Chips N Dust

Bits and Spindle Speed

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I was wondering how do you choose the proper bit when you are setting up your tool paths? Also, how do you choose the RPM of the spindle and feed rates? Is most of this just trial and error?

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I started to type up a quick answer.  A book later I deleted it all to leave this:  Trial and error may be your best strategy.  

 

4D

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If you've been using routers, handheld and table mounted, then you already have a good background for using a CNC.  The CNC IMO gives you too much fine control of too many variables.  VCarve and Aspire have default settings in the tool table they come with.   Good place to start.   Listening to the thing cut I can hear/see/smell if it should slow down or speed up.   There is a feed speed slider in most control software where you can vary the speed while a cut is in progress. . 

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99% of what I cut is at 18,000 rpm mainly because that's where my VFD is set and I have rarely seen the need to change it.  So that's sort of a 'fixed' speed for me although I can go from 6,000 to 24,000 rpm very easily.  When I dipped my toe into the CNC waters my feeds were very slow - 1/4" upcut end mill at 50 ipm - and depth of cut very shallow - typically 1/16" with the same bit (that only lasted a week or so before I began speeding things up).  Fast forward a few months and now my typical starting point for the same bit is still 18k rpm but feed rate is 150 ipm and depth of cut, depending on material, is 1/8" minimum and sometimes up to 3/16".  One time I have cut 1/4" deep and it was just fine.  Walnut is what I cut about 90% of the time but I cut Sapele, Maple, and Cherry the same. I've only cut Pine a couple of times and it seems like I pushed my envelope a bit on that - deeper cut, if I recall.

 

I have a water cooled 3kW (4 HP) spindle on a very rigid machine and I've had people tell me I can cut 250 ipm at 1/4" depth of cut in Walnut with no problem, but I can't bring myself to dial it in that fast.  One day, maybe.  Many of the jobs I cut are short runs of 10 minutes or less so going much faster is only going to save a minute or two so there isn't much incentive to speed things up except for bragging rights.  But there's always someone who will push a machine faster and harder so I wouldn't win much on the bragging rights contest.

 

David

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Me rambling...

.

As "smart" as devices are getting these days, this RPM/speed/pass question should be more automatic IMO.  CNCs can probe for a surface.   Many routers now come with load sensing circuits.  It seems like the CNC itself should be able to start any tool path then dynamically sense/adjust RPM and feed speed throughout the cut. 

 

I have a drill press now that prompts me for bit style used, diameter of bit, material being drilled into, then it chooses/sets an ideal RPM.   It senses the load on the bit and will stop it once I've drilled through.  It knows how deep the bit is and will stop or reverse the bit once my desired depth is reached. 

 

All CNCs/controller programs should be this smart!    Routers/spindles should be improved to communicate their load sensing back to the controller PC. The web already has lookup tables of info for each bit and the material you'll be cutting.  The CNC itself should be idealizing each cut!  JMHO.  

 

4D   

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36 minutes ago, 4DThinker said:

Me rambling...

.

As "smart" as devices are getting these days, this RPM/speed/pass question should be more automatic IMO.  CNCs can probe for a surface.   Many routers now come with load sensing circuits.  It seems like the CNC itself should be able to start any tool path then dynamically sense/adjust RPM and feed speed throughout the cut. 

 

I have a drill press now that prompts me for bit style used, diameter of bit, material being drilled into, then it chooses/sets an ideal RPM.   It senses the load on the bit and will stop it once I've drilled through.  It knows how deep the bit is and will stop or reverse the bit once my desired depth is reached. 

 

All CNCs/controller programs should be this smart!    Routers/spindles should be improved to communicate their load sensing back to the controller PC. The web already has lookup tables of info for each bit and the material you'll be cutting.  The CNC itself should be idealizing each cut!  JMHO.  

 

4D   

The software to do what you are talking about would probably cost several times the coast of just one of the best hobby CNC machines that the backyard woodworker can afford and there may be some large commercial machines already running something similar.  There are too many factors involved in the mix of hobby machines to make it feasible for hobby use.  

 

For now the best way I have found to set feed and plunge rates for the bits in your data base is hold your spindle speed, calculate what the feed should be for a bit, then test and adjust that feed rate until you get the best cut/speed for that bit.  Enter those values in the data base so when you pick that bit you know it will cut correctly in most materials you use.  If you do a job with softer or harder materials then you can edit the feed for that job.  So trial and error for the hobby user is the best way to go in my opinion.

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I won't worry about the software side until some company comes out with a router/spindle standard that can feed back data about the load it senses.  Photo/video cameras have surprised me lately by coming with wifi/bluetooth/nfc and are capable of sending live video they see over wifi to a phone or tablet. Far less info, perhaps  2 bits of data (for 4 load ranges) would be enough to tell linuxCNC that it can/should increase or decrease feed speed.   No point in figuring out the software side until the hardware side speaks up.   I have faith in the open-source community to add router communication support to LinuxCNC once it is available.  I can already imagine a simple subroutine to handshake with a router.  I've got programming experience though. 

 

I personally don't have trouble setting appropriate pass depth and feed speed for all the bits I commonly use. I set and leave my routers at one number under their max dial speeds.  If I get a new bit then I set the pass depth to 1/2 the bit diameter, and feed speed to 200ipm (My CNC's hardware limit).   I listen while it cuts, and keep my mouse cursor on the feed slider to slow the feed down if the bit seems challenged.  Whatever I end up at is what I update the tool database with in Aspire for that bit. 

 

It is the novice users that would greatly benefit from a smarter CNC.  In our college furniture shop the older dumb drill presses are scarily abused. The many student users (who have all been "trained" before granted access to the shop) can frequently be caught almost killing themselves using the wrong RPM for a big forstner bit.   Those drills are difficult to change (belts positions usually) so our shop tech sets them somewhere in the middle of their range and locks them there.   The new drill press tech not only picks the RPM for them, but teaches them that different speeds are appropriate for different bit styles, diameters, and material being drilled into. Win, Win. 

 

4D 

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