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IndustryArena Forum > Tools / Tooling Technology > CNC Tooling > MDF cutting bit that can accept low feed rates
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  1. #1
    Join Date
    May 2008
    Posts
    18

    MDF cutting bit that can accept low feed rates

    Hi there,

    Pretty new in the CNC world I just finished building my first CNC router and made some test cuts.
    My machine is kinda similar to Grunblau's Platform CNC except is has rack&pinion on the X axis and it's bit larger (82" x 24" machining area). I use an Hitachi M12VC router on it.
    Here's my questioning : I'm the middle of a small project that involves cutting MDF sheets (5/8" & 3/4"), I bought a 1/4" MDF bit from Amana (#46170) but I'm really not sure it's the best choice for my setup. Let me explain.
    My machine is only driven by NEMA 23 low inductance 320 Oz-in steppers, I rapid at 300in/min on X (dual R&P) and 200in/min on Y and Z (ACME screws). Since my first cuts I only feel comfortable cutting at feed rates around 60in/min even though I successfully made some cuts at 100in/min and could probably go a bit faster on straight cuts. My concern is the rigidity of the machine, with faster feed rates I'm worried the machine will "deform" when changing direction... but I have no proof to support that gut feeling.
    For the first batch of MDF parts I cut, my setup was : Hitachi router at position 5, DOC 0.1", feed rate 60in/min (tried the last cut boosting the feed rate to almost 100in/min, no problem but it was a straight cut)... lots of passes, lots of time but quality was good. Dust collection was also pretty damn good and the parts once cut where clean and almost ready to use.
    Yesterday, wanted to speed thing up a bit for the 2nd set of part, setup was : Router at position 5+, DOC .25", feedrate 60in/min, same AMANA 1/4" MDF router bit. The cut felt very "rough" and one thing that changed was the cut stayed full of MDF dust + my dust collection system was having a hard time keeping dust under control, I had quite some MDF dust airborne in the shop... also the edges of the cut were not as clean as the other parts. Dimensionally the parts looked OK though (still need to clean the edges and check...)

    So here are my questions :
    The bit I use is suggested to run @20000 RPM, which is close to what my router speed should be at position 5. Problem is at that speed, to get an acceptable clip load I should be feeding waaaays faster than what I'm able to do.
    Are there any bits (MDF specific or not) that would be more suitable for slow feed rates ?
    Am I asking too much to my machine when cutting .25" per pass with a 1/4" tool ? Maybe I should just go with higher feed rate but keep shallow passes..
    Is the fact that the cut stays full of dust/chips an indication of something ?

    For sure I'd prefer cutting slow in 1 pass (less cutting marks on the edges) but I guess the bits won't allow that and will just burn...

    Any suggestion /advice / anything will be much appreciated !

    I Hope I made myself clear with those explanations, English is not my first language...

    Thanks !
    Ben

  2. #2
    Join Date
    Mar 2003
    Posts
    35538
    Well, if the machine is flexing, then yes, you're asking too much of it.
    I'd try using the tool you have now at the same 60ipm, but as slow as the router will go. (8000rpm?)
    But keep in mind that the lower rpm will increase chipload, which puts more forces on the machine, which can lead to flexing again.

    You can try a single flute bit, which should reduce heating.

    An alternative would be to take shallower passes at higher speeds. However, this will wear the tip a bit faster.

    If you want to contain the dust, you need a good dust shoe, with brushes that seal down to the table to prevent the dust from escaping.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

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    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  3. #3
    Join Date
    May 2008
    Posts
    18
    Well I have a KentCNC style dust shoe + the exhaust deflector for my router and it seems to be working OK, I just believe yesterday I was producing too much dust for my small dust collector to keep up.
    I will try to reduce the RPM, increase the feed rate as well as taking a bit less of material with each pass (I've read somewhere about 1/2 the diameter of the tool as a rule of thumb, but don't know if this info is reliable.. many "professional" shops can cut through 3/4" MDF in 1 pass with a 1/4" bit)
    I think I really need to take some time to make some deflection / accuracy tests at different speeds on my machine too, you never know and maybe my gut feeling is wrong and the machine is perfectly able to handle 150in/min, which would make a big difference too ! After-all, even when I increased the speed on some parts I didn't noticed any deflection or difference in the shape being cut... on straight cuts I'm pretty sure my machine is OK, it's for the quick direction change I'm worried but maybe I could tweak that in Mach3 to limit the acceleration/deceleration.

