Little help needed please...

[jedgerton]

OK, just when I thought I was getting the hang of this pen making thing, a recurring problem has got me stumped. I'm getting cracks in my acrylic turnings at the end of the tube. Usually the cracks are between 1/16" to 3/8" long and they are along the length of the tube.

I usually don't see them until I finish polishing the blank and then they stick out like a sore thumb. I've seen these in acrylic blanks from Woodcraft and in Inlace acrylester blanks from Woodturningz. I'm using Gorilla glue for gluing in the tubes and I use sharp tools with a light cut. In fact, my most recent problem pen, a Sierra, the blank was so uniform after turning that I was able to start with MM1500.

My polishing procedure is MM1500 - MM12000. I do this dry and frequently move the MM to avoid any buildup.

Any ideas?

John

 

[stevers]

OK, just when I thought I was getting the hang of this pen making thing, a recurring problem has got me stumped. I'm getting cracks in my acrylic turnings at the end of the tube. Usually the cracks are between 1/16" to 3/8" long and they are along the length of the tube.

I usually don't see them until I finish polishing the blank and then they stick out like a sore thumb. I've seen these in acrylic blanks from Woodcraft and in Inlace acrylester blanks from Woodturningz. I'm using Gorilla glue for gluing in the tubes and I use sharp tools with a light cut. In fact, my most recent problem pen, a Sierra, the blank was so uniform after turning that I was able to start with MM1500.

My polishing procedure is MM1500 - MM12000. I do this dry and frequently move the MM to avoid any buildup.

Any ideas?

John

Are you saying the cracks are perpendicular with the tube or are they in a ring around the blank?

 

[alphageek]

I read that these cracks are the long way - is this before or after assembly? If its before, how hard did you have to push the bushings? Did they slip in or press tight?

 

[jedgerton]

Direction of cracks

The cracks start at the end of the tube and extend along the length of the tube. I hope that helps.

The bushings slipped in easily and the crack is present before any assembly.

John

 

[stevers]

I wounder if the GG could be cracking the blanks. It does expand as it cures.

Are the tubes clean on the inside when you press in the parts? I've had blanks crack because the tube had glue in it and the part pressed in hard and cracked the blank. ????

 

[fernhills]

Try wet sanding, just because you can`t feel the heat, doesn`t mean it is not there. Speed is a factor to.

 

[Skye]

My guess would be:

A. When you're squaring the tubes, use a light pressure. I let the blades do a combination of cut and heat the ends of the blanks. If you get chipouts on the ends, you're probably going to get cracks. I'm sure the GG will do nothing but make this worse as it's an expanding glue.

B. When you're turning the blanks, be very gentle with the ends. It's the easiest place for the tool to catch and chip the acrylic. Be sure the tools are sharp and always work your way out to the ends from the middle until you're touch is controlled enough to start from one end and take it to the other.

That's what I'm thinking anyhow.

 

[maxwell_smart007]

I don't htink it's chipout, Skye, as he said it's running lengthwise...

My guesses:
A) Gorilla Glue is a polyurethane that expands - it could be that it's too constrained by the size of the hole you're drilling, and putting excess pressure on the blank - hence the cracking. Try a different adhesive, or drill a slightly larger hole.

B) Perhaps you're snugging the mandrel nut up way too tightly? I suppose that could also have the same effect, by putting excess pressure on the blank, but my guess is still the first...

Andrew

 

[george]

I once noticed crack blank on kerrock when the pen was almost finished. I belive that it happened while drilling the blank ( I drilled it the same way as wood blank - all the way threw), and the drill crack it while exiting. Since than I make little longer blank, stop the drill before exiting the blank and cut away the rest of the blank. If the crack is made when drilling, the PU glue could make it bigger while expanding during curing time.

 

[heinedan]

John,

I have had this happen to me when turning wood blanks. The problem was that I was cutting the blanks a little bit longer than the tune. When I mounted on the mandrel, and cranked down the knurled knob, they cracked and split on the ends. Make sure your blanks are cut/trimmed to the exact size of the tube.

Dan

 

[jskeen]

I'd be willing to bet that this is happening when drilling through the end of the blank. Even if your careful enough to not have the blank explode when the drill exits, it still stresses he material at the point where the flute of the bit first catch the end of the blank that is at 90 degrees to the direction of drilling. These cracks can run parallel to the axis of the blank for a good distance, then they usually curve off to the right (kind of like my golf shots) but if you are being really gentle, only the straight part may be showing up. Only way I know to avoid for sure is to stop short of the end of the blank and cut off the undrilled portion.

 

[DurocShark]

I rarely have a clean exit hole when drilling. I always allow some extra length to the blank when cutting then I can trim off any chipping or cracking.

EDIT: When drilling acrylics and similar materials. Wood is usually fine.

 

[jedgerton]

Good advice

Thanks everyone! I've concluded there may be multiple factors but I believe the most significant issue was the drill bit size and the pressure from the expansion of the Gorilla Glue. What I figured out tonight is that I used a 10.5mm drill bit instead of the recommended 27/64" bit. The tube does fit but its tighter and I believe the expansion of the Gorilla Glue just created too much tensile stress.

I tried another tonight with the same results (both drilled to 10.5mm). I'm thinking of sticking with the 10.5mm drill and then use medium CA. Sierra's get so thin that I'm worried about an uneven pattern showing up from the glue itself.

If its not caused by the issue above, its probably something else

John

 

[george]

If its not caused by the issue above, its probably something else

John

Good one. Perhaps try with nickel tubes, if you are worried that ordinary tubes will be seeen threw. Good luck.