Blanks cracking

[Smokey7385]

I have cast several blanks using both the resin from Hobby Lobby and the kit that Craft Supply has. In every case the castings have had cracks in them. I can't tell if the cracks occur during the casting process or during drilling. My first thought was that I was cracking them when I drilled them, but I've been very careful while drilling and still end up with cracks. I don't have this problem with "store bought" blanks only the ones I cast. Anyone have any advice??

Thanks,
Rusty

 

[ed4copies]

Sorry this is gonna sound snotty, it's not MEANT that way!!!

LOOK at them before you drill. Isolate your problem. IF they are fine before you drill them, start changing drill bits (I have 3 10.5 mm bits, all bought at the same time, from the same source - two work for spit, one is GREAT -- I would NOT have known that if I had not had THREE to test).

Once you know WHERE your problem is, we can be more help in solving the problem.

 

[cnirenberg]

Do you use a centering drill first or just hog out the final hole? I have found out that using a centering drill then a smaller diameter drill bit works well. I then drill the final hole with the necessary bit. Works good for the larger sized holes(There is a joke there, I'm sure). Make sure the bit is sharp or new if you do not use the drill doctor thingy.

 

[Smokey7385]

No offense taken...

I looked over very carefully the blanks I was turning last night prior to drilling and found no visible flaws. In each case I only encounter the cracks after turning near completion and always where the blank is the thinnest. I did not notice any cracks after drilling, but will admit that I did only a cursory look at that point.

Obviously (or not) I am suspecting my casting technique, but I follow the directions to the letter.

 

[spitfire]

I have done a casting or 2 where I used to much catalyst and the casting cracked during curing. This was when I first started out and really didn't count how many drops of catalyst I put in. Kinda just squirted it in there and called it a day.

Maybe that could be part of the reason for the cracks in your case? Does resin cure from in the inside out? If so, maybe that is why you only see the cracks during final turning because the are in the center of the blank and don't make it all the way to the surface?

I don't know, just throwing some things out there LOL

 

[ed4copies]

So far, I am suspecting your turning technique.

But a high intensity light will answer our questions. IF need be, oil the outside of the blank and use the light - IDENTIFY the stage that makes the cracks.

 

[Smokey7385]

Do you use a centering drill first or just hog out the final hole? I have found out that using a centering drill then a smaller diameter drill bit works well. I then drill the final hole with the necessary bit. Works good for the larger sized holes(There is a joke there, I'm sure). Make sure the bit is sharp or new if you do not use the drill doctor thingy.

I don't use a centering drill. I was using a 7mm brad point bit in a centering jig on the one that cracked last night.

 

[Smokey7385]

So far, I am suspecting your turning technique.

But a high intensity light will answer our questions. IF need be, oil the outside of the blank and use the light - IDENTIFY the stage that makes the cracks.

I can't say that it isn't my turning technique, but I don't experience the same problem with the "store bought" blanks. I have maybe 10 pr/acrylic pens that I have turned sitting in my display stand none of which have cracks (I've done maybe 50+ pr/acrylic pens), but I always get cracks in blanks I cast.

I'm wondering if I may be using too much catalyst as suggested by spitfire?

 

[cnirenberg]

I am wondering if excessive heat created by drilling would cause the blank to crack?

 

[aggromere]

Yea, i vote for heat. I have had that problem in the past. I drilled a wood blank so fast and it go so hot while drilling, when it cooled completely down the tube wouldn't fit.

 

[hewunch]

is it cracked more on one end? Are you drilling all the way through your blank? Or are you drilling and then cutting the last little bit off?

 

[ed4copies]

I can't say that it isn't my turning technique, but I don't experience the same problem with the "store bought" blanks. I have maybe 10 pr/acrylic pens that I have turned sitting in my display stand none of which have cracks (I've done maybe 50+ pr/acrylic pens), but I always get cracks in blanks I cast.

I'm wondering if I may be using too much catalyst as suggested by spitfire?

Which store are you buying from???

CSUSA, Berea, Beartoothwoods all sell acrylic or "celluloid acrylic" blanks. These are NOT the same as Polyresin. Unfortunately, people on IAP use these terms interchangeably --- they are NOT interchangeable.

Woodturningz "inlace acrylester" is Polyresin. MY "metallics" are Polyresin. Apart from those, I know of no major manufacturer selling polyresin. So you are, inadvertantly, comparing apples to grapefruits.

Re catalyst -- if you have turned "inlace acrylester", that has about as much catalyst as you can have and still turn the blank. IF this is what you are comparing --- it is a GREAT comparison and should help you isolate your cause of difficulty.

 

[ed4copies]

I am wondering if excessive heat created by drilling would cause the blank to crack?

Yes, it can. Unusual, but possible.

