Cutting Acrylic Pen Blanks with a 1/32" Micro Bit

[coolhammerman]

My wife asked why I need a 4X8 machine to make something 1 inch wide and 5 inches long. I'm still working on that one.


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My question is the proper (1) feed speed (2) RPM and (3)Step Down/cutting depth for cutting inlay pockets into acrylic pen blanks. I do fine with 0.0492 cutters, approximately 1/20". Having trouble with the 1/32" though.

I know how to calculate chip load and total chip load, but I'm still getting lots of breakage. I'm also using a precision collet from Precice Bits. I use a hand held compressed air hose to clear the swarf. These don't take very long to cut.

The three pen blanks in the attached photos cost me 7 cutters.


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I'm really hoping to hear from those who have actually successfully used the micro stuff and how they managed it.

Thanks,
Coolhammerman
The Other Ron Brown

[magic]

Can you detect deflection?
Is the bit designed for wax?
Is the speed too high even though a chart tells you otherwise?
Is the bit as far into the collete as possible?

Try a 2 step pass, as a test.

[shoeshine]

you might want to try using the stub end cutters.
(like MN208-0313-005F)

They are designed for metal but the geometry isnt really that different and they are an order of magnitude stronger. Of course you sacrifice depth of cut these only cut .05" but they might work in your application. I use these a lot for fine detail. I use max pass of .01 takes longer but again in your application, not a real issue.

[gabepari]

Why not use a v-carve bit?

Gabe

[coolhammerman]

Magic:
No noticeable deflection. I'm using PCB router bits for now due to the affordable (up to now ) cost of less than $1 each. Works great down to .0492", but not so good at 1/32". Yes I'm the original cheap skate.
I've tried slow, medium, fast speeds. Haven't found the just right speed yet.
Bit has a stop collar.
Only cutting .03125" now.

Chris:
I'm cutting .15" deep. The bits are breaking when very shallow so It's not like they are breaking because I'm cutting too deep. I cut .240" with the .0492 with no problems.

Gabe:
The cuts must be perpendicular to the surface with realtivlely significant depth. These will be turned round and made into pens with typical diameters of .500" for slimline, .5625 (9/16") for european and .625 (5/8") for Cigar pens. The whole idea is to provide enough depth in the inlay so it will not disappear when turned to a very tight radius by the pen maker.
A V-carve bit would disappear about 1 second into the turning process. Laser engraving is done after the pen is finished. That is not what I'm doing, but thanks for the suggestions.

Here are some samples done with the .0492 cutters. Most of the finished diameters won't be much over 1/2".



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[navigator7]

Ronco wrote: My wife asked why I need a 4X8 machine to make something 1 inch wide and 5 inches long. I'm still working on that one.

When you secure an answer to that profound question would you repost?
;-)

Ben Hogan was complaining to his wife about his putting. His wife suggested he just hit it closer to the hole.

Are you doing these inlays with a ShopBot?

[coolhammerman]

Chuck:
So are you saying that if I pick the right speed, depth of cut and rpm's that my breakage problem will be solved?

I have a 2004 vintage PRT G4 upgrade PC router and 5 stepper motors rather than the usual 4. It actually works pretty well. This machine started out as Eric Francy's first Bot. I have modified it some and am extremely happy with its performance overall. I may have found it's lower limits, but haven't given up yet.

All of these except the Masonic emblem were cut with bits smaller than 0.050" (fifty thousandths) with complete success. Those I can do 7 days a week. The white inserts are Corian. Now that is some tough machining with little tiny bits. Corian is about twice as hard/brittle as acrylic.
Ronco

[navigator7]

@ Ronco,
I've got six-pac abs laughing over your wife's comment and your reply.

I don't have an answer for you. I had no idea somebody could do such detailed work on a Bot. Your choice of cutters is smaller than anything I've seen except at the dentist.

But Yeah....feed speed and depth of cut is a holy grail of machining.
I was machining acrylic in a one-off Cleveland mill.
It wasn't going good. A one-off job not worth buying the proper cutter. The chips weren't clearing. The idea came to me to chuck up a broken cutter. It only had one flute.
It worked.
I don't know what the answer is but expecting different result from the same methods probably won't get you where you want to go.

Nice work!

[coolhammerman]

Chuck:
I modified my profile after I looked at yours so the inspiration goes both ways. I also do all the usual stuff, like cabinets and slotted sheet goods. I have even made a few dozen marble signs. But alas, I am afflicted with that skin flint disease when it comes to cutters. I entertained the idea of paying $5 for each cutter, then $11 for better ones. I showed their web site to a friend who said, hey, that looks exactly like the Printed Circuit Board Cutters I used when I was in the business. Turns out I found some resharpened PCB cutters on the web. As I've previously stated, they work fine for me down to 0.049" and cost around $0.70 a piece. Even I can afford to break a few of those while I'm learning.

