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Dave · 03-20-2005, 07:53 AM

I just wanted to share some experience I have with plastic cooling block "cracks".
I have been reading on this board, about cracks being caused from some liquids, thermo difference between fittings, etc.

It is more likely the cracks where already there.

Please I am not trying to argue with you guys, I just want to explain what I found out myself.

I was making a prototype at AVT when Dan came up behind me and said "don't do that", which happens often. I had machined a piece of acrylic with threaded ports. Dan indicates that you can not cut threads in acrylic without causing cracks.
As is the norm, he does not explain why so I tried to figure it out myself after hours.

I used dye on my first piece and sure enough there were cracks all around the threaded area. Assuming it was only a machining issue, I cut several samples to test with different tools and speeds.

For the first test I used a profile thread mill, and cut samples at 3000,6000,10000, 20000, and 30,000 RPM all at low feed rates. Dye showed cracks in all with little to no difference between the samples.

I them repeated the above test, this time with a single thread very sharp carbide mill. This was sharp enough to scratch glass so I figured it would be the answer. The dye showed I was clearly mistaken, although there was a small improvement.

When I finally had Dan's attention I asked why and all he said was "it is the shape of the tool".

Amazing what I learn at work on a daily bases, that should have at least been discussed at school. I have seen people machine plastic at collage all the time.

Just my experiences please no arguments. I suspect someone in here will explain what is happening in more detail then Dan has.

03-20-2005, 07:53 AM	#1
Dave Cooling Savant Join Date: Oct 2004 Location: Canada Posts: 260	About the "Cracks" I just wanted to share some experience I have with plastic cooling block "cracks". I have been reading on this board, about cracks being caused from some liquids, thermo difference between fittings, etc. It is more likely the cracks where already there. Please I am not trying to argue with you guys, I just want to explain what I found out myself. I was making a prototype at AVT when Dan came up behind me and said "don't do that", which happens often. I had machined a piece of acrylic with threaded ports. Dan indicates that you can not cut threads in acrylic without causing cracks. As is the norm, he does not explain why so I tried to figure it out myself after hours. I used dye on my first piece and sure enough there were cracks all around the threaded area. Assuming it was only a machining issue, I cut several samples to test with different tools and speeds. For the first test I used a profile thread mill, and cut samples at 3000,6000,10000, 20000, and 30,000 RPM all at low feed rates. Dye showed cracks in all with little to no difference between the samples. I them repeated the above test, this time with a single thread very sharp carbide mill. This was sharp enough to scratch glass so I figured it would be the answer. The dye showed I was clearly mistaken, although there was a small improvement. When I finally had Dan's attention I asked why and all he said was "it is the shape of the tool". Amazing what I learn at work on a daily bases, that should have at least been discussed at school. I have seen people machine plastic at collage all the time. Just my experiences please no arguments. I suspect someone in here will explain what is happening in more detail then Dan has.