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seeking advice on milling 3/4 in plywood
hogman:
Hi fellas,
I am new to cnc. I run a woodshop that makes a chair. I sell chairs at art shows. Each chair has 70 pieces of 3/4 ply stack laminated. I did this by hand and to sculpt curves I started with a 5 hp grinder using 16 grit and moved up to the finish.
My objective is to cut out 90 % of prefinish grinding and sanding out. I have cut very nice parts to date.
I made my machine. I just upgraded Z for more rigidity. Table is 48 by 48. spindle is 3.5 hp.
My issue is RPM of spindle. Calculators tell me that for ply using a 2 flute end mill speed is 3000 for 100 in per minute feed.
My machine chokes on this. I run 17500 and cuts are good without burn. But this bothers me due to discrepancy. I use fusion 360 cam and adaptive clearing does not give me option to cut in small steps down.
I use 5/16 and 1/2 inch ball end mill. I dont need speed of feed. I can go slower if advised to. Please tell me how to do this if you will.
thank you, JR
philf:
Hi Hogman,
For metal cutting I use FSWizard ( https://zero-divide.net/?page=fswizard )to calculate feeds & speeds. I have no experience in cutting wood on a CNC.
Selecting wood (no plywood option in the free version) with a 5/16" ball ended cutter (carbide) and 0.5" depth of cut it suggests 10,039 rpm and a feed rate of 178 in/min. (1.71 HP). With 0.25 DOC it recommends 8,816 rpm with 163.2 in/min (1.28 HP)
For a 1/2" cutter and 0.5" DOC the spindle speed reduces to 5,596 rpm and the feed recommended is 109 in/min. (Spindle power 2.61 HP).
These figures are for taking full width cuts.
What's the maximum speed of your spindle? If there aren't pulleys to give you different speed rates if you're running a 20,000 rpm spindle at 5,000 rpm you may find your spindle is only capable of less than 1HP.
Phil.
Spurry:
Generally, when machining wood, the governing factor is holding the stuff. Without knowing how you are holding your material to the bed it's difficult to advise. Usually I just depend on the vacuum pump, so with a carbide 2 flute 6mm cutter I would not cut more than 3mm deep per pass. Revs would be circa 22,000 - 24000, and max speed 4m/min. The machine could cut deeper per pass, but that would mean taking extra time/effort to secure it to the bed first, so I take the easy way out.
If your software cannot step-down per pass, a possible solution could be to duplicate the contours with each consecutive contour to an increased depth. For a good edge at 18mm, I would step down in 3mm intervals for 17mm but with increased offset (0.2mm - 0.5mm.) Final cut would be to 18mm deep at the correct offset. I never use tabs either, as they seem to be detected in the final passes.
hogman:
Thanks for the info fellas.
My spindle is 220 3.5 HP 24000rpm. My table is 1500 mm by 1500mm and I do have issues securing a 48 inch board. I clamp edges, keeping operations into a 47 by 47 box. I have tried a vac pump. I build a small box with hose to pump. this box holds well if I just put it on wool yet when I try to get it thru 3/4 in mdf waste to ply does not hold.
I have learned alot in past 9 months. One is the build of CNC. This one runs on wheels. And I see the inherent deflection. I have put Z on rails and it is much better. Yet it is reliant on X which is still on wheels.
I cut 9/64 holes an inch apart on mdf. I am thinking of one larger hole in middle of MDF to secure center of ply. That would be hugely helpful.
JR
hogman:
Hi Spurry, can you please explain what offset is and how to use it? JR
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