Standard tolerance scheme for lasercut stainless

[loughnane]

Looking at some suppliers I see people claiming plus/minus .003 to .005 on positional dimensions.

The question I have is "for a lasercutting machine, is that over the entire part? For the first 6 inches?

I'm working with a 16" wide sheetmetal part that mates to some plastic parts, and just need to verify my dimensioning scheme.

Chris Loughnane - Product Design

http://www.pdnotebook.com

http://www.twitter.com/DesignNotebook

 

[MikeHalloran]

In my experience, mostly with stainless steel, the roughness of a lasercut edge makes a .003-ish tolerance seem a little silly.



Mike Halloran
Pembroke Pines, FL, USA

 

[loughnane]

I guess I'm thinking more hole center to hole center.

Chris Loughnane - Product Design

http://www.pdnotebook.com

http://www.twitter.com/DesignNotebook

 

[MintJulep]

Firstly, any such claims are in regard to their manufacturing capability, not tolerances.

Tolerances are a design function. Manufacturing processes have error.

As engineers we should understand the difference and use the terms correctly.

Now, to your question.

When your drawing is converted to a set of instructions for the machine, all the coordinates are converted to an absolute reference scheme based on the machine's known origin point.

The cutting head has no idea what feature it last made, or what feature it is going to make next, and it has no idea of the dimensioning scheme or tolerances on your drawing.

 

[loughnane]

Isn't manufacturing capability expressed in tolerances?

That's kind of like saying I'm talking about height, not inches.

Semantics either way...

Now, to your response.

That makes sense, but still confuses the issue a little bit. If a machine starts at the origin, and goes to one hole, then the next, then the next, etc. It seems that the error would add up between each.

Assuming it creates the features closest to the origin first (I think likely), that would make features further away have a greater possible variation in their position.

The only way I can think of them holding .003 (or .005) throughout is if, after each feature is made, the machine comes back to the origin and recalibrates it's position.

I doubt that the latter of those two methods is true, which gets back to my question... does the variation increase with it's distance, or is it the same over the entire workpiece?

Chris Loughnane - Product Design

http://www.pdnotebook.com

http://www.twitter.com/DesignNotebook

 

[MintJulep]

Let's start with, how can they say that they can achieve +/- and then give a range. Can they achieve the small side of the range, or the large side? If they're going to give a range then then need to explain the range.

Perhaps they have more than one machine, and different machines have different capabilities.

So for the moment, let's pick a number. .003

What they think they are saying is that if they command the tool head to a specific position then the tool will go to that position with an accuracy of .003.

With only the information given so far, we don't really know if this accuracy is composite, or applies separately to each axis of motion.

The machine knows the tool position based on feedback from some sort of encoder. Maybe its error is a function of distance from the origin, maybe it isn't.

In all likelihood the people making the accuracy claims don't understand what they are claiming.

 

[TheTick]

Basically, most laser cuts are within .005 of the ideal line. Varies with thickness and material. Nitrogen blanketing helps.

 

[loughnane]

Thanks everyone... I haven't selected a vendor yet but will definitely be pressuring them for specifics once I have.

Chris Loughnane - Product Design

http://www.pdnotebook.com

http://www.twitter.com/DesignNotebook

 

[MikeHalloran]

Pressuring vendors for specific claims about what they can or cannot do is pointless.

Give them a drawing with tolerances, and buy (or don't buy) a part made to the drawing.

The resulting part passes inspection, or it doesn't. Binary function, returns one value, 0 or 1, no ambiguity to resolve.





Mike Halloran
Pembroke Pines, FL, USA

 

[fcsuper]

Mike, I don't really think that asking vendors about tolerances they can hold is pointless. Actually, it's quite the point. Many vendors even try to add PO modifying comments to their invoices that state their own tolerances regardless to drawing requirements. This kind of information is very important before one issues them a PO.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

http://groups.yahoo.com/group/solidworks & http://twitter.com/fcsuper

 

[MintJulep]

Many vendors even try to add PO modifying comments to their invoices that state their own tolerances regardless to drawing requirements

Which brings us back to my earlier point about people needing to understand the difference between tolerance and manufacturing capabilities and process deviation, error and distribution.