Burning out 3/8" bits

[lcolburn]

Hi all,

I'm a fairly new ShopBotter, and working out the kinks from my process, as I've been burning out lots of bits.

I was cutting 3/4" ACX plywood last week with an Onsrud 3/8" downcut bit, 14000 rpm, 4 ips, .4" per pass climb with a .03 allowance, and cleaning up with a .8" deep conventional pass. At this speed the bit was screaming- I burned up two of them pretty quickly. All the while, the bits were leaving fairly rough edges that required sanding.

I slowed down to 1.7 ips, and got better results, but I'm stumped as I see lots of folks cutting much faster. I've got a 48x48 PRSAlpha w/spindle, no vacuum, so I've been using screws and tabs. The only thing I can think of is that the plywood is flexing and vibrating at the center of the sheet, causing extra wear. Should I lay out screws in the center of the sheet, or do I just need a vacuum to cut that fast? Any tips would be great as I have the potential for a large job, and it really won't be feasible unless I can cut faster and get the bits to last longer. Thanks!

[dlcw]

Lewis,

Slow down your spindle speed to about 10000-11000 RPM. The screaming is the bit wanting more material to eat. I normally run a 3/8" bit through plywood at 5" ips at 10000 rpm and have not blued a bit yet.

As far as the rough edges, create a profile path that first cuts in a climb cut leaving about 1/32" of material. Create a second profile path that cuts conventional the last 1/32". What this does is removes most the material on the first pass with the bit being pushed away from the cut line. The final pass removes very little material and cuts right to the line and cleans up the cut.

Hope this helps.

[gene]

Like don said , a screaming bit is yelling FEED ME , go faster . Cutting @.4 deep at 14000 you should be pushing 8 ips . When a bit screams feed it. I cut with a 1/4 " bit 2 flute comp in mdf @6 to 7 ips @12500 full depth 3/4

[thewoodcrafter]

I was going to say the same thing as Gene.

Double your feed rate at least. Maybe even to 10IPS.
ACX ply is pretty soft.

Look at the cut chips (dust) at your speed. Then double your feed and look at them again.
You want the larger chips. These larger chips carry the heat away from the bit.

[lcolburn]

Thanks for all the help- I did a test and the 8 ips speed is great for the bit- produced big chips just as described. I'll just have to add a few more screws to the hold-down pattern as warps in the plywood cause deflection and chatter at those speeds.