[rich_cree]

Hi everyone

I'm having a specific problem with my cnc taig. During a longinsh run (over an hour) of milling pockets into 7075 aluminium the pockets suddenly get deeper.

I don't think this is an issue with the motors/programme/software. i think it's potentially the tools slipping in the ER collets I use. Unfortunately both times its happened I've not measured the length of tool protruding from the collet prior to cutting so I couldn't just measure and check.

So my question is >>> If a collet looses its grip on an endmill does it tend to come out of the collet whilst milling, or would it go further into the collet?

Thanks in advance

Rich

[packrat]

The helix on the cutter tends to draw it into the work, until it dulls to the point that friction between the cutter and work pushes it up in spindle. So the short answer is- yes. it will do both

cary

[lsces]

One of my customers has just been fighting a similar problem! He was using a nice new 6mm ball nose cutter, but was loosing position vertically. In this case it was the mount for the Kress spindle that was moving very slowly down the V plate. But it does show that the cutting pressure puts some strain on the vertical axis. Adam just nipped the mount up a little more and things came good, so it is more than likely that you have a similar problem and it's worth checking both collet, and the head stock.

On a slightly different tack, it is worth pointing out that as the tool gets blunt, then the weight of the motor ( the Kress is a lot lighter than the ER16 assembly ) is not enough to hold the tool down, and so backlash on the Z axis is as important as X and Y

[Menacer]

Hey Rich,

I've had this happen to me a bit to much. now please keep in mind I do this as a hobby... No school... This is what i have guessed. I seem to have ordered every collet but skipped one. So collets can only get so tight and from what I understand the more down-force put on the collet the tighter/ harder it grips and maintains center of tool.

This is were the problem is... If your collet is to big for your tool theirs a point that it can't grip any longer (Look at the end this is what the slots are fore ) and because of this the end mill wants to pull downward BUT! the end-mill is not like a drill if you look at the tip of drills there made to remove material from the lowest point and toss it upwards.. end-mills are more for cutting wile traveling on a X or Y.. so if it slips its not just going to dig in as it will slowly fall.. try this with a drill and watch it snap!

Sorry for all this extra info... basically I think you need to use the next size collet down and force the tool in.

As you can see I'm not to good at weighting so if someone else who knows what there talking about can explane this please help me out here.

[Jeff-Birt]

basically I think you need to use the next size collet down and force the tool in.

Bad advise, you should never force a tool in a collet. If the tool does not slip in easily then you have the wrong collet. ER collets have a 1mm grip range so the shank of your tool must fall in that range.

One common mistake when working with collets is to over tighten them, doing so can distort the closing nut and damage the collet. If the wrench your using on your ER16 collets is more than 6" long then you stand a very good chance of over tightening (in other words do not use a cresent wrench or large end wrench). Also you need to keep your collets and spindle clean, google 'ColletCare'.

One way to diagnose the problem is to measure the 'stick out' of the tool before starting, measure it again after your done. This way you know if the tool is slipping or not. I have found that carbide tools will slip quite readily in worn out collets.