Chatter with 1/4" onsrud spiral cutter

[jerbear66]

We do a lot of 2D cutting in various thicknesses of MDF with 1/4" spiral downcut bits from onsrud. We are getting a significant amount of chatter on both the finished piece and the scrap piece. We cut at 13,000 rpm and a feed rate of about 1.5 ips. Anyone have any idea if it is the bit, the spindle, the speeds or anything else I should be looking at.

[paco]

Gerald,

you may speed up the RPM (not much at a time) to see if it get to less chatter... but the first thing I would consider/recommend about chattering is lenght of tool; use the shortess you can... always if you want to push the machine to maximum possible feed. You might want to consider a larger tool CED if the job allow it...

Do you have a good hold down on thoes sheet?

[richards]

Gerald,
Paco's recommendations are right on: short cutter length and good hold-down. In addition, the Chipload Calculator that I use shows that a feed rate of around 6.5-ips should be used for a 2-flute cutter and about 3-ips for a 1-flute cutter at 13,000 RPM. If you're really running at a true 13,000 RPM, you might try increasing the feed speed. If you're using a router, instead of a spindle, then the 13,000 RPM setting might be giving you something much lower than 13,000 RPM (sorry, but I've experienced that myself with the Porter-Cable 7518 router before getting a 3-hp spindle).

Personally, I get a much better finish using a 3/8-inch cutter than I do with a 1/4-inch cutter, and a 1/2-inch cutter is even better, at the expense of wider kerf and higher router/spindle hp requirements.

Also, ramp speeds played a big role with 'chatter' on my Alpha. I've set mine quite a bit lower than the factory default.

-Mike

[paco]

A word of caution with thoses "Chipload chart"; they're mostly appropriate for large (I'd say 3/8" CED) bit... I've snap of bunch of 1/8" and 1/4" CED ones trying to follow 'em...

I've observed that long thin tool tend to chatter under load, so one have to work with more respect. Keep 'em short, you'll be suprised of how much you can push 'em more the shorter they are... and remember, as for drilling, that small CED bit need to be rotate faster (high RPM) in order to have the right velocity.

One other thing; I have never been a fan of thoses downcut... I'd recommend straight or upcut... or better yet compre$$ion!