[victorbl]

I'm using my machine to cut a new mount for a router out of 3/4" MDF. I was planning on using a 1/4" carbide end mill that has a cutting length of 3/4". Is it safe to do the cutting of the 3/4" MDF in a single pass as long as my feed speed is slow enough?

I was thinking of doing ~7imp. Should I be concerned?

[ger21]

It will probably burn. With a 1/4" bit, I'd make at least 2 passes, maybe 3. And go as fast as your machine can go. Contrary to what a lot of people think, when cutting wood, if your using at least an 1/8" or 1/4" tool, there's no reason not to cut as fast as your machine can. With smaller tools, you might have to worry about breakage if you go too fast. Going too slow causes excessive heat, and your bits will get dull much faster from the heat. If your machine can cut at 30ipm, make 3 passes at 1/4" and it will get done faster than 1 pass @ 7ipm.

[coherent]

It also depends on your spindle speed, and the specific cutter (composition, style, 2 flute, 4 flute, spiral, etc etc). Most bit/tool manufacturers will provide you with the recommended spindle & cutting speed for your exact bit and type/thickness material if you give them a call. Some have the info on their web pages. If nothing else, find a cutter with the same specifications and get the info for it... you'll be in the right ballpark! We could all take an educated guess, but if you want an answer thats the "most" correct, that's the route I'd take.
-marc

[ger21]

Marc. You're entirely correct. But I think you'll find, as long as your talking about router bits designed for wood, that recommended feed speeds will usually be around the 150-200ipm rangeand up, at about 18,000 rpm. Since few people here can cut that fast, saying "go as fast as you can" should cover most situations.

If anyone wants a good source of information, go to http://www.onsrud.com and download their complete catalog. In the back it has recommended chip loads for all their tools, broken down by tool diameter, and material. They have charts for softwood, hardwood, softwood plywood, hardwood plywood, mdf, partical board, plastics, aluminum, ....

[nda22]

I was cutting some letters out of 3/4" MDF tonight running 48 ipm and 24,000 rpm cutting through in three 1/4" deep passes. I was using a Grizzly C1465 1/4" single flute 3/4" long 1/2" shank router bit. After about 6 linear feet of cutting the bit snapped off. I was wondering if anybody has had any luck using these cheap bits from Grizzly? The bit still felt sharp, it broke off where the carbide is brazed on to the bit.

[nda22]

Just thought I'd post a couple of pictures of the broken bit and my machine.

[dfmiller]

A couple of questions on your bit breakage.
Which pass were you on, first, second or third?
How much crap was in the groove? I don't see any dust extraction system in place.
I have found in my limited experience that using a 1/4" bit the slot clogs up pretty qucikly. Now that I an using a dust hood and a dust collector the groove get clean out much better. Beat my shop vac.
The downside is you can't watch the cutting. :-(
I find watching a machine mesmorizing. :-)

Dave

[louieatienza]

Just thought I'd post a couple of pictures of the broken bit and my machine.

Never used Grizzly bits before, but 24K semms awfully fast. At that speed you're probably just heating up the bit and making MDF powder.

The only other thing I can think of, is that with your long z plate, it's possible you got some vibrations that resonance that caused the z plate to shake around, and the jolt snapping the bit. You might want to make an auxiliary raised table to keep you z as short as possible.... I run 1/16" 1-flute up spiral bits at over 40 ipm in wood...

[tkubic]

lots of reasons it could of broken.

• Vibrations lead to high stress, due to maybe machine rigidity, or part not clamped down.
• stuff in the groove and it bound
• bit dull (you said it was sharp though)
• ramped into the work too fast
• too fast speeds (48 seems slow to me, i cut at 120)
• too much heat (possible with 24k and only 48 ipm)
• many others I am sure I am forgetting

I've broken so many bits, it can be frusterating. Most of mine were due to parts getting bound or ramping into work too fast (often aluminum)

[Phife]

Just thought I'd post a couple of pictures of the broken bit and my machine.

Those single flute bits are not so good for wood, also they are cheap. Get a good double flute up cut bit and you shouldnt be able to break it as easy.

[nda22]

I was on the First pass about 9" into the cut, I was holding a shop vac up to the bit and the groove was clean. No noticable vibrations while cutting, the machine is quite stiff even with the long Z. Yes I was making MDF powder, but I find that just about any tool cutting MDF makes powder. The bit was hot to the touch after it broke but the paint on it was not burnt and neither was the slot I was cutting.

[dfmiller]

That bites.
Any chance the bit had a crack in to start with? It sure did not last long.
My limited experience is that the bits are not that hot if things are correct.
Just asking a few more obvious questions.
Turning the right direction?
Have you verified that your are actually turning 24K. I know with my spindle and VFD, it took more than one try to get the correct speed?

Dave

[louieatienza]

That bites.
Any chance the bit had a crack in to start with? It sure did not last long.
My limited experience is that the bits are not that hot if things are correct.
Just asking a few more obvious questions.
Turning the right direction?
Have you verified that your are actually turning 24K. I know with my spindle and VFD, it took more than one try to get the correct speed?

