Acrylic: Drilling Holes and Newbie Questions about Feed formulas

[roamingdrone]

So, I have a CNC router, and I'm somewhat constrained on the variables at my control presently. Namely, I am limited to a spindle RPM between 10k and 23k, and a maximum feed rate of 80IPM.

We'll get to that in a moment -- but, as a newbie, I have a (very important to me) equation to figure out before I really get started routing acrylic. I'd like to get feedback on it _before_ I start to avoid the same mistakes I made in wood ($40 in bits broken in 5 minutes == a stop to any work until the next payday).

So, my question is pretty straight-forward: I need to cut a number of small holes (think 0.196" and smaller in diameter) for screw pass-throughs, I have a 1/8th single-flute up-cut spiral bit, and a whole slew of drill-bits. For holes like the the .196", would it be wise to use just a standard drill bit (knowing I bottom out at 10k RPM), or to use the 1/8" carbide router bit?

So, in that light: should drilling in acrylic be done by drill-bits, or by router bits? In whichever situation, does anyone have any advice on plunge rates?

To the constraints I mentioned earlier, everyone out here seems to like very low spindle speeds and moderate feed rates, or very high spindle speeds and exceptionally (to my limited budget at least =) high feed rates. I'm a bit confused, as when I calculate out for the "optimum" rates, using the formula on plasticrouting.com, I always end up with something in the range of my machine... That is, given I'm forced to choose one of the factors, I choose RPM at 10k, and if Cl = Feed/(RPM * CuttingEdges), and I'm using a single-flute helix, and an optimal range (according to the above website) for Cl is 0.004 to 0.012, I get Feed == 0.008 * (10000) == 80IPM. Of course, as 80IPM is my "maximum" speed, I consider that I could tune for "worse" finish (theoretically) and choose 0.006 which gets me closer to 60IPM, and staying well away from my upper limit and relaxing the possibility of positioning errors.

But why do I get such drastically different ranges using that formula than what people talk about on here, and shouldn't bit size play a role in the formula? I.e., shouldn't my 1/4" bit be able to be pushed faster than the 1/8" bit?

Thanks for any advice!

!c

[SCRAPWOTSCRAP]

How thick is the acrylic you're putting holes in? Given your machine the two flute carbide 1/8 " bit should do nicely. Try the holes at 11k/50 IPM. If it's thick acrylic peck the holes and keep the bit clean, blow it off with compressed air if you have it. Routing edges might be a problem at 80 IPM, you may have to make a few passes depth wise, melting the acrylic might be an issue at that slow speed, maybe try 14k rpm.

[roamingdrone]

Thanks for the advice, I'm primarily dealing with .25" and .125" cast acrylic sheet, making small housing for electronics and mechanical components. Most parts will require all of: facing (part cut-out with tabs), pocketing (creating groves for panels to slide together, and counter-sunk holes), and straight drill operations.

For keeping the bit clean, at the moment I am stuck with hand-holding the ol' shop-vac, as until I complete the enclosure, I can't have acrylic dust all over my office, getting into computers and printers and such. (I realize the contradictions here, but I mean "I like to keep it reduced" vs. "blowing it all over the place" =)

Thanks for the info - note: I have only single-flute cutters, I presume those would work fine as well? (They are Onsrud 63-744 and 63-712)

!c

Edit: I think I figured out why I was confused by the formula, namely, that it was only part of the equation (i.e. it gave me IPM, but RPM had to be "guessed"), I found this link through another post on here, and it cleared everything up for me: http://www.cyro.com/methacrylates/MC...ef5Routing.pdf

In light of that, it looks like I can get pretty acceptable values at 15k RPM and 60IPM, yielding a SFM of around 340 or so, and an IPT of 0.004".