Intro: Custom Made Knobs on Your CNC Mill
Here is an easy project for all those CNC metalworking beginners.
This project came about as one day I, somehow, lost the knob on my metal vibrator / tumbler. I remember said knob was so lousy, it used to get unscrewed and fly away while the unit was tumbling. What a "fantastic" design...
As you can imagine, this knob is of some importance. Unless you enjoy from the misery of spewed tumbling media and whatever can possibly follow... Sounds like a malady I would prefer to avoid. Luckily, one of the first things you learn when dealing with metalworking is how to make threads, so I decided to make my own knob and in turn, enjoy from some CNC milling.
This instructable will take you from designing to manufacturing your custom made knob. No need to try and get to Michelangelo proportions here, as after all, what we ultimately need is just a fastener. But if you want to be known as the ultimate 21st century Knoboneer, hey!, be my guest! I am just going to go with a simple 5 spur knob.
Let's get started!
Step 1: Design Your Knob
Here is where you get to be creative. There are so many SW suites you can use I am not even going to go there. I must point out I did the 3D model in Autodesk Inventor. For the step-by-step-profile illustration, I am using Corel Draw. You could design the profile in Corel Draw (or any other SW) and then import it to your favorite CAD drawing package. Or you can do it all within the CAD package. Either way works.
Note most knobs out there are 4 prongs. Maybe there is an usability reason for this, but I just felt like going with 5 prongs. Hence, instead of spacing everything 90 degrees apart, I used 72 degrees. This did pose a problem when centering the object as the center of a pentagon when you encase it in a rectangle, is not the same as the center of the same pentagon when you encase it in a circle. It is possible I was missing some setting on my CAM tool, but at the end I solved the problem by modeling the item within a larger circle (as can be seen in the last picture)
Step 2: Preparing Material
Cut a piece of your favorite material. For this part I cut some 6061 T6 aluminum rod with a 1 5/8" diameter. The actual length of the stock does not need to be overly precise. Don't cut it too short, or you will waste too much material. Don't cut it too long or it will cause unwanted vibrations or even worse, it may not fit on your machine Z axis envelope. I just eye balled about 8 inches or so.
I like facing material on the lathe in order to make it square. I recommend this step.
Place your stock on the mill vise and pinch it by using a V Block. Another alternative would be to use soft jaws, but this is an instructable in itself!
Here comes the tricky part. Centering it! There are two venues which I use:
1. Edge finder
2. Coaxial indicator
If you are going to use the edge finder, you will need four points. Get two points in X (without moving in Y). Zero your DRO after finding the first edge. Move to second edge and whenever you find it, divide by two on your DRO. You will now be at the offset from X = 0. If I type "G0 X0" on my CNC machine controls, the spindle will move to X = 0. On a manual mill, you can now move to X = 0 and you will be at the center.
MEGANOTE!!!!Make sure the edge finder has been raised in Z and can't collide with the part when you are moving to find the other edges. Sounds idiotic, but it feels even more idiotic when you forget this dinky little detail!
Repeat the exact same procedure for Y and then you will be at the round stock's center.
On my little diagram here, I show how ideally we would already be at the Y center when we take the two edge measurements in X (right diagram). Anybody might imagine this is the only way to find the true center of the part in the X axis. Assuming the part is perfectly round, however, we can actually make this measurement whenever we want (as shown in the center diagram) and you would still find the center in X.
What you cannot do is take two edge measurements in X while you have moved in Y. The reason is that then you will be at an angle and whatever you deem your center will most likely be have an offset.
If you use the coaxial indicator, then this is way simpler! When the needle doesn't move too much, you are there. How close do you want to be depends on how much room you have left. For this particular knob, my stock is 1.625". I made the knob outer diameter 1.55" which gives me +/- 0.0375" worth of offset. For any of these centering methods, such a head room is gargantuan! (I am usually off by 2 or 3 thousands of an inch.)
If I had made my parts diameter equal to 1.625", however, then I would have needed to ensure I was dead center, or any offset would have transferred into the stock's boundaries. Not the end of the world, but certainly not very elegant.
