In this final article in the series I’m going to look at what happens when you put an expensive lump of carbon fiber on your cutting table and hit the big green button…

Disclaimer

Okay, so you must be getting the idea by now. I’m not an expert at this stuff. Most of what I know I learned the expensive way (as we’ll see shortly).

If you follow any of my ideas you do so at your own risk. That said, give it a go and if you need to go ahead and ask questions at the bottom…

Dogbone Dilema

ToolpathIn the last article I mentioned the use of “dogbone” holes to overcome the inner radius on the corner of close fitting parts.

It turns out that using a 1mm end mill is not a great idea. I cut the frame to the right 4 times before I worked this out – a pretty expensive lesson to learn.

The 1mm tool simply can’t cope with cutting a component. As it followed the outline of the camera plate I was getting vibration in the drill and this was knocking the stepper-motors out of sync by as much as 3mm.

As a result I have moved to 1.5mm dogbone holes and try to cut everything using a 1.5mm tool as far as possible.

Let’s get on with it then…

So, we’ve got our design and we’ve got our toolpaths, now we need to get the thing cut. Here is pretty much how I go about every project…

Preparing the blanks

I think I already mentioned that I use one of two blank sizes. The smaller size is 250mm x 200mm and this fits most miniquad size frames. The larger plate is 250mm x 400mm for the larger frames.

The sheets come from the factory in 500mm x 400mm sheets and I cut them down with a hand-held circular saw with a thin 40 tooth blade. I cut the plates down in this way for a couple of reasons:

  1. My CNC machine isn’t that big so I couldn’t fit a full size sheet on the cutting bed if I wanted to.
  2. It is easier to work with smaller plates and creates less waste if I cut one frame per blank.
  3. Using the thinner materials, larger plates can bow and vibrate causing problems with cut depth and deflection.

The next thing I do is stick the blanks to the sacrifice boards. As most jobs use both 1.5mm and 3mm plates I stick one of each either side of the sacrifice board.

Hang on. You “stick” the blanks to the sacrifice board? (I hear you ask)

10731871_307782739405995_1401339119_nI spent a long time trying to work out how I could cut components without having to have nasty tabs all the way around the edge. In the end I settled on actually fixing the blank plates to the sacrifice board using spray adhesive. The image shows early tests using foam board blanks but I have since moved to MDF blanks as they are both cheaper and stiffer. Here’s what I have learned:

  • Cut a piece of 3mm MDF to the same size as your material blanks
  • Leave the protective coating on your material
  • Spray one side of both the MDF and the material with spray contact adhesive and press together
  • Put cutting material on BOTH sides of the MDF. This reduces waste and also adds a lot of stiffness to the surface to be cut. If you are using two thicknesses of material (e.g. 1.5mm and 3mm) put one of each on your sacrifice board and cut the thinner material first to maximise the stiffness.

The last thing to do once your blanks are stuck is mark the centers by drawing a line from corner to corner over the protective coating.

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Clamping & Setting Up

My machine was supplied with four screw type clamps that can be moved anywhere around the cutting table by way of square nuts that lock into the channels which run full length.

I marked a straight line (with a set square) across the bottom of my table so I had a datum from which to align my boards and then use the screws clamps on each corner. Alignment is important if you are trying to maximise cutting areas and minimise waste as you don’t want your tool to slip off the edge of your material during a cut. I find that by having the second thicker sheet on the back of the sacrifice board I can clamp right on the corners and still not see any bowing in the center.

TIP: Use small plywood tabs under the clamp tightening screws (if you have them) to prevent damage to the cutting table and also prevent slipping on the metal surface.

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With the correct end mill securely clamped in the chuck I now manually wind the point into place so that it is on my center mark, just touching the surface of the material. I do have an electronic z-axis setting tool that I could use but I find it just as easy to find the material surface by hand.

With that done we are now ready to start cutting!

A Short Word About Safety

2014-12-21 18.11.57-1920CNC Machines are designed to cut hard materials at speed using very sharp tools. Once the tool bit is in motion it will keep trying to reach is pre-programmed coordinates regardless of whether it is fiberglass, the metal clamps or your child’s fingers that are in the way.

ALWAYS make sure cutting is done in a safe environment, secure from prying eyes and fingers. If you are going to be near the machine during cutting then make sure at a minimum you are wearing eye protection. You need your eyes to fly so don’t risk them!

The jury is still out on just how dangerous carbon fiber dust is but I know for sure that I don’t want to be breathing a lot of it. If you are running a small bench-top machine like me then make sure you keep a filtered vacuum handy and regularly suck up the dust. If you are going to be doing a lot of cutting then maybe even a cabinet with dust extraction would be  good idea.

Let’s get on with it then!

Turn on the power to your machine and then move to the computer. The first thing I do is hit the software “Reset” button to make the connection between the computer and CNC machine. Then I zero the three cutting axis – this is REALLY important as not doing this means the software could think your center point and material surface is anywhere! Now I load my G-Code file and I’m ready to cut.

I’m kind of assuming you have got your machine all setup and calibrated by this point. If not you should refer to the manual and get it all done.

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If I am cutting a very full sheet I will sometimes load the final cut (the component outlines) and “jog” the tool bit around the table at a safe height to check the extremities and make sure everything fits and the clamps won’t get hit.

Only when I am 100% sure I am ready do I hit the green “Cycle Start” button and cross my fingers.

Note: Most cutting software can control the spindle speed through the interface. If yours can’t then you will need to manually start the drill before starting the cutting cycle.

Here’s a couple of short videos of my machine in action…

 

 

After each cut I check everything is okay by vacuuming off the dust and checking the tool has returned to my center mark correctly. If I need to change tools then I find it is safe to switch the machine off and manually raise the drill to change the bit so long as you remember two things:

  1. Only touch the Z-axis. Don’t touch the X or Y otherwise you might knock it out of alignment.
  2. Reset the Z-axis zero in the software so the machine “learns” the surface point for the new bit.

And that’s it really! Just repeat the process until all your cuts are complete and you have a finished set of components.

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Finishing Off

2014-12-21 18.46.30-1920Once your cuts are complete you should be able to tell (after vacuuming) if the cuts have gone all the way through. Now is the time to find out as it is easy to make some changes to the toolpath and repeat a portion of the job whilst the material is still in place and the zeros are set. Finding out once you have taken the plate off the machine is disastrous!

To make sure I actually remove the components from the blank whilst it is still clamped. To do this I get a small flat headed screwdriver and pry the parts loose. With a bit of pressure the glue or protective sheet will give way making it easy for you to lift the component out.

I can then peel off the protective sheets, check the finish and move on to the next job.

At the end of the day I always take some time to vacuum around the work area to get up any stray dust. I also check the set screws on the CNC machine, give it a wipe over and lubricate the moving parts.

I only have one machine and it is busy making me some money. Look after your investment and it will look after you!

THE END!

I hope you’ve enjoyed reading this series of articles as much as I’ve enjoyed writing them. I’m not sure the title of the series was correct. Maybe it should have been “CNC Cutting BY a Dummy” as I could probably have saved myself some frustration by reading a bit more and clicking a bit less at the start.

CNC machining is a bit of an art and once you get the hang of it it is a total addiction. You’ll have some failures and frustrations along the way but they’ll all be forgotten the first time you put a frame you’ve designed and cut together and feel the satisfaction of the components slotting perfectly into place.

If you have any questions, please add them below. I’ll do my best to help if I can! If you like the idea of an own designed frame but don’t own a CNC Machine, don’t forget the I am also happy to take on custom jobs for pilots all over the world (bit of a plug there)!

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