Stock center not true center

[Anonymous]

Today I messed up a whole run of parts.

Ive been making these parts for 3 years and never had encountered this issue before.

I was hoping someone more qualified than me could take a look at this attached project and tell me why my parts came out off-centre. 

to brief: I set up the machine on center, I set up in cam environment at what I thought was center and I machined the parts from opp 1 - using the centre hole from opp1 as a centered reference in opp2. The parts came out wrong for the first time in 3 years for me. Ive been doing this same thing for runs of these parts for 3 years and never got any off-centre parts using this same method.

Now issue is, I choose stock box point as centre for X + Y orientation.  and I choose fixed size box 6x6 as a stock set up *MOST IMPORTANT POINT TO NOTE* is that I choose for there not to be additional offset in my set up. 

Turns out this was not centered in my model as I verified by drawing a measurement sketch. with lines and points creating a center point in the middle of the center circle.. I used the measure tool to verify the points in the sketch were in fact on center of the model and then I opened the set up which showed "stock box point" to be off-centre even though I selected for there not to be additional offset in the stock set up.

The model was drawn on center and I drew a sketch once I found the parts came out wrong ontop of the model to be sure it was centered.  

ive attached the model to show the issue. 

why did this happen? im not very bright so I felt this was something I needed to figure out.

I assumed that no additional offset in stock with the model selected - the center would be the model center.

why was it not defaulted to the model center? 

WOULDNT THE STOCK CENTER WITHIN THE PARTS TRUE CENTRE? 

[Anonymous]

if you cant understand my frustrated rant from above here is a snip showing what i mean.

WHY DID THE STOCK NOT CENTER WITHIN THE MODEL IF THERE WAS NO ADDITIONAL OFFSET?

[daniel_lyall]

It is bad I have tried and failed even using a joint to the world or local origin makes no difference.

@jeff.pek this is a bad one please have a look thank you.

[jeff.pek]

Hi -

I took a look at this model, and I think we have a slightly off-kilter model, which is throwing off the positioning of the model within the stock.

I created a new setup, and set it up as a relative-sized box, with zero offsets. Note the X & Y extents that it calculated, based on the tight bounding box of the model. I would expect these values to be the same, but they are not. I'm guessing this is due to the way that the teeth of the sprocket are modeled; if you removed those, and just used the circular bit, I'm guessing you wouldn't see this.

Jeff

[jeff.pek]

Can you use the model origin as your CAM origin? This would be guaranteed to be where you want it, if you can account for the height offset.

Jeff

[porsbym]

It is in fact using the center of the model... but because the model is slightly irregular this creates a slight offset as can be seen in this sketch with the 0.008 dimension to the right between the center of the rectangle and the model origin:

Notice that for the rectangle I have set up tangent coincidents with the most extreme arcs on the model in each direction. Notice also that the rectangle is not actually square.

This is the same way that we calculate the bounding box for the stock in the CAM setups, and the center of the stock box is always the center of the calculated stock. You can see the calculated dimensions like this - it match that of the sketch (although shown with more decimals here):

We don't currently have an option to specify the stock exactly like you are looking for here, but you have the option to set the WCS origin to "Model Origin" or a selected point which would give you the exact origin you are looking for.

If you also need the simulated stock box to be perfectly aligned with the WCS origin you will have to draw a solid box and use that with the "From Solid" option.

We can of course consider adding additional options to control how the stock is positioned, but it's always a balance between adding options in CAM and just using the modeling environment.

With all of that said, I must say that I can totally see how one would miss this slight skew on a model like this.

I hope this is helpful.

/Mark