precision engineering practices to woodwork.

 

[Lons]

A bricklayer, however, would be working to something like a millimetre or two for his best work, and would regard that as "spot on", because most buildings would be perfectly fit for purpose built to those limits (or even wider ones!).

In principle a good argument but poor example and can't be compared to engineering or reasonable quality woodwork.IMO

A brickie wouldn't get anywhere near 1 to 2 mm as he is not materially altering the products he works with. Bricks, blocks and other clay and ash type products are characteristically far from a uniform size due to the nature of their manufacture, compare a couple of dozen bricks from the same batch and the differences will be obvious from varying size to banana shaped. The differences are accommodated by the mortar but only to a degree.

Squaring a building withing those limits is usually done by using a 3,4,5 method and building to a line gives you a close as you can get but still is subject to potential line sag or accidental line movement via obstruction.

 

[Cheshirechappie]

Well .... whatever.

I thought I'd try to give some thoughts on why some people talk about 'spot on' and some people get very picky about accuracy and tolerances.

I clearly didn't do a very good job of it, and I wish I hadn't bothered, now.

 

[Lons]

Well .... whatever.

I thought I'd try to give some thoughts on why some people talk about 'spot on' and some people get very picky about accuracy and tolerances.

I clearly didn't do a very good job of it, and I wish I hadn't bothered, now.

I wasn't meaning to criticise tbh as in general I agree with you. My squares are as accurate as I can get them and that'e enough for my needs.

 

[transatlantic]

Not to deter the thread, but it also really depends on the size of cut. It should be pretty easy to get a 6" square accurate enough by eye, simply by trial and error. So long as you only use it for 6" or less.

I really dont see how those new track saw 90 degree guides can work as well as they're advertised though. You see them being used with 1.5+ meter track, with what? 200mm reference surface? You have a lot of leverage on that track and the smallest of movement at the reference surface will make a huge difference in terms of how square the cut is. Yet because of the crazy tolerances and the aircraft grade aluminium , people just assume its always going to give accurate results. No point in having the guide be super accurate unless the track meets similar standards

Sent from my SM-J510FN using Tapatalk

 

[MusicMan]

It always tickles me how often the same conversations are repeated. How-square-is-square is one we do every few months here, and although the personnel change, the discussion points remain the same.

Yes ... I keep reposting the most precise and cheapest way to check it at home and people keep ignoring it and rabbit on about 'standard' squares and the meaningless 'spot on' ...

Would you like explain what is meaningless about spot on?

CheshireChappie explained it very well.

 

[MusicMan]

Well .... whatever.

I thought I'd try to give some thoughts on why some people talk about 'spot on' and some people get very picky about accuracy and tolerances.

I clearly didn't do a very good job of it, and I wish I hadn't bothered, now.

I wasn't meaning to criticise tbh as in general I agree with you. My squares are as accurate as I can get them and that'e enough for my needs.

And that is a perfectly good and meaningful statement!
Keith

 

[MikeG.]

.........Maybe I should have worded that a bit better - yes, in theory, perfection is possible, but in practice, it doesn't happen..........

No, I'm not having it. Of course it happens. The issue is that we can't be absolutely certain that it has happened in any one particular instance.

 

[That would work]

I have had quite a chuckle reading (the latter parts) of this thread as if Alan Bennett were reading it :lol:

 

[Cheshirechappie]

Well .... whatever.

I thought I'd try to give some thoughts on why some people talk about 'spot on' and some people get very picky about accuracy and tolerances.

I clearly didn't do a very good job of it, and I wish I hadn't bothered, now.

I wasn't meaning to criticise tbh as in general I agree with you. My squares are as accurate as I can get them and that'e enough for my needs.

Sorry, Lons. I shouldn't have snapped at you - you did make a fair point. My interpretation of building practice wasn't as clear as it should have been. No offence taken on my part!

 

[Cheshirechappie]

.........Maybe I should have worded that a bit better - yes, in theory, perfection is possible, but in practice, it doesn't happen..........

No, I'm not having it. Of course it happens. The issue is that we can't be absolutely certain that it has happened in any one particular instance.

Well, I don't want a fight over it, Mike.

 

[Cheshirechappie]

I have had quite a chuckle reading (the latter parts) of this thread as if Alan Bennett were reading it :lol:

Now I REALLY wish I hadn't bothered ....

 

[That would work]

I have had quite a chuckle reading (the latter parts) of this thread as if Alan Bennett were reading it :lol:

Now I REALLY wish I hadn't bothered ....

No no, I don't refer to the thread itself it's a fair point to raise.

 

[Cheshirechappie]

I have had quite a chuckle reading (the latter parts) of this thread as if Alan Bennett were reading it :lol:

Now I REALLY wish I hadn't bothered ....

No no, I don't refer to the thread itself it's a fair point to raise.

Ah! With you now - I think. "Sorted" as they say!

 

[That would work]

Spot on.

 

[G S Haydon]

Well done Andy T! Good practical advice that hopefully the OP would find useful. CC, good attempt at trying to give context. Without context people often loose sight on what is actually required given any set task.

 

[bridger]

"Perfectly accurate" just means more accurate than you can measure in your shop.

I once bought a 7' section of aluminum bar at a salvage yard. Something like 4" wide by 3/8" thick, hardened extruded. It was perfectly straight until my jointer plane technique improved enough to detect that it had a few thousandths of an inch (like 10) of bow in it's length. Now it's marked for direction of bow although I do not have metrology enough to reliably ascertain exactly how much bow it has. A few thou is accurate enough for my needs at this time.