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IndustryArena Forum > MetalWorking Machines > Uncategorised MetalWorking Machines > A bunch of random questions about new CNC mill build
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  1. #1
    Join Date
    Mar 2019
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    13

    A bunch of random questions about new CNC mill build

    I'm building a fixed gantry CNC mill with 40"x18"x12" (X,Y,Z) travel. Budget is around $6500.

    1. Machine rigidity

    Can anyone with a non-DIY 3 axis mill like a tormach or HAAS help me out here please?
    I am wondering how rigid these machines are so I know what I am getting into when building my own. That is how many Newtons of force required per micron of deflection or something along those lines.
    I know people have measured bridgeports by sticking a dial indicator on the spindle and putting it with a bungee cord with force gauge. (they were reporting about 30N/micron)
    Right now my machine frame is sitting at about 60N/micron in all directions (based on Inventor FEA and manual calculations for sanity check). Add in deflection of ballscrews and stuff and it's probably down to 50.

    2. Spindle

    Is it a good idea to use spindle cartridges taken from tormachs and driving it with a DC motor? I want to cut steel at a reasonable rate and those 10000rpm spindle motors on ebay don't seem like the right choice.

    3. How much thrust (pushing) force do I need in each axis?

    I already bought 2 ball screws. The one for X axis can handle 22kN and the one for Z can do 15kN (I know they are huge and overkill but they were pretty cheap and really nice NSK C2 precision ground screws).
    I want to know how strong the motors need to be (I will be using steppers for now). I know most HAAS can do 15kN but I doubt I need that much.

    4. small 5 axis trunnion (future upgrade)

    I know very few DIY cnc mills are 5 axis but just how crazy is this? It looks pretty straightforward to me mechanically but I know there is definitely a lot more to it than I know, otherwise everyone would be doing it. Pretty sure fusion and mach 4 can do 5 axis just fine?

    5. What would it take to cut grade 6 titanium alloy?

    I know titanium and DIY don't usually come in the same sentence, but what would it take to cut it? Probably won't be doing any production runs on this thing but I imagine adaptive toolpath with very light cuts while blasting it coolant can probably make a dent. I mean people were able to make shapeoko cut aluminium.


    Thanks

  2. #2
    Join Date
    Jun 2015
    Posts
    943

    Re: A bunch of random questions about new CNC mill build

    It is easy to machine titanium, titanium has better machinability parameters than for example C45 steel.
    The only problem machining titanium is that it requires a massive vibration free machine and you need to select your feedrates and spindle speed for your tools properly because titaium is hard material and that requires an expensive tool which can quickly break if you plunge too deep or too fast into it.
    If you want to machine titanium then make sure to build a massive machine possibly from cast iron frame.

  3. #3
    Join Date
    Apr 2005
    Posts
    629

    Re: A bunch of random questions about new CNC mill build

    Ummm, titanium???? Seriously???

    Dream on brother, very few home built mills can machine Aluminum.

    Chris D.

  4. #4
    Join Date
    Mar 2019
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    13

    Re: A bunch of random questions about new CNC mill build

    Well that's some pretty contradicting information. What is the biggest difficulty when it comes to machining titanium alloy?

  5. #5
    Join Date
    Jun 2015
    Posts
    943

    Re: A bunch of random questions about new CNC mill build

    Do you even read comments bro? Or you just typing?
    I'm not dreaming bro, I've already manufactured a few titanium stuff including titanium tooth implant screws.
    As I wrote previously you need a very massive and vibration free machine to machine titanium. I worked on a hobby built but robust cast iron frame machine which is similar vibration free as a HAAS or other machining center, but it is still hobby built.

    If you don't beleive me about machineability of titanium then do your research about titanium machining and you will see that what I'm telling you is right.

  6. #6
    Join Date
    Jun 2015
    Posts
    943

    Re: A bunch of random questions about new CNC mill build

    Dfficulty is to select your speeds and feeds properly for your tools and do not do large plunges because your costy tool will quickly break and this is because titanium is really hard.

  7. #7
    Join Date
    Aug 2004
    Posts
    780

    Re: A bunch of random questions about new CNC mill build

    I will Try to help as much as possible.

    Screw thrust or yield is actually right.
    Every decent 5x machine has similar 2-30 kn thrust rigidity on the linears.

    But the rotationals must have 100 Nm+ of rigidity, for 20x20 cm workpiece size.
    Look at haas rotational tables 2x of 200 mm size, circa 30k, circa 300 kg.

    I have qualified, sold, delivered, 5x stuff to 100mm sub work size.
    Needing about 1-300x less rigidity vs 200 mm.

    It is not "hard" to do 5 axis.
    Use high-end planetary gearboxes, or similar zero-backlash stuff like harmonic drives.

