Cut Outs for Electrical Boxes in Plywood

I haveHello group,
I am a builder in Albuquerque NM. I just built my Avid CNC machine (4 x 8 ). We use 4 x 8 sheet goods in a variety of way. At the moment I am trying to figure out the best way to cut out electrical outlets, recessed lighting cutouts, etc.
I've made some test cuts but the width of the bit makes my hole bigger than needed. And I'm having difficulty getting to the right spot on the plywood to make my cut. I am new to this whole process.
Here are my questions:
1. What is the best way to get to specific measurement on my plywood to make my cutout for an electrical outlet?
2. What is the best way to set up the cut to get the exact size of an electrical outlet?
3. What size and type of bit should I use to get the best accuracy and cleanest cut? Currently I am using a 1/4 up cut bit.

I am currently using VCarve Pro

Any other suggestions would be appreciated. Thanks!

Well, I could try to answer your questions but it wouldn't help you much. You know the old saying, "teach a man to fish..."

So here is a good pond to learn in; https://www.cnccookbook.com/

I would buy their speeds and feeds calculator. You will need it.

I would also invest in a good set of digital calipers and a micrometer. I would measure your tools and use the number you see with your own eyes. Even a cheap set will have better resolution than your AvidCNC machine but get a nice set as they will be your best buddies in this brave new world of yours.

I would also start watching the how-to videos on YouTube. Especially the touch off plate how-to video here; https://youtu.be/CnAlZiYjeiQ

Then you will know where you are on your stock.

Good luck!

Quote:

---

Originally Posted by Cncbuilder310

.......
Here are my questions:
1. What is the best way to get to specific measurement on my plywood to make my cutout for an electrical outlet?
2. What is the best way to set up the cut to get the exact size of an electrical outlet?
3. What size and type of bit should I use to get the best accuracy and cleanest cut? Currently I am using a 1/4 up cut bit.

I am currently using VCarve Pro

Any other suggestions would be appreciated. Thanks!

---

1) You would measure the location just like you would measure it if you were going to use a saw for the cutout. You would measure from 2 edges of the plywood to locate the cutout, which means that the X0,Y0 would be at one corner of the plywood.

2) Normally when you are setting up the job in CAM, you tell the program the diameter of the tool bit and the program subtracts (or adds) the radius of the bit to cut the correct tool path. So for a cutout, you would subtract the radius of the tool because you want the tool to cut inside the outline. So using a 1/4'' bit, the tool path is 1/8'' smaller, per side, than the desired cut.

3) You might try a down cut or compression bit, but you can not plunge in with these bits, you have to ramp in. Alternatively, you could just cut from the back side where a little fuzz/splintering on the cut edge would not make any difference. Should be cleaner than a saw cut.

Quote:

---

Originally Posted by Jim Dawson

2) Normally when you are setting up the job in CAM, you tell the program the diameter of the tool bit and the program subtracts (or adds) the radius of the bit to cut the correct tool path. So for a cutout, you would subtract the radius of the tool because you want the tool to cut inside the outline. So using a 1/4'' bit, the tool path is 1/8'' smaller, per side, than the desired cut.

---

So in vcarve you don't specify the tool diameter and then it figures all that out for you? Crazy stuff. Glad I uninstalled that mess!

Quote:

---

Originally Posted by subnoize

So in vcarve you don't specify the tool diameter and then it figures all that out for you? Crazy stuff. Glad I uninstalled that mess!

---

My statement might be a bit confusing. I assume vCarve will allow a tool diameter input, but I have no experience with it.

Quote:

---

Originally Posted by Jim Dawson

My statement might be a bit confusing. I assume vCarve will allow a tool diameter input, but I have no experience with it.

---

Understood, but you might be onto something. In a more advanced CAD/CAM package it is readily apparent on which side of a line the tool will be cutting. Could be that he didn't realize that the CAM part reversed the selection, thinking that the cut out was the final part and put the tool on the wrong side of that line.

1. The zero setting tool shown above is a wonderful addition. So, what I do first is measure my workpiece and enter that into Vcarve. I typically use the lower left corner as 0,0. Using the tool, that point is easily identified as work 0,0. I use the top of the workpieces as Z0 and get all three at once (using Avid's Mach4). In Vcarve, you can use guidelines for locating your holes, or locate the rectangle when creating it. There are many good tutorials. You can also use dimensioning or measurement in Vcarve to double check your location. You should expect very tight tolerances with this machine and Vcarve will tell it to go exactly where you tell it! Are you homing your machine?

2. For an outlet hole, I would use a profile toolpath. Make sure you have the correct tool profile chosen (1/4" diam) and also make sure you chose the setting to cut INSIDE the hole. Vcarve will offset the tool path so that the 1/4 tool will cut the inside out. Now, when cutting a profile, there will be a leftover waste piece. You can use tabs so it doesn't fly, then just punch it out after you remove the workpiece from the machine. Also, some folks cut to a depth that leaves an "onion skin" layer at the bottom of the cut. That is usually enough to hold the waste and it prevents your spoil board from getting abused. With this machine and Vcarve, you should be able to get a tolerance so good that the box will hold itself in via friction! Pocket cuts can be used too, but that takes more time and is a lot more wear on the bits.

3. The upcut will leave splinters on the upper surface for sure. A downcut will help. I use those when it matters. They tend to cram sawdust into the cut though. I would suggest that any of the name brand bits will work well. I use a lot of Amana. Toolstoday is a good source and resource. Some of the surface treated bits might last longer in plywood. Also, check your feed rate and depth of cut, etc.

Hope this makes sense. Hope it helps a bit. Lots to learn at first, but it will soon become second nature. Remember, the machine needs to know where to start and it does exactly what you tell it! Use the preview in Vcarve every time. Once you get the hang of it, you might benefit from templates for similar setups, fixtures, etc.
Oh, one last thought. Plywood is not always flat. Make sure it is held down well and flat to the table. I often screw into areas that are safe from tool bits. Vac tables are nice for this.

Tony