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Thread: Some observations on running CNC.

  1. #1

    Some observations on running CNC.

    I have been running 2 CNC routers for about 4 years now. I cut all the parts for my products on them and run them for thousands of hours. I can see that there is an interest in this technology and thought I might scatter some of my fairly random and very personal observations in this post in the hope that they might be useful to others .

    Workholding -

    This is massively important. My production system uses vacuum workholding. Each machine has its own vacuum pump and the workholding plates are made from phenolic ply and I use neoprene based tape for the seal. There are lots of ways of using vacuum - mine is pretty specific but if you are at all serious about using CNC invest some time and money to make a good solution for your own needs. For one offs where vacuuum isn't appropriate I generally make some drill holes in the workpiece away from the cutting lines and run a drilling program before the cutting program and screw through the workpiece into a spoilboard. There are other ways to hold workpieces but for any real volume some manner of vacuum system has to be the best solution

    Cuttters -

    I've gone through several evolutions on this matter. Most of my work is profile cuts, in other words cuttting out the edge of a shaped workpiece.

    I started out with 1/2" straight cutters. Disaster. Expensive, wasteful, noisy, limit the minimum internal radius of cuts. Not suitable for drilling pilot holes or cutting small holes. Disaster.

    Then I tried 6mm spirals. Litttle bit less of a disaster. Expensive, wasteful. Tiny bit better for cutting small holes and slightly tighter radii.

    I'm now on 1/8" (3.175mm spirals). Best solution so far. Cheap, relatively quieter. Perfect for drilling pilot holes and small radii. I have to make more passes at a shallower depth but for me the trade off is well worth it.


    I'm cutting 3 identical components at the same time. If something goes phutt then I will usually lose three workpieces. I try hard to program for reliability. I would rather have 99.9% succcess in 10 minutes than 75% success in 5 minutes. Here are my rules for that.

    1 - Don't speed. Things go wrong when you push the edge of the machines performance envelope. My machine will rapid (G0) at about 8M/min (call it 25 feet/min). But it will rapid alll day without a hiccough at 6M/min. For actual cuts I usually run rough cuts at 2.5M/min (8 feet-ish) and final cuts at 1.25M/min (4 feet-ish)

    2 - Don't cut too deep. Most of my cuts are 23mm total. I typically take 5 passes of 4.6mm. Much deeper and I get too many broken bits. Much less and I'm wasting time. I nearly always program so that the component gets a rough cut to full depth in 5 passes but 0.1mm (work it out yourself) oversize. I then make one or more final passes at full deph without the oversize allowance. If I have the time I make 3 or 4 of these final passes which means that the commponent comes off with basically no need for any further edge finishing.

    3 - Climb cut. All my cuts are made on the climb. I used to make the final cut "conventional" but I found that it resulted in frequent grain tearout or splittting along the grain which kills my vacuum and therefore the workpiece. Climb cuts are massively more reliable in the timbers I use with the cutters I use.

    4 - Clear waste. Most of my SNAFU's came back to a loose waste offcut lying on the table snagging on the cutter head. All my programs now have a pause between the rough and final cuts so that I can safely remove loose waste. It only took me three years to work that out.

    Sorry - got to run now - will try to pick this up with more thoughts tomorrow. If any of this doesn't make sense or if anyone has questions about running CNC let me know and I'll do my best

  2. #2
    Join Date
    Mar 2007
    Southeast Pa
    Great information. Your Cnc could turn my CNC into saw dust in about 40 seconds I think even with a small cutter and several passes. However much of the information carries over. I am always seeing people on the net trying too get that extra IPM yes we use inches per minute and you are using FPM but the results are the same. Even worse when most of us are using steppers that have no idea they fell behind..

    I suspect you have servo's. What are those spindles? They look as if they would work great on my little machine.

  3. #3
    Join Date
    Jan 2007
    Sacramento, CA
    Great stuff, Ian!

    Lots of great information in here
    Jason Beam
    Sacramento, CA

  4. #4
    Join Date
    Dec 2007
    Billings Missouri near Springfield Mo
    A Turn N Time
    Components for John Smith Organs and the Hobby Organ Builder

    Frog Pond Guitars

    Before you criticize someone, you should walk a mile in their shoes. That way, when you criticize them, you're a mile away and you have their shoes.

  5. #5
    Join Date
    May 2007
    Kansas City, Missouri
    Great advice Ian.

    To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault

  6. #6
    Join Date
    Feb 2008
    East Freeetown, Massachusetts
    Information is always valued

  7. #7
    Join Date
    Oct 2011
    Cortland NY
    Ian, you have clearly stated what I have issue with with our Thermwood CNC. Vacuum is by far number one issue and hold down for small parts very critical. 90% of work being cut is good but 75% is not. Vacuum is the key to that. We use a waste board spoil board setup pulling vacuum from below the table. For cabinets that works fine. for large areas that works fine. Can pocket, drill, index, and cutout with out much care. With small pieces 12" or smaller the vacuum is far more critical. Because we have such a large surface area on our machine. 5' x 12' we often cover the table with scrap to max the vacuum to the area of work. Still we sometimes even tape down. That is time consuming but in the long run less material loss and production continues.

    Cutters another issue. We do use spiral cutters in our shop. Our shop cuts 85% solid surface material 1/2" thick single pass. We are using a 1/4" spiral bit. We seem to work fine in single pass with that. Bit breakage is minimal to the amount of cutting. I'd have to get time to do a feed rate check to give a cut rate. With 3/4" mdf or melamine we are as fast at that video you posted.

    As to reliability I can't add more that what you have posted. I agree with you on what you said although part 2 I am cutting full depth.

    I'd like to see what others are doing and how they use their CNC's. It's nice to know I not the only one here.

    Here's a video of our machine. Not ours but the same machine cutting.

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