Question about precision


(Paul) #1

I’m currently weighing up purchasing a nomad and had a quick question about milling very small holes. One of my colleagues would like to drill an array of holes into some ABS or similar. The holes are 0.15 mm diameter in a honeycomb pattern with a pitch of 0.4 mm. Given a suitably small end mill, would such precision be possible with the nomad? If not, how small can holes be and how close can they be to each other?

Thanks in advance


(mikep) #2

You want an array of holes, .006" in diameter, every .016"…

I guess the good news is that such an end mill exists (this one is ball):
https://bitsbits.com/index.php?main_page=product_info&products_id=490

Note that the cutting length is pretty short, only .018".

Maybe @ApolloCrowe can chip something in here.


(William Adams) #3

This would definitely be pushing the envelope. Since it’s plastic it’ll be a bit easier — I believe it ought to be possible — I suspect you would have to carefully plan out the toolpaths, always starting from one edge, to do one row or column then returning to that same edge so as to minimize any issues with backlash, repeating this for each row, and over-travelling to the original edge for the staggered ones, and maybe even continuing past the cutting point for the others.


(Paul) #4

Thanks, now I’m excited to give this a try once I have the basics mastered


(William Adams) #5

One other thought — there ought to be an underlying grid of full-step, half-step, quarter-step, and eighth-step positions — if you could set this up so that the hexagons align against that grid, it would at least theoretically help.


#6

To really answer this requires more information, such as: How deep? Through holes or to a bottom? What tolerance for the hole size and position? To a first approximation, I’d say it is worth a shot.

This is, in my opinion, getting to the limit of the positioning capability of the Nomad, but, it the tolerance isn’t too tight on hole location, very achievable. The spec for positioning is 0.0005" (0.013mm), with repeatability 0.0015" (0.038mm). As WillAdams suggested, toolpaths running from the same direction each time will probably give the best result. The machine is pretty good with regard to backlash, but no machine is perfectly rigid, so positioning all features from the same direction minimizes the effect.

As to the diameter, you are into specialty tool territory. The smallest DRILL I have used with this machine is a #80 carbide drill (0.0135", 0.34mm) for FR4 (fibreglas circuit board). Anything smaller doesn’t show off the shelf from any of the larger suppliers I use (McMaster, etc), but they are available. Even at this size, runout is showing it’s head. There is no spec for the spindle (that I can find), and typical for the ER collets is 0.0002" or so (0.005mm). The runout can be significantly more if there is any dirt or grime on the collet, in the spindle, or on the tool, or the torque isn’t right on installation, or for a number of other reasons. At 0.15mm, this could be significant, both for hole size and for tool life. In mounting a tool, I would VERY carefully check for runout. Any imbalance in the tool (uneven stop collar, crud on the tool or spindle nut, etc) will add to this as vibration.


(William Adams) #7

We do have a statement on the spindle runout:


#8

Ah. Thanks. I was looking at the spec sheet. Didn’t think to look in the forum.