CNC Milling

Further Tapping Nightmares Resolved

Sometimes I pose the following question: “Why couldn’t things be simpler?”

And here is the answer: Because if they were, what would there be to fill our life with grief, ragged tympanums, sandy throats and broken walls?

After gathering all of my data on how to tap, I decided to go into Sprutcam and enter the threading operation only to find out it was compiling without doing anything at all. That is, the process was being generated with an empty set of steps. Needless to say, this wasn’t going to work very well…

I do have to admit that the ability to see a simulation is crucial! To have had to go through this nightmare without actually seeing what the program was attempting to do would have taken way much more than the close to two hour that it shaved from my life.

But anyway, here is what happened:

After selecting my centers as you normally do (Double click on one of the half surfaces of the holes you want to apply the Hole Machining operation and press Center under the Job Assignment tab) I chose the tool as my #8-32 Tap. It looks something like this:



If you are a CAMing expert, you will be able to spot what the problem is by just looking at this page. If so, you will be able to correct the issue in seconds. Perhaps this is why it took me anywhere in between 3600 to 7200 seconds (across two days of pulling my hair) to finally figure this out. I will get back to this…

Next step is to select the Feeds and Speeds. This is where most of the documentation on the web is available and it is also one of the topics from my previous post. Pretty much everything out there will tell you to use 300 RPM, but after reading on some forums, this appears to be too slow a speed rate. Anyway, there are tables and there are pieces of software which will tell you what RPM to choose. Then you divide this number of RPM by the thread pitch, and that is your feed. In my case I chose 800 RPM, and since this is an 8-32 screw, 800RPM/32TPI = 25 IPM.


Finally, we need to select the actual strategy which is the tab where we specify the “tapping” operation. There really is not that much to it. By following Tormach’s recommendation, all we need to do is make every value equal to zero, except the dwell time which should be 0.3 seconds. Everything else (not shown on the picture for some weird capturing reason) is as default.


If everything is right, you should just be able to press the “RUN” button and get it over with, right? Err, wrong! Because as it turns out, everything is NOT right. As I detailed earlier, there is a bug in one of the previous screens. Were you able to spot it? Let me show you the result, which is so frustrating here is where the wall banging session begins…


Notice there is nothing below the Threading operation. You may be wondering what do I mean by nothing when clearly there is something there. Well, not really. The stuff you see here is basically a preamble. In this case, a preamble to NOTHING! What you want to see is this:


As you can see, Sprutcam has now applied the Threading operation to my 4 holes, whereas on the previous instances there were no holes to be found. What an A HOLE!!! But as it turns out, it was me the A Hole, or in this case the idiot not realizing there was a bug in my programming.

Let me give you a list of what it isn’t so you don’t waste your time trying all of these alternatives on your next tapping endeavor:

1. It is not a bug with Sprutcam.

2. It is not a matter on how you select the hole centers. You still do it like you have been doing it before and you do not need to select both half surfaces of the screw. One is all you need!

3. You do not need to modify the hole properties on the Hole Editing window.

4. You do not need to…

You know what? I bet what you want to know is what is the frigging bug and what are all of the things that I tried to no avail.

The bug is actually quite simple! If you go back to the tool selection picture, you will notice my tap has a diameter of 0.099″. This is a copy+paste error from when I copied the #6-32 tap into the new #8-32 tap. The problem is that to make an #8-32 threaded hole, you need a 0.140″ drill. If you tell Sprutcam to use a tap that is way smaller than this already existing hole (e.g. 0.099″), then Sprutcam is going to say:

“Well, I would run this operation but there is no material to be removed, so I am just going to skip it.”

WHAMMO!!!!!! I would be pissed at Sprutcam, but as it turns out this is the right answer. Why would you work arduously in doing something which is already done? The solution then is to specify the tab to have a diameter larger than the existing hole so that Sprutcam can be happy and feel like it is removing some material with the tap. Hey! SW suites also want to be happy!

What eventually solved the problem for me was to use a tap diameter of 0.14″. And with that I can now finally go to the machine and get the darned part manufactured. I was supposed to finish all of this on Friday, but Sunday will have to do. Long day ahead, so let me get busy!






Tapping The Heck Out Of The Heck!

AHhh, Tapping! That activity invented by the human race which is destined to provide countless hours of hair pulling, carpal tunnel, broken taps and all sorts of vocal maladies launched into mid air while fueled by disproportionate demons.

Unless you do it right…

Except it is not that easy to make it right!

One of the beauties about using CNC machines is that it removes the burden of tapping from your shoulders. Is the tap at 90 degrees with respect to every possible axis on all parallel universes? CHECK! Is the right force being applied? CHECK! Will I break a tap? Well, it won’t be you, but the machine. CHECK!

The only problem with using a CNC machine to do your taps is if you, like me, do tapping every three months. Although most of the time I do it I get it right, the truth is next time I need to do it I have COMPLETELY forgotten how on Earth I did it. When that happens, I find myself looking through all of my reference material and wasting about two hours of time before I can let Sprutcam compile the G Code.

And that is where I am today as I write this post. I have completely forgotten how to do this, so I have decided it won’t happen again. Next time I have forgotten (which will be in a couple of days…) I will simply tune to this blog and WHAMMO! Problem solved!

First place I go to check how to tap the crap out of my material is Tormach’s Blog. Although this posting is practically ancient (more than 4 years old), it has some of the nuggets which get my memory cells jump started:

Unfortunately, there are still a few questions I can’t answer by looking at the 4 pictures they have provided. For example:

1. How do I choose the RPM?

2. Are Aliens visiting us?

And so I then tune to John’s (from NYCCNC) Youtube channel and watch the next video where I still don’t get much of an answer on how to select the 300 RPM but I kind of answer question #2. Just kidding man! This video has a bunch of very useful information!

NOTE: I didn’t ask John for his permission to link to this video, so I truly hope he doesn’t send a horde of lawyers to pursue my sorry butt. John, if you want the link removed feel free to let me know!

I did a quick search and found this table which gives a bunch of RPM rates for different materials and depending on the screw. It gives us the same equation we have seen over and over (Feed = RPM / Thread Pitch). The RPM values, however, is what blew me away! If I have been using 300 RPM for aluminum, I have been doing this wrong!

Another option is to use GWizard. I thought of giving this a try and found that 300 RPM is also a big mistake! Intriguingly, the GWizard output is not identical to the table I linked above. Then again, this table may be meant only for those taps. Right? How would I know? What I found interesting is that according to GWizard, the same RPM applies for both aluminum and plastic. At first I thought the program was not updating the selection, but choosing Stainless steel quickly modified the values to smaller numbers, which makes sense. Why is GWizard different to the table?

And why on Earth is everybody assuming 300-500 RPM for aluminum!!!!

Well, no fret. It is time for me to give this a try and hopefully I don’t send my scarce number of taps into hades. At least now I know that a few weeks from now, when I need to tap again, a guide on how not to soil my underwear will be available!