how to tell where the cutting bit will start?

[BrassBuilder]

Hey DolphinCadders,
I have another problem I cannot figure out.

Is there a way to tell DolphinCad where I want the cutting bit to start. For instance take this crude representation of a Cad drawing:


O O
x
O O

I need to mill four circles like above (only bigger). Is there a way to tell the bit to start right in the middle of the four circles (where I have it marked with the "x")?

Right now it starts to the left and above center of the four circles. If I could start it right at center, that will make things much easier to line up.

If I cannot tell the bit to start at a certain place, how do I get my work lined up so that when the bit does start cutting, it will be in the right place?
Thanks
Mike

edit: arrrrgh. The x moves over to the left when I load my thread. The x is supposed to actually mark where the center is of the four circles. Just move the x over to the right about 1/4".
Mike

[metalworkz]

Hi Mike,
It sounds like you want to set the datum for the drawing and part. Just select the Datum button(blue quadrant circle) from the top menu and and you can either click on the drawingto locate it or usethe X Y text box to locate it precisely. This should move the starting point for all operations and demensions to the new datum. Hope this is what you wanted to do. Sometimes it is easier to work from the center of the part.
Regards,

[BrassBuilder]

Originally Posted by metalworkz Hi Mike, It sounds like you want to set the datum for the drawing and part. Just select the Datum button(blue quadrant circle) from the top menu and and you can either click on the drawingto locate it or usethe X Y text box to locate it precisely. This should move the starting point for all operations and demensions to the new datum. Hope this is what you wanted to do. Sometimes it is easier to work from the center of the part. Regards,

That is what I want to do. I had tried that earlier and it still put the starting point about .030 off to the left and to the above of where I wanted it to start.

I am finding center by drawing a line kitty corner from the center of each circle. I then mark where the two lines cross as the new datum point.
Mike

[metalworkz]

Hi Mike,
I think if the snap is set to 'snap to the nearest point' the Datum will not always go right where you want it either, so you might want to click on the snap 'none' button(arrow without crosshatch). The method you are using to find an intersection will probably work good and you can snap to that point.
Regards,

[BrassBuilder]

I'll have to take a look at this again. My method isn't working quite what I thought it would. The center is still off a few thousandths.
Hmmmmm
Mike

[metalworkz]

If the circles are not all the same diameter or if any are not located equidistant than you may try drawing a box tangent to the outside of the circles and then 2 diagonal lines from the corners to find the intersection close to the center.

[BrassBuilder]

The circles are the same diameter and they are parallel with each other (side to side and up and down).
But I'll give your suggesting a try. I am out of ideas.
Mike

[metalworkz]

Hi Mike,
I don't understand why the datum location is so critical unless the part is going to be rotated or otherwise relocated, but then the setup will need to have a definte scheme for maintaining the part location also. Maybe flipping the part etc?

[BrassBuilder]

Originally Posted by metalworkz Hi Mike, I don't understand why the datum location is so critical unless the part is going to be rotated or otherwise relocated, but then the setup will need to have a definte scheme for maintaining the part location also. Maybe flipping the part etc?

Let me try to explain what I am doing. I have four small holes that are going on the side of a piece of aluminum that is 1/2" tall and about 1-1/2" wide. The holes are appoximately 1/4" apart and are just under 1/8" in diameter. They are representing soft plugs in the side of a cylinder head assy for a model engine that I am building. I need these holes evenly spaced in the allotted area (1/2" x 1-1/2"). The bit/DolphinCad/the mill does not quite understand what I need done. It wants to start a few thousandth to the left and above the center of the part, which then puts these holes off center. If I can tell the bit to start at the very center and it then moves over and cuts the first hole, I will be good to go.

OR...how can I place my part so that the mill will start cutting in the right place?

Mike

[metalworkz]

Hi Mike,
The starting point or datum of the drawing should not affect the actual hole locations. If the hole locations are not right I would recomend that you check the actual centers of the holes to be drilled on the drawing. You can move the datum where ever it seems to work best for your setup and it should not change the coordinates of the holes. Also how do you know the part is centered on the datum with the tool? Did you check this? If a hole looks like it is off it is also possible the part is not in line with the tool path or machine datum. You need to line the part up with the datum of the program somehow.
Regards,

[metalworkz]

You could even use an actual center of one of the holes as the datum and then you might be able to more easily layout the hole and center punch it to use as a method of locating the part on the worktable with the spindle when it is at the Datum position?
It seems it would be easier to do this with the definite center line of a holw than trying to find the center of the pattern and lining it up on that. If you start at one of the centers of a hole the rest should drill correctly if the part is positioned right and the centers are drawn in correct positions. I think it might be easier to make a drill jig to use for drilling and then any mistake in making the jig does not ruin the part.
Regards,

[BrassBuilder]

Originally Posted by metalworkz You could even use an actual center of one of the holes as the datum and then you might be able to more easily layout the hole and center punch it to use as a method of locating the part on the worktable with the spindle when it is at the Datum position? It seems it would be easier to do this with the definite center line of a holw than trying to find the center of the pattern and lining it up on that. If you start at one of the centers of a hole the rest should drill correctly if the part is positioned right and the centers are drawn in correct positions. I think it might be easier to make a drill jig to use for drilling and then any mistake in making the jig does not ruin the part. Regards,

Ahhhh...ok....I'll give this a try. I should have added this is not a simple drilling job. There are two holes. One goes in about .1" and is .093 in diameter. The other one goes in .03 and is .125 in diameter. The last one is right on top of the first one. This gives a little "lip" that simulates the soft plug in the head. I am quite picky on the details that I put in. LOL.
Mike