Is it my blanks or technique?

Signed-In Members Don't See This Ad
S

soligen

Member
Joined
May 11, 2010
Messages
1,090
Location
Sterling Heights, Michigan
Started turning my first fully cured blanks. First try blew out big time when drilling. then I remebered you are supposed to stop short then cut the blank - much better.

In both these you can see the pits, which is my biggest issue. I just got the Woodchuck yesterday, so was trying it out on these. Is my technique bad, or are the blanks too brittle? My rest is too big so I have trouble getting real close to the blank.

Both these were put in the hot car to cure. The orange one was cast Friday evening, then put in the car all day Saturday while I was out with the wife, then turned Sunday AM (today). It is a 3 color pour (red/orange/yellow) of castin craft PR with Mixol and Mother of Pearl. I poured the colors togehter way too soon, so have crappy seperation - learning!

The blue (single color with the MOP) is from fiberglass resin and was cast last weekend. I had turned a sample of this before it was fully cured and had a smaller issue with tear out - this is much worse.

I had a 3rd (not pictured) blue with MOP from PR that blew out real bad when drilling. It had the pits like these too.

Is over baking by putting it in a hot car for a day making it too brittle? Or i smy technique a problem? or both?

Other observations:

1) Drilling through the center was excellent - much more accurate than wood
2) A standard twist bit drilled much easier than the brad point bit
3) It helped somewhat to try to "ride the bevel" on the woodchuck, although thats not what the instructions said to do

As it stands now, I would need to do quite a bit of final shaping with sandpaper - which I would rather do with a tool.

Any advice appreciated.
 

Attachments

  • IMG_0112.jpg
    IMG_0112.jpg
    21 KB · Views: 255
Signed-In Members Don't See This Ad
My only advice based on the info given is, SHARP skew with a LIGHT touch. I've turned some extremely brittle acrylics before. Some were more fun than other but I've manged to get through them all eventually.(Not without a blow up or two)
 
By "pits" I think you mean the chips that are visible on the upper ends. Those aren't very big chips, you still have plenty of room to turn it down as needed. I often chip them up worse than that before I pick up a skew.

Use a sharp skew and a light touch for the rest of the turning.
 
I agree... skews seem to be the best choice for acrylics, especially PR.
Gouges can be a bit agressive. When using a skew, the PR should peal off in ribbons. I've not used Castin' Craft but reports have been that it is a bit brittle. If a skew is not an option, then try a round nose scraper or even a negative rake found nose scraper once the blanks are turned close to final size.
Do a good turn daily!
don
hunter-27 said:
My only advice based on the info given is, SHARP skew with a LIGHT touch. I've turned some extremely brittle acrylics before. Some were more fun than other but I've manged to get through them all eventually.(Not without a blow up or two)
Click to expand...
 
Dennis, are you getting any chatter noise when you are turning? we turn some brittle PR and get some pock mark chipping until we get it nice and round, try using a corner of the insert untill it's round, them use the face of the radius , I only have a 12 inch tool rest for my new lathe, I'm going to get some short ones from Rick Herrell, It really shouldn't be necessary to "ride the bevel" with a Woodchuck, the 2 inch radius insert is about the best for brittle acrylics and Acrylester, usually the Woodchuck is great on acrylics. also try turning at a higher RPM, but if you are sitting very far over the tool rest, it could be possible to build up some harmonics that would help cause chatter but you'd have to be hanging over a couple of inches and turning slow like under 1500 RPM.
 
Dennis,

I've only casted Simlar41 so I'm not sure if my experience is helpful, but so far I have not had any issues turning my blanks. I use a very sharp roughing gouge rather than a skew, but the key no matter what the tool of choice is, is light cuts and proper tool presentation along with a very sharp edge. If your tool rest does not allow you to get up close to the blanks, you need a smaller tool rest. You're dealing with very thin material, when turning pens and so you need total control over your tools. any chatter at all can cause some of the problems you mentioned.

With Simlar, I have just followed the instructions and things have turned out just fine. I am finding that as I cast outside in this heat, I can use less catalyst to give myself more time to mix and pour. I don't need to put them in a hot car to cure them. In fact, I'm not sure that adding heat to the curing process helps at all. I think that if they cure too fast, that could be where the brittallity (that's my word for "brittleness") may be occurring. My blanks can usually be pulled from the molds in a few hours, and then I let them finish curing overnight.