  8. #8
    Join Date
    Mar 2019
    Posts
    13

    Re: A bunch of random questions about new CNC mill build

    How about machine rigidity? I am having a really hard time determining what I should aim for. Tormach P440 which weighs 440lbs and seem to cut steel and titanium just fine, but others are saying I need way more rigidity. Do any of you have some actual numbers? I downloaded the CAD model of the P440, simplified it, and ran a couple finite element analysis on the frame only. It seems to be giving me about 30N per micron of deflection when Z axis is halfway up (I would say realistically 20). Can anyone confirm that for me?

  9. #9
    Join Date
    May 2016
    Posts
    316

    Re: A bunch of random questions about new CNC mill build

    You don't need massive rigidity to machine titanium. Rigidity helps, of course, but it helps with everything.

    I can machine Grade 5 Ti no problem on a Tormach 1100-sized mill that has a fairly floppy column. The trick to Ti in a hobby mill is sharp tools, appropriate speeds & feeds, no 'rubbing' and flood coolant (or an aggressive mist.)

    Ti is not actually very hard - even Grade 5. Tool steels, pre-hard 4140, nickel alloys, etc - those materials are all significantly harder, and harder to machine. The problem with Ti is that it work-hardens very easily and is terrible for heating. The cutting heat doesn't get in to the chip so it stays in the part/stock and your tool. Hence the recommendation for aggressive cooling.

    With a dull tool or inappropriate cutter engagement the Ti will flex out of the way and spring back - and now it's work-hardened. A second pass without getting below the hardened surface will go poorly.

    I suspect that most people who claim Ti is miserable, or difficult, or whatever have experience limited to drilling or tapping titanium - or they just 'wing it' with the feeds and speeds. Dull drills and/or overly-light feed rates will work-harden the Ti and then you're trying to drill in to granite. And most non-Ti specific taps have a generous land behind the cutting edges to help smooth the thread surfaces. The titanium flexes out of the way of the cutting edge and then your tap gets grabbed by a work-hardened Ti hole - snap.

    Threads should be made with Ti-specific taps or threadmills. Either one of those will result in beautiful threads.

    Because I don't have tons of rigidity or HP, I've found that uncoated, polished end mills designed for Aluminum are the sharpest ones and they work extremely well in titanium. Lack of coating means they don't last as long, but I'm more about not pushing the limits and buggering a part up.

    I'd suggest not fisting over rigidity numbers, unless that sort of enginerding OCD makes you happy. Steal basic designs from pictures of machines on the internet, and just build something.

    - Try to stick with steel where you can. Aluminum is easy to work with, but steel is much stiffer for a given dimension component. If you are doing a fixed-gantry thing, then you don't care about gantry & carriage weight (think router or high-speed plasma/laser). Build the gantry out of steel box tubing and fill it with concrete, epoxy granite, or even oiled sand.
    - Stick with bolted connections instead of welded where possible.
    - Surface grind any mating connections if you can, and use epoxy as a grout between bolted connections to increase contact surface area for less distortion and more rigitiy.
    - You want stiff? Buy a cheap used surface plate for the machine base. Use a diamond drill to poke holes in it and epoxy threaded sleeves to bolt the various motion components to it.

  10. #10
    Join Date
    Jul 2018
    Posts
    6248

    Re: A bunch of random questions about new CNC mill build

    Hi Henry- 50N/um is a very stiff machine. If you can achieve that in practice then you will be able to do the Ti. Follow the Maximus thread its all about stiffness of machines. There is a link in it to a thesis on machine stiffness that is exceptional information. If you have bolted connections and or bearings in your FE they will not be accurate in terms of deflection of you use a simple FE model. Usually we aim at 10x the required stiffness in the FE to achieve the target stiffness unless you are using FE that includes bolted joints, bearings etc and their friction.

    Steel construction is a good idea but it is not necessary to fill it with stuff. Make it heavy wall and big with lots of triangulation. Only solve vibration issues if they appear.. Cheers Peter s

  11. #11
    Join Date
    Mar 2019
    Posts
    13

    Re: A bunch of random questions about new CNC mill build

    Thanks guys, I've switched my design to a conventional milling machining layout (similar to tormach machines). Work volume reduced to 30"x10"x14" (xyz) and it weighs 1300lbs. Right now my stiffness is about 30N/um when Z is at highest and 90N/um at lowest. Realistically it's likely a lot less, but is probably still way more than enough.
    I'm trying to figure out what spindle I should use and how much torque I need. I'm thinking buying a BT30 spindle cartridge from Tormach and driving it 1:1 with a 2.5kW motor with peak torque of 70Nm at about 500rpm. Most people just went with those 15000rpm engraving spindles and I don't recall seeing any DIY machines using an actual milling spindle. Is this a good idea?

  12. #12
    Join Date
    May 2016
    Posts
    316

    Re: A bunch of random questions about new CNC mill build

    Until Tormach comes out with the 1100MX kitted with a BT30 and an ATC, I'd wait. The BT30 spindles they've been supplying are for threaded drawbars and I don't think they have internal provisions for a gripper and spring stack.

    Aliexpress has a bunch of ready-to-run BT30 spindles available with different RPM ratings - up to 10-12k if I recall.

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