Depth of Cut and Speed Questions

Hi All,

First off, A big thank you to all the contributors on this forum! The experience and troubleshooting help from here has been a HUGE help with getting started.

I am new to CNC so I have a few questions…

  1. With depth of cut being .08 (3/4" Red Oak / Dewalt 611 / 1/8" Down-cut Bit) - What speed should my Feed Rate be set at?

  2. For Dewalt 611 users… What speed would you set your router at using above info?

  3. When routing should I be removing a lot of saw dust of larger pieces (1/8" bit)?

  4. When pocketing a large area, what is a good Step Over % (Again 1/8" bit)?

I’m sure I’ll have more questions as I go but any info for a “Newbie” would be greatly appreciated!

Thanks Again!
Morgan

Between the variety of modifications, V-wheel tightness, belt tension, and everything else, each machine is different. Also, in my experience red oak is a royal pain, so it’s hard to give a good speed estimate. I’d recommend you try doing a feed/speed test with the wood and the bit and find out what works best for you. You can find a good test design at http://precisebits.com/tutorials/calibrating_feeds_n_speeds.htm, just modify as needed. Keep in mind that most of the community members will recommend not cutting deeper than half bit diameter in a single pass.

When I do my feed/speed testing, I start at 1/2 bit diameter cut depth and 60% of the calculated recommended speed. Then I’ll cut a line (or box .001" thick and 4" long) and examine the chips for .003" thickness. I’ll also examine the sides for chatter, burning, or other problems. From there, I’ll drop the cut depth by 10% if there’s chatter, especially for straight flute bits. I’ll also adjust speed up and down in small increments until I go from “good” to “not so good”. It may take a dozen passes, and you should do this with each bit/material combination, but once you’ve done it once for a bit/material combo you can write down the numbers you used and use them again.

As for your questions, let me see if I can help:

  1. Do a feed/speed test and find a feed/speed that gives you the cut quality you like.

  2. In general, it’s best to keep your 611 set to 1 or 2. The flexibility of the machine limits your feed and speed, and the RPM’s at higher settings are high enough that you will likely end up either breaking a bit or burning the wood (and ruining the bit from heat). Also, the brushes in the 611 last a lot longer at low speeds. Unless a specific material calls for it, there’s not much need to run it fast.

  3. If you are removing sawdust, then you are not moving fast enough (or your RPM’s are too high). Sawdust doesn’t remove heat, which is what dulls and ruins bits. You should aim for chips. My general goal is .003" chip thickness, which I get through doing feed/speed testing.

  4. Stepover should generally be 40% or less, unless you plan to go super slow (and heat up your bit). There’s something called radial chip thinning, lots of detail about it elsewhere, but it basically says that if you’re cutting less than half bit diameter, you’re not cutting chips thick enough to remove heat AND you’re bit is likely to deflect toward the inside because it rides along the wood instead of cutting it. Too much deflection will cause the bit to snap.

***Edited to change “deviation” to “deflection”

3 Likes

Thank You! Great reply and very helpful.

On a side note… Your videos have been EXTREMELY helpful. I watched everyone before I even turned on my machine. I am having some issues with pocketing but I think I know the cause (level and square).

Thanks Again!
Morgan

Thank you for the kind words! I’m always glad to know that they’re helping someone. :smile:

Thanks Phil!

I was close… I am running at 55ipm but I think the speed on my router in way too high. I have it turned all the way up for everything I router (Dewalt 611 / set at 6). lol

When I pocket, I am not getting “smooth” pockets. I am getting light steps in the tool-path. Will a slower speed, plunge rate and router setting help with this?

My waste board is square and level along with my gantry and router. It only seems to happen around “curved” areas (ie - pocketing around a letter “S” or “O”)

Thanks again… You have also been a big help on this forum.

Morgan

Speaking of a non-square Z-Axis…

Using Easel to move the Z-Axis up and down, I find that it’s moving just fine on the bottom half of the Acme rod. I get a grinding near the top. I’m assuming it’s not square and therefore the motor is bogging down?

hmm, ok, will have to have look at that.