Help with adding new bits and custom feed rates, cutting depths, etc.

  • Advertise with us

« back to CNC Woodworking forum

Forum topic by koorey3 posted 12-08-2017 02:16 PM 1531 views 0 times favorited 3 replies Add to Favorites Watch
View koorey3's profile


1 post in 942 days

12-08-2017 02:16 PM

Topic tags/keywords: custom cutting settings tweaks box core cnc rpm feed rate step over step down

Hello all,

I have 2 questions about tweaking cutting settings, questions I have investigated myself, but just cant seem to get the straight forward answers I am looking for. I am hoping a direct dialog with someone can help.

I apologize if this information is listed elsewhere, but after doing a lot of searching I joined this forum to just ask the source – people who know what they are doing.

  • my experience – I would say I am a competent CNC operator, I know how to use the machine, design, and cut. However, I am basically self taught and have only learned how to do what I need to do. I cut profiles at work and have played with engravings and reliefs. I have added bits before and I am familiar enough with the feed rate calculators and equations to derive my settings for striaght forward bits – 1/4 and 1/2 end mils.

First Question:

custom Feed rates, RPMs, SFM, chip load, etc…

ok so for most of the bits I use I can locate manufacture specs and get the suggested RPMs and SFM and chip load per tooth for the bit I am using relative to the material I am cutting. And if I can find that data ok great, the formulas pan out and I start easy and ramp it up as I see fit. But, I am starting to cut more detailed pieces at work, specifically long 3D rails out of solid pine and MDF and foam. sure the traditional 10% step over for finish cuts and 40% for roughing works fine.. but waiting 2 hours for a cut is getting bothersome when I know I cut through 3/4 plywood in 3 passes with 1/4 and 1/2 bits. I also know that I dont need the detail to come out perfectly smooth I can just sand it flush. We only have one CNC machine and we get backed up in the shop a lot so I want to sacrifice the finish cut for a quicker run time. Now that i want to do this I run back to my formulas and notice… they dont account for cut depth and step over, so I am not really sure how tweak things. I can increase my step over and get a bump in between my passes – which I am ok with – but I am also taking off more material with each pass and that changes with the detail in the shape i am cutting. I can increase my feed rate if i increase my RPMs but now i am also taking more material away with each pass and my equations dont seem to account for that. and where all of this really became a problem is when I ran into my second question and i am hoping by walking me through this issue we can answer my above questions.

Setting up 2in box core bit to cut a 2 – 1/2 diameter by 1in deep ‘cove’ into a finish cut piece of pine.

My problem:

Adding the bit into my database with the correct step down (or cut depth) and step over units. As well as setting the correct feed rates and RPMs.

I am not as comfortable simply plug and chugging and tweaking the setting off the sound as I cut when dealing with a bit this large. normally I am working with 1/4 ball nose and 1/2 str bits.

My main concern is the general rules of thumb seem a bit – stupid – for a bit this size. For instance, a cut depth/ step over half the diameter of the bit used. Ok, sure that holds up for 1/4 and 1/2 bits, but a 2” bit ? I dont think an inch step over would give me what I need and I am not dumb enough to run this thing with an inch step down each pass lol but I cant find what the ‘chip load’ should be for the bit, it has a MAX RPM, but I know with larger bits you need to slow your RPM down so I dont want to touch the max RPM. all of the equations use at least one given variable and then the rest build off of eachother. use chip load to get RPM use RPM to get feed rate etc.. but i dont even know where to start given that at even an 1/8th down I am making a very wide pass clearing a lot of material.

All I want to do is cut this cove as quickly as possible with enough detail so that I can sand it smooth within a good 10-15 minutes. I was going to use a 1/2 str bit to rough it out then do a finish pass with a 1/4 ball nose, but we have this 2in box core bit and logic tells me the larger the radius the larger the step over I can do to achieve this 2-1/2in diameter cove. So I am thinking 1/2 rough and a finish path with the box cove or just using the box core for the rough and the finish. but I do not know how to set up the RPM and the feed rate for that beast.

Sorry for the long winded question. I know I am getting into the more nuances of cutting, but thats my point, by the standard metrics it should take 2 hours to cut some of these arced ‘rails’, but in my head im like I cut 3/4 in plywood in 3 passes, this rough should be much faster and the finish, i dont need the thing to be smooth just get me the shape and I can sand it. granted most people who are using a CNC want the wood to be pretty much done after the cut, but I am looking for basic shape.

its a soft wood, I want to increase step over, rpm and feed rate to reduce cut time and I would like to know how to set up a box cove bit.



3 replies so far

View CherryWood's profile


22 posts in 4011 days

#1 posted 12-09-2017 01:02 AM


There are books written and college classes to cover your question(S) – MANY.

There is SOOOOOO much in what you are describing here. I am a manufacturing engineer near retirement, so I have 40 year of experience doing what you are asking here.

Part of the problem online is – deciphering who “really” knows what they are talking about and who “thinks” they know what they are talking about.

I cannot answer everything you asked about in a forum like this – it is simply too much content.

Just a couple of tid bits.

1) Who ever said you cannot rip out 1” or 2” stepover? That is dependant on the machine, setup, tooling. Of course you can. Not with a Rockler Shark of a hobby level machine, but with an industrial machine with some muscle yes you can.

2) It’s more to do with the machine and the hold down, fixturing.

3) Not really knowing what you are doing – Rough cut step over in MY book is 75% to 90% of cutter. Finish stepover can easily be 90% – BUT – it depends on what you are doing

4) You can go 1” deep with a 1/4 mill – BUT – it depends——————

5) Watch the chips – they tell you all about performance.

6) On ball nose—use the BIGGEST you can fit into the curves and increase your stepover. Use a 2” or 3” if you can. Do a CAD layout so you can see the radius of the cutter and the effect on stepover.

DO NOT be making dust – DO be making chips.

Try to keep the RPM’s down to control heat. Speed is friction – friction is heat. Control the chipload with feedrate. Make chips not dust.

Soo – do some “trying” of different things in scrapwood. Test – experiment – DOCUMENT – try more test more – develop your process. I do that all day every day in my job. That’s how you learn.

View DS's profile


3503 posts in 3192 days

#2 posted 12-11-2017 06:39 PM

Generally speaking, a roughing cutter is used to remove the majority of material, then a profile bit comes in and puts the finish profile on it.

On my setup, I use what I call a hogger bit to do all the heavy material removal.
It stays at least 1/8” away from the final profile.

Also, don’t overlook the value of your other machines to prepare your cnc blanks for processing. A strategically placed dado cut on a table saw can save a lot of time on the cnc machine roughing the blank.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View oldnovice's profile


7558 posts in 4139 days

#3 posted 12-11-2017 09:07 PM

What you want is my youngest son but he is not for sale!
He is, and has been a certified CNC machinist for about 18 years, so when I have a question about speeds and feeds he looks up in the air for a pensive moment , as the answer is obviously there, and answers my questions!
However, he is not available to me at all times so he gave me a Web site from Onsrud that is a good site to calculate chip load but it is not as fast as he is.

When I use my old router bits I just rely on what I did with my hand held router, and, for most wood cutting with router bits this does not require calculations.
However, when cutting plastics, brass, alminum, or composites I rely on the Onsrud site as most of the machining bits I have are from Onrud …... they also make some darn good bits.

The nicest thing about using a CNC is what Doug, post #2 above, referred to as roughing out and finish cutting which makes a world of difference with the end result.

-- "It's fine in practise but it will never work in theory"

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics