Clean Cuts

[nick]

Hi Guys,

Not real happy with cut quality on soft / hard woods and was wondering about RPM's. I've gotten into the habbit of using MAX rpm 18000 of my spindle (HSD 2.25 HP) as a constant, keying in the desired chip and away we go, then decreasing / increasing feedrate as needed. Should I be using different RPM's for different bits. Using the starter bit collection and their is no RPM other than 18000 that I can see in any documentation, so I'm asking the pro's. What RPM works best as a rule of thumb and especially on V Bits and engraving bits as well.

Also...I know the books say +/- .005" for positioning on a standard...getting +/- .020". Is this the best under load or can I improve this...does RPM effect this.

Thanks

Nick C.

[richards]

Nick,

I rarely run my spindle that fast. My usual speed is between 12,000 and 14,000 RPM so that I can keep the feed speed more moderate.

V-bits are going to be a problem - at any speed. Because you're going from a point to 1/2" or greater diameter (with a 1/4" depth of cut), you're asking that single cutter to perform better than it was designed to perform. Add to that the grain of the wood and you can easily have 'chatter' and tear-out when cutting hard/soft woods.

Absolute positioning error is greatly related to speed. The faster an axis goes, the more the cutter flexes (and the machine flexes), so, for critical positioning, go slow. When I cut circles for indicator lights, I often cut at 1" per second or even slower. At those slow speeds, the cut is nearly perfect. At faster speeds, I sometimes get elipses or off-sized holes.

When I'm cutting lumber, my usual method is to first orient the workpiece so that I'm not going to tear the grain of the wood and then I make multiple passes at slow speeds with the RPM adjusted to give the proper chip load.

(When I first bought my PRT-Alpha in 2004, I expected to be able to run everything at 8" per second at full depth - kind of like expecting my Dodge Dakota to run at 120 MPH with a full load. It's just not going to happen. After getting familiar with the machine and tweaking the hardware a little, I'm getting quality cuts at moderate speeds. My customers are happy so I'm happy.)

[scottcox]

Hey Nick,

I agree with Mike that 18K is too fast, even though Onsrud designs their bits for that speed. Slow it down along with the move speeds and you'll get cleaner cuts.

[knight_toolworks]

for hardwoods I have found straight bits tend to cut the cleanest. but thats in 1/2" sizes. I bought a couple of 3/8" straight bits from whiteside and they sucked. they chattered and had a lot of blowout.
down cut or roughers are the next choice. I usually cut a max of 1/2" deep on tropical in one pass.

[rhfurniture]

OK, I have a theory on this one that works for me in practice. Looking back at my pre-shopbot days, I cut a lot of shapes on the spindle moulder with a template. The best finish came from a 5 inch (125mm) dia cutter running at 6k. To get the same knife speed at 10mm dia would need an rpm of 75k! I compromise on work with hardwoods needing the best finish by bandsawing within 2mm first then using a straight single knife 20mm dia Replacement Tip cutter at 18k. This means I can keep dead sharp for low cost, and keep up a reasonable chipload. I know it is using CNC to mimic ancient technology, but it gives me a very good (almost as off the spindle/shaper) finish.

[nick]

Mike,

Thanks so much for your input...taking the RPM to between 12000 - 13000 and changing my feedrate to be relative is working out very nice...I see a big improvement...

Recap.

Feeds are around 1 - 2.5 ips and R's are between 12 - 13.

I'm very pleased with the quality.

Thank you

[Gary Campbell]

Guys...
As an add-on the above posts... When we are not completely sure of the feeds and speeds. We set the feedrate and RPM in the middle of what we guess it should be and Then use the keyboard to move the feedrate up or down. The RPM can be changed using the VFD Keypad. Using these tools you can fine tune your settings to match the material. Your Eyes and Ears will let you know when all is cutting well.
Gary

[laura]

Nick,

Even though it sounds like you're happy with the results of 12000-13,000 rpm, 1-2.5 ips, check out Onsrud's site. They have a pdf on feeds and speeds, which are based on chipload. They explain that as well. There are so many factors that determine that, but they have info on different woods with base guidelines which could help you in the future.

I've had success with a carbide-tipped straight double-flute for mdf, but I'm considering using solid carbide upcut spiral for harder woods.

Laura