Machining Void in Blind hole 1

[shinjukus]

Hi Guys,

I have a product which is a honeycomb shape with lots of round blind holes. The quality issue that I am facing right now is that I am seeing void (some can be as big as 5mm long and 2mm wide) on the vertical side wall of a random blind hole.

I did a search and found that to reduce such defect, it is recommended to
1) Apply cutting fluids by drip, mist or through the tool to minimize the machine fluids present. Avoid flood application. Use air or vacuum to remove chips.

2) Use low viscosity, low surface tension fluids that will drain easily.

Questions:
1) Is this defect (void in blind hole) inherent in the blind hole process?
2) Should I drill a pilot hole first before the blind hole process in order to eliminate these void?

Thanks.

 

[unclesyd]

What are some sizes?
What is the L/D.
What is the material?
Number of holes?

 

[BobM3]

Is it a casting?

 

[arunmrao]

How did the void appear originally. Like bobm3 mentioned is it a casting? Else difficult to get a void in a rolled plate.

 

[shinjukus]

Hi guys,

Thanks for the quick reply. Here are more details:
1) What are some sizes? The random void size can varies from 5mmL x 2~3mm W and it is usually shape like a snake. This appears on the vertical wall of the blind hole. It looks like a casting porosity but it is not.
2) What is the L/D. Blind hole Dia 4.5mm, depth 7.89mm
3) What is the material? Aluminum 6061-T651
4) Number of holes? 96 holes.
5) Is it a casting? No, it is a full machining from a standard block say 120mm x 75mm x 10mm.
6) How did the void appear originally. Like bobm3 mentioned is it a casting? Else difficult to get a void in a rolled plate. According to my supplier, the chip got spot welded onto the cutter and caused the void. Is this true?

Need your expertise. Thanks.

Cheers.

 

[MikeHalloran]

Chips getting stuck to a cutter is not unusual if the cutter is rotating too slowly. Ideally, the cutter will produce short, friable chips that will literally fly out of the hole. Which shouldn't be difficult; that's not a particularly deep hole.

I have to ask; how did this become your problem?
If you're buying the machined part from a supplier, why is it not the supplier's problem?



Mike Halloran
Pembroke Pines, FL, USA

 

[shinjukus]

Thanks, Mike. The reason that the customer is involved, due to bad yield at the supplier who is unable to feed the pipeline. Usually the quality guy is tasked to go to the supplier to understand the problem. So, i came to this forum to seek your experiences and expertise to see if the supplier is telling the truth and also to work with them to get out of the current situation.

Questions:
1)Other than the r.p.m. of cutter and the coolant, does the feed rate of the cutter play an important part?
2) Do you think a pilot blind hole (slightly smaller than the actual spec called out) prior to the actual hole size will give considerable leverage in reducing the void in the blind hole.

Regards

 

[arunmrao]

Can you try EDM or other techniques for one piece and see if the problem persists. If it has gone away then the chip theory holds good,else blame the stock.

 

[unclesyd]

I would checkout the literature from Guhring on drilling Al. They are other people that also make drill bits specific for Al that you could checkout. There are a couple of approaches to drilling Al based on the geometry of the drill bit but we normally stayed with Guhring. The key point in with all approaches is that you don't want the bit to dig in, keep the material cool, and get the chips out.
Another approach might be no more than changing you cutting fluid for the current setup to something specific like the Tap Magic product for Al or any other fluid specific for Al.

http://www.guhring.com/ProductsServices/Aluminum.aspx

http://www.tapmagic.com/TMaluminum.htm

 

[ornerynorsk]

I highly doubt it would be a material problem, that is very rare. Sounds like cutter deflection from the initial plunge feed in the Z direction. 2/1 is fairly deep for that small of dia cutter. You have very little rigidity when you take into account the flute relief, the root of the cutter does not have substantial cross section. What kind of accuracy do you require? Perhaps a flat bottom drill would work instead of milling.

 

[BobM3]

Chip welding to the drill is very common with aluminum. But I'd think a chip welded to the drill would produce a void around the circumference of the hole as the drill and the material on it rotate. I'd bet that a loose chip is periodically wedging itself between the drill and workpiece.

Try running it on a machine that can apply high pressure coolant through the middle of the (special) drill. Or try a secondary reaming operation or secondary cleanup with an end mill.

 

[saberblue]

You might eant to look at a dimond coating on the mill or drill to kill off the BUE

A.R. "Andy" Nelson
Engineering Consultant
anelson@arnengineering.com

 

[unclesyd]

What is happening is that the two materials are galling and when the Al welds to drill bit it tears out a chunk. Al tends to get very hot at the point of contact with the tool edge which in turn exasperates the galling. Al will also weld to it's self by rubbing, which is the reason to get the chips out of hole. Working multiple holes chip removal gets to be chore.
The galling is the reason that some operations use highly evaporative coolants, such as alcohol. We used Trichloroethylene + oil for years until we found out that we could have an explosion.
As stated in my previous post there are different approaches for getting the chips out by using drill bit geometry in drilling Al. A lot of manufacturers use a shallower point with high helix flutes while people like Guhring uses parabolic flutes with sharper point. There are three and four flute drills. Most Al drill bits don't really bit into the metal as they tend to shave if off as on a lathe. Speeds and feeds are all over the map but one should go with the manufacturers recommendation.
Another point often missed when drilling Al is the setup must be rigided.

 

[btrueblood]

Using peck drilling techniques also helps to clear chips from the drill bit, and provides a method for re-lubing the point. Time consuming, but it works.