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the truth about CNC machining...

Discussion in 'CNC Routing & Laser Cutting' started by OADesign, Jan 28, 2006.

  1. OADesign

    OADesign Active Member

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    here's my question?

    I think the CNC machine is one of THE coolest additions to any sign shop.

    But how much extra work goes into finishing? For example when you cut say 1/2" acrylic. i know when you cut fast your going to get tool mark. but is there any feed rate/tool speed that would allow me to go from cut to install with no steps in between?

    another example. we make ALOT of bead foam letters with acrylic faces. right now we mount them with silicone and cut with a band saw. (don't laugh). but i heard that you cant cut the bead foam and plex together? is that true? you cant tell the you cut them separate and mount them after do you?

    and with metal and wood what do you do to get rid of the hatch/island fill tool marks?

    and with bits i know they are pricey for the quality but how often you you replace them? or do you just have them sharped?

    i know that was allot of info there...

    thanks in advance for the help!:thumb:

    -Oh
     
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  2. andy

    andy Active Member

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    Cnc has quite a learning curve to it and you should expect to spend a few months getting to grips with the system you buy.

    Cutting "fast" is a relative term- some materials won't cut fast as they weld back or snap the router bits- this is very easy to do as in sign work you need to use smaller cutters to get into tight spots- serif fonts etc.

    Using small cutters to get into the tight curved areas in letters etc impacts on how deep you can cut- I work in metric but you will get the idea.

    Standard cutter is 4mm diameter. If I'm cutting 15mm Plex then I need to do 2 passes at a push- 3 to be safe. This will be cutting down a set depth on each run, dropping deeper down on each pass. The rule of thumb is you can go twice as deep as the diameter of the cutter- so for a 4mm cutter the max depth is 8mm per pass.

    I tend not to use expensive router cutters because they are a consumable item- it's very easy to snap a cutter and cheaper ones make this less of an expense. I have great results with CNC bits that are £15 a pop- a lot better than £45 or £90 a pop.

    I hear all the time that you can "wizz" through this material or that but in all honestly Plex won't machine well other than in straight lines or long curves much above 200-250mm P/M. You can try a run a lot quicker but you will snap bits and you will get rough edges and weld back. Accuracy and good edges are what's important to me so I'll sacrifice some speed as it works better for me overall.

    Things like composite and MDF will machine faster and with good edges but there is a hell of a lot of testing to do yourself until you get techinques you can rely on everytime- my CNC system came with processing speeds based on Onsrud cutters and even using this brand I found them way, way to fast.

    If you run a fatter router bit you can run faster in some materials but this won't work too well if you are trying to cut detailed parts- the fatter cutter will be too big to get into the tight spots without destroying the design you are trying to machine.

    The best thing to do is talk to a machine supplier and possibly send them the materials you want to cut and ask for test pieces from them- then you can see if you get the results that you need.

    CNC is a whole new ball game and does allow you to do lots and lots of wizzy things- but you need to plan in time and money for testing and toolpathing strategies that work for you- once you have these it gets easier.
     
  3. Westcoast Sign Guy

    Westcoast Sign Guy Very Active Member

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    what he said! I can laminate all materials together and cut
     
  4. openwood

    openwood Member

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    I agree with andy.I only have a few things to add.I use 1/16 to 1/2 bits routing all different materials.the clean-up and edge finishing is sometimes a problem.match the bit to the thickness of the material that you are cuttiing.do not let the bit stick out more that it has to.long bits(sticking out more than 2") will "chatter" and leave not a very clean edge.I like to use the 1/2" bit when cutting things over 2" thick.useing the finish cut option sometimes does the trick (make sure you have evnough LOC to do this)and it can clean up alot of the mill marks.certain bits work better on some materials than others. I like to use a straignt flute for cutting wood,it does not chip as much as a up cut sprial bit.up cut sprials work great with foam and help clear the "chips"better.bit quality is also important.I like to use solid carbide bits.it is a huge learning curve.take andy's advice on software reccomended speeds and feeds,they are way to fast.

    the reality is that i sand almost all the edges milled on the table,to smooth out and remove any webbing that is remaining.the same process that has to be used when cutting material the old fashion way.

    Good Luck

    Brian
     
  5. OADesign

    OADesign Active Member

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    whoo. sorry for the typos and grammar. i guess i was tired when i typed that...

    so yeah. thank you for all your input.

    so the resounding fact i keep hearing is that the learning curve is steep. i hope to hit the ground with my wheels spinning (well turning at least). i am a fan of the scanvec's flexi so ive been looking at their enroute. i think im going to take a look over to shopbottools also. i wish i could just walk over to that other guy's shop that has that cnc machine and say, " Hi, can i take it for a spin?". any tips on how/where i may get some hands training?
     
  6. andy

    andy Active Member

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    For hands-on experience I would be looking to get this from someone who has the SAME machine as you are considering buying.

    The principles for CNC are pretty universal but... each brand of machine will have it's own control software- the front end that controls toolpaths etc. If you get training on a system you aren't going to buy then you could just confuse matters more than you need to.

    In terms of buying a system my advice is to go for the biggest table size you can get- I have 10x5 but would have likes 3m x 2m. There are stacks of folks with the 4ft table sized machines- with a big table you can do full panels not just cut out letters.

    The shopbot system looks pretty popular in the states- I was thinking of importing one but decided to get a "proper" system instead- more support and a more industrial type system.

    I would also go for a proper industrial spindle not a mounted router- options on a bot setup. I have an Elte spindle and can't speak highly enough of it- brilliant bit of kit- best part is that the guys in Italy will support you direct with fault fixing and spare parts quick to get hold of.

    If you have the bucks I would say go for it- after all a CNC is way more useful than a digital printer- PLUS it won't be out of date in 18 months time- it'll do whatever you need day in day out for years.

    If you get stuck or need quick advice just post here :)
     
  7. Bobby H

    Bobby H Very Active Member

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    I don't have much to add to what others have posted already, but can offer these observations.

    Every CNC routing table seems to have its own "personality." A method that works on one may not work exactly as well on another. The best thing to do is keep lots of "drops" (scrap material) around and experiment. You have to find the right balance of feed rate speed, tool bit size and router RPM speed for each kind of material. It took us awhile to get really good at routing acrylic with our current table. We can cover sheets of White acrylic with translucent vinyl and cut certain types of channel letter faces cleanly without messing up the vinyl and take them directly to the trim-capping stage.

    Normally I would not recommend laminating two different types of material together and then attempting to rout letters or other objects from the laminated materials. Different materials have different levels of density, heat up differently, melt or not melt, etc. If you have the scrap material to spare, it won't hurt to experiment. But I would not try it on a large job where you have only so much material budgeted into the project.

    It can even be risky to rout two or more layers of identical material laminated together. Whatever glue is used to bond the layers is its own wildcard. If the bond isn't consistently strong throughout the sheets, and if the router isn't cutting as smooth as possible you have to risk of cracking material or breaking down the laminating bond between layers.
     
  8. mgieske

    mgieske Member

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    Get an Onsrud catalog. It has a very useful feedrate chart & tool application guide. They also have full time techs to answer questions.
     
  9. FlambeauMan

    FlambeauMan New Member

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    What mgieske said, get the catalog. I learned alot from it in regaurds to speeds on certain materials. I have a Kongsberg i-XL figured out pretty well, but it took alot of time and testing. Onsrude offers good prices on bits, and a very wide variety. Have fun and good luck.
     
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