What am I doing wrong - breaking bits....

[ErikS]

I've recently had two MLCS spiral upcut bits break - I can't help but question what I've been doing wrong.

Bits in question are very small, 1/8" & 5/32"

What I was doing -

I was cutting circular grooves in pieces of acrylic (cast). The grooves were 1/16" deep. I was using a home made circle jig in a router table. The jig is pretty much a turntable (for those old enough to remember records). Process is to start the router & drop the piece onto the bit & spin. After breaking 1 bit (the 1/8") I made doubly sure not to feed/spin the material fast. The 5/32" bit broke after less than 1" had been cut.

Bit was in a 1/4" collet, seated with more than enough depth, and spun at max speed. No signs of plastic melt (grab the bit) - just a snapped off bit.

What's wrong with the technique?

(I'm assuming it's me as I've not had any previous problems with MLCS bits).

Thanks in advance

[LCHIEN]

I've broken a 1/4" spiral upcut bit in plywood before, I know the spiral groove in the bit makes a thin section that is pretty weak. It broke when I did not expect it. MLCS advised taking a smaller cut. (like 1/8").

Now disregarding the difference between plywood and acrylic, the shank is only 1/4 as thick (going fom my 1/4" to your 1/8") and maybe you need to take 1/4 as deep a cut (1/32nd, I know that's small).

The melting/grabbing thing was my first thought but you said that's not the case.

I would call or e-mail MLCS, and ask their opinion. Being a router bit specialty house, I think they would be one of the most expert sources you have access to. I find them friendly and helpful. And they sent me a free replacement bit.

[ErikS]

Thanks - I did contact MLCS & they have woodworkers that are available for help.....but I trust you folks a bit more - what can I say.

It's got to be depth of cut, I was going by a "less than 1/2" the dia of the shaft but I forgot that spirals are so thin in the non-cutting area.

Might have to do 1/32" multi passes - that's no biggie, just spin & crank up the height.

Thanks again.

[Holbren]

Vibration is a killer on those smaller bits too, make sure everthing is solid.

Brian

[Tom Slick]

I believe that a straight cut bit supposed to work better on plastics.

[ErikS]

Thanks again.

No vibration that I'm aware of. It's in a router table, fastened down, and the jig is held down to the table with t-nuts & knobs = pretty stable. Haven't noticed any vibration is the router (Bosch 1617ESV)....hmmmm.....

But then I must be dense - how did I miss the 1/8" spiral bit on your site Brad....that's always the 1st place I go - lol.

I think I'll try a single flute straight bit, might be better though the finish won't be as smooth as the spiral.

[Holbren]

I believe that a straight cut bit supposed to work better on plastics.

I personally would never carry a steel bodied bit with a cutting diameter so small. I see that MLCS has a single fluted bit 1/8" which may be worth a try but I can't image how it wouldn't break. If they are going to send you a new spiral, maybe for 7 bucks it's worth having them throw one in the box.

Brian

[vaking]

I am thinking of your jig - the turntable stile as you described. Is there a plate sitting on top of your router table under the acrylic that you cut? If so - how thick is the plate? How long is the bit length total (not cutting length) and how much is still sitting in collet?

[ErikS]

Thanks Brad - I talked with some acrylic fabricators that I know.....answer was spiral bits are the #1 choice. Single flutes = grab = snap = not good.

Thickness of the jig is .354. it's two .177 pieces laminated (chemical weld). Overall bit length is 2", cutting length is 5/8" & bit is just about down to the shoulder (router is all the way up to the stops pretty much)...........guessing about 1" in the collet.

[Stytooner]

I have had better luck with two flute solid carbide bits when they are this small. I have broken spiral router bits and end mills very easily at this size. The two flute straight cutters that are carbide will cut through steel, aluminum, plastic and even wood.

[LCHIEN]

alternatives

Erik,

let me ask this. Is there a reason why you want such a small kerf? Is this a thru cut? I read your original post and you said you wanted a 1/8th groove 1/16th deep. I guess that explains it.

But, just thinking outside the given parameters, if you could somehow go with a 1/4" spiral bit you would have it made. Or if you use a V-bit 1/8th then the material and forces are 1/2, in fact you could use a 1/2" or 3/4" wide V-bit. which would never break. Even a round bottom bit would reduce the forces quite a bit.

[ErikS]

I thought of using a V groove bit (for the same reason, strength), just not sure it would work.

What I'm building is attached (from one of my other hobbies - reef tanks). You can see the piece in question, the black top. The o-ring (orange) rides in this groove & provides a water tight seal with the top. I'm not sure if a V groove would work - not even sure how I would test it, that would require building a unit & that ain't cheap (acrylic tube goes for over $20/ft) - lol.

I think that water would leak under the o-ring with a V groove (at the tip of the V). The material is silicone & gets squished by the top plate....some....but I don't think it will squish into a V?

The o-ring is 1/8" thick & the real trick is to get the slot cut just deep enough so that it will sit in without popping out (if it pops out you'll never get the screws in the top lined up).

And to further complicate matters the pieces are only .236 thick = I don't have a lot of material to play with (the next thicknes up - .354 - more than doubles the price tag)

[Tom Slick]

I personally would never carry a steel bodied bit with a cutting diameter so small. I see that MLCS has a single fluted bit 1/8" which may be worth a try but I can't image how it wouldn't break. If they are going to send you a new spiral, maybe for 7 bucks it's worth having them throw one in the box.

Brian

you are absolutely correct, I completely missed the tiny bit part. a straight cut would not be very good that small.



you could also try these guys. http://www.onsrud.com/
they specialize in industrial cutters for many materials including acrylic.

[Holbren]

Good point on Onsrud Tom. They make some nice HSS bits that may work out better than the carbide.

[ErikS]

Thanks for all the help folks, apparently there's no clear cut answer.

Onsrud's main page recomends HSS 2 flute cutter, their plastics page recomends solid carbide 2 flute - lol - I think I'll be okay if I just remove the material in smaller increments.