I’m not sure where to post this, so hopefully here is appropriate.
I’m setting up to make guitar bodies, but that means I need a cutting depth of almost 2". To get there, I bought a 4" x 1/4" spiral upcut bit with a 2" cutting depth. I used it to successfully make a test guitar body from MDF. I used a feed rate of 800mm/min, a depth of cut of 4mm, a plunge rate of 180mm/in and a spindle speed on my Makita 700 of ~3.25, or so.
I now want to try cutting some scrap mahogany but I’m a little apprehensive about how to set the speeds with such a long bit. I’m assuming one needs to be more careful than with a more typical shorter bit. I’m hoping there are a few guitar builders here with experience that can offer some advice, or anyone that works with long bits and thick slabs of hardwood to get me started in the right direction.
I almost exclusively run “long” bits, with a 1"-1.25" flute cutting length. This is because I am cutting hardwoods that are 1in thickness, and I am doing a part cutout. I think the combo of long bit and hardwoods is a very difficult one, but also very achievable. For slot cutting I try and stick to 500mm/min (roughly) and have had decent success. One thing I have considered doing is using a shorter bit, like .5in, as deep as it can go first, and then switching to a longer bit for the final half. Not sure if the time saved would be worth it with the tool change, however. I have both 1/8in and 1/4in and the 1/4in is noticeably studier and cuts cleaner than the 1/8in, probably because it is not flexing as much.
I will be interested to hear other responses to this to see if there is a better method.
if you are using 1/4" bits you are good to go, as i said 25mm+ 1/8" bits flex too much (ive never cuted hardwood but my guess is that it is harder than mdf ) if you have some off cuts you can try to find the limits for your bits by dialing up your speeds with the override section tab, first limit your rapids by firmware to something safe ( i use a hard limit of 3000 mm/min for rapids in my machines for x/y and 480mm/min for z ) that way you are only upping the cut speeds.
I normally only go about 2-3mm, which is a very small amount. I have never been able to get decent cuts without the machine slowing down or the bit struggling at the normal 1*diameter (6.35mm). I would really like to, but unfortunately I have found that small depths of cut work best for me. I recently purchased a shorter 1/4in bit, and I plan on testing that out to see how big of a difference the tool stick out makes. I am hopeful I will be able to get around 5-6mm DOC, but if I am still struggling then there must be another variable slowing me down, because I have seen/heard of several other people on here cutting at those feeds/speeds with relative ease. My only other guess as to why its a struggle is just the material, and I have been cutting hard maple, white oak, and generally tougher hardwoods than your standard MDF/plywood.
I cut a pickup cavity in a piece of scrap cherry as a test for my first attempt with a hardwood and it went very well. The bit was completely cool as soon as it was finished with no signs of burning or slowing during the cut. The only thing I noticed was a few splinters chipped out, but nothing that affected the final cut. Nonetheless, I was using a 45% stepover and I think I will try reducing that just a bit, maybe to 40%. I think mahogany is more splinter prone, especially near end cuts, and I want to be conservative on my first full mahogany body attempt.
Overall, the test with this 4" long bit at 800mm/min and a 4mm DOC ran very smoothly and I’m tempted to try again at either 900mm/min and/or a 5 or 6mm DOC.
I should have added the scrap cherry was 1.75" thick and I cut pockets a little over 1" deep. I don’t know if the results I got are good or not compared to what is achievable, but they are good enough for the few guitars I plan to make - I’m not a production shop. I only want to go a little faster during a test cut to ensure I’m not running the machine at its limit at 800mm/min and 4mm DOC.
I ran the Maktia 701 at around 3.5 on the speed setting. The manual indicates 3=17K rpm and 4=22K rpm, so I was around 18-19K rpm.
Here a link to another CNC forum thread where someone measured and posted the speeds at various settings with a tachometer. I found that thread after the fact, but it confirms what I think I’m running mine at.
Also, if anyone is interested, the Z limits on my LR4 meant I had to raise the Makita in its clamps about 4mm in order for the 4" long bit to clear the 1.75" work piece with <1mm clearance.
Larry i know this isn´t the nicest thing you want to hear-read: each machine is different (it will deflect more depending of the size you built your machine, materials used, what you are cutting and what bits you are using (brand, lengths, shank diameter, #of flutes etc)
you should make your own table for feeds and speeds based on what materials you are using -with scraps- and annotate your maximums and dial back a little just to be perfectly safe. you wont regret the time spent. i only cut HDF/mdf (the green stuff), extruded PVC and acm/acp/dibond using 1.5mm bits and 1/8" and flat tip vbits for acm 90-135 degrees bends. my numbers certenly wont work for you
Hey Larry, have you cut out any guitar bodies yet? I’m finishing up my LR4 this week and one of my first projects will be a mini guitar for my kid. So I’m interested in hearing more of your experience.
Not yet, other than the MDF test piece. I did test cut a PUP pocket in a piece of cherry to see how that would go, and it worked fine. I plan to test cut some full thickness mahogany scrap pieces this weekend to see how that cuts. If it goes well I should be able to cut a full body next week.