The basics of bits and spindle speeds

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The basics of bits and spindle speeds

Postby LonePine » Sun Jan 22, 2012 2:37 pm

Is there a good resource to learn the basics of bits and spindle speeds? This week I got my cnc machine setup, and have been learning Aspire, but I'm really unclear on which bits are appropriate for which scenario, and have no idea even where to begin selecting spindle speeds appropriate for the job.

There are hundreds of possible bits available? How to know where to begin.

Any links, advice, PDFs or sites would be great. Thanks!
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Re: The basics of bits and spindle speeds

Postby StuartC » Sun Jan 22, 2012 2:58 pm

Check out pages 4 and 12 of this PDF on the Hartlauer Bits siite
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Re: The basics of bits and spindle speeds

Postby Adrian » Sun Jan 22, 2012 3:25 pm

There's a few references I use but at the end of the day you can only ever get a starting point from charts and formulae. Once I've got the basics worked out I listen to the cut. I use spiral bits so if it's screaming I up the feed rate or slow the RPM's. ... speeds.htm ... and_speeds
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Re: The basics of bits and spindle speeds

Postby Rcnewcomb » Sun Jan 22, 2012 11:51 pm

There are hundreds of possible bits available? How to know where to begin.

The reason there are hundreds of possible bits is that there is no single perfect bit for all job and materials.

Start simple and ask yourself two basic questions:
1. What material am I usually going to be cutting? (plywood, MDF, solid hardwoods, foam, plastic, metal, etc.)
2. What kind of designs: 3D, V-carving, cutting out parts? i.e. are you usually cutting all the way through the material or just carving on the material?

You can buy starter kits of bits
Centurion has a number of starter sets

Shopbot sells two different collections of Onsrud bits in starter sets:
Router Bit Starter Kit (13699)
Shopbot Desktop 7 Piece Bit Kit (13698)

Some basic terminology:
Straight: Neither upspiral nor downspiral (see below)

Downspiral or Downcut: Down spiral bits push the chips downward back into the slot that was just cut. They are designed to leave a clean top surface of the material. They work well with vacuum hold down becauyse they place a downward force on the material and, at the right feed rate, they will pack the chips back into the groove that was cut so there is little loss of vacuum.

Upspiral or upcut: Up spiral router bits pull chips towards the shank and are great for plunge cuts. Good for doing fast cuts but can have some tear out problems on the top surface of the material. Not great for machines with vacuum holddown since they lift the material as they cut.

Up/down or Compression: The part of the bit near the shank is downspiral, and the part farthest from the shank is upcut. These bits are designed to cut sheet goods such as plywood and MDO in a single pass with minimal tearout. They tend to be more expensive and, because of the complicated geometry, they cannot be resharpened.

Spiral O, Super O, or O flute: Available in up or down cut, the Open flute allows for easier chip formation and evacuation, but also provides for a sharper cutting edge that delivers a better edge finish on a variety of materials. For certain materials these are the only bit that will produce a good result.

Flutes: The number of surfaces that cut the material as the bit rotates one revolution. For wood a single or 2 flute design seems to work best. Bits for working in metal may have 3 or 4 flutes.

If you don't buy one of the starter bit sets here is how you can put your own set together:
A 1/4" bit is pretty handy general purpose bit. Start with a straight bit and later try upspiral and down spiral bits. Buy a good brand such as Onsrud or Centurion, but you should also try a super cheap bit. That way you can understand the difference between a $2 bit and a $40 bit.

A 90° bit is great to try out V-carving. Try the ones from Centurion and CMT. Later you can try a 60° bit for smaller lettering or a 120° bit if you are carving larger signs.

A 1/8" ballnose bit is great for doing 3D work. If the carving is less that 1/2" deep then you can get away with shorter, less expensive bits from MSC Direct. For carvings deeper than 1/2" buy the bits from They are beastly expensive and worth every penny.

A large diameter bit (1.25" or larger) for leveling your spoilboard is handly. You won't use it often, but when you do it will be glad it is there.
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10 fingers in, 10 fingers out - another good day in the shop
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Re: The basics of bits and spindle speeds

Postby zeeway » Mon Jan 23, 2012 12:22 am

Randall's reply is about the best summary I have seen on this topic.

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Re: The basics of bits and spindle speeds

Postby metalworkz » Mon Jan 23, 2012 3:45 am

Along with all the great links and information already offered here, there is a program that will help you figure the speeds and feeds of various tools as needed and more here:
Best regards,
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Lathe, Horiz. Mill, Shaper, Leaf Brake
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Re: The basics of bits and spindle speeds

Postby dansfoundry » Mon Jan 23, 2012 1:48 pm

Thanks Wes, that is awesome software. Dan
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Planing with a leveling bit

Postby dtorney » Wed Aug 30, 2017 6:35 pm

I plan to use the large (1.25") bit I bought for leveling spoilboards for planing the large top
and bottom surfaces of square blocks, prior to carving. I would remove at most 1/8" of
material, fixing bowing and sloping.

I'd use the recommended spindle speed. I could compute toolpaths, or I could move the bit
manually. The latter would be slow, and it might yield a rougher cut. I'd like to do this routinely.
I could compute finishing, leveling toolpaths for blocks of different sizes, and I'd be set.

I'd be grateful for your suggestions
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