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magickpurple
07-31-2014, 04:21 PM
I recently took delivery of a Desktop with the spindle option and upgraded to Aspire. I noticed that the parameters in the tool database in Aspire are way outside the limits in the Chip Load Calculator. Here is an example:

The 1/8" Solid Carbide Two Flute Upcut Ballnose (Onsrud 52-240B) in the tool database has a Feed Rate listed as 0.83 inches/sec with a Spindle Speed of 12000 r.p.m. Plugging this into the Chip Load Calculator (Spindle rpm=12000,2 flutes, Cutting Speed=0.83) it calculates a Chip Load of .00208 which is way out of the range of .005-.020 listed on the Chip Load Chart for the calculator.

If I take the average chip load of .0125 with and rpm of 12000 and 2 flutes, the calculator gives me a cutting speed of 5 inches per second.

This is a big difference. Was the data that came with the software correct? If so, why was it so far outside the range. Can I trust the chip load calculator and change my speed to 5 inches per sec?

adrianm
08-01-2014, 05:39 AM
Don't forget that Aspire isn't a shopbot only program so the parameters in the tool database are probably set to be ultra safe so if someone doesn't change the settings they are unlikely to do any damage to their machine. Some people have small machines made of wood with fixed speed routers and steppers that can only do 20ipm.

Chip load calculations are a reasonable starting point but will almost always need adjusting for the particular machine/material/bit combination.

Ballnoses are a special case as well as they are generally used for 3D finishing cuts which take a very small amount of material (if the toolpath is set right) and, due to the number of Z movements in those toolpaths, rarely get anywhere near the programmed speed.

It's really a matter of experimenting and recording what works for you.

magickpurple
08-01-2014, 09:12 AM
Yes Adrian, I agree. I was just afraid of breaking a bit by moving too fast but I don't want the job to take hours to complete if I don't need to. Have you ever operated at over 3 inches per sec with a ball nose? Or any bit for that matter?

scottp55
08-01-2014, 09:14 AM
David, Like Adrian said. Take it safe to start with wood. I found all the defaults were too fast for my taste as I hate sanding and love quality of cut. For hardwoods a good rule of thumb is pass depth=1/2 Diameter of bit. Grab a scrap of wood you do most of your work in and make some sample lines. Start at say 1.1-1.2IPS for maple and check for good chips(NOT sawdust) and gradually increase speed using Shift-> until good chips/cool bit/quality of cut falls off and then back feed off a smidge. Then you can increase feed AND RPM until you are happy. Each wood/bit is a little different. Maple you will probably wind up around 1.5,.5,10-13K for that bit at 1/2diameter pass depth.
This is good article to read.
http://www.precisebits.com/tutorials/calibrating_feeds_n_speeds.htm

adrianm
08-01-2014, 10:46 AM
David, I have a PRS Alpha so I regularly cut at more than 5ips. Most of my cutting is done at 8ips and I could push faster than that if I wanted to.

I wouldn't recommend it on a desktop though!

hipstomp
01-01-2015, 08:55 PM
David, I have a PRS Alpha so I regularly cut at more than 5ips. Most of my cutting is done at 8ips and I could push faster than that if I wanted to.

I wouldn't recommend it on a desktop though!

Hi Adrian, just out of curiosity, why does the smaller overall size of the Desktop not lend itself to cutting at the speeds you'd use on your Alpha? Also, what would happen if one were trying to run the Desktop at those speeds? Any info appreciated!

thanks,

- Nick

Kyle Stapleton
01-01-2015, 09:41 PM
You would lose step, they are not made to run the fast plus you would never make it up to that speed on a desktop it is just too small.

scottp55
01-02-2015, 05:03 AM
Nick, Our Move Speed is supposedly maxed at 4IPS. JS is 5. Have seen TJ do a 3D finish pass at 5,5 , but because of the 3 axis moves it never gets anywhere near that speed.
Have done a lot of surfacing/dimensioning With Desktop and run at 4IPS taking light cuts (.125" passes against grain) with a 1.25"mortising bit in Hard Maple, etc., But last pass is usually less than .05" with the grain at 3-3.5IPS.
Most hardwoods don't like those speeds if you want a good finish on the cuts though- Rarely get above 1.6IPS unless I don't care about the finish and suddenly develop a strange urge to sand a lot.:)
Kyle, Have you ever lost step on Desktop doing a reasonable cut? Only time I have, is doing my "Stupid Moves" tricks(3 times this year---Usually when I forget I referenced to spoilboard and have the dust foot on and can't/don't see the cutter height):(.

Kyle Stapleton
01-02-2015, 02:39 PM
Not yet, unless you count being dumb.

scottp55
01-02-2015, 03:53 PM
:) Yup! Pretty amazing when a 1.25" bit ramps into an 1/8" cut and it just Sloooooows doown because you forgot to turn the spindle on :)
Of course I'm not POSITIVE I lost position, because I slapped the spacebar so many time SB3 faulted out:( Wheelchair wheels were locked and I was trying to "Hop" the whole chair so I could slap the E-stop:)
Good thing I had a 5" ramp stuck in......BAD thing it was Teak!
LOUSY finish on the cut too!