Kress Companion Spindle - Maximum Torque?

[GJeff]

I'm cutting materials that need extreme SFM (hardwoods that want 9k SFM and soft that want up to 15k!) that I can't reach even with 1/2" diameter cutters spinning at 29k rpm. I'm possibly nearing or exceeding the limits of my 800w Kress companion spindle considering these materials also call for aggressive chiploads to prevent rubbing. I can't for the life of me find any info on what is acceptable torque for this spindle, however. All I see when I RTFM is that the Kress will automatically LOWER its RPM if it is getting overloaded, which could be disastrous. Can anyone recommend a safe amount of torque for this spindle? I use FSWizard which has me at around 10 in/lb for the cut I want to try.

Appreciated!

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[tmarks11]

Since you are doing dedicated wood cutting, how about building a companion spindle mount for a Porter Cable 892 router motor? More power, and with speed control built in to keep rpm constant, plus it is relatively cheap.

Others have done similar things:
http://www.cnczone.com/forums/tormac...n-spindle.html

http://www.cnczone.com/forums/tormac...ml#post1625824

[GJeff]

I'm not actually doing dedicated wood work. I typically use my kress for aluminum with <1/4th diameter end mills, but if I bust it... I'll probably get the higher powered Kress >< I use very small end mills at times and run out is a major concern with them.

[cajbr]

According to the mfr, the maximum output power is 420 watts and at rated max speed of 25000 rpm that is 1.42 in-lbs. That's really the only rating I could find in the manual. If you assume constant power, at 9000 rpm that is 3.94 in-lb.

STP Conversion Calculator | Torque Calculator | Groschopp

cajbr

[GJeff]

Thank you so much for that info - now I know how to convert output power and RPM to available torque! I wish you were not correct, but it seems like you are. I would have really overloaded it doing what I was planning. Back to the drawing board.

Great first post ... if your post count can be believed. You've really been lurking for 9 years?

[Zetopan]

"... now I know how to convert output power and RPM to available torque!"

The conversion of Watts to torque is relatively simple. Since you are in the USA I will use the common units used there for this example. If you start with Watts and want to get to HP (Horse Power) then there are about 745.7 Watts per HP (the metric HP differs). So the first step is to divide by 750 (close enough for what you want). Now that you have HP you only need to know that HP = Torque * RPM / 5252, where Torque is in units of pound feet. Using the number provided by cajbr above we get 420 / 750 / 25000 * 5252 = 0.118 pound feet, and multiplying that by 12 inches per foot yields about 1.4 pound inches, which is cajbr's result. If you use 745.7 Watts per HP the result is more accurate but for what you wanted 2 digits of precision are plenty. So the end formula for this example is Torque in pound inches = motor output power in Watts / 745.7 / RPM * 5252 * 12. By the way, the 5252 conversion constant comes from 1 HP = 1 Pound Foot * 5252 RPM (i.e. HP and Torque [in Pound Feet] are numerically equal at 5252 RPM - something that you can see on any dynamometer plot).

[ger21]

I'm cutting materials that need extreme SFM (hardwoods that want 9k SFM and soft that want up to 15k!) that I can't reach even with 1/2" diameter cutters spinning at 29k rpm

What feedrates and rpm's are you using? 29,000 rpm is much too high for cutting wood, unless you're cutting at 2000ipm.

[GJeff]

Good point. At 29k it's wanting 350 IPM to get a reasonable .006 APT. I'm going to have to reduce the RPM just and APT just to get within my machine's limits.

The lowest APT that is recommended for hardwood that I'm seeing is about .003. This is Purple Heartwood. Any suggestions on if that is reasonable? Your 2000IPM estimate makes me think you might favor much higher chiploads.

[ger21]

For a 1/2" tool, .003 is much too low.
Wood can be tricky, and sometimes you have to lower the chip load to get a better quality cut, at the expense of tool life. A good starting point would be around 10,000-12,000 rpm, somewhere between 100-150ipm, with a 2 flute tool.

[GJeff]

Thanks I'll give that a shot. I'll have to do either shallow or narrow cuts though to keep the torque down, and I'm limited to 110 IPM on my Tormach, so at 10k RPM I can get closer to .006" APT. Keep in mind, this is a straight fluted router bit and I believe only the flutes are carbide so deflection is substantial even with my wimpy cuts. Maybe I should mess with the 1 flute downcutter I have coming, but it's only 1/4".

[ger21]

The straight flute router bit should be fine.

[GJeff]

I ended up using the 1/2" straight carbide router (2 flute) at 12-24k RPM, 110 IPM feed, beautiful finish. I had to limit WOC to 9% of dia with a depth of cut of .45". When I tried 15% WOC and .6" DOC, it definitely bogged down. Hard to say which RPM looked better, but the higher RPM sounded better (at the expense of tool life). All looked good.
Also tested O-flute (shape) downcut 1/4" carbide 2 flute 20k RPM 110IPM 15% WOC, .375" DOC, also beautiful finish. Honestly I couldn't determine which looked better, so I stuck with the higher material removal on the bigger tool.

Both were Freud. This hardwood is a bit more forgiving than I expected.