Bit Choice and Feed Rate for Cutting Small Parts

The machine never reaches full speed when cutting lots of small parts, so adjust accordingly. October 20, 2013

I need to make a lot of small cutouts in 3/4" MDF, as fast as possible, using a 1/4" tool. Top edge quality is important. I was going to use a 3 flute downcut, making 2 passes at 500ipm and 18,000 rpm. Can I do this in one pass, or go even faster? Machine is a Morbidelli 636 with vacuum table.

Forum Responses
(CNC Forum)
From contributor J:
I would mirror the part and use an upcut bit. You will want to clear the kerf as much as possible.

From the original questioner:
I can't because there are a few small shallow pockets.

From contributor M:
Not sure of the acceleration/deceleration of the servos on this machine, but you say the cutouts are small. If they are small enough, you will never hit programmed speed anyway. The servos might start the deceleration for the corner, or next move before it can even accelerate out of the last position. Anyway, just something to consider. Going at a slow, reasonable feed rate at full depth may take less time than two passes at any speed, especially if the tool never hits those feed rates anyway.

From contributor D:
A down cut will run slower than an upcut for the reasons listed above. Try a 1/4 compression. Also, MDF quality is a big deal. I use Plum Creek with a 1/4 upcut and while the top edge fuzzes, it wipes off and the edge is still decent. Cheaper stuff won't do this.

From contributor A:
I agree with Plum Creek brand being the best, as it is the only product used by a MDF door manufacturer that does roughly 100,000 doors a month through three Wemhoner presses. As for feed rates, this customer uses strictly diamond bits on his 8 four head machines cutting at 12000 rpm and 25 meters per minute. They have been running these rates for the last 10 years. I would say, to be exact, get the weight of the MDF that you are using as it will make a huge difference to your feed and speeds.

From contributor C:
You need to try the 3 flute slow spiral upcut. We have had great luck with this tool. The slow spiral is not as aggressive and does not chip the top as bad, but you still get the benefits of the spiral and upcut in getting the chips out. That is one of the tricks in CNC routing.

I agree with the others - the machine will more than likely not get close to the feed rates in the machine on small parts.