chips not dust

[vassilm]

Do I need to be looking for chips not dust with straight 2 flute cutters as well?
(I am cutting MDF)

[dana_swift]

Generally you want chips from any material. If you do the feed rate calculation for a 0.010 advance with each cutting edge in the material, expect to see chips around that size.

MDF is only unique in that the dust it gives off is nasty. Have a good dust collector or a mask. Note that the dust hangs in the air for hours after the cutting finishes.

D

[beacon14]

MDF doesn't really give off chips, just that nasty dust - yuk.

[widgetworks_unlimited]

Actually, David's comment above is incorrect - most MDF will make huge, curly, soft chips.

I don't want to start a fight by contradicting another post, so let me explain... For years I cut MDF without seeing any chips. The dust was horrible, just as David said. I was running an underpowered 2 1/4 hp router, with a 1/4" 2 flute bit, on a slow machine/slow feed rate (<2 ips.)

These days I cut MDF at 5-6 ips, 19,000 RPM, with a single flute 3/8 cutter. My chips measure .025-.030" thick. They look very similar to the chips I get from plywood - little scoops/shavings 1/4" long x 1/8" wide, like a mini version of what you get from a block plane.

These chips are easy to contain/collect with the dust shoe and I don't get a film of dust building up on the rest of my equipment, glasses, etc.

I will say that my last order of MDF cut differently than any of the other product I've cut in the last 2 years. I believe it was Trupan brand, though it wasn't ultra light. This material gave off large dust instead of chips - like course sand/corn meal. It cut more like 15 lb urethane sign foam than MDF.

Best of luck...

[Gary Campbell]

I agree on the chips... dont cut much MDf, but it does shoot out the curly's. 480 ipm, 13500 rpm 2 fl compression.

[beacon14]

I guess I have some tweaking to do the next time I cut MDF, which fortunately is not very often.

[vassilm]

Thanks a lot, guys.

But when you are talking about long curly chips, are you talking about straight bits or spiral flute bits? Because I do get chips when using an upcut bit, but I get dust when I use a 2 flute straight bit and was wondering whether with the straight bit I am to expect the nice curly chips at all.

Also, and I feel this is truly a lame question but I couldn't find it spelled out clearly anywhere so I need to ask, when talking about chipload size - is this the thickness of the chip or the length of the chip?

V

[tgm]

Vassil,
Remember, when you use a staight flute or downcut type bit the chips are staying in the cut line and getting 'chopped up' by the cutter, kind of like mulching grass clippings.
You have to get the chip out of the cutting area as the chips carry away the heat from cutting and allow the bit to run cooler. Heat is the bain of any carbide cutter.
Use the proper O flute upcut on MDF and the long curly chips will come flying out. You will be amazed.

Good Luck,

Tom in PA

[dana_swift]

The thickness of the chip is the width along the curl.

http://www.talkshopbot.com/forum/attachment.php?attachmentid=10571&d=1286029027

Most materials respond to being cut by curling, cracking and breaking, so chips don't often look like the moon sliver that is the shape of the missing material from the cut.

Feed rate calculations are a fancy way of figuring out how far each cutting edge advances from the previous one at a given RPM and number of flutes. Upshear, downshear or straight do not enter into the math (or physics).

The shearing forces will make it more likely that an individual chip breaks, cracks, or curls. That doesn't matter of course, it ends up in the dust "chip" collector.

I look at chips that get past the dust skirt to quick check them.

Feed rate is determined by optimum chip size: 0.01 to 0.02 are good numbers for most materials. Depth of cut is mostly determined by the cutting motor horse power, bit flexing, precision required, finish, and the ability of the CNC motion system.

I determine depth of cut in a material from experience and listening to the router motor, if it is struggling to maintain RPM, the depth of cut needs to be less. For a quick fix until I can re calculate the SBP file, I can drop the feed rate, but that is just a band aid. The depth of cut is one of those things that you learn from experience, same as best chip size for a material.

If the feed rate is too low and you are getting dust, you are risking fires. The bit is dwelling in the cuts too long from the shallow cuts generating heat that can only go into the dust which is easier to ignite than chips.

Hope that explanation helps-

D

[Gary Campbell]

Vassil...
I use mostly compression cutters for edge quality. They are spiral cutters, but configured with both up and down flutes.

To add to Dana's explanation, here is some workshop math to show that Russ's chipload and mine are virtually the same.

He cuts at 19000 with a single flute cutter @ 5-6 ips (say 5.5)
I cut at 13500 with a 2 flute @ 8ips

my 8ips/ his 5.5ips = 1.45

take his 19000 / 2 (single vs 2 flutes) get 9500

take 9500 * 1.45 (increase in speed ratio) and get 13,775

13,775 would be the hypothetcal rpm that gives the same chipload, assuming similar tool geometry, and my 13,500 is very close so you can see why we both are "shooting chips" rather than blowing dust.

I prefer to see how slow I can make the spindle run and how fast I can push the bit (move speed) rather than the other way around. I feel that a bit cuts best in sheet products about 10% above the speed where it breaks from not being able to evacuate the chips. You will increase your bit life and make dust collection painless. Breathing is easier too!

Others have good results from speed/feed combo drastically different than mine, but I say... "Let 'er Eat!"

[jdervin]

This is turning into one of those threads that ought to be kept at the top of the category no matter how old it gets. Super information! Thanks all.

[vassilm]

Again, what a great place this forum is. I can't stop being amazed at the support in this group.
Now, usually I use spiral cutters too and with some tweaking I am usually able to generate chips. My question was provoked by a job that I described in a different post where I need to do a lot of shallow pocketing in MDF with depth of cut of 3mm (1/8") and some very intricate shapes of the pockets. I was given the advice to use a 2 flute straight bit. I did it and for the most part it is coming out very well, but I can't generate chips and I was worried that even though the pass depth is very shallow I will soon damage the bits. On the other hand, If I go too fast the edge quality suffers as the toolpath follows a lot of small curves.
By the way I've followed Brady's advice and changed the ramping values, which tremendously improved edge quality of these engravings.
http://www.euromebel.com/stuff/pocket02.JPG

[widgetworks_unlimited]

Shallow cuts like you're describing are easy work for a dust collector - you may be make chips and never see them if everything gets sucked up.

Also, because of your depth of cut, you may get chips that look like little pieces of string or curly straws/springs.

And... the chips get smaller in size when you use a smaller diameter cutter. They are also "mulched" more (as TG explained above) because smaller flutes have a hard time ejecting large chips from a deep cut. Hard to tell what you used in your picture.

I wouldn't worry too much about heat on this job. The cut is very light and there's no way for chips to pack around the bit. If you burn your fingers while taking the bit out of the machine when the jobs done, you'll know that something is wrong. When I'm cutting well, my bits are luke warm after 1+ hours of constant cutting pockets/profiles.