Feed/speed adjust 1/4 to 1/8" bit

I’m cutting polycarbonate but I guess this question relates to other materials, as well.

The recommendations for a 1/4" straight bit are .08" DOC, 18K-RPM, 55 feed and 25 plunge.

Now say I want to use a 1/8" bit instead, where should I start w/ adjustments? Should I increase RPM and keep everything else as-is? And where would you go w/ RPM?

Funny I did this the other day and had no problem cutting (w/ 1/8" bit), but I can’t remember what speed I had the router set to. This evening, I tried the same thing and lost Z steps somewhere and didn’t get all the way through the material after the third profile. Both times I was using a DOC of .125", and feed of 50 and plunge of 15. This evening I also hit the unit with my hand while it was cutting (LOL, don’t ask).

I found this table when looking for some specs on some Freud bits I bought. People say to follow the recommendations of the bit manufacturer. I’d like someone else here to look at the Freud chart and comment on it. http://www.freudtools.com/admin/manuals/SolidCarbideSpeedsinParticleBoard.pdf

My comment would be that recommendations from bit manufacturers are good starting places, but there is a machine-dependent component that they cannot consider.

In the meantime, anyone have suggestions for 1/8" but cutting polycarbonate?

There are four settings for cutting polycarbonate with 1/8" endmills on the wiki:


The last seems the most relevant.

Machining holes: 2-flute 1/8" end mill, 40 IPM, DWP611 speed at 1, depth per pass 0.04", and following up with 2 finishing passes (one climb, one conventional)

I’d like to try working up a way to get from the official 1/4" values to appropriate ones for 1/8", but it’s hard to view them as a useful beginning point when the numbers in them are inconsistent in a way which offends my OCD. I suppose I should accept it given that I’ve pointed out the inconsistencies twice now, and no one else seems to have noticed or been troubled by them.

Or, we should all just buy a copy of G-Wizard from CNC Cookbook.

1 Like

I support you Will, I see the inconsistencies and have been troubled by them also, but I swayed that observation with the fact that I know nothing about proper feeds and speeds. I don’t like the G-wizard option because I have not been able to comprehend using (adjusting) the 5,000 (as it appears in my brain) variables. It is far to intense for my personal disabilities for “me” to be able to make any use of G-wizard. You on the other hand have a tenacity for completeness and accuracy that (as you say) borders on OCDism. That is not a handicap imo but a tool you utilize to everyone’s benefit. I say “Go for it”, too bad I have not the ability to assist (except by trying your recommendations and reporting) your efforts. “Yu khan du et!” (cajun inflection??) Good Luck.

Well it turns out the explanation for my problems with repeating this cut are due to a bad motor.

Funny thing when I first got the machine, I had real difficulty plunging plywood and polycarbonate. Trying to drill holes didn’t work and someone told me drilling is difficult. So I switched to a smaller bit and profiled my holes, figuring that as long as it worked, I was okay with it.

So last night I’m trying to cut the polycarbonate pretty much exactly as I had before, using the same feed/plunge rates and only unsure of my motor speed (trying the DW611 at 3 and all the way up at 6, etc.). And I am just dropping steps like mad.

So this morning I’m trying to adjust the plunge rates, etc., and not getting anywhere, still dropping Z steps like nuts, and then I just start to cut air and see the motor isn’t reliably moving the Z-axis. So I start jogging and see the same thing, it is just getting worse and worse.

So I try to turn the shaft of the Z-axis motor and sure enough, it spins easily (it should be locked, the machine is powered-up. So I unplug the power and swap the X and Z axis connections to the controller board and power back up, and the Z still won’t lock (so I know it isn’t the controller board).

So it looks like my Z problems aren’t over yet.

I sent an E-Mail to support@carbide3d.com, hoping they can sell me a motor.

It gets funnier. I used the troubleshooting guide to check the stepper motor continuity and it seemed fine.

So I took it off the machine and turns out, no set screws holding the pulley.

I’ll go to the hardware store and buy some.


Always use the machine operating checklist:


(for those who are curious, it could be viewed as a laundry list of pretty much every stupid mistake / oversight / neglectful act of carelessness I’ve ever experienced at the machine, save for the fire and spontaneous combustion stuff — that’s just my paranoia — there was one day, when each and every thing which could go wrong, went wrong, but sequentially, so that each time I tried to cut, a different problem arose — finally ran out of daylight and patience…)

Thank you for the checklist. I had gone over the assembly instructions to familiarize myself with the machine but (and I may be wrong) the pulleys come installed. So my suspicion is that the previous owner, with all the Z-axis trouble he was having, removed the set screws to try to figure it out.

It is surprising how well it worked, when you think about it!

I’m printing your checklist, it will go up in front of the machine.

Okay, as a start, I’ve regularized the data from the official feeds and speeds and am beginning to add the calculated values at:


If anyone sees any further typos there, or at https://www.shapeoko.com/wiki/index.php/Materials#Router_Speed_Chart — please let me know. For the router speed chart, the intermediate values are currently just interpolated directly — it would be nice to work up a math function (or chart them?) and interpolate them in some fashion which would better model the actual RPM (or would a couple of folks w/ tachometers be inclined to measure theirs with more detail than we currently have?)

Continuing this at: Tutorial on feeds and speeds

The bits #201 and #202 are 3 flute in Carbide3d store , and that’s what I got when I ordered, maybe they used to be 2 flute. Your wiki chart shows 2 flutes for these. Thanks Will


I pulled that from Carbide Create — going to file a bug report now.