general rpm and feed rate query

[denmanmarine]

Hi all,

I have a PRS Alpha with a 5hp Colombo spindle and wanted to ask a general question on rpm/feed rate.

I was trying out a new compression bit today (Onsrud 60-126MC) in 5/8" MDF (cut in two passes) The initial cuts were at 6 IPS and 12,000 RPM and were noisy.

I consulted the chip load chart for this bit which advised chip load for a 3/8" bit was 0.019 - 0.021 indicating a feed rate of 12 IPS which seemed pretty fast to me so I though I'd try 10 IPS but using the formula for RPM, the RPM dropped to 10,000 which seemed low and actually caused the gantry to jump of the rails as the machine bogged down trying to take too big a bite at too low an RPM.

The last pass I tried 16,000rpm and 10 IPS and it sounded pretty sweet and cut nicely but if I plug those numbers into the relevant formula, it indicates I should be feeding at 16 IPS which really seems too fast.

Anyone have any suggestions on rpm and feed rates for this cutter and material?

TIA

Andrew

[chiloquinruss]

I don't have an alpha but I do have a spindle. I was always confused by all of the feed and speed charts and all of that stuff. Early on I saw a simple post that said we should be making CHIPS not sawdust. Secondly the chips should be warm and the bit remains cool. It seemed to make sense actually but I was not convinced. So I took a piece of scrap (which I seemed to be making a lot of at the time) and made a test file that had several test cuts. All at various speeds, rates, and passes. When running the test file, when the noise level dropped everything looked great. If it bogged or screamed or . . then all was not well. My cutting these days seems to be mostly 3/4 inch HD cabinet (big joke here) plywood, 1/4 in compression, 4 passes, 13-16k 3-6ips. Warm chips, no sawdust, cool bit (after it stops). Russ

[knight_toolworks]

with a downcut it is often sawdust though even with a new bit so it is hard to go by that when using them. the smaller the bit the worse it is.

[dlcw]

With a 2.2hp spindle on a PRSAlpha I cut most plywood with a 3/8" compression at 6ips at 12K rpm. Used to use a 1/4" compression bit but got to much chattering and vibration and had to slow down my feed rates (not what I wanted to do).

First pass is a single climb cut all the way leaving a 1/32" skin. Second pass is a single conventional cut 1/32" thru the material to be cut. This method gives a super clean cut. My spindle doesn't bog down with these numbers. I cut about 400 to 500 sheets of plywood a year ranging from 1/4" to 3/4" thick and everything in between. These are the numbers I've zeroed in on for MY machine. Your results may vary.