bigger doesn't mean better

[twelchPTM]

just an observation I though I'd share.... Since I first got the bot I have been doing almost all my general cutting with a 1/4" bit. Well I was cutting some test pieces the other day from mdf and hdpe and needed to use my 1/8" bit for some of the inside corners. Out of sheer laziness I have been using the 1/8" bit to continue cutting parts for my prototype and I have been getting much cleaner edges with significantly less noticible toolmarks left behind. I just ran some pockets in the hdpe and the only marks you can see are with your face inches from the surface and the light hitting just right...

[gene]

What are you making ? Is it small parts or big Parts?

[Steve M]

There can be other benefits.

I was able to increase the yield by 15% by using a .125" bit instead of .1875", in addition to reducing the cost of bits from $6.50 to $2.80.

[twelchPTM]

I produce parts if all different shapes and sizes, yeild is not a huge issue, 90% of what I do is one-off production, I build interactive exhibits for kids at the Please Touch Museum here in Philly. Sometimes I have to buy a full sheet of something even if I only need a quarter for the project. I do try to get the most out of my material because anything left over is used for prototyping and experimenting (the fun part!). Its great to be able to cut test pieces from the final material because there is no concern about variation inthe product.

[shilala]

Pay close attention to your stepover values and you can do away with tooling marks in most anything you do, regardless of bit size.
Stepover should be set under 20% for your finishing work, I like it under 10% in most instances, someone told me "14% as a rule of thumb", but it depends. If I lose a half hour tooling something that'd sand out in a minute, I'm not helping myself. So it's something to be smart about, and use to my advantage when I can.

There's a good little blurb on stepover values right here (http://www.cncsharktalk.com/viewtopic.php?f=4&t=1672).
It's not something to carve into the brain or anything, but it does help get an idea of where the line of diminishing returns is. It also helps me somewhat towards choosing a bit size because for some reason, I too hate changing bits.
No clue why. It's super easy and not even a bother. It's just one of the many things wrong with my head. :)

[Brady Watson]

40% is the defacto standard for 2D pocketing work, unless cutting very dense material. It is the fastest setting without leaving material in the corners. 8-12% for 3D work with a ball end mill. 15-20% for 3D coarse finishing.

-B

[bobmoore]

Thomas;
Saying you are using an 1/8 or a 1/4 inch bit doesn't tell us much as there are so many styles and geometries of cutters. A 1/4" single O flute needs a much tighter step over and far different speed and feed rates than a 3 flute end mill and there are many more options available than just these 2.
Bob

[twelchPTM]

well bob i really wasn't trying to be to technical, mearly observing that while working with two bits who's only geometric difference is the diameter and cutting the same material, the smaller of the two produced a much better finish. But since you asked....They are both 2 flute up-spiral center-cutting endmills for plastic from harvey tools.
A buddy pointed out that the situation is similar to saw blades in that the teeth are closer together on the smaller bit much like the closer teeth on a fine-cut blade...

[steve_g]

As a rule of thumb... a larger diameter bit gives a smoother cut. Think about it... for a specific micro-step of the CNC router, the larger diameter bit will "blend" with the last step more smoothly. Your best cuts are from the largest and shortest bit that can do the job. If your larger diameter bit is cutting poorly perhaps something else is going on... like a dull or bent bit? I've attached a drawing to illustrate the principal.

SG

[twelchPTM]

I think there are way to many variations in cutters to really say for certain what is "best" there is more then one way to skin a cat. But in this situation with the specific tool geometry i was using the smaller bit provided the better cut.
Steve- I never said my larger diameter bit was cutting poorly, just that the smaller performed better. Also while your point makes sense, your drawing is definatly not to scale. It also does not take into consideration that the smaller bit would move at a slower speed. I know I didn't mention that I set the 1/8" bit to a slower xy feed rate but again I knew from the get go that there are to many variations in tools and material to say that this occurance applies to all situations i simply shared that on this occurance even thought my parts could easily be cut out with a 1/4" bit with good cut quality, using the smaller bit produced a better cut and hope that maybe some one will be having trouble in the future getting the cut quality they want in the future and find this helpful.

also the size of the micro-step might not be so large to present the situation presented in your drawing

[steve_g]

Thomas...

I knew that the drawing was not to scale... I wanted to show an extreme case to make my point. My drawing assumed that both bits were moving the same IPS. More importantly the XY speed needs to be calculated from the manufacturers chip load recommendation... If the speed is too slow the bit can chatter or vibrate in the kerf giving a poorer cut than a faster speed would. Too fast and the bit can't eject the chips fast enough... this increases the lateral thrust and either breaks the bit, tears the wood or moves the work piece. The manufactures recommended chip load is determined by many things and you can't make any generalities by bit size or type.
I started getting consistently good cuts when I adopted the onion skin cutting strategy... It made more difference than bit geometry!

SG

[twelchPTM]

I apoligize for not providing enough information initialy. The speeds and feeds were adjusted proprtionally to the bit size. the speeds were previously found to produce the best cut in the material in question. Now I am certain that there are .25" bits that will produce a better cut then the standard 2 flute I am using and the same can be said for the .125" bit. again I was only trying to share that in this one situation the smaller bit produced a better cut and maybe someday someone will be having trouble getting the cut quality they want and upon reading this find that the same thing works in thier situation. I was not in any way trying state that as a fact or as a rule smaller bits cut better. Frankly there as so many variables and options in tooling and material as well as holddown and cutting methods and feeds and speeds and so may more that while there are some standards we can consider "rules of thumb", there is no one right way to do anything on the bot.
Also I use the onion skin on damn near everything, or I spray super 77 on my spoilboard to create a sticker like temporary bond to hold my piece in addition to my vacuum