How to locate existing part to engrave

Hi,

I’m completely new to the world of CNC and looking at purchasing a Nomad 3.

In most of the videos I’ve seen of people using these CNC machines, people are taking a big lump or sheet of material and carving the full part from that. But what if I have an existing part (some aluminium hex bar stock in my case) that I want to perform some operations on? How do I go about “locating” that part so that the machine knows exactly where to start cutting and what orientation the part is in etc.

Is it a case of lining the part up with the X or Y orientation of the bed as accurately as I can with an engineers square and also measuring the distance to the first cut and including that offset in the CAM program? Or is there a more sophisticated and automated way of handling this?

Thanks

For odd shapes, I’ve found it most expedient to machine a fixture w/ a pocket to place the part in.

Forgive my stupidity but how exactly does that help? Do you make some kind of marking on that jig that you always set the machine to zero on so you can use that as a reference for the cuts?

In my case the part isn’t really an odd shape as its perfectly straight down one axis (see image), so would you just line that up with one axis of the bed as best as you could with a square or something?

To line it up accurately, attach a piece of (for example) mdf to the wasteboard, then use the machine itself to create an accurate edge you can place the item against.

For this case, I would create a low, long ‘L’ shape, so you can push the item into the corner.

Now, in your design program, create a simple representation of the object. Lets assume the lower left will be the X-Y zero (ie, the inside corner of the ‘L’). In this case, I would use a rectangle, and place the rectangle so the lower left corner of the rectangle is at X-Y zero. Create your design using the rectangle as a reference for placing the desgn and operations.

In order to machine, first place a square block into the corner of the ‘L’, and use the edges of that to set X-Y zero. Now place your item, and set Z zero as appropriate.

2 Likes

Leave the origin where it was after machining — I set it at the corner of the stock for the fixture.

1 Like

I’m not sure what your current level of knowledge is about how to use a CNC, I thought I might cover a few bits of knowledge that it’s easy to forget about.

On a CNC, when you start it up, the machine tends to go and find it’s ‘zero’. This is generally achieved by moving to one corner until the home switches activate and tell the machine where it is. On big expensive machines they have ‘absolute’ encoders which always know where they are, at the hobby level, the machine needs to go find zero. Now that it knows this it knows where the limits of physical travel are etc. It has no idea where the thing you want to cut is.

When you produce some CAM from your CAD the software calculates a toolpath, which is a bunch of moves at specified speeds. These have to start somewhere, this is the ‘job zero’ or ‘workpiece zero’. When you create the CAM you tell the software where your workpiece zero will be, this is the place on the workpiece, or fixture, where you will zero the machine to tell it where the workpiece is.

To set up a part such as yours there’s a couple of major options as described by Will and Michael.

  1. Create a fixture for the workpiece, this is frequently best done by having the machine cut out a space for the workpiece to be held. In order to do that, you’ll have to choose a point on the fixture to be ‘workpiece zero’. To use the C3D zero tool that’s frequently a lower left corner. You’ll then use the same point on the fixture to set zero for your actual cut, this way you know where the workpiece is relative to the fixture, and the zero point.

  2. Align the workpiece and zero directly from it. If you want to go this route it’s best to buy a dial indicator and a mounting arm of some sort which you can attach to the spindle. I have a dial indicator which fits directly in the collet on my Shapeoko. Using this you can set the workpiece up straight by eye and then ‘dial it in’ by jogging left / right or forward / backward with the dial indicator on a straight part of the workpiece and adjusting until the reading doesn’t change and the workpiece is in line with the machine X or Y. You’ll then need to zero from some part of the workpiece, you can to this by eye / fine piece of paper method or using the bitzero. Your workpiece is not well suited to the bitzero however as that is designed to sit on a flat, square corner.

HTH

3 Likes

Ah ok thanks, that does indeed help a lot and clarifies what the other guys had described.

One thing you mentioned though, the Carbide 3D Zero Tool - is that just a tool within the software, or do you mean the physical BitZero product that they sell? I saw that but noticed it only mentioned the shapeoko and not the nomad so I assumed either the nomad already had something similar built in or its just not compatible

I’m a Shapeoko user so my Nomad knowledge is limited, this video by Winston Moy suggests that the bitzero works with the current Nomad machines however.

2 Likes

oh nice, it sounds like its included with the nomad now anyway :slight_smile:

But yeah like you said, it probably won’t work directly with this particular workpiece due to the shape so I guess I’ll either go with the dial indicator method or just machine an L shape to push the part against and then use the bitzero on that.

Thanks for the fast and useful replies everyone! Much appreciated

1 Like