[p.melanocephala]

Hi all!

I'm new to the whole CNC thing. I've been building guitars as a hobby for years, then I came across a CNC machine on craigslist for pretty cheap, so I bought it with the hope of building a few guitar bodies with all this new fangled computer technology. It's a Zenbot 1624 -- 16"x24" HDPE construction, unsupported X-axis on gantry, Gecko G540, 2.5 Amp 0.9deg NEMA 23 motors (270 oz) with a toothed pulley system for linear movement, 19v 6A power supply.

It seems to work alright for MDF and most of my shallow hardwood cuts, but things get iffy from there. Whenever I get to a pocket that's more than about 3/4" deep, the bit is prone to digging into the side of the pocket and pulling into the wood. I've tried various depths and speeds (1/2" depth to 1/16" depth; 60IPM down to 10IPM), but it's always the same: the bit digs into the sidewall of the pocket, pulling the spindle down and further into the sidewall of the pocket until I smack the E-Stop or the router jams. I can usually hear when it's about to have a problem as the bit sounds like its bogging down and chattering (There's usually a few wavy scallop marks on the sidewall just before the bit goes for a dive). Also, it seems that curves that use both X and Y axes at the same time is where it really has trouble -- straight cuts along either axis in the deep pockets don't seem to be a bother, but when the X and Y start turning at the same time, that's when things get dicey.

I'm using:

Hitachi M12VC router
2 flute 1/2" diameter, 1.5" LOC, 3.5" OAL spiral endmill
conventional milling path, 3D contour roughing with parallel finish
Gecko G540 controller
Mach3 with supplied .xml on a Windows XP desktop
Zenbot 1624
Clamping via woodscrews into spoilboard, which is bolted to table
Shopvac for dust/chip control


What causes this sort of thing to happen? Is there a workaround or technique for these deep pocket cuts? Am I doing something stupid? Could the machine just not be up for the job? If that's the case, could stronger motors/power supply be added and the gantry stiffened, or is it best to just cut my losses and go for a better system? Thanks for your time!

[Spk64]

All your problems are from a lack of rigidity. Using a 1/2" cutter with this type of machine will make matters worse along with the stick out on the cutter.
I really think you are going to struggle with guitar bodies on this type of machine.
Plastic is at the bottom of the list for materials to build a rigid machine with.


Steve

[ger21]

It's mainly just a case of your machine being too flexible. Changing motors won't make any difference, but anything you can do to make the frame stiffer will help a little.

Try climb cutting your pocket, starting from the center and working outward. If possible, leave a small amount of material and do a climb cut finishing pass on the pocket.
I'd cut about 1/8" depth/pass, with a 1/8-1/4" stepover.

With climb cutting the bit will try to push away from the wood, rather than pull into it. It still may pull into the corners a little, though.

[p.melanocephala]

Thanks for the replies!

That's what I was afraid of. Luckily I didn't pay too much for it -- $600 with a Mach3 license included -- so I guess I could always resell it on the cheap and just buy a more rigid machine.

I'll give climb cutting a try, maybe that'll help!

[sharp-shooter]

You might also try a smaller bit, such as 1/4" down cut spiral 2 flute. It will resist the temptation to "suck up" the wood or dig in.The down cut feature helps eliminate end grain blow out. When you get into deep pockets, it may be necessary to blow them out with compressed air to eliminate chip compacting.

[BuckNaked31]

I'll second what ger21 said, I cut guitar bodies all the time. I have a CNCrouterparts type machine, but the concept is the same. I cut with a 1/2 inch bit .25 inch at a time at 100ipm, but my finish pass is full depth at .020 at 75ipm. Another trick is to cut the pocket with a g64 (constant velocity), and do the finish pass with a g61 (exact stop). This gives you the speed while you are clearing the stock out of the pocket, but the accuracy for the finish pass. Guitar parts have to be spot on. This is easy if you make 2 tool definitions in your cam software. One that limits the tool pass depth for the pocket clearance pass and another that allows it to cut full depth for the finish pass, but limits the speed. I have to add the g64 and g61 by hand, though. I've found that with these settings my machine doesn't care whether it's cutting hard maple or basswood, the finish is spot on. I still have the problem of the router bogging down slightly in the corners of the pocket, but the .020 finish pass minimizes that. I can get away with the slower speed without the cutter heating up during the finish pass because it's barely taking anything off. Hope this helps, it took me a while to figure it out. You may need to play around with your settings a little, these are what is optimal for my machine. Good luck!

Sent from my SCH-R720 using Tapatalk 2

[inventor83]

Using a smaller diameter bit should reduce your warping problems. Maybe try a 1/4 O flute single flute bit. The chattering might be caused by a number of things but your speeds and feeds might be causing the initial chattering and your machine rigidity just amplifies it from there. Rule is never plunge deeper than the bit diameter, there are many exceptions to this but its a good starting point.

[ger21]

Rule is never plunge deeper than the bit diameter, there are many exceptions to this but its a good starting point.

My rule is never plunge ever, unless your drilling.
Always ramp in. It's easier on the tool, the spindle, and the machine.

[inventor83]

My rule is never plunge ever, unless your drilling.
Always ramp in. It's easier on the tool, the spindle, and the machine.

I guess plunge was the wrong word to use here. Cut or machine would have been a better word.

In some scenarios plunging is the only method available. I primarily cut mfd, which is soft enough that plunging strait down is not too hard on the machine. Very small pockets and shapes are sometimes not large enough to perform a ramp into.

[zenbot]

Just thought I'd weigh in. The machine is obviously not a super heavy duty, ultra rigid machine, but the performance is really good if you program within its limits. a 1/2" cutter in hardwood is going to put a lot more force on the machine than a 1/4" bit. what depth of cut are you using? with this machine, take lighter cuts and more of them at higher feed rates to get good material removal. the machine is very precise, so you will get good tolerances on your parts if you take a light finish cut. I have 2 customers who are guitar manufacturers who use our machines, and they like them so much they bought multiple units, so you can get good results if you experiment with bits and programming. feel free to contact me if you need any help. [email protected] or 559-901-8329

[zenbot]

Also, make sure your belts are really tight. loose belts will really reduce rigidity and performance.

[p.melanocephala]

Thanks everyone for your help! After switching to a 1/4" bit, the Zenbot cranks out guitar bodies with no trouble. With this approach, the machine runs just fine at up to 1/8" depth of cut with a feed of 50-60 IPM in softer woods. No idea if that's good or bad as far as these machines go, just what I've found with a bit of experimentation. Also it seems that it can handle a two-flute 1/2" bit if I cut with a climb direction as was suggested and then follow up with a 1/4" finish cutter. Still chatters at deep cuts, but instead of pulling into the wood it just bends outwards slightly and scallops until I can adjust the cut speed or it leaves the problematic area. The finish cut takes care of these problem areas.

As far as the machine's rigidity, I think it's just a matter if knowing its limits and building a cut with that in mind. I've also just finished tightening the timing belts, which were slack in two axes, tightened all the bolts, and now I'm using shorter bits. All in all, it's night and day and mostly the result of pilot error.

Thanks again to everyone for your help! I'll post some images when I'm at my computer.

[zenbot]

Thats great to hear. Im glad your machine is working well for you. If you ever have issues, feel free to contact me. We support our machines whether or not you are the original buyer.