Speeds & Feeds - slot vs. edge cutting
I'm trying to pick the best speeds & feeds for cutting 1/2" acrylic (0.472 to be exact) on a ShopBot Alpha. I'm mostly concerned with the finish on the final edge cut. I'm actually rough-cutting these on a laser but the laser I'm using is too low power and is giving a terrible finish on the 1/2". But its convenient to get the rough cut, then moving the piece to the shop bot to profile the finished piece.
This is probably machining 101, so pardon my ignorance on the subject!
Based on what I read elsewhere I went out and got a few Onsrud cutters made for hard plastics.
Now to actually set up my cuts: I have 3 sources of information.
1. The Onsrud web site lists chip loads for the cutters, and have formulas for deriving cutting speed at a selected RPM.
2. Onsrud has a web site where they list surface finish results at different speeds for different types/manufacturers/thickness of various plastics. They quantify the surface finish so this info is really useful. Unfortunately they don't list my acrylic manufacturer but I selected a few bits that seem to have the best results on a variety of acrylics they tested. I got a 3/8" single flute (that seems to have the best results of all), a 1/2" two-flute and a 1/2" 3-flute which has consistent good results but not the absolute best. (I also got a 1/8" for some tighter cuts but I'm not so concerned of the finish on that one)
3. I have a speed/feed calculator (GWizard)
The first area of confusion is the depth of cut. These bits are spec'd for a 1xD depth of cut at the given chip loads. But I'm just not sure how that relates to edge cutting. For sure, my cutting depth is the full thickness of my acrylic (.472) but I assume the cutting width is relevant here too. So it seems to me the Onsrud site is assuming a slot cut? It seems pretty common for end mills to just specify a depth of cut- so how does that normally translate in edge cutting? (Or even surface removal after you've made the first slot, you'll mostly be edge cutting after that right?) So I think I'm just not understanding the basics here.
Now on the plasticrouting.com web site (where Onsrud lists all the results for various plastics/cutters)-- you can see that all these cuts are also edge cuts just like what I'm doing. They have a general recommendation of doing a rough cut then a finishing cut of 0.015" on the full thickness of the plastic. They also use 18,000 RPM on all of their cuts. They vary the feed rates on these plastics anywhere from 100ipm up to 300ipm for some. What isn't clear to me is: do they just use the specified feed speed for the finishing cut? At 0.015" cut width won't that be well under the spec'd chip load? But does that matter for a finishing cut?
Using just the chip load specs and calculating feed rates at various RPMs the numbers come out much higher than what they used in the tests (and also like I said I'm not sure exactly what type of cut this is calc'd for). For example, the 1/2" 3 flute cutter with a chip load of 0.010 (it is spec'd .006 to .010) at 18,000 RPM the calculated speed comes out to 540ipm!
Using my GWizard calculator, it likes to specify a much lower RPM. This calculator also takes width of cut into account and of course when I specify my 0.015" cut it gives me huge cutting speed recommendations to get my target chip loads. But if I specify 0.472 as depth of cut and something reasonable like 0.1875 width of cut (1/2 the cutting diameter of the single flute cutter) I get reasonable feed rate recommendations- but low RPMs such as 5600. If I force it to use a higher RPM like 18000, the feed rates it gives me get more in line with the Onsrud calculated values - but this calculator also shows HP (horsepower) and these cuts tend to jump up to around 2hp. I'm not sure if the shopbot can deliver that kind of power or not.
I'm getting ready to run a bunch of 0.015" test cuts at speeds from 50 to 300ipm with all 3 of these cutters to see what results I get. But I'm a numbers guy and it would be good to have an actual understanding of what I'm doing & why. So basically I have a few things I just don't understand:
1- When cutter manufacturers specify chip load and depth of cut, how does this apply to edge vs. slot cutting?
2- Whats the cutting power capability of the shopbot alpha? If they specify a cut at 18,000 RPM and 300ipm through acrylic (0.015 edge cut in this case) will the shopbot sweat it?
3- What to make of the GWizard speed/feed calculator recommendations vs. Onsrud?
4- When making a finishing cut do you use the same feed/speed or do you adjust to keep the target chip load on the finishing cut?
Sorry I know this is very long. Any thoughts/advice/insights would definitely be appreciated. Oh and the Onsrud cutters I got are 60-249, 56-650, 61-121 and 650-013.
Cheers,
Eric
