Help with feed rate and pass depth

[Mimbler]

I would really appreciate some advice from some of you experienced folks. I am new to CNC having purchased a piranha. I have successfully modeled a classical guitar bridge I want to cut out of rosewood. It needs to be cut with .125 endmills (flat and ballnose). I have tentatively set the pass depth as .0625 and speeds at 30 inches/minute which I "think" is conservative as I need a good surface finish. The calculated times are pretty high for such a small piece, but I can live with that if these seem appropriate to those with experience. The wood is pretty expensive so I would like to be in the ball park on the first try. Does this seem reasonable? I just don't have a feel for these tiny bits!
Mike

[drueth]

Mimbler

Rosewood is expensive and very hard. So I would do some practice cut's on a much cheaper wood, I do not have a piranha so I do not have a feel for what will work best on you machine, but I think 30 inches/minute should work fine to start I would think it should be able to do 50 with no problem. What I have found to work is to set it at a faster rate in Vcarve and than move the fro down before I start the cut. That way if it is running OK you can move the speed up. If you can find some hard maple or cherry I would work with them until you are happy with how the cuts turn out.

Good luck.

[Mimbler]

Thanks for the help Drueth. I think the fro suggestion is a great idea to allow for easy speed optimizing. Now I just need to poke around and find how to set it on Piranha! The machine uses a pendant and cannot yet be controlled by a computer so there is no control panel software, and directions for the pendant are less than complete. OTOH, the machine appears to be pretty simple to operate, and I can fumble around without complete directions,
Mike

Mimbler

Rosewood is expensive and very hard. So I would do some practice cut's on a much cheaper wood, I do not have a piranha so I do not have a feel for what will work best on you machine, but I think 30 inches/minute should work fine to start I would think it should be able to do 50 with no problem. What I have found to work is to set it at a faster rate in Vcarve and than move the fro down before I start the cut. That way if it is running OK you can move the speed up. If you can find some hard maple or cherry I would work with them until you are happy with how the cuts turn out.

Good luck.

[Rando]

The other way to get a good starting pair of "Feeds and Speeds" is to use a purpose-built calculator. I'm using the one the G-Wizard from CNC Cookbook. It has far more capabilities than I would ever hope to use, but it's completely stopped bits breaking in the Aluminum I'm doing, clogging with melted metal, dulling the bits too fast, and so on. It can be spendy, but I've found it well worth it.

[HappyWoodworker]

The other way to get a good starting pair of "Feeds and Speeds" is to use a purpose-built calculator. I'm using the one the G-Wizard from CNC Cookbook. It has far more capabilities than I would ever hope to use, but it's completely stopped bits breaking in the Aluminum I'm doing, clogging with melted metal, dulling the bits too fast, and so on. It can be spendy, but I've found it well worth it.

Hey Rando, I am just now trying to get G-Wizard set up but running into problems with the Machine Profiles. Where did you get the info to correctly fill in on it?
I have a Shark HD3 Anniversary Edition.
Are you still using the G-Wizard?

Thanks for your help.

Ron

[Rando]

Hey, Ron .

I'll first reference their tutorial on doing the setup, just in case it somehow was missed.

http://www.cnccookbook.com/Slides/GWFir ... pHome.html

The following two screen shots show how I have my machine configuration done. Note that in the spindle-power derating page, I'm showing the **recommended** setting for a machine like our's. I, on the other hand, find that annoying, since I already check that the HP, torque and other numbers make sense, so why limit the calculations. With the small-machine limits turned off, at least then I can see just how far off the values are. Again, more sanity-checking of the numbers. (Oh...and yeah, I changed the VFD programming to let my spindle go to 36K RPM, for using 1/16" endmills in aluminum)

CNC Shark HD2+ config settings in GWizard

CNC Shark HD2+ power derating in GWizard

To lend a hand, I quickly drew up a cheat-sheet that shows the general flow of steps I use to get a set of numbers. It's here:

GWizard Brief Cheat-Sheet

One of the techniques I use to make the project more efficient of my time is to find multiple feature-level toolpaths and run those together. I don't have automatic spindle-RPM control ( ), so what I do is define two, maybe three related cuts. They all use the same bit, and I'll calculate one for the main material-removal (aka "roughing") pass, one for a more-careful pass with good surface quality, and then a third fine-finish cut that I use to clean up the surfaces. I'll calculate them with the bit choice and RPM locked, but adjust the DOC, step-over, the slider-thingie, and so on to get the cut parameters. That way, as long as they can be physically run one-after-another, I join them and run them as a single operation. But hey, that's me .

Hope that helps,

Thom

[Rando]

(Limit of three attachments, so adding them here....)

Nearly all my work on the Shark is in Aluminum these days. And breaking bits in aluminum is EASY. Painful and expensive, but EASY. So, I spend a lot of time verifying feedrates and cut parameters, ramping rates, and on and on. For your feedrates, I highly recommend you create a "cut catalog" where you record the calculations you come up with, arranged in families of cuts or by bits, whatever makes sense for you. You'll eventually get into the habit, once you have a well-established cut catalog, of only using known-good cuts that come from the catalog in for-reals projects. Unproven cuts must be marked experimental, and should not be included in "production" toolpaths. You know, the ones where you can confidently walk out of earshot and know it will finish properly . That discipline, maybe is just OCD talking, but at $50 a pop for a 2" extended-length 3/16" endmill, thorough verification is well worth the time.

The first attachment is just an image of the table header, showing how I do mine.

Cut Catalog table heading screen shot

The second one is a PDF file of my ALUMINUM 6061 cut catalog. Don't try using these on wood....

DrillAndCutCatalogMarch2017Aluminum6061.pdf

Drill and Cut PDF for ALUMINUM 6061

(658.35 KiB) Downloaded 256 times

Again, hope that helps get you started!

Regards,

Thom