Feed / Speed / Chipload confusion

[sagreen83]

I am a bit confused at what the chipload calculator is telling me...

I am cutting bamboo plywood. The supplier tells me that you want to setup for the same chiploads as solid oak. I am cutting at 1.9 IPS, full depth pass, at 15500 rpm, 4hp HSD spindle, 3/8" downcut 2 flute cutter. I am not happy with the edge quality. I hear very noticeable oscillations in the machine (like it is pushing way too hard). The edge quality is not what I would like to see.

Ok, so I figure that my chipload is wrong. I check Onsrud's site and it is calling for a chipload of .02 per flute. Using the shopbot chipload calculator If I enter 1.9 ips, and .015 for chipload (-25% for full depth cut) The calculator tells me I should be running 3800 RPM.

Seems to me that the recommended speed is way too low, and my cut quality is going to get worse. I would expect to raise the RPM to make the work easier on the machine. Am I wrong? at 15500 it sounds like the machine / spindle is going to fly apart...

Any recomendations would be greatly appreciated!

Scott...

[Brady Watson]

Scott,
My suggestion would be to use the chipload calculator as a guide...not as the last word on feeds/speeds. It boils down to edge quality. So, you can reasonably turn up the RPM and slow down the move speed to say, 1.2 IPS and see how that goes. If there is no burning of the material, then you are OK in that respect.

As far as edge quality goes, some woods really benefit from Climb Milling. That is to say, instead of going around the part in a counter-clockwise direction, the Bot moves the router around it in a clockwise motion, and the clockwise-spinning bit 'climbs' on the surface of the edge reducing tear-out. (Climb = CW on outside profiles & CCW on inside profiles) Give this a try and see if it helps your cut quality. Conventional chips tend to look like flat square flakes, and Climb chips look like long strings (most of the time).

A few test cuts using different speeds should get you where you need to be. If the material is burning, increase move speed or reduce RPM. If the tool is bogging down and giving a coarse finish, increase RPM or reduce move speed. For bamboo ply (bamboo is a grass by the way, not wood), I would try starting out at 17,000 RPM with a move speed of 1.5 IPS. Put your safety glasses on and get in there where the cutting is happening and really look at what is going on. See, smell, listen. The tool will tell you what it wants. Start out with multiple step-downs, as full-depth passes tend to complicate dialing in a good baseline chipload. If you want to monitor your actual chipload, just do some cutting and let the router rooster-tail some chips into the palm of your hand (or pick a few off of the table). Then take a digital caliper and measure the thickness of the chip(s) you collected. This is your actual chipload.

-B

[knight_toolworks]

I think it is the bamboo. I never get as smooth a cut as I do with other ply's and solid wood. I always get larger oscillations then any other wood. the last I cut at 1ips or 1.5 and still had that. purpleheart cuts better. I think it is the nature of the wood well grass with the really hard ends and the different faces. but it sure cuts clean though. wonder if a single or double flute straight bit would work better?

[sagreen83]

Brady,

Thanks for your comments. I ran a pass at 1.4 ips, 12k rpm, Climb Cut, and like the quaility a lot better. Just to see, I ran a pass at 6ips, and wow! I was starting to see bit deflection with a 3/8" bit... first time I have ever seen this in a 3/8" bit. I forgot to measure the thickness of the chips, so I will have to do that next time.

Scott...

[seana]

Scott,

I have been cutting a lot of bamboo and for being a grass it does play with your head on feeds and speeds.

I have an older prt so my max speed is 1.9 ips and i only have a router not a spindle.
I'm using a .25 compression.

I cut in two passes.
First pass offset of .04 to a depth of .6
finish pass on profile to full depth. I'm getting a smooth edge with no tool marks.
Dados and pockets i have to use a smaller step down so to eliminate the bit defection.
I cut .5 material the same way, in two passes

I have found that using compressed air blowing on the cut to get the chips out along with dust collection helps in the cut and the prolonging of the bit.
There is a lot of silica in bamboo and it will eat up the bits.

Hope this helps.

Sean