 

[Smokey7385]

Ed,
I've used blanks from CSUSA, Arizona Silhouette, Woodturningz, Woodcraft and Augum's. I've found the blanks from Woodturningz very "chippy". I've turned several bottle stopper and cosmetic brushes, items cast by Ed Davidson (Yo-Yo spin), sold through Arizona Silhouette that are PR.

I'll have to cast some more blanks up tonight and take a very careful look at everything. I'll also document each step along the way to help troubleshoot.

I appreciate all the responses.

 

[Smokey7385]

is it cracked more on one end? Are you drilling all the way through your blank? Or are you drilling and then cutting the last little bit off?

Hans,
Yes it is always near one of the ends and usually only one end. I do drill all the way through with the blank sitting on top of a piece of waste wood.

Being only one end would certinly indicate heat, I'm thinking. If that is the case, why would my castings be more susceptible to cracking over other blanks?

 

[ed4copies]

I forgot about Ed, but that is also a GOOD comparison.

If you can make bottlestoppers from him, your turning technique is likely to be very good. More info sent to you by PM.

Good luck!!

 

[DozerMite]

Heat can vause cracking whether during curing or drilling or even while turning. Try feeling the blank as you turn it and see if it is getting warm.
If the bit grabs as it comes out of the blank, it can crack it. You may not notice it if it grabs, it doesn't take much sometimes.
Also, if the bushings are tighted too much (I say bushings because I don't know if you turn with or without a mandrel) it can cause it to crack, especially if it is getting warm while turning.
Use a damp paper towel to keep the blank cool.

 

[hewunch]

try this then, leave your blank a little long, drill almost all the way through then cut off the rest. See if that helps.

 

[ed4copies]

try this then, leave your blank a little long, drill almost all the way through then cut off the rest. See if that helps.

This SHOULD be SOP (standard operating procedure) with Polyresin blanks. Will cut down your waste factor --- at least it did, mine.

 

[JackTheSquid]

A contributing factor could be how the ends of the blank are squared after drilling and gluing the tube. I had some trouble with cracking on the ends of the cut blanks and discovered that my squaring tool was dull and causing small cracks at the end that were not visible until I had turned down to that point.

 

[Smokey7385]

Thanks to everyone who responded. There are three things I can take away from this:

1. Drill less than all the way through and cut off end.
2. Keep the blank cool while turning and sanding (I actually just started wet sanding from start to finish).
3. Make sure the barrel trimmer and drill bit are sharp.

 

[sbell111]

A contributing factor could be how the ends of the blank are squared after drilling and gluing the tube. I had some trouble with cracking on the ends of the cut blanks and discovered that my squaring tool was dull and causing small cracks at the end that were not visible until I had turned down to that point.

That's a good point. Also, if it hasn't been milled all the way to the tubes, then the bushings are putting pressure directly on the thinly turned plastic which, while typically causing more spectacular damage, could cause cracks, I suppose.

 

[HawksFeather]

I really have no clue as to the cause of this, but I have a question that might or might not go along with this topic. Does anyone use transmission oil to lubricate their bit when drilling these blanks or similar blanks? I know that when making acrylic calls (from cast acrylic) you just about have to use it or something similar to keep it from getting too hot and checking and glazing.

Jerry

 

[ed4copies]

I really have no clue as to the cause of this, but I have a question that might or might not go along with this topic. Does anyone use transmission oil to lubricate their bit when drilling these blanks or similar blanks? I know that when making acrylic calls (from cast acrylic) you just about have to use it or something similar to keep it from getting too hot and checking and glazing.

Jerry

I have used oil, cooking oil, "pam' spray, water, water with dishwashing soap - they will all help. Cast acrylic is more temperamental than Polyresin OR "penturning acrylic blanks".

IF you use ANY of these techniques, remember the liquid is an accelerant to CA. So, if you are going to glue your tubes in with CA (as I do), dry with compressed air first (this will also COOL the hole so the CA does not set instantly and lock your brass tube in place, sticking out of the blank--DAMHIKT- I read it somewhere!!)

 

[dgscott]

I'm wondering if you are successful at getting out all the air bubbles after you pour. I set my molds adjacent to a pad sander and turn it on for a minute so the vibration causes the air bubbles to rise to the top of the pour. If a dull bit catches on an air bubble, it might also cause it to crack slightly. Just my $0.02.
Doug

 

[CSue]

[In each case I only encounter the cracks after turning near completion and always where the blank is the thinnest. I did not notice any cracks after drilling]

Maybe you should allow a tad extra in the length and stop drilling short of the end of the blank. Then cut the blank to length after that - cutting off the part not drilled and a bit of the drilled end.

Sometime I get blanks where the heat builds up quicker than others. When I do this, I avoid that thin, hot end of the blank cracking.

Sometime if you use too much of that stuff that gets the PR to "set" (forgot what its called) it can make the blank more brittle and tending to crack.

 

[seawolf]

As mentioned above water will help. Fill a old dish soap bottle and keep it at the drill. As you drill wet the bit often this will keep the blank a lot cooler.
Mark