Vibration Ramping

I'm using an R8 collet in my Myford VME Mill.

I'm trying to reduce the thickness of some gauge plate, and then add numerous holes.

I have mounted the plate on two parallels, the gap bridged underneath is 150mm. I have been told that 15mm thick plate can bounce quite easily, however, I proceeded being that I need the gap underneath in order to drill my holes through the work.

As I was reducing the thickness over the area of the finished piece with a carbide center cutting 12mm endmill, going from side to side etc, I noticed the tool was making a progressively loud vibration noise. Closer inspection of the work revealed that the endmil had been slipping down the collect and thus been cutting a downward ramp into the work instead of a flat cut.

Questions.

Although I'm inexperienced at milling I'm fairly confidant the collet was tight enough, I mean you shouldn't have to get them so tight that they are close to striping threads on the draw bar do you ? So I'm guessing this was down to the work piece bouncing and thus grabbing the tool and pulling it into the work ?

Can bounce really do this ?

Also, in order to stop bounce should I have a third parallel bar in the middle, and then remove this to drill my holes at the end ?

Any help or advice would be much appreciated.

Many kind thanks.

David

My R8 can take threaded shank cutters to stop this sort of thing happening.

I've never had a cutter slip in an R8 collet, even the largest ones with aggressive cutting, so I'm surprised that you have this problem. It is the cutter slipping and not the vertical feed? How are you applying the latter? Does the cutter fit the collet properly? - it should be a light interference fit when you slide it in before clamping.

I would be inclined to clamp the plate flat on the table for thinning it, on a piece of thin card or thick paper. You will have to demount it and clamp it again - I always use a bit of MDF for drilling flat plates, better support, clamp the plate flat on the MDF on the table, drill through into the MDF..

Is the cutting speed right? Carbide tools should run quite fast I think.

If I don't lock the quill on my bridgeport clone it will slowly pull in giving the same effect.

David

Hi Richard,

Applying basic beam theory, your assembly would be more than eight (8) times stiffer (in the downwards direction), with a central prop.

So your idea of introducing a third parallel makes a lot of sense.

Good luck,

Sam

It wasn't the quill, It was clear to see that the endmill had moved from where I had originally set it.

I don't have enough experience to comment on the fit of the endmill in the collect, however, had it been loose when it was in my hand before attachement common sense would have told me something wasn't right.

Definitely a 12mm collet.

It can only be that the collect wasn't tight enough. I suppose by the low response to my question of vibration/bounce, we can rule this out. Back to the machine and a much closer inspection of the collect and drawbar.

Kind thanks for everyone's help.