acrylic blank

[Bigblackdog]

Well, I turned my first bit of acrylic last night- a blank from timber bits.

Comments so far:
* what a stink.
* the best finish came from the freshly sharpened scraper rather than my carbide tool.
* i turned it to round so that the chuck gripped it better, and then tried to drill it. I tried to drill first with a 7mm bit, which was fine, but then when i tried to expand that hole to 11.9mm for the tube it exploded. I presume that this was the wrong way of doing it- the drill was new so it must have been user error.
* the smell was almost enough to make me stop!

could somebody tell me the basics of drilling these- lathe speed, sequence, equipment etc.?

Thanks
Mark

 

[Vic Perrin]

I drill mine on the Lathe. Use a pin Jaw chuck to hold the square blank. I use the finished size drill only and drill with a slow speed and feed making sure I clear the cuttings regularly. Only had one break on me doing it that way and that was on a 37/64 blank.

Vic

 

[Bigblackdog]

thanks Vic. i felt it grab on the second drill- I think that pre-drilling it with the smaller size probably caused the issue.

 

[Terry]

Try drilling slowly Mark at around 450rpm and keep withdrawing the drill bit to clear the swarf. If the swarf builds up inside of the hole it will create internal pressures which will cause the blank to overheat and explode. Also let the drill do the cutting and try not to force the drill bit through. !!!

 

[Woody]

As Terry said also it sounds like you got a lot of heat build up creating the smell and causing the grab if you do as Terry said them problems will disappear

 

[Bigblackdog]

the smell was actually from the turning rather than the drilling. i did the drilling on the slowest speed, and withdrew the bit every half inch or so.

I will have another go though, making sure that I follow the advice to the letter.

 

[Lons]

Wot Terry said.

Make sure you wear a visor when turning 'cos plastic swarf and chips can be sharp, also hot if you're a bit heavy handed

Smell is easy to cure, just needs a clothes peg on your nose.

 

[Dalboy]

As already stated above plus this extra bit.

Also don't drill all the way through. Just deep enough to take the tube and a little extra. Cut the end off before glueing the tube as you need it to measure the hole depth.

 

[Woody]

Another tip When I drill acrylic I always cut them to long and dont exit with the drill especially on round blanks just measure the tube on the drill bit and put a bit of tape on the drill as a stopping point then cut the excess off after good luck

 

[Woody]

Oops Derek got in before me LOL

 

[Penpal]

Look carefully at which drill to choose initially.

Peter.

 

[Bill Mooney]

Should be good to go now. Look forward to seeing the finished pen.

 

[wm460]

All of the above.

 

[chas_41_uk]

As already stated by Terry, Woody and Derek

 

[EStreet]

Several things.

Some plastics love high speed drilling, others demand low speed. Generally the more fragile the blank is the higher the speed you need.

When you drill plastics the act of drilling will put bubbles into the material as it is drilled. Many shops will drill undersized and ream to the proper size to avoid this. The type and style of drill bit matters greatly in this regard. A forstner bit is very good on plastics that has bubble problems when drilling.

I would say drill first then make it round. By rounding it you could have imposed stress fractures in the blank which made it ill fated when drilling. Depending on the material mind you some blanks are very prone to this problem.

Heat is your enemy. As you withdraw the drill bit if it's to hot to the touch then you are in the 'no no' zone, the goal is to keep it cool to the touch and fluid may be needed at times. Also metal expands when it is hot, the hotter it gets the more inaccurate the drill bit becomes.

Explosions are often the result of bad catches and you have it under load so something has to happen, the blank being the weakest point in the chain and it goes SNAP. The events leading up to the explosion will tell you exactly what the cause was. If you look at it you have the feed rate and the stress forces applied to the blank when drilling or turning and the bonding of the blank material can only withhold a certain amount of stress before it yields. This is a very fine line to walk.

As for the scraper vs carbide tool a good and sharp traditional chisel will win hands down vs a carbide tool any day of the week simply because the carbide tools are *NOT* sharp. Even being brand new they are not that sharp.

 

[paulm]

All of the above and the main think I find is don't let anything build up as you're drilling, thats where the BANG! factor comes in.

 

[Penpal]

Ed I understood the newer carbides had overcome the sharpness aspect by refining the metal to create that sharpness that was lacking before.

I have a mate a furniture maker who changed the heads C/W these cutters from the blades using his surface planers with huge success with and across the grain specialising in burl surfacing. I agree 100% with taking the edge off the blanks to lessen that stress for sensitive blanks such as ebony, snakewood and desert ironwood as well. There are so many plastics involved in pouring blanks these days some appear to be more brittle or susceptable than others certainly the order of drill then round makes perfect sense to me. It would also from Marks description of drilling the 1/2 inch before backing off is fine initially but deeper in shorter distances as well as backing out and clearing the swarf can ease drilling heat and pressure is critical IMHO.

There are so many very experienced makers and pourers of cast blanks I too will be interested in their experiences, Talking to a friend of mine this morning who makes his own blanks he only pours with material from one source chooses crystal clear that both drills and turns without problems I will ask when I see him next the type, brand etc.He does not vibrate or shake this mix