3 months by cncdivi

In a previous piece, I discussed the comparative advantages of twist drills and hole interpolation using an endmill. The article was primarily concentrated on the necessity of the initial hole that’s required for starting pocketing. Traditionally, this involves spiraling the endmill to the requisite depth, then moving forward. What I sought to investigate was:

Although, profiles and pockets often begin with the need to get the endmill down to proper cutting depth. Given that you know how long a tool change takes on your CNC mill, how many such plunges do you think are required before you’d be better off to use a twist drill to do the initial plunge and then let the endmill interpolate off of that?

I include an Excel spreadsheet to help calculate the two scenarios in the article, but the bottom line was an example that showed that as soon as you had more than one such pocket-starter-hole to make, the twist drill came out ahead.  I also discussed how a number of machinists keep a 1″ or larger insertable drill in their toolchanger for just such occasions.  In fact, here’s a neat little video showing exactly that approach in the making of a timing pulley:


Let’s turn to the flipside.  When is it a better idea to interpolate?

I had a recent conversation with a moldmaker who was eager for G-Wizard to help figure speeds and feeds for helical interpolations.   He was doing some relatively small holes (I had always thought of interpolation as being more for bigger holes) of under a half an inch, and they had to be precise and of good finish.  As an aside, his issue was that if he used standard G-Wizard feeds and speeds (which are not designed for helical interpolation, but rather for normal milling), the holes were not accurate enough.  This is an issue of the acceleration capabilities of your machine and its servos.  If it can’t change directions fast enough, the servos will lag and accuracy suffers.  This is particularly acute for small holes.  At some point I will add a function to G-Wizard that deals with this.

Getting back to the moldmaker’s thoughts, he made a good point for why he was doing interpolations instead of twist drills.  It was really a function of toolchanger slots coupled with his setup time to get the machine rolling (never overlook the latter!).

The molds he was making all had quite a few different hole sizes, and the holes frequently required a counterbore.  Accuracy had to be high as some of the holes were used with precision pins to locate the mold halves precisely.  When you look at the sequence to drill a hole such as this correctly and accurately, you’re looking at 3 tool changes:

1.  Spot drill

2.  Twist drill

3.  Reamer

While we can probably make do with one size spot drill for many jobs, we’d need a twist drill and a reamer in the appropriate size.  For a mold with 5 different hole sizes, that’s 10 tools that have to be slotted into the changer and dealt with.  That equates to:

–  More time at the CAM program managing all that.  Some CAM programs can recognize the holes automatically and will do the work of cycling through all 3 tools automatically as well.

–  More time setting up the machine.  The right 10 tools have to wind up in the right slots according to what the CAM program expects.  If you need to do a different part, you may wind up with a whole different set of tools.

–  You’re adding tool changes, so it takes more holes of a given size to beat the interpolated endmill.

For your own jobs, try thinking out of the box.  What are the different ways to machine the feature and is one more favorable to your current job?  I like to use the G-Wizard G-Code Editor to compare some of these different possibilities very quickly, but for some parts, you have to resort to CAM as they’re too complex to mess with at the g-code level.

It was a great discussion with my friend the moldmaker and a reminder that machinists have to consider all the angles.  There are very few hard and fast rules to “do it this way always”.

That’s why it takes a lot of skill and experience to be a great machinist!


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