3/4" MDF feed/speed question

[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