speeds & feeds / excessive flexing

[MacMarty15221]

I'm helping a buddy explore the limits of his Shark pro Plus. Right now we're having chatter trouble cutting a 2D sample block that we've concocted for this purpose. (see attached .crv) I'm hoping that the Great Hive Mind on this forum will have trod this path before.

Life was good when we worked with 3/8" birch ply and a Rockler 1/8 straight router bit. Things are also reasonably fine if we restrict ourselves to v-cutters for simple signs. Now we're stepping up to 3/4" solid stock, using a 1/4" carbide up-cut spiral bit, and we are getting regrettable amount of chatter, even with a pass depth of only .02" . This happens even if we dial the feed rate down to almost nil. As you might guess, the low feed rate is also giving us some burning. :/ We've tried cherry and pine, both have the same amount of chatter.

It's apparent that one factor is the amount of flexibility in the Y support rods and Z assembly. (I can pitch the Z carriage up and down with moderate hand pressure.) I should also mention that we've moved our material to the centerline of the table, since we have also observed that the entire gantry can be rotated in the yaw axis. (Thinking was that the yaw effects would be minimized if we were directly over the gantry's feed screw. Of course, that's where the Z support rods have the GREATEST flexibility. (Sigh.)

Are we pushing the Shark Pro Plus past its limits? Or are there some feed / speed or end mill choices that will allow us to cut 3/4" pine cherry at reasonable rates without chatter?

[jeb2cav]

At face value you have loose couplers (motor axis to lead screw) on the z and perhaps x and y axis. The key indicator of some mechanical defect is that you're getting this chatter at 0.02 depth, you can move the gantry up/down in z, etc. With the ProPlus I experienced some yaw with moderate pressure, so perhaps this is similar. You should not be able to move the gantry in any 'straight line' direction with the power turned on. If you can - check the couplers. There are a couple of good posts here:

http://cncsharktalk.com/viewtopic.php?f=9&t=1422

http://www.cncsharktalk.com/viewtopic.php?f=9&t=722

http://www.cncsharktalk.com/viewtopic.php?f=2&t=684

I've made some nice things out of black walnut and oak with a ProPlus running 30-40 ipm and had great results. Roughing pass for 3D component in hardwoods using 1/4" end mill (not a router bit) at 1/8" depth of pass at 30-40 no problem.

If you have more than 3 mounting holes on each side of the gantry bracket, and aren't using the 'top 3' - you may want to do this as well (unless you really need the maximum clearance possible more often than naught).

While I think you have some mechanical problems to sort out - I've also seen this kind of chatter even with no gantry mechanical problems - when the cutter wasn't really 0.25 in diameter for example - so it was moving around a bit in the collet.

Hope this gives you some starting points to look at.

[drueth]

MacMarty15221

How sure are you that the bit is sharp if you put it in a plane old router and just try to do a cut slot does it cut clean or does it also chatter. I did look at your tap file and the feed is way down I usualy set it to 50 and if I feel it is a bit to much youse the FRO to slow it down. I would try another bit and set the feed rate to 50. The other thing I would check it that the router is not cocked some way and the bit is not straight up an down to the table.

It would not be a bad idea to check the couplers as jeb2cav has sujested but it sound like a bad bit to me.

[rungemach]

Something to check is the runout of your collet.

I assume you are using a Colt router and they have a great variability in the accuracy of their collets. I bought several OEM collets to test them, and they were all over the map as far as runout was concerned. One out of 4 was acceptable, the rest were not up to the quality cutting you would like to do. If you are not using a precisebits collet you may want to consider that.

Since you are not using a straight cut bit, you might also check for free play up and down in the router shaft itself. The bearings in the colt are held down by a wave-spring, and after a while , plunge cutting pushes the lower bearing loose in the housing and the router shaft can float up and down.

From my experience with the older shark design, the "flex" in the bearings did not contribute to chatter significantly, but did greatly impact dimensional accuracy of the cuts.

Chatter was usually a result of the bit/collet combination, as well as any free-play in the system. If you can feel any free play or rock in the router, it will need to be tracked down and tightened up for you to get good surface finishes. FWIW

Bob