Upcut, Downcut Or Compression Tools

Which is better? It depends on the use (and your technique). April 10, 2005

We are purchasing a new flat bed CNC router and are wondering if there is much of a difference in the finish from the different tool types. We will be working with 18, 25 and 40mm double-sided veneers on mdf or chipboard. Has anyone tried the different types and noticed the difference between them (up, down and compression)?

I understand that compression is the recommended tool type but at double the price are they really worth it? Any advice would be appreciated.

Forum Responses
(CNC Forum)
From contributor A:
In my experience an upcut spiral will chip the top surface of material and a downcut spiral will chip the bottom. Hence a compression which is part upcut and part downcut, leaves the material chip free. I have had good luck with an Amana carbide tipped opposite shear (which is the same as compression) at a lower cost of purchase and of sharpening.

From the original questioner:
Thanks for the feedback. The reason I'm asking is that when using a hand based router on double sided material, this chipping does not seem to be a problem.

From contributor B:
As contributor A said, use a compression bit to do through cuts on double-sided material. This bit produces chip free cuts on both sides. Use a down shear bit for cutting dados, rabbets, pockets, etc. This will leave the top surface chip free (a compression will most likely chip the top surface).

From contributor A:
You need to check the length of the upcut on a compression bit. The dado needs to be deeper than the upcut to get a chip free dado. What I usually forget is to make the first pass deep enough on a multiple depth cut. There are bits (mortising compression) that have a smaller upcut than a standard compression spiral.

From contributor B:
You also need to check the upcut length of a compression bit to ensure that it is not higher than the thickness of the thru-cut, otherwise it will chip the top of the material.

From the original questioner:
Thanks for all this information - it will save me a fortune in stock and experimentation.

From contributor C:
Are compression tools or any solid carbide tools really worth it? Not if you are running them incorrectly. On a CNC a carbide tipped tool will do the job, but at slower feed speeds and more tool changes. With a solid carbide tool, it will last a minimum of 4 times longer and increase feed speeds also by the much if not more, with a superior edge finish. If you are using a hand held router solid carbide is probably not the way to go, as you will not realize the benefits.

Here’s a little story. 15 years ago I was hired as a consultant to decrease cycle times at a very large furniture mfg. The tooling we designed reduced not only their cycle times by 70%, but reduced their need to sand the parts. The sanding dept. had 240+ employees. The company decided against this as it would put a few hundred people out of work, (it could have deployed these people to assembly or other positions) and their brothers, sisters, etc. would have been out of a job. Since then the company has closed numerous plants, let people go and are importing their parts from China. Over 60% of firms using a CNC are not utilizing them as were intended.

From contributor A:
My limitation for router speed is based on vacuum holddown more than anything else. I can't go as fast as a solid carbide should be run without moving parts, so I use the carbide tipped at a slower speed.

From contributor D:
Yes, the compressions are worth it if you are running the correct feeds and speeds. Keep in mind also that the collets are just as important.