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Old 06-04-2015, 06:00 PM
Javierunzueta Javierunzueta is offline
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Default Any reason for high RPM's on wood?

To all the gurus out there,

Although I've only been at it a month or so, Is there any possible reason to go faster than 10,000 RPM on wood?

I wish there was a primmer listing various woods and feeds and speeds in a simple chart rather than making me calculate chip rate. And may I add, while chip rate seems to be important for keeping a bit cool, I have found it to have very little impact on how clean the cut is. Am I wrong?

While I'm still goofing around with speeds and feeds to find the best bit/feed/speed/wood/finish combo, I have yet to find a reason to go over 10,000 RPM. In fact, I'm considering inserting a dial to turn down the speed into the 6000-10,000 range. Unless I'm carving out a HUGE chunk of wood in one pass, I can't imagine ramping up to 16,000 let alone 24,000. And if I was carving out that fast, I suspect the cut would not be clean. As I have read the forums and everyone recommends small bites, I have not tried this. Am I justified in this line of thinking? Or am I missing something?

Thanks.
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  #2  
Old 06-05-2015, 08:33 AM
BradyWatson BradyWatson is offline
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Wood is soft compared to metal. You need higher RPM to get a clean finish. Very generally speaking, the softer the material, the higher the RPM needs to be in order to sheer off chips cleanly. Harder materials generally machine at lower RPM, with hard plastics being the exception.

Many metal workers who come from milling machine experience try to run their lightweight CNC routers at low RPM, because that is what they are used to, only to get poor edge quality. The spindles/routers on these machines are designed to run at RPM suitable for cutting non-ferrous materials. In fact, most spindles on these machines are optimized to run in the 12,000-18,000 (or 24,000) range.

Just for a visual exercise, take some pink insulation foam and machine it at 10,000 RPM. Note the edge quality. Then crank your spindle all the way up to 18,000 and machine the same exact part again noting the edge quality. Which one looks better? Softer materials get 'grabby' and will yank the material out if the RPM is too low or the chipload is too high.

-B
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  #3  
Old 06-05-2015, 09:20 AM
Javierunzueta Javierunzueta is offline
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Brady,

Thanks for for info. In light of this, I'll experiment some more with higher speeds on different wood types. Again, I wish there was a detailed chart or primmer on wood carving feeds and speeds.

I have only tried RPM's over 15,000 a couple times on wood thus far. Since they produced similar results (with more noise) to the 10K projects, I stopped running at those speeds. I also read numerous posts warning against higher speeds saying it wasn't necessary and would only serve to burn out bits.

I read so many articles in forums and received advice stating that I should be making chips, not dust. Were they referring to feed as opposed to speed? Trouble is, it's often hard to distinguish whether the operators were referring to working on woods or metals. One would assume there are similarities as well as differences between the two.

Thanks again.
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  #4  
Old 06-05-2015, 09:49 AM
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Gary Campbell Gary Campbell is offline
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Javier...
There is no firm rules or docs for what you ask. Each combination of operator/designer, machine, material, hold down, feed range available, rpm range available and bit geometry will most certainly be unique. The resulting combinations are infinite in number. And that doesn't take into account natural occurring variations in each.

There are guidelines and advice, which you should use to perform YOUR testing on YOUR machine with YOUR material and YOUR bit to get the best balance of what works best for YOUR product.

Testing would involve varying the RPM and feedrate along with the bit style or geometry on scrap material that is exactly what you will be cutting. Always start conservative, and look to higher feedrates if production of multiples are required, but NEVER leave quality to find speed.

Even tho you may not end up using them daily, when looking to new or possibly difficult items to cut, go to the high end bit suppliers and find the right bit for the job and test cut with it. Then look around for less costly alternatives.

Remember: You can search for speed if you have quality, but you may NEVER find quality when you have speed. Its like a teeter totter, when one goes up, the other goes down. Its your job to find the balance that works for you.
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Old 06-05-2015, 09:56 AM
BradyWatson BradyWatson is offline
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Don't get too hung up on chipload numbers. This is for production and to get the longest life out of your tools. Carbide lasts a VERY long time. You'd think bits cost a million dollars the way some of the garage engineers adhere to chipload numbers. Unless your parts are glued or bolted down, running chipload numbers will defeat your hold down in many cases.

Having been in this business for 15 years, I recommend you focus on how the cut quality looks first and foremost. Nobody is going to pay you for running chipload...or if your parts look ragged or burned. Find what speeds/RPM give you the best looking parts and make that your yardstick. If you cut the same parts or material day in and day out, that's different.

