Import the 2 models in Repetier.  In the example below, I simply centered the 2 models.  Since they are both the same height, the negative volume should displace the the letter shape from the block when sliced, leaving a nice cutout.

And here are the results

Here is an alternative way.  You can reposition the negative shell model (the letters so it only partially intersects the other model, giving you an engraved look.

That's it for now.

Some asked on another thread how to add text to an already existing object.  The response primarily suggested using a modelling program (like Cubify) to start from scratch.

There are 2 problems with that approach.  One, it assumes the user is capable or willing to invest time and money to learn the program, and two, it assumes the model he has can be imported and modified.

There is however another approach, and luckily it involves simple, easy to use, and most importantly free programs:  ScketchUp, NetFabb and Repetier.

SketchUp will be used to create the lettering, NetFabb will be used to create the negative volume object, and Repetier will be used to blend the two.

In the example below we'll assume the 2 objects, text and other model (in this case a rectangular block) are already done.

First we import the text STL in NetFabb, then we select it, and click the repair button.

Then we use the "Shell Selector" tool to select all the separate letters.  If the text is cursive and one pieces, it may just be a single shell.  Once all the shells are selected, click the "Flip Selected Triangles" to invert the face orientation.  This effectively creates a negative volume shell.  Click "Apply Repair", when prompted click "Delete Old part".

You can tell the part is now in error, as it 's completely highlighted in red.  Export it out to an STL.

Next Up, we'll use Repetier to merge the two objects.

couple questions on Netfab when doing the fast repair with pull down giving 2 options either default or simple. whats the dif and can others be added to list?

Take a look at the Repair Scripts tab to see the difference between the default and the simple scripts.  The simple script only fixes fliped triangles and closes trivial and all holes.  The default has 9 separate actions

If you don't remove the old part, you'll simply have both the original and the repaired file.  They'll both be occupying the same space, so it may be hard to tell there's two part in there, but when you save them they will be saved separately.

When you have a model made up of multiple bodies, you can save each part as a separate entity.  You can even select parts and pieces of your model and save them as separate STL's  This is helpful when you want to disassemble a larger more complex model into small pieces for easy printing.

This is what I think the tech was referring to.  As far as putting them back together as a single model, unfortunately, that is a Boolean operation and requires the "Pro" version.

Here is an example of this.

The is Tails from Sonic the Hedgehog.  The model is made up of many separate parts, tails, hand, feet, body head, etc.  You could print this as a complete model, but it would require quite a bit of support.  Or you could separate it into individual, printable parts, then glue it together.

So we go to Repair, then, using the Shell select tool click on a part (the head for example).  This would highlight the complete part.  If you wanted to select other parts to go along with the head (like the body and arms, all you need to do is hold the SHIFT key for multiple selection).  Click the Mesh Edit drop down from the top menu, and select Extract Selected Triangles as Part.  This will create a new part that can later be exported as a separate STL.

The Boolean operations allow you to merge together multiple parts, objects into a single one.  Additionally it allows you to fix intersecting parts. 

For example lets say you have a cube, and a rod going through it.  Those intersecting bodies create unwanted errors, and unwanted geometry.  The parts of the rod that are inside the cube are not needed or wanted.  The Boolean operation will cut the part of the rod that is inside and get rid of it, while at the same time stitching all the edges of the rod where it intersects the outer surface of the cube. 

That is a very powerful function that NetFabb wants us to pay for.  Can you manually duplicate that function? yes, to an extent, but it's very laborious.

BTW, here is the Tails model.

then when applying repair it asks if you want to remove old part or not. when would you not want to?

also I had a Netfab tech fix a model once for me and he said to "explode' parts to repair sub assemblies inside etc then re-assemble with some features in Netfab.

I know one feature disabled in free version is Boolean stuff what ever that is. but like others here I havnt begun using over 90% of the free versions tools and Netfab seems to be able to do about any fix needed if we know how to use it. one thing I have not found yet is how to cut or separate parts of a group of objects. or delete unwanted parts e.g. clean up flash on a scanned model etc.

these are a few things a short tutorial could be made for.
I have most other programs available for repairs/editing stl files too but even more lost on how to use parts of them. and Netfab is much easier/user friendly.

Here is an alternate method to the repair above.  Depending on the complexity of the model, this might be faster and simpler.

Here we start with the upper cut of the previous model.

As you can see from the first slide, the triangulation process did the same thing to this piece as it did to the other one, namely it filled everything in.  You can even see in the second slide that where the opening is, we have a single wall, open surface, which is the error  that requires repair.

Click Repair, flip the part upside down so the bottom is facing up.  Using  the Surface Select tool select the surface and delete it. 

BTW, this is what you would get if you unchecked the Triangulate Cuts option when you make the original cut.

Next we will use the Add Triangles tool to close up the holes manually.  Hover the cursor over one of the yellow edges, and when it turns blue click it, then move the cursor to the opposite side, and when it turns blue click it.  Do this as the arrows show, until you closed most of the main surface.