I also mounted a Jet mini lathe on the end of my bot and use the X & Y cars as a CNC lathe of sorts. I mounted a different cutting device rather than the router motor. I did 800 wooden golf club handles out of 1x1x42 hickory. That was fun! At least the first 100 or so. Have done production furniture parts out of 6x6 SYP with the same setup; a few hundred pieces. Thank God I have another job that actually pays the mortgage. Read my profile and you'll see what I mean. Merry Christmas.
Ronco

[curtiss]

Ron,

What takes up more space at your house ??

Your 4 x 8 machine, or your wife's shoes ??

[coolhammerman]

Curtis:
With all humility, I a the luckiest man in America. I have an almost perfect Proverbs 31 wife. After over 40 years of marriage, if I could choose anyone in the world, I would pick her all over again. My junk takes up far more space than any of hers to the point of embarrassment. I often tell her when I'm gone to get a real husband worthy of her.

In fact, we were discussing that just the other day. I asked if she would let him run my Bot after I was gone. She said probably yes if he knew how. Then I asked her if she would let him drive my new pickup truck. She didn't see anything wrong with that either. Then I asked if she would let him play with my golf clubs. She said, No! Because he is left handed.

Ronco

[clueless]

I think you need to be looking at engraving cutters, not router bits or endmills.

http://www.2linc.com/engraving_tools_conical.htm

would be an example. It's what engravers use all the time.

[coolhammerman]

Steve:
That would work if I were doing the engraving after the turning. I'm doing the inlays while the blank is still square. I don't know what the final diameter will be. I'm supplying pen blanks to pen turners who will make all manner of pens as the final product. Sizes vary widely. Also the depth is from .24" to .15". the image must not change regardless of final diameter. Engraving with V bits does not lend itself to this application.

Thanks for the thoughts.

Ronco

[eaglesplsh]

I've cut brass with 1/32 2 flute end mills before and didn't have problems. Unfortunately, I don't have the files anymore so I can't give you the feed/speed.

You may have a problem with your plastic melting/clogging up the flutes of your cutter. I didn't run into that with the brass, of course.

I'd call these guys for small cutter advice:
http://www.bitsbits.net/index.php?main_page=index&cPath=38

They have helped me out in the past with great service. They grind their own bits and know their products. Unfortunately, they're bits cost a lot more than the PCB bits you've been using.

One last thought... to avoid most of the side-load cutting that's giving you problems, you could rough out the pocket by drilling a bunch of holes in a grid pattern that fills your design. That would leave you with a profile operation to clean up the outside edge - much less material to take off in that pass. Use shallow ramping to reduce shocking the tool. I'd use carbide PCB drills for the roughing part - they might do a better jog extracting the chips. I'd also try it as a peck drilling operation, to see if the improved chip extraction helps.

[eaglesplsh]

Here's some more info to add to the mix - not exactly an answer, but maybe it will help things along...

I just did a bunch of pocketing in trupan with an 1/32 2 flute end mill. Ran it at 3 ips (2ips plunge ramped at 22.5 degrees) 24,000 RPM (PC router) 0.050" depth of cut without any problems.

The pockets were short in length (1/2" long, 1/8" wide) so I don't know that it ever got up to full speed. Also, this was a short bit - 1/16" long for max depth of cut.

Obviously, trupan is low density and cuts like butter compared to acrylic. It doesn't melt/clog the flutes either. But, based on what I just saw, I'd start with these numbers if I had an acrylic project.

I keep pushing my bits faster and faster as I'm feeling out the capabilities of our new machine and I keep surprising myself with how fast they can go compared to what I used to run things at on our old machine. The chips from our larger bits keep looking better and better + their thickness matches the chipload calculations and things "sound" better too.

I used the same 3 ips feed/speed settings to do larger pockets with a 1/16 2 flute end mill at 0.250 depth per pass. Cut great! Previously I would have run it 0.0625 depth per pass and 0.5 ips.

That works out to 24x faster cutting than I'm used to - I'm a happy camper today!

[clueless]

Doesn't matter if they are round or square, those cutters will cut it. They've been used for a long time to engrave just about most things out there. ShopBotter's some times forget that the world was already making things before the bot came along


Lots of reinventing of wheels going on in the CNC routing world.

[navigator7]

LOL @ Steve,
Can you imagine if the Romans had CNC and CAD files?
Masada would still be in existence because all the soldiers would be AWOL earning great money making parts, signs and trinkets and selling them to the locals.

"I built a huge ramp to Masada and all I got was this lousy tee-shirt!"


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