cutter breaking inside collet

[Greg VN]

We have a project going right now where we are using a carbide .25 spiral down cut mill. We are making mahogany grills. The cutter is triming the inside of the opening removing about .05 of wood .75 thick with a feedrate is 125" at 15000 rpm. Actually the feedrate is slower as we are not running at 100% at the controller. I have had at least 5 break in the same spot. Not where the spiral begins but about a 1/4" into the collet. I have replaced the collet with a new one and it still happens. Does anyone have any ideas why this would be happening consistantly in the same spot? I'm running out of cutters.

[Mike Gates from Barefoot CNC]

Is it breaking on a straight move or in a corner?

If in the corner, you may want to slow down your feed rate just before get to corner, then speed back up as you come out of it. If that doesn't help you may need to make 2 passes, as a .75 depth cut puts a lot of pressure on the bit even thou you are only cutting .05.

[Mike Gates from Barefoot CNC]

Also are you using an upcut, downcut, or compression cutter?

[Greg VN]

I don't know where it is breaking in regards to a corner. The part has several hundred openings that it is triming and there are 30 panels. Since I programed it using sub programs I can go in and edit the program to slow it down in the corners fairly easy. I like that suggestion.

Thanks

[Mike Gates from Barefoot CNC]

Ah.. a downcut, I missed it. It might help to try a compression cutter. This may put less pressure on the bit than the downcut, but still give you the clean cut you need. Also make sure you have plenty of vaccum under the grill to prevent vibration. Poor vaccum is another culprit sometimes in machining small parts.

[Greg VN]

It is a 2 flute downcut. I don't have any compression cutters on hand. I don't like a upcut even though it makes the most sense but it has too much chipping.

[Mike Gates from Barefoot CNC]

Another option is to reverse the direction of your cut. Sometimes this can help with the pressure but make sure cut quality is still o.k.

If nothing else helps you may need to have maitenance check the spindle for excessive vibration(bearing going out, etc.). This can definitly cause cutter brekage in small dia. tooling.

[smokey_44]

is the cutter being pushed up into the collet then breaking ? I've seen this before.

 

 

Dave

[Greg VN]

I'm using a climb cut if I go to a conventional I will get a undesirable cut in the wood.

My cnc is a 5 axis and the B axis is set at 9 degrees. It is not plunging into the wood it is plunging into an open area and than moving into the wood.

I ordered 10 down cuts and 5 compression cutters, they will be here tomorrow. I have 1 new cutter left.

I guess what seems odd to me is where the break is - inside the collet not at a weaker part of the tool.

[gti_jay]

Could be:

 

1. Faulty collet

2. Faulty holder

 

If it's breaking inside the holder, it's possible the holder/collet itself is not distributing the clamping force equally along the shank and rather locating it all at one point.

 

thus, breaking all your tools at the same point...

 

Also, don't rule out a bad batch of tools...If all the broken tools are from the same manufacturing lot, they may be faulty themselves..

[Mike Gates from Barefoot CNC]

That is a very valid point. I had it with a batch of 3/8 compression cutters once. Come to find out the grind in this batch was different than the ones I usually got. Tooling companies have the same employee turn over problems as the rest of us.

[Greg VN]

Thats what I was considering. The cutters are from the same company. They were purchased at two diferent times but they could of easily been manufactured from the same batch. When I told the supplyer what was happening he said he would pass it along to the manufacturer (Onsrud). A previous project I had the same thing happen to the same cutters. At that time as well as this I replaced the collet with a new one and the problem continued. This time I will save the broken cutters. I have a plastic bag with 3 in it from yesterday. I have about 15 more panels to cut out and they take about 3 hours each. At this rate I will have a few more to add to the bag at $40. each. I'm going to look over my program to see if I can find any way to help reduce stress on the cutter.

Thanks

[robk]

I've had the same problem with Promax cutters. At $200.00+ per cutter it was getting pretty expensive. Tried new holders, new collets, different machines and all were breaking in the same place, and thet was on our "standard" programs that are about 8 years old. Promax admitted to having a bad batch, but they would not work with us... Fu*k 'em I say... Try different brand.

[les]

this may sound dumb, but , measure the tool and see if it is undersized. I have some Onsrud .25 down sprials with the cutter&shaft at .24 something { can't remember exactly }.

 

 

Chimo Les

[Greg VN]

The shaft measures .247 is that enough to be a problem?

[robk]

quote:

---

The shaft measures .247 is that enough to be a problem?

---

As far as I know all cabide cutters are undersize...

Unless you're plunging the cutter into a corner it shouldn't be an issue. You could and should relieve the corners by drilling them out first, but you also said that you don't know where the cutters break.

[George_Gerhard]

Onsrud has had quite a few complaints lately the shear of the cutter goes far up the shank. Is there any shear inside your collet?

This allows them to use a bigger wheel when grinding, but the use too much shank causing breakage at the collet

rgds

George

[Greg VN]

The 3 broken cutters that I have there is at least .25" of full round shaft between the break and the machining for the shear.

I called the supplyer about the diameter and he was going to call Onsrud.