Notice that I'm staying away from the circular edges.  The reason for that is that there are hundreds of opposing edge pairs, and to do that manually would be a really time consuming task.  What we're trying to do , is close most of everything else, so that when we apply the Close All Holes action, NetFabb has no choice but to follow the correct edges. 

The simplest way I can explain this, is that by manually creating a surface in the direction you want, you remove NetFabb's option of making the wrong decision.

So now that we did just that, click on the Close All Holes, and the model should be fixed correctly.  There shouldn't be any open edges, but if there are, simply use the Stitch Triangles action prior to applying the repair.

On a side note, you really don't have to close all this manually.  Simply close a few edges around the areas that Netfabb seems to get wrong, to remove it's "wrong" options, then click on Close All Holes.  Experiment and you'll get the hang of it.

Have fun.

Fantastic work Pirvan
Again I would request this become a sticky.
Have you considered putting it in the WIKI?  It truely would be (already is) a great contribution.

+100

Many times we have a model that we need to cut into one or more pieces prior to printing.  NetFabb does a good job of cutting the model, but sometimes fills in holes that should be left open.

Here is how to fix that:

First we load the model we fixed in the previous tutorial, using the "Z" slider select a place to cut the model (about half way, or 6mm in this particular case).  Click Execute Cut, and new tab appears with information and some options.  By default the Triangulate Cuts box is checked.  Leave it checked. Another available option is Only Selected Parts".  If your model is made up of multiple parts, you can select to cut all, or only a few by SHIFT selecting the parts you want to cut and checking this box.

Click Cut.  Your model is now in 2 separate parts. Unfortunately, both your new parts have errors (The yellow bang on the main window and next to each part in the upper right parts list window.

While in the parts windows, click the Eye icon next the part name to make that part invisible.  You can now see we have errors.  NetFabb attempted to triangulate the cuts, but also closed the center rectangular hole as well as the semi-circle holes under the 2 domes, which should have remained open.

NOTE:  The reason for the error is that the openings were filled with a single "wall" open surface.

Right click on the part and Select Repair. Using the Surface Selector click the top and you'll see that the entire top is a single surface, including the areas that should be open.  You can also see the yellow outlines of the openings.  Those are actually the edges of the openings which the triangulation process didn't properly handle.

So we'll use the Triangle selector and manually select the triangles inside the center square opening (holding the SHIFT key), then hit the Delete key.  We need to do the same thing for the other openings.  Because of the circular nature of the opening, the surface is made up of dozens of small triangles. In fact the higher the resolution of your model, the more triangles there will be.

Holding the SHIFT key, start on one of the edges and simply drag the cursor slowly across the surface.  The faster you go, the more likely it is it will skip a triangle or two, so slow down.  Zoom in as close as you can get to see the edges and click the missed triangles until the whole area is selected.  When you have it all, delete them.

The only thing left to do is stitch the open edges, apply the repair and save the repaired part.

Next up, an alternate way to do this same repair.

Here is a simple way to repair a model that is supposed to be solid, but it turns out it's not.

Here we have a model with 2 domes.  One dome is properly merged int the main body, but the other is not.  It actually intersects the body on the main surface.  In this tutorial we'll reattach the dome to the main body.

Right click the model and click Extras/Repair Part.  Using the Shell Select tool, click the body of the model.  You'll see the main body turn green (selected), but not one of the domes.  That's because it's not contiguous with the rest of the model.  You'll also notice there are 4 open/border edges.

Rotate and zoom the model to get a better view of the side as shown.  Using the Surface Select tool, click the side of the body, and click the Remove Selected Triangle button, or simply hit the Delete key to open the side.

Looking inside the main body, you can now see the bottom surface of the dome intersecting the top surface of the main body (highlighted in red).  Using the Surface Select tool once again, select each one of these and delete them (you can also  multiple select by holding don the CTRL key).  Once deleted you can see the edges of the now open bottom of the dome (yellow dashed lines).

Next select the top surface of the body.  Rotate the model around to verify that only the surface you wanted was selected.  if not, simply click outside the model to deselect , and do it again.  Once you have the right surface selected, hit the Delete key.

You're now left with a bunch of open edges.  If you were to simply tell NetFabb to close all the holes, it would literally revert back to the original.  The fill holes algorithm is not very intelligent.  So we going to close the holes manually.

Select the Add Triangle tool from the toolbar. then move your cursor to one of the yellow edges.  It will turn blue.  Click it.  Then move the cursor to the opposite edge (follow the arrows).  The open area between the two edges will be highlighted.  Click the opposite edge and the area will be filled in.  Do the same for the surface under the dome, and then for the side which we opened in the beginning.

Last thing we need to do is clean up the few opened edges by stitching them together.  Click the Actions tab, then click Stitch Triangles, then OK on the pop-up.  That's it.  Click Apply Repair, Remove Old Part and then export the repaired model.

The Add Triangle tool is very powerful because it allows you to not only repair a model, but change the entire surface geometry.

Let me know if you have any questions.