    Thanks a lot for your answer Gerry !

  4. #4
    Join Date
    May 2007
    Posts
    35
    Try using a 1/4 2 flute spiral upcut. 1/8 pass. 80ipm. That seemed to work well for me on 3/4 mdf. Router was running at10,000 rpm.
    Any straight bit I used left too much dust in the cut and started burning regardless of DOC or feed.
    The spiral upcut even worked well on a manual mill at 3000 rpm and 20 ipm.

    Sent from my SAMSUNG-SGH-I717 using Tapatalk 4

  5. #5
    Join Date
    May 2008
    Posts
    18
    I have a 1/4" 1 flute upcut on hand, I'll give it a try too ! thanks !
    Running the router at such low speed (8000-10000) was not the first idea that came to mind but I'll try ! : the noise changes when I cut at lower RPM and my confidence level lowers with it

  6. #6
    Join Date
    May 2008
    Posts
    18
    Well, it looks like my machine can handle .125" passes in MDF @150IPM with no problem (I put the router speed at approx 16000 RPM). Cuts are not perfect ("finish-wise") but I made some tests to compare the same operation (cutting a square) @60IMP VS @150IPM -» same thing !

    On my "square" test I found something interesting : I'm doing multipasses so I have cutter marks from the successive passes, what's weird is I have those marks on the "inside" square (the square cut, inside the toolpath) almost all around, but the leftover (the square outside the toolpath) shows no tool marks !
    Also I made a little test : I cut the lines of a small drawing I had on hand (engraving operation in CamBam) : (2) .125" passes at various feedrates. In all cases I had discrepancies between the first and the second pass on some curves, sometimes as much a 1mm of deference ! It's not like I'm going too fast and the machine is losing steps as if I run the g-code a 3 or 4 times in a row it stays exactly in the previous cut. I checked for some play in the machine, everything looks solid, the only thing that was not 100% solid is the coupling between my stepper and my ACME screw on Y, it's a "coil style" coupling I bought from CNCrouterparts (it looks like it's not for sale on their website anymore) and I can totally see having some backlash from that... maybe switching to an Oldham coupling would help ! While I'm writing I just thought I should try to cut this drawing super slow and see how it goes...

    Anyway, here's where I'm at !

    Anyone has some kind of procedure or a g-code test routine to check the accuracy/repeatablity/stiffness of a machine ?

    Thanks for all the help !
    Ben

  7. #7
    Join Date
    Mar 2003
    Posts
    35538
    On my "square" test I found something interesting : I'm doing multipasses so I have cutter marks from the successive passes, what's weird is I have those marks on the "inside" square (the square cut, inside the toolpath) almost all around, but the leftover (the square outside the toolpath) shows no tool marks !
    Try cutting in the opposite direction, and see if they switch to the other side.
    If you're talking about horizontal lines at the bottom of the bit, and they go all the way around, then my guess is that the spindle is flexing in it's mount, or the entire Z is flexing.


    Also I made a little test : I cut the lines of a small drawing I had on hand (engraving operation in CamBam) : (2) .125" passes at various feedrates. In all cases I had discrepancies between the first and the second pass on some curves, sometimes as much a 1mm of deference !
    Can you post a pic of what you mean?

    I checked for some play in the machine, everything looks solid, the only thing that was not 100% solid is the coupling between my stepper and my ACME screw on Y, it's a "coil style" coupling I bought from CNCrouterparts
    If the coupler is supporting your leadscrew, that's very bad. The coupler will fail, probably sooner than later. Your leadscrew should be mounted with fixed thrust bearings to keep it from moving.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  8. #8
    Join Date
    May 2008
    Posts
    18
    The leadscrew is not moving side to side, I do have thrust bearings on both sides of it. What I meant is I can slighty turn the leadscrew (a few degrees) even if the stepper is powered (= not going anywhere unless Mach3 says so).