Dave

You might be on to something. It's one thing for a carbide endmill to snap off, but since the bit in question is carbide-tipped, the body is probably HSS, which probably would have bent way before snapping off. I have snapped 1/16" 1-flute upcut spiral endmills in the past, and it's always where the flute "fades" out where it transitions to 1/8". Mainly, this happened because in aluminum, my max depth with the 1/16" is only .032", and the shortest flute length I could find of this type is 1/4". If you only cut 1/4" deep, the cutting length of the bit acts like a big lever against the thicker shank. Provided your spindle can take it, the deeper the bit is, the stronger it will be. Of course this may not match up with your spindle's power under load, so you have to compromise by taking the shallower depth so you can keep the feedrate up, knowing that your bits won't last as long.

Even when cutting aluminum I can grab the bit (after I shut off the table of course) comfortably. Even with wood you should aim to remove the heat away from the bit with the chips. This also makes the bit last longer. When I know that there will be some heat (like profile cut in aluminum) I have a cold air blaster set up and run it intermittently; but it doesn't get used too often.

Ridgidity is relative. My phenolic machine is eons more ridgid than my previous wood machine, but definitely not as ridgid as cast iron (though I feel it's more ridgid than some extrusion machines I've seen.) You could add backlash, backdriving, and bearing "slop" into the mix. As far as bit ridgidity. I'm not sure if the Grizzly bits are designed for CNC use, especially considering that a CNC can push a bit with way more force then most humans can ever dream to. Also, most bits designed for hanheld routers have a more closed-flute design which prevents kickback, but limits the radial depth of cut, and ultimately, the speed at which it can be fed safely. CNC specific bits have more open flutes to take more aggressive cuts and produce bigger chips, as well as clear them out. Maybe a solid carbide spiral bit costs more, but it's cheaper in the long run and less time-consuming than snapping cheap bits! Onsrud has great bits and endmills but you'll pay for them. Amana sells great wood bits, and for a "bit" less than Onsrud...

edit: I should add that if your spindle is limited to only 24Krpm, then the single flute is probably the best choice, though a better quality one would help. If you have spindle speed control, then a 2-flute at way-lower RPM would probably be better....

[dfmiller]

I am surprised it busted off.
I did this to one of my bits a while ago.


Its demise was caused trying to cut a deck screw I was using as a holdown. ;-)
The shanks are fairly soft.

Dave

[ger21]

It's one thing for a carbide endmill to snap off, but since the bit in question is carbide-tipped, the body is probably HSS, which probably would have bent way before snapping off.

I've seen carbide tipped bits break like that lots of times. Those small ones are pretty weak at the point where it broke.

I've also seen steel insert bits break, rather than bend.

I think Dave's bent bit might be the only one I've ever seen. Imo, it's a lot safer for a bit to break, then bend. Broken bits tend to just stay stuck in the wood, while a bent one would cause extreme vibrations, and could lead to a projectile.

I would think that 100ipm at 24K would be better, but that's a fragile bit, and for $7, you get what you pay for.

Was that bit making a high pitched scream before it broke?
Is there any play in your Z axis, when extended to cutting depth? If it starts chattering, the cutting edge can "grab" and put enough extra load to easily snap a bit like that.

It could have just been a defective bit.

But, the feedrate was definitely too low.

If using similar tools, I'd take shallower passes. But a 2 flute carbide spiral will let you cut at least 5x faster, and it takes a lot more to break them.

[dfmiller]

Gerry,
I was not impressed either with that bit. ;-) I have broken a few solid carbides in my time.
It was a Lee Valley bit I am quite sure. Not a real cheap one.

The strange thing was it was not vibrating at all. I cut over 12 inches more after it bent. Given the unresolved backlash in my Y I would have expected some vibration. I shut it down because it was going to take another chunk out the the screw next time around. It seemed to be cutting just fine. I was rather shocked when the spindle stopped and I saw that.

NDA can you take some better pictures? I could not see them that well. I always like to have a good look at the failure for interests sake. It would be nice to figure out if it was a brittle fracture or what.

I am sort of intrigued at the look of the failure.

Thanks
Dave

[louieatienza]

I am surprised it busted off.
I did this to one of my bits a while ago.


Its demise was caused trying to cut a deck screw I was using as a holdown. ;-)
The shanks are fairly soft.

Dave

Nice! Though I haven't bent one with my machine, yet, I've bent a few during hand operation in my time. Mainly from a router falling off a table, and usually with a flush trim bit (1/4" shank with 1/2" radius cutter and ball bearing pilot). The bits I've snapped on the CNC were clean breaks; once when I was rapid jogging and ran into a fixture I screwed down..

In theory, that bend is normally what I'd expect. I expect carbide bits to snap since the blanks are sintered, as opposed to carbide tipped bits where the bodies are cut from HSS rod. It's possible the Grizzly bits are made with cheap steel that might havev a ligh level of sulfur or whatnot. Of course where that transistion is, is the weakest part of the bit.

I tend to look at a bit carefully before I decide to use it. I check to see how finely ground the cutting edges are; bits with edges so finely ground they look polished is a good indicator that quality control is good. You can "feel" the heft of a good solid carbide bit. I'm also beginning to think that most serious CNC bits are NOT painted!

[nda22]

I took about 6 pictures but this is the best one. It's a pretty clean break. I think I'm going to try an Onsrud 48-700 and see how it goes.

[dfmiller]

I cant really tell form the picture but it almost looks like part is a brittle fracture and part is polished. If that is the case I think that supports the theory of a crack.
I am sort of rusty on my failure analysis. I will have to look at my text books. Its only been 30 years.

Dave