Step 3: Run Your CNC Job
Now we are ready for some knobbing experience. Run your job and you should see your knob being extracted from the aluminum stock clutches. As many machinists like to say: "there is a knob trapped inside this piece of aluminum!" Rejoice in the fact you are the movie hero and you have saved this damsel in distress. Don't get married with the knob, though, as this is usually frowned upon...
In order to be able to take these pictures, I divided the job in two pieces of G Code. One for the round and then another for the prongs. The truth is all of this part can be processed in a single step.
Nonetheless, I did this part in four Contour operations:
1. Contour 1 runs at 4000 RPM, 25 IPM. It executes from Z = 0 to Z = -0.375. It leaves a stock of 0.01" and uses helical machining with 4 passes. I used the round outline for this operation.
2. Contour 2 runs at 5000 RPM, 20 IPM. It executes from Z = 0 to Z = -0.375. It removes all stock with no helical machining and 2 passes. I used the round outline for this operation.
3. Contour 3 runs at 4000 RPM, 25 IPM. It executes from Z = -0.375 to Z = -0.75. It leaves a stock of 0.01" and uses helical machining with 4 passes. I used the 5 prong outline for this operation.
4. Contour 4 runs at 5000 RPM, 20 IPM. It executes from Z = -0.375 to Z = -0.75. It removes all stock with no helical machining and 2 passes. I used the 5 prong outline for this operation.
This part took less than 5 minutes of machining time to run.
Step 4: Back at the Lathe... Parting the Knob
Place your stock material in the lathe and select your parting tool. In the event you don't have a lathe and or a parting tool, you can alternatively use a band saw to separate the knob from the stock. Each method has its own advantages/disadvantages. For example, parting wastes more material than the band saw. However, parting is WAY quicker than separating on the band saw.
If you go with the parting route, be certain to use plenty of cutting fluid to cool down the part. There is a lot of friction going on!
One step I like to take towards the final steps on the parting operation, is to get a large cup with water close to the part. What this does is ensure the part lands in a soft spot, versus the lathe bed which may nick it. The water also helps because rest assured this part will be HOT! Chances are you will see steam raising from the cup, specially if you are quite aggressive with the parting as I usually am.
NOTE: I really advise against trying to grab the separated part with your bare hand! Although it will not pierce through your extremities like a lightsaber, there is no reason to tempt fate here. Also, this part may have burs. Remember, this thing was spinning at high speed shortly before it was parted. I see a lot of people catching parts with their bare hands and whereas in most cases nothing horrendously bad will happen, there really is no need to play brave when a free cup will get you there safely.
Step 5: Finishing the Knob
Expert machinists may be able to part the knob out of the stock and face it at the same time. There are ways to sharpen the parting tool blade so that it almost leaves no center nipple on the part. My hat off to you if you happen to be one of these machinists. The rest of us mere mortals, will need to flip the part and face it. It is really not a big deal!
Do note that this part poses a small complexity when facing. If you parted so that round stock was left, there is a point in which the facing goes from continuous cut to interrupted cut. In other words, continuous cut would be when you are facing the round, and interrupted cut would be when there is no more round and your cutter is now meeting the knob prongs.
What I have found is that when the round portion is thin enough, you will start to hear a mild "click-click" sound which tell us we are on that region. At that time, I turn the lathe off and simply grab the extra material with pliers. It will come right off!
Step 6: Threading the Knob
This knob is already pretty but it is in essence quite useless. What is truly useful about this knob is the internal threads and we can do that on the lathe as well. For this particular knob, I needed an M10x1.5 thread.
There is a trillion ways to do this. You can take the part on a drill press, drill it and then thread by hand, but since the part is already in the lathe chuck, why not take advantage of that? There are basically three simple steps:
1. Center drill
2. Drill through at final tap's recommended size (for this tap the tap/drill size table recommends an 11/32" drill bit)
When tapping, remember to use the respective cutting fluid. Tapping is one of those tasks where you can't imagine the system getting hot but rest assured it will! The amount of friction is ridiculous!
I like to put my tap holder's back resting on the lathe's tailstock with a live center. This allows me to ensure the tap is centered with regards to the part. You really want to maximize the tap being perpendicular with the part as any level of crookedness will haunt you later on.
And there you go! Now you have a lovely knob you can show others or simply use on your workshop!