    I will try to cut the square test thingy in the opposite direction tomorrow and will see what it does.

    Here're a couple pictures of what I meant with the curves, I cannot take a larger picture because the said drawing was a joke from a friend of mine so decency prevents me from posting it
    As opposed to what I said earlier it was a 3 passes job,
    On this first pics you can see the 2 first passes were the same, but the third one was slightly off.
    Attachment 198428

    On this other part of the cut, the 2nd and 3rd passes are matching but are slightly off from the 1st one
    Attachment 198430

    One thing I remember from when it was cut is the 3 passes were not made in the same direction.. I don't know why CamBam made the g-code that way but that could be a source of mis-match in the cuts if my machine is flexing..

    I'm just back from the shop : I put a micrometer on the table and checked my Z axis, close to the router but not on the router-mount itself nor on the router. when putting pressure on the bit (by hand) I get a deflection of +-0.005" both on X and Y that would mean cutting "pressure" make my Z axis deflect, but not that much... (I put quite some pressure on the bit)

    Thanks

  9. #9
    Join Date
    Jun 2013
    Posts
    1041
    Have you checked to make sure your z axis is aligned correctly. If your not going straight up and down you will get steps like this. Put a precision pin in your router. Then put a indicator on the end of the tool. If it moves while your jogging down then your alignment is off. Check at the front of the pin and the side also.

    Ben

  10. #10
    Join Date
    Jan 2007
    Posts
    1795
    ben

    hundred of people will say hundred of different opinion..

    make testcuts, and don't worry if you have to make more passes.. if it doesn't work of 3 passes, then make it of 4-5-6...

    make sales, and of income you can reinforce your router or go for a ready built better.. it never worth to pushing to limits a machine..
    most important you have already a machine, and it is working.. you can make cuts..

    it is my opinion..


    edit,,,, yes on picture it showing, the different directions makes the steps on different sides..
    make more passes, and make one direction.. example make 3-4-5-6 toolpath, in one direction and merge files together
    or load after each other..

  11. #11
    Join Date
    May 2008
    Posts
    18
    Ben -» Yes I know my tool is not square to the table, that's one main issue with my machine, while designing it I didn't account for all those alignment/squareness adjustments, they're now pretty tough to do. I already took some corrective actions as far as tool squareness goes, it's still not perfect but it's allright for what I do. I don't really mind the cutting marks especially as they're not "deep", a quick and light pass with sand paper takes care of it easily.
    What concerns me a bit more is the mismatch between passes in different directions, but as victorofga said, easy fix is to always go in the same direction ! the difference between 2 tool-path in opposite direction is not that much so I can assume if I go in one direction only, I'm off from the theoretical perfect toolpath, but not from much.
    Victorofga -» You're absolutely right, I have a totally functionnal machine which I'm very proud of since it is my first one and I designed it entirely. It's not perfect but it's working well for what I plan to do with it. I make skis and the machine is gonna be used to cut and shape wood cores, as well as cutting templates for making molds. Before that I was doing it by hand... so even if not perfect, the end result will be ways closer to perfect than it was by hand !

    Thanks again !

  12. #12
    Join Date
    Jan 2007
    Posts
    1795
    difference is the climbing conventional method..

    my concern is use it as long as you get enough resources to get a ready built machine..

    around 4500 you get a 4x4 and bout 5700 you get a 4x8 ... for this cost you can add about 1500 as other fees including custom fee..

    CNC Router XJ1224 47 X94" Milling Engraving Cut Machine | eBay

    so just invest the minimal in your exist machine, because I believe you make more money to making those cores than building routers.. to cutting with cnc an arc or a profile a whole new level compared to cutting by hand..

    on routerbits also can save money..

    yourtoolroom | eBay

    for small bits look for drillman1 on ebay..


    I have an 4x6 feet machine works well and overall cost was 6700 at my door.. including the extra 300 dollar for 8 inches Z

    I just bought spareparts for.. powersupply costed 140, a pci controlcard cost 50 dollar.. this is what affordable..

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