Countersink Scratches

[asw20c]

I tend to get these weird scratches on the aluminum when countersinking. It obviously can't be the bit, and the location puts the scratches somewhere between the bit and the cage. The only thing I can figure what is causing it is probably the chips coming out of the hole somehow get swept around the countersink between the bit and cage. I would expect to see concentric circles, which are there, but the irregularly shaped scratch is the real mystery. Is my technique messed up somehow? Does anyone have any insight into how or why this is happening?

[asw20c]

[vernon smith]

These came from the countersink depth guard spinning against the work. Either hold onto the guard with one hand so it doesn't spin or buy the adjustable countersinks sold by Pan American Tool;

http://www.panamericantool.com/count...e-bearing.html

They have a Teflon protective ring around the surface that contacts the work preventing the guard from spinning and from scratching the work.

[asw20c]

Nope. My countersink cage has the nylon lip that contacts the work and it wasn't spinning. If you look closely at the photo you will see what looks like a dusty ring. That ring is where the nylon lip contacted the work and it is outside the scratches.

[VA Maule]

I don't think it's the depth guard,the scrap is the same size as the Cleco to the right. Looks to me that you're applying too much pressure at the start and making too large chips and curls coming off the bit faster than they can exit and subsequently they drag around in the cage making the scratches.

[asw20c]

Thanks. I thought it might be a technique problem. I'll try it without so much pressure next time.

[rvbuilder2002]

Using an air drill running at high speed can also be a factor in scratches caused by swarf (chips).
Slower speed and medium pressure also reduces the chance of cutter chatter.

[XOverZero]

I had several instances of concentric scratches at countersinks. It was caused by my using a single flute bit in the cs cage. One flute creates one long continuous chip that bunches up and spins with the cutter. I switched to a multi flute cutter. No more swirlies. Short chips just fly out and away.

[StressedOut]

I had terrible problems with massive chatter the first time I tried to countersink. It was on the VS doubler which is 0.125 thick. Besides screeching like a banshee, I got small aluminum particles and the characteristic wavy looking surface.

The only way I could solve the problem was by going very slow (about 120 rpm) and using very light pressure. Only then did I start getting smooth cuts and the long, curly chips. The resulting finish was superb as if it was polished.

I was using a three flute cutter from Cleaveland Tools. I also bought a single flute cutter, but before it arrived I solved my countersinking problems so I've never had a chance to use it.

[asw20c]

Here's another clue that may contribute to the conversation. This is a multi-flute countersink bit, and sometimes the chip bunches up like a tumbleweed and sticks to the bit. Usually I can just blow in the cage to clear out any chips, but sometimes I have to "dig" it out with something like a punch. In other words, it seems to be snagged, or stuck on the bit somehow and I have to break it up to get it out. This must be how those weird scratches are created, but again I would expect concentric circles and not the meandering scratch like in the photo.