1/4" HSS bit discolored after "long" MDF job

Hey guys,

Last weekend I noticed my 1/4" HSS 2 flute upcut spiral bit ended with its tip discolored after a long (for me at least) 2.3 hour session cutting MDF at an average of 50 IPM, .060" DOC and default 40% stepover.

Here is a picture of the “burnt” 1/4" bit:

Picture of the completed 2.3 hr long MDF job (5 of the center cylinders were removed by me):

Now the bit was not brand new when I started this job. It had already successfully cut Oak and Pine wood for me for a total of 3.9 hours without trouble. However, it showed no signs of discoloration until this MDF carve came along.

One thing I know doesn’t help the situation is the fact that I don’t have a dust boot and have an air diverter installed in the Dewalt to help minimize the spread of dust. Today I will be buying the smaller dust collector from Harbor Freight to continuously clear/suck in the chips and provide some air flow to the bit while cutting.

So is a total of 6.2 hours the typical life expectancy of a HSS bit?
Is this how you know when your bit is officially “done”?
Is a carbide bit a better choice for MDF?

Your observations and feedback are welcome.


in composite woods never use HSS tools only carbide or diamond tooling

also yeah once the bits turn black like that they are toast and will never hold a edge again

just try and remember

Natural Wood= HSS OR CARBIDE

Composite woods= Carbide or diamond

also speed up your feedrate you are feeding to slow I would start at 80 inch/min and .125" depth of cut in the mdf 16,000 rpm on the router as make sure to ramp into the material but that job should take 30 min max on the x-carve

you will have great luck with a Onsrud 57-910 tool try and get one of those and you will love it

here is the onsrud catalog make sure and check out the chip load charts in the back
OC-12-ProductionCuttingToolCatalog.pdf (2.8 MB)

also here is a little guide made by onsrud thats worth a look
LMT-Onsrud-CNC-Prod-Routing-Guide.pdf (1.1 MB)


Thanks for that info… I never knew that either.

What speed setting was the Dewalt running at?

over the past year I have tried both whiteside and onsurd with great results. Also have some yonico bits that have worked really well. Haven’t checked out freud though… that will be the next to try I suppose.

not to bad plus onsrud has great support and resources for all of there tools I can only find 1 pdf for freud tools explaining chip load

Wow thank you all for the excellent info! Learning as I go here…

I have some more MDF projects planned, so I will try a faster feedrate and deeper depth of cut with one of the recommended carbide cutters next to see how things go. I’m glad to hear this job should have gone way faster!

I also just picked up my 13 gallon dust collector from Harbor Freight… need to get that up and running next and eventually couple it to a dust boot.

Thanks for the links to those guides WorkinWoods. They seem to be very well worth the read. Will do so as soon as I can. I promise you that I will never forget that HSS is no good with composite woods. Carbide it is.


Allen, I was running it in setting 1… so about 16,000 rpm.



are you in the usa if so what state are you in?

Your RPM is as good as can be with the Dewalt, you should increase your DOC to .12 and increase feedrate to at least 65 or 70. If you machine is rigid enough, ideally you would like to get your feedrate up to 80 or 90 ipm in MDF.

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Yes in the USA. Living in central Florida.


oh well you to with the warm weather lol

okay so yeah give onsrud a call @1800-234-1560 and see if there is a representative in your area you can get in contact with him/her and they are a wealth of info plus a really good person to build a relationship with

I was born and raised along the northern coast of Colombia (South America), so yes I love and NEED the warm weather. I always say Jacksonville, Fl is as far “north” as I am willing to move to… lol!

Will do so. I’ve read and heard good things about Onsrud.


True true all true

Allen, so far I’ve stiffened up the Y and X axes as best I could. It is way stiffer than it was when stock. I do have the X-Carve upgrade kit on order with the X-Controller and 1-piece makerslide.

.12" doc and 80 to 90 IPM sounds like a good goal to hit with MDF. I am willing to stiffen up the Z-axis too if need be.

Have you had to stiffen your Z to achieve these speeds and feeds with MDF?


I had the 500x500 Xcarve so it did not require as much enhancement on the X axis. I was able to cut MDF at 90ipm with no trouble using a 1/4 inch tool. I even accidently ran at 140 ipm in MDF with no trouble. As long as your DOC is around .12 you should not have any trouble.

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recently ran test with a 1/4" end mill (think it was whiteside)

doc was .14
speed was 105ipm
setting on dewalt was 2 or 3 I believe.

ran well overall. think I might start running roughing passes at that speed with finishing at around 85-95ipm.

my xcarve only has a simple mod to the x rail. just bolts going in between that are tightened. In the near future, I think i’ll get the single piece rail.

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@Chachin nice stiffening brackets you have on your X-carve,what are they?

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Thank you Christoph. I’m assuming you are referring to the black pieces stiffening the Y-axis. If so, they are 3D printed prototype stiffeners I designed and made in order to eliminate most of the Y-axis flex that I could easily induce with my hands in the stock configuration. I did this and the X-axis stiffening mod so that I would feel more comfortable milling aluminum for the first time with this machine.

Each bracket consists of an upper and lower piece that fasten to each other. The upper piece bolts to the Y MakerSlide and the lower piece bolts to the aluminum extrusion that supports the waste board at the bottom of the machine. Both upper and lower pieces have slotted holes which allow the mating parts to “self-align” during installation without inducing (if you are careful) any vertical or side loads that would otherwise slightly bow or bend the MakerSlide.

I made the brackets this way only because I installed them after I had already assembled and squared my machine. So this design allows the bottom frame of the machine to drive the position of the brackets, not the other way around.

The next step is to mill these out of a single piece of aluminum and let the brackets drive the position of the bottom frame (as I believe is more commonly done). But the plastic prototypes are working so well with my conservative cuts that I haven’t bothered with that yet.

I hope this makes sense.


Y-Stiffening Hardware (I had to double up the upper washers):

Close-up of installed hardware:

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I just cleaned up the burnt carbon crust from a bit that I (obviously) ran too slow. After soaking in solution and scrubbing with a Dremel tool I didn’t noticing any discoloration. I haven’t had chance to test to see if it will hold an edge yet, but am hopeful.