[Analias]

I'm in the process of finally putting a decent work surface on my CRP-4848. I'm cutting 3/4" red oak plywood. I choose red oak thinking it would be more dimensionally stable and would provide more strength for the t-bolts.

Attachment 257180

While cutting the 3/8" slots for the 5/16" t-slot cutter I noticed the feeds and speeds I used from GWizard was causing the end of the cutter to discolor and I was getting more smoke than I was expecting. I was hoping someone with more experience than I could help point out what I did wrong and give some advice for the future.

Attachment 257182

The cutter is a Vertex 2 flute .300" down spiral. It has more stick out in my 2.2KW spindle than I would like, 2.3". I was cutting 3/8" slots with a final depth of 0.52" with a step down of 0.1644". GWizard originally recommended 0.0177" step down at 11941 RPM @ 127.593 IPM with a full .300" cut. I thought that was ridiculous with 30 passes to get to 0.52" depth, especially when I just successfully ran 3 passes at 0.1644" step down on an array of 1" holes with a depth of 0.52".

I tried allowing GWizard to lock the step depth of 0.1644" while cutting the full width of the cutter (0.300") for the initial cut of each step down on the 3/8" slot. GWizard recommended 10456 RPM and a feedrate of 72.713 IPM, but showed the deflection was greater than 0.001". I used these values anyway.

I used Artsoft's Mill Wizard to generate the g-code for the slots. I was surprised to see that Mill Wizard generated g-code that cut the initial slot at full width straight down the middle of the 3/8" slot and then went back and cut the remaining .032" on each side of the slot. Mill Wizard did this for each of the four step down passes. With so little to cut, this strikes me more rubbing the cutter than cutting. I got a lot of smoke when these passes were ran. After the first five or six slots I noticed the cutter was starting to change color. I suspect this is from all the rubbing and I was worried the cutter was dulling. I would expect that cutting once side of the slot at full width of cut and then cutting the remaining .075" in a single pass would be better. Or better yet, make the four step down passes at full width of cut and then do the remaining 0.075" at full depth of cut.

What should I have done? Were my feeds and speeds completely off? If I was seeing smoke, how should I change the feeds and speeds - decrease the spindle speed, or increase the feedrate?

I'm going to be cutting a lot of red oak plywood in the future for kitchen cabinets and tools for the shop (router table and a cart to hold it and the table saw). I'd like to do this right and avoid burning up cutters. Most advice I found online suggested 3/8" or 1/2" down spiral or compression cutters. I'm willing to spend reasonable money on decent cutters, I'm just not sure where to find them. I have had some luck with buying old stock Onsrud 2 flute cutters of of ebay.


-Freeman

[ger21]

causing the end of the cutter to discolor and I was getting more smoke than I was expecting.

I hope that you don't ever expect to see any smoke, because you shouldn't.

I think that there are 2 issues here. Feedrate, and the material. As I mentioned in your other thread, plywood is very abrasive, and hard on cutters. They won't stay sharp for very long. The best way to combat the wear is to increase the feedrate, to minimize heat buildup.

When cutting wood, with bit's larger than 1/8" diameter, you should have no issues cutting at depths/pass of 1x diameter. The limiting factor will be machine rigidity.
When rigidity is an issue, then I'd reduce the depth of cut.

My starting feedrate recommendations would be about 12,000-14,000 rpm and 150-175 ipm.

Be aware that downcut bits will dull even faster that upcut or straight bits. But, they have an advantage due to the downshear action is that even when they start to dull, they'll still give clean cuts. But you can't baby them, you really need to keep the chiploads up.

One other thing to be aware of, is to keep the bits clean. If there's any discoloring or residue, then clean the bit with a blade and bit cleaner. And remember that most likely, the cause of the buildup is a low feedrate.

For reference, on a big industrial machine with a powerful spindle, I cut with 1/4" downcuts at 400 ipm and about 18,000 rpm. With a 3/6" compression spiral, I'll cut 3/4" plywood in one pass at 750ipm and 17,000 rpm.
The bit's can take it, if your machine is up to it. Don't baby them, push them as hard as your machine is capable of.

[ger21]

Forgot.
My preference for slot cutting would be to cut the center first, followed by a cleanup on both sides. The reason is that the tool (and machine) will deflect on the first pass. By cleaning up both sides, you'll get a more accurately sized slot.

Rubbing shouldn't be an issue. Just use conventional cutting.

A good strategy for cutting slots like this would be to cut the center of the slot in 3 passes, back and forth, followed by a full depth profile pass to the finished slot width.