Advice on finding the right feed rate

[rcnewcomb]

I'm running into difficulties finding the right feed rate & RPMs when I and trying to remove large amounts of material. I do fine with shallow carvings and 3D work. But when I need to hog out a large area to 0.5" deep I'm burning up bits. This tells me the feed rate is too slow and too high on the RPMs. The problem is that my Porter Cable loses substantial power at 10,000 RPM. About the only way I could compensate would be to use extremely shallow pass depths and take 16 passes.

Other thoughts besides a large horsepower spindle?

The material is limestone and parts of the 3D carving require hogging out large amounts of material 0.5" deep.

[courtney2018]

What size bit are you using? Can you go larger?

[rcnewcomb]

I've burned up a 0.75" and a 0.375"

[Gary Campbell]

Randall...
Sorry, but you seem to already know the answer. One of the main advantages of the spindles is to maintain torque at lower RPM's. This is a huge advantage when machining materials other than wood.
Gary

[ron brown]

In addition to burning bits ..... I'll recommend using the Her-Saf insert bits. Their 12mm square insert with 4 usable edges is $3.61 (price checked on 8/02/2009). The carbide is also of a higher grade than can be braised on.

It won't keep you from burning bits BUT, it will make it less expensive ....
Ron

[richards]

Cutter life greatly depends on heat and heat is increased when the chip load is low. Just use the chipload calculator furnished with SB3 software. I generally use 0.015" as a starting point. Then I enter the feed rate and let the software calculate the RPM that I need to use. When the RPM is more important than the feed rate, I enter it and let the software calculate the feed rate. Finally, I run some test cuts to see how deep I can make the cut using that feed rate and that RPM. With the correct feed rate/RPM dialed in, the chips will carry off the heat and your cutters will live a normal life.

Even with my 3hp spindle, I rarely make cuts 1/2" deep. Making two passes climb cutting (leaving a 0.025 'skin') and one final pass full depth conventional cutting gives me accurate sizing and adequate edge quality.

With my PRT-Alpha and the two FEIN vacuums that I use (and my 4,500' altitude), trying to push things harder just leads to part slippage and frustration. Just like any other tool, I let the Shopbot tell me what it can do and then I stay withing its limitations. By doing that, I end up with the fastest *good* cut possible.

[gene]

Ron,
How much is the " holder" for the carbide inserts that you are talking about? Does it hold them at 45 deg or 90 deg to the work? Who is your supplier?
Thanks Gene

[ken_rychlik]

Have you thought of a diamond bit for the limestone?

[ron brown]

http://www.hersaf.com/shop/index.php/action/category/id/7/subid/57

Gene,

That link should answer your questions.

Ron

[rcnewcomb]

quote:Have you thought of a diamond bit for the limestone?
Yes - though I don't care for the prices (ouch!)
I'm also thinking about one of the Vortex coolers.

[eaglesplsh]

Has anyone ever used a tachometer to measure the actual RPMs of a Porter Cable router under load?

The chip load calculation is simple, but if you don't know your actual RPMs it may give you totally bogus recommendations.

I've always wondered how well my actual RPMs under load compared to the settings on the dial.

I know you have to use your eyes and ears to dial in each individual tool path, but it would be nice to have some verified numbers to back up what you're hearing.

[courtney2018]

Here's something that may be helpful:

http://www.precisebits.com/tutorials/calibrating_feeds_n_speeds.htm

I talked to a guy there about some bits I ended up buying. He also turned me onto the nut and collet replacement kit that they sell for the Porter Cable 8517 router.