I have not had any blowouts while drilling, but I always go very slowly whether on the lathe or on the press and I go even slower as I get to the end of the blank. I try not to punch through the bottom, but rather slowly wear away the final layers of material.

I can't imagine that I have made all perfect blanks, so at least some of mine must have been brittle. I think that if you work on your technique, and take some more time on each step of the process, you will find that your brittle blanks turn just as nicely as your professionally made acrylics.
 
I just came back from the shop. Tried the woodchuck on wood - its amazing. On the PR I'm struggling. On painfully light cuts I got some smooth areas, but I can't sustain cuts that light accross the whole blank. The barest hint of a mistake and the pits would happen. I had an eyeblink of inattention near the end, and the end broke out down to the tube - blank ruined (oops - i mean blank became a designated practice piece :biggrin: ). The blank was not even very thin yet.

With a freshly sharpened skew - burr side up things worked better for me. This seems to be the better option right now.

I've never turned a purchased acrylic blank. I think I need to go buy one for comparison so I can compare and see if mine just really suck.

My current score: Blanks: 3 - Pens: 0 - the Blanks are winning.

I have 2 blanks left of my own casting (the orange)
 
If you do go out and purchase a couple blanks, get atleast 1 acrylic acetate and 1 inlace acrylister (spelling :confused:) Inlace is very brittle and the acrylic should turn nicely. This would give you a sort of benchmark for how your blanks turn. Also, if you'd like to try other pr blanks I'd be happy to do a trade with you. Another thing, like Chris said. I'd leave out the curing in the hot car. From what I can remember, too hot too fast can cause the blanks to become brittle as well. Add your catalyst like normal and let them cure throughout the day/night. If the fumes/smell is another reason to put them in the car you can seal them up in a 5gal bucket to keep it to a minimum. Just try to keep everything level so you won't get lop-sided blanks (Don't ask me how I know this)
 
Last edited:
Dennis, bear with it.

I'd say your blanks are almost certainly not at fault. These look like my first few. The dents in them are most likely where the tool has just dug in for a nanosecond and chipped it. If I were you, I'd get these two back on the lathe and have another go. There's still loads of material left.

Make sure your skew is extremely sharp. Turn at 1600-2000 RPM. Try cutting with the trailing / heel edge of the tool. That's how I got my first ribbons. I have a metal turning lathe, so the advantage of being able to very accurately set the tool rake and the angle it "slices" in. Remember you're kind of "shaving" the piece.

My buyers have had mixed results. One guy managed to turn out these two cracking pens:

1_P1050132.jpg
1_P1050134.jpg


...yet, the same turner (14 years experience, including acrylics) managed to smash two identical blanks cut from the same slabs.

So take heart!! Ther's a in infinite amount of stuff to learn with an infinite number of variables. If it was easy, everyone would be doing it!
 
Scott,

Thanks for the link - I never read through that whole thread until now - vwery informative.

Ken,

Definately will be working more on those blanks. Took the pics while tubes were drying. I alwasy keep turning my mistakes to try to learn something - at elast until they break. I have 2 left that can still be a pen.

From teh video in Scott's thread, Part of my problem is definately skew technique. I've tried the planeing cuts, but have no control lol.
 
Dennis; Make sure you use the Woodchuck BELOW THE CENTER LINE of the blank. The tool rest will look like it is way too low, but you should not try to cut anywhere near or above the center line.

Hope this helps...
 
Randy,

That helped tremendously. I had set up the rest to what looked like a hair below center (per the woodchuck instructions). Either an optical illusion made this not the case, or it just needed to be even more below center. Now it is quite obviously below center and my test piece gave me a finish without pits, and I think I could go stright to MM with.

Next blank has tube glue curing. Hopefully it will be a pen or pencil tomarrow evening. Now I'm excited!! :biggrin::biggrin:
 
Dennis,

If you are still struggling with turning your own castings, let me know and we can do a trade. I'm curious to see if there is any difference between our blanks. Let me know if you're interested.
 
Dennis , the castin craft is more brittle then the GP resin you have and the Silmar we all use . As everyone else has said Scary sharp skew doing a shearing cut (not like a scraper) and very light cuts until you get used to the technique . Chip out is ok untill the blank is round is ok but try to keep it to a minimum . Also keep a good grip on your tools , if you loosen your grip you will get more chip out (no comments from the dirty minded) , control is key .
 
You must log in or register to reply here.
Back
Top Bottom