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Author Topic: CNC engraving  (Read 14028 times)
oldskoolpunk
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« on: January 10, 2010, 08:58:18 am »

Any tips on engraving when you have a CNC mill available?  I have a membership at TechShop, and can use the big Tormach mill (overkill for engraving) or the little Techno mill. The Techno mill is a tabletop machine with 3 axes with about 0.003" precision. The spindle is a Bosch router, like an oversized Dremel tool.  Collets for 0.25" and 0.125" are available.

The problem is that this is a rigid machine. Most engraving is done with something that has some "give" in the vertical direction. I tried engraving with a 1/8" ball-head Dremel bit, but it didn't work too well. The plate being engraved had a wood block underneath, and the wood wasn't flat enough. So the lettering faded out towards one side. Worse, even the good letters weren't deep enough to hold ink. A ball mill was the wrong tool for the job.

I probably need to get a real engraving cutter (about $12), but I'm wondering if there's some way I can give the thing a little vertical compliance so the process won't be so touchy about  material thickness. Assuming I'm machining sheet brass or nickel-plate ("german silver"), any ideas?  A layer of rubber under the material, maybe? Anybody tried this?

The goal is to do elaborate Victorian scrollwork. I can get the machine to do that; it's just a cutter problem.
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« Reply #1 on: January 10, 2010, 02:30:51 pm »

You need to have the work piece like any other work piece when milling,perfectly flat on a solid bed no rubber wood ect. I use 60 degree carbide engraving cutters and they work a treat up to around 0.5mm deep. any deeper and i use carbide ball nose cutters.
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Otto Von Pifka
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« Reply #2 on: January 27, 2010, 03:33:49 am »

a floating engraving cutter would help considerably.

if you have access to a machine shop, you could make something for yourself.

what we did to make quick work of stuff was to use a thick piece of mdf as a pallet for the piece to be engraved. lay out the work in marker to see your outer edges of work and then use sturdy dogs to hold down the sheet, screwing into the wood. also use a few extra screws around the workpiece to keep it from sliding under pressure.

don't try to get your full depth in one pass, creep up on it with multiple passes. cutters dull fast even in relatively soft materials. lubrication helps.

carbide cutters stay sharper longer, are more expensive, and are quite unforgiving (they're brittle and snap easily) so unless you have a microscope and a steady hand for sharpening, high speed steel are short lived.
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oldskoolpunk
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« Reply #3 on: February 09, 2010, 09:36:56 am »

CNC engraving is starting to work. Here's a recently cut nameplate.



Workholding

I put a piece of pine board in the CNC mill and milled a pocket the size of the brass plate I was going to engrave. Then I put double-sided machinists tape (paper tape, rubber adhesive) on the back of the brass, and stuck the brass plate into the pocket. This holds the plate flat without bending it, and provides side support to keep it from slipping. Because the board was machined just before using it, and not removed from the bed, it was reliably flat relative to the cutter. For repeat work, machining a pocket plate out of aluminum or Delrin would be more effective. Wood won't hold dimensional accuracy.

Materials and equipment

Brass sheet, 0.040" (I've tried 0.025, unsuccessfully - cut-through is a problem.)
Techno desktop mill, which is basically a Dremel-like tool under computer control.
Software: Vectrix Cut2D
Conical engraving cutter, 0.250 shaft, 60 degrees included angle, 0.020 tip.
2linc.com part: C-250x2.0-60-.020-G -- $16.95 each

Speeds and feeds

Speed setting 4 on Bosch router. (10,000RPM?)
Feed rate 16 IPM.
Plunge rate 8 IPM.
Cut depth 0.015"

Finishing
The letters are filled in by spray-painting the whole plate with black bare metal primer, letting it dry, and then sanding the plate with a flat sanding block.



Notes

Text was cut as profiles on the line, not as pockets. Use a font which is either single line or fixed line width. Text 3/8" or larger looks good.

Installation

As installed on the Aetheric Message Machine.



 


  

« Last Edit: February 11, 2010, 10:26:57 pm by oldskoolpunk » Logged
Otto Von Pifka
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« Reply #4 on: February 09, 2010, 10:08:54 am »

fantastic work!
looks like you got things figured out quite nicely.
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shoeshine
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« Reply #5 on: April 27, 2010, 09:40:52 am »

Check out,

http://www.precisebits.com/products/carbidebits/scoreengrave.asp

I use the Vcarve bits by the 10pack for all sorts of incredibly precise engraving. from $9 -$14 ea (uncoated is cheaper, just a matter of cutter life)

I'm on a shopbot with a PorterCable #75182  3.25hp router. Vectric PartWorks 2.5 (shopbot branded version of VCarvePro)
speed and feed for brass = .3ips feedrate, .1ips plunge, router at @ 16k rpm, passdepth of .01 -.02" (I usualy ramp my toolpath entry cuts over .5" travel, helps to keep from snapping the tip off the bit)

and -YUP- surfacing your spoilboard is key.  It doesnt matter what the table is like.  what you need is a surface that is parallel to the travel of the cutting head.

all sorts of tricks for holddown to work thinner metal:
-my favorite is to epoxy shimstock brass (down to .003) to a piece of scrap 6061 alum... mill, then hit it with a torch. pops right off.
-alternate is polyacrylite (superglue) to the same alum. then flood with acetone to release.  less danger of heat warping, lots more fumes and planet damage.

Feel free to hit me up... I have been playing with metals on the CNC for about a year now.

see http://brassgoggles.co.uk/forum/index.php/topic,15364.25.html about the end of the page. made on my "bot"

PS: TechShop hunh?  You in the Bay area? going to MakersFaire in May? Im in San Diego but I hit the Faire every year.

Shoeshine
« Last Edit: April 27, 2010, 09:55:42 am by shoeshine » Logged

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« Reply #6 on: May 07, 2010, 08:33:29 am »

That is a really useful resource for cutters, thanks Shoeshine.
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Miles (a sailor)Martin
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« Reply #7 on: September 27, 2010, 02:28:29 pm »

another trick for thin stuff is spay adhesive, and acetone for removal
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Miles (a sailor)Martin
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« Reply #8 on: November 24, 2010, 03:01:08 pm »

also have used double sided tape, over a vacum tray,shop vac source then when milling complete sparying wd-40 in from the back to dissolve glue on the tape worked. this was for part .100 thick flycut to .020 in center then engraved.005 on back side. parrallelisin of the two side was called out.003 ,we were holding .0015 of an inch parrallel acording to the best we could measure.... Sandia National Labs liked it,so we must have got it close enough I guess Grin
                                                 Miles (a sailor)Martin
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