I've cut just about every material you can cut on a CNC router (woods, plastics. composites, non-ferrous and ferrous metals, stone, etc) and I have never referred to a chipload chart or even calculated it for a job. The tool and material will tell you want to run if you have eyes to see and ears to hear. There aren't chipload charts around for new or exotic materials that my customers send me. I have to do it old school - observe & adjust.

Do lots and lots of your own testing. Just get in there an cut out some stuff, observing your speeds/RPM and how the edge looks. Don't be afraid to crank up the RPM, to break tools or to ruin material. Just get in there and listen to what the machine and part tells you is going on. Make some adjustments, and cut the SAME part right next to the previous one. Write down your cut speeds and RPM on each one and try another one. Keep pushing as hard as your hold down allows and see what looks best.

You'll be confident in no time...but ya gotta do the work. Nothing replaces actual hands-on experience. You'll quickly figure out on your own what runs true in your shop, on your machine in your material. What's the worst that can happen? Think about that...then get to it! :)

EDIT: Didn't see Gary's post...

-B
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Old 06-05-2015, 10:14 AM
Javierunzueta Javierunzueta is offline
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Gary,

I completely agree with you regarding quality vs. speed. Perhaps I didn't communicate my intent correctly. My current goal (in my CNC learning process) is consistent quality. As I'm learning to master the machine, I'm finding a quality in the cut that is often sub par and I'm trying to do everything I can to choose the right combination to reach a certain level of quality in the end product. (Agreed, the combinations and results are near infinite.)

I'm coming from a background of Laser engraving where the cuts are extremely precise as well as rotary engraving where I'm working at a smaller and finer scale. I've grown accustomed to a piece being done the moment the programs finishes.

Similarly, using my hand and power tools, I've learned a level of control whereas any wood I work with will come out of the process requiring minimal sanding if any.

Prior to working with the CNC machine, while I expected a bit of a learning curve, I naively expected every piece to come out of the machine flawless and ready for finishing like I'm used to. (I forgot to take into account the years of mistakes that got me there.)

To be more specific...
In your (all forum members) experiences in working with soft woods such as pine and cypress, what has worked best for you for detailed work?
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  #7  
Old 06-05-2015, 10:21 AM
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Gary Campbell Gary Campbell is offline
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Brady is correct. The term "chipload" is one of the most over and erroneously used in CNC routing. I teach its use in my class, but only as a guideline to a starting point for testing AND as a way to understand how to keep toolpath parameters within the capabilities of a given machine.

Many less experienced users "get hung up in the decimal points" and quote chipload numbers or calculations. That chipload number quoted is usually as accurate as "fast" or "slow". For example:

60 ipm, 6000 rpm and 2 flutes
80 ipm, 8000 rpm and 2 flutes
240 ipm, 24000 rpm and 2 flutes
120 ipm, 24000 rpm and 1 flute
15 ipm, 3000 rpm and 1 flute
300 ipm, 20000 rpm and 3 flutes

ALL RETURN THE SAME .005" CHIPLOAD CALCULATION!

Chipload numbers are much more likely to tell you that you are doing something wrong than they ever will that you are doing it right.
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Old 06-05-2015, 10:27 AM
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Gary Campbell Gary Campbell is offline
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Javier...
Because I learned much of what I know from Brady in years past, what has worked for me the best has been to develop a method to test multiple bits, feeds and speeds until I had the quality that was required for the job.

The tolerance and finish required for a 3D water pump impeller that is cut as a mold plug will be substantially different that that required for a birdhouse in the backyard or a 50 sheet kitchen job.
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  #9  
Old 06-05-2015, 10:28 AM
Javierunzueta Javierunzueta is offline
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Thanks Brady,

As I'm typing this the machine is running and I continue to test new speeds/bits/processes/designs/woods combinations etc. I just put the questions out there just to make sure I'm on the right track and not doing something incredibly stupid.

My first real boss gave me the best advice I ever got from anyone: "I don't care of you make a thousand mistakes in this job as long as you learn from each one, but if you do nothing for fear of doing something wrong and losing your job, you're fired!" -Dave Goode, Radio Shack manager, (1981)
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"I'm a firm believer in learning from my mistakes and failures, but I prefer to learn from others'!"
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  #10  
Old 06-05-2015, 10:39 AM
Javierunzueta Javierunzueta is offline
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Gary,

And THERE'S the reason I ask questions. Thank You. You just pointed out something to me so obvious that I had not considered it. In all my years of troubleshooting issues with computers (and just about everything else), I always had a process. I've been so anxious to find the answer this time that I jumped head first into trying bits/speeds/feeds/woods/etc in a completely random order with no accounting. I neglected the most important step of a systematic incremental process starting from scratch. Back to the drawing board.

Now I'm pumped. You raised my spirits from "What the hell am I doing wrong? This should work." to "Time to get to work! Excellent!"

Thank You Gary and Brady
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