Best way to dimension plates with radii

[DerbyLoco]

What is the best way of dimensioning plates with radii at intersecting edges for NC manufacture?
Imagine you are drawing a triangle with a radii at each corner. Is it better to dimension the triangle by defining each corner location then fitting in the radii or would it be better to define the radii centers then add the triangle profile as tangent to the radii.
When actually creating the drawing on CAD it does not make much difference. However I am interested in what is best for the manufacturer.
If I was marking the shape out with traditional tools by hand I would do the radii first then mark out the edges but which is best for NC.
The part could be made anywere and the manufacturer would only have the drawing to work from - no electronic file transfer.
I usually draw parts as if I hade to make them myself with traditional tooling but I suspect that I am getting out off date in my thinking

 

[kingnero]

Does not matter really, as the drawings on paper probably will have to be redrawn into the manufacturer's software, or are "translated" into code by people who deal with this on a daily basis and know the easyest way to input this specific problem.
I'd think there is a preference towards marking the center of the radii, however more experienced people will certainly correct me if I'm wrong.
I's say it largely depends on the used software.

 

[DerbyLoco]

I imagine that the cutting machine software input is very like creating the drawing but I have never seen it done hence my question.

 

[MintJulep]

A drawing of a part should show the part. Since the pointy parts of the triangle aren't part of the part they have no part in the part drawing.

 

[DerbyLoco]

Good arugment MintJulep - as I said I always tend to define the centers but most of the other drawings I see dimension the points.
In reality the parts are not triangles but I thought that was the easiest way to phrase the question

 

[TheTick]

One good reason to dimension to intersections is that those are the points that definte the linear edges. In most cases, a fillet is defined by radius and tangency. If you dimension radius and position of a fillet, then it takes more dimensions to define the fillet, even though those dimensions don't really drive the design.

As far as inspection, it depends on methods. Again, measuring to tangencies can be a problem no matter what method. A CMM can be programmed to measure location of straight edges and determine intersection.

 

[drawoh]

DerbyLoco,

I dimension to the sharp corners, usually.

Quite a long time ago, I asked some machinists what they preferred, and they preferred that. With most stuff going to CNC nowadays, this probably is less important. Still, it is easy for everyone to layout and draw a pointy triangle, then apply fillets.

A second issue is geometry definition. If your radii are there because they are required for machining, then your primary interest is the straight edges. You want to describe your geometry. The sharp corners do this.

An alternate possibility is that you are interested in the radii. If that is the case, then you should show the centre and radius.

Note how futile it is to control this stuff with +/- tolerances. This is an excellent application for GD&T profiles.

JHG

 

[mrainey]

It's also pretty common to see specified the width of the radiused triangle and the radius size. The width would be measured across the nine o'clock point (left radius) and three o'clock point (right radius).

Not to say that's the best way, just that I've seen it a lot.

Software For Metalworking

http://closetolerancesoftware.com

 

[IFRs]

Send the shop your cad file in a format they can use. Problem solved.

 

[berkshire]

Similar problem in a way to dimensioning a bend at an angle greater or lesser than 90 degrees in sheet metal.
the flat sides are carried to the theoretical intersection and measured from there. Any variance in the radius does not alter the dimensioning of the part.
B.E.

 

[DerbyLoco]

Thanks for your replies gentlemen, they have been helpful - I agree that it would be best if the CAD file was sent to the manufacturer but my company does not do that for some reason unknown to me.
The parts could be made anywhere in the world by several different suppliers with whom I have no contact.
As it is they were originally created in England, drawn in India and checked by me in England to go on a train in Asia. I don't know were they will be cut out before fabrication. Neither will I ever see the finished product unless I go to China - such are the joys of the global market place

All I really know is that if I had done the job I would not have done it like it has been done but I cannot actually say that it has been done incorrectly