How can Laser cut plastic discs have a tolerance of +- 1mm?

[flanajb]

I recently spent £60 on having some acrylic discs machined. 10mm and 20mm diameter to be precise.

When ordering on the web site it asks you what size disc you want in mm. The actual wording is below.

Please enter the overall diameter of the Plastic Discs you require in Millimetres

Minimum 1mm - Maximum 800mm



All our plastic discs are made to the nearest millimetre

They are not suitable for applications where very tight tolerances are required

Now the 3rd paragraph seems to contradict the "T&Cs" that I failed to read :-( which states accuracy is +/- 1mm. The mind boggles how a laser cutting machine can be so inaccurate! Surely, it should read "All our discs are made to the nearest +/- 1mm"

Bloody annoyed as a tight tolerance in my eyes would be +/- 0.01mm or even +/- 0.1mm, but not +/- 1mm and the company are not interested in resolving the issue.

 

[paulm]

Bloody annoyed as a tight tolerance in my eyes would be +/- 0.01mm or even +/- 0.1mm, but not +/- 1mm and the company are not interested in resolving the issue.

That'll be because there is no issue, they appear to have supplied as per their descriptions, T&Cs and stated tolerances.

It's not their fault you didn't bother to read the details and/or call them to check

Cheers, Paul

 

[marcros]

how large are they? I could use a few at 11mm if they are of no use to you,. whilst it wont recuperate much of the outlay, it may help.

 

[Phil Pascoe]

It would be interesting if you order a 1mm disc with a machining tolerance of up to -1mm :? :lol:

 

[RogerP]

It would be interesting if you order a 1mm disc with a machining tolerance of up to -1mm :? :lol:

That was my first thought, maybe you'd just get an empty box back?

 

[blackrodd]

It would be interesting if you order a 1mm disc with a machining tolerance of up to -1mm :? :lol:

Well spotted Phil.p does rather make a nonsense of + or-.
Rodders

 

[AndyT]

Just thinking aloud here, but trying to answer the question of how come the tolerance is so large.

I imagine that the that laser is burning away some of the plastic. Assuming that the power setting of the laser is fixed, the width of the cut might vary measurably according to the temperature of the material being cut - if it was relatively warm, I imagine that the melting would continue a bit further into the plastic. If it was cold, the heat from the melt could not travel very far from its original path, and so the width of the 'kerf' would be less.

Does that make sense?

Also, if the cut goes right round a disc, then the margin of error on the finished piece is double the margin on a single cut. So the cutter must be twice as accurate as the stated tolerance.

 

[flanajb]

Frustrating, as I usually use a different company for the discs. They laser cut too, but theirs do come out bang on 20mm and 10mm.

Lesson learnt.

On closer inspection of the T&C's I cannot now see reference to a +/- 1mm tolerance.

http://www.candlmailorder.co.uk/terms.htm#terms and conditions

 

[doctor Bob]

I'll leave this thought here free of charge...............bespoke tiddly winks........... nice little earner.
Too big?.... garden tiddly winks :wink:

 

[flanajb]

I'll leave this thought here free of charge...............bespoke tiddly winks........... nice little earner.
Too big?.... garden tiddly winks :wink:

Haha!!

I'm always looking for a new business idea.

 

[Lons]

As someone who has experience selling semi finished plastics including ICI perspex, I would say that it's virtually impossible to get tight tollerances and TBH I'm surprised they're quoting +/-1mm.
Thermoplastics expand and contract according to temperature which is why fixing holes have to be oversize and allowance for expansion included when fixing into frames. They aren't know as "thermoplastic" for nothing. Some of course move more than others.

Bob

 

[rafezetter]

I know it's obvious but my respose to the company would be if you cannot see any direct reference to the +/- 1mm in the T&C, I would point this out to them or ask them to provide directions to where this information is displayed on either the product info or the ordering page info, because to my mind unless it's obviously and clearly stated, then the "nearest millimetre, and "not suitable for tight tolerances" could be taken as ambiguous and up for dispute.

Is the "nearest millimetre" rounded up or down (and whichever it is where is that stated so the buyer can take that into account)? Is it a partial or whole millimetre? do they do it to 12.5 > 13.5 or 12.5 >13 tolerances?

How tight is the "tight tolerance"? Aircraft machining tolerances or barn door tolerances.

And as mentioned how can they say they can make it from 1mm and up if the tolerance is that large? What do they do if someone actually orders a 1mm disc?

I'd send them another email to this effect, or find head office and contact them, and also contact CAB about this and ask them to refer this to Trading Standards, because they should either make it very clear, or accept fault and redo if they have not.

Lastly you might wish to see if you can get them to compromise and have blanks remade a bit bigger and then have them sized as you need - maybe a router and template?

Edit: I've also worked in plastics, so I agree with Lons and what he says about plastics, but this is about the fact that the conditions of an order and sale are a little ambiguous, and thus open to interpretation by a customer, whom they must assume knows little about the intricacies, technical data and behaviours of plastics, and thus make it absolutely obvious what may happen BEFORE any order is placed.

also waterjet cutting.....