What size bit question

[mikejohn]

I wish to cut complete through 20mm softwood.
Which is better to use, a 6mm bit or a 12mm bit?
With the ShopBot moving at the same speed in both cases, the 12mm bit removes twice the material that the 6mm bit is removing, but the tip is moving twice the speed, so both bits remove material at the same speed. So the force on the bit tips appears to be the same in both cases.
As the 12mm is a stronger bit than the 6mm it seems the bettr choice.
Can I therefore cut deeper with the 12mm bit?

......Mike

[benchmark]

Mike

I use 8mm as a compromise between a weak (6mm) cutter and removing a lot of material.

Paul

[gerald_d]

We have pretty much standardised on 6mm and 10mm bits. For cutting through 20mm softwood, we would only use the 6mm only if more detail was asked for. The customer will then have to pay more because it will take longer to cut than if a 10mm was acceptable.

(Purchased 35 cutters (10mm) approx $1000 last week for one particular job in panga panga (http://www.pangapanga.com/english/pangapangas.asp))

[wemme]

Hi Gerald,
how much would you cut using a 10mm cutter per pass?
what typical feed rate would you use in the above mentioned setup.
cheers

[gerald_d]

We are drifting off Mike's question.... The 20mm panga panga is being cut 3 passes at 7mm per pass, 25mm/s feed. Cutter life 2 hours. That "wood" is nasty - it has silica in the grain

[mikejohn]

Gerald
You say "because it will take longer to cut than if a 10mm was acceptable. "
Does that mean I can cut deeper or faster with a 10mm bit than I can with a 6mm bit?

.........Mike

[gerald_d]

A 10mm bit can take a lot more load (before snapping off) than a 6mm. So you can go deeper and faster. For us the big difference is the reduction of the number of passes, while the feed-rates are about constant.

[mikejohn]

So the answer to my question is "Yes, you can cut deeper with a 12mm (or 10mm) bit than with a 6mm bit, everything else being equal."
Of course, for cutting power you spindle guys have it made over us router guys
.

...........Mike

[charles_o]

Mike,
So you have come to the conclussion that; "Yes, you can cut deeper with a 12mm (or 10mm) bit than with a 6mm bit, everything else being equal." But things are seldom equal. Typically a smaller diameter bit, say a 6mm vs. a 12mm, that have the same geometry, will have a smaller optimal chip load. So if you use the formula, [Feed rate = RPM * # of cutting edges * chip load ], the smaller chipload of the 6mm bit will necessitate a slower feed rate to achieve the same quality edge finish and the same tool life.
But can you cut deeper with the larger bit? Maybe. Most of the time this is true, but if you balance a bits recomended chip load with the spindle RPM and feed rate, you can sometime cut through 3/4" material in a single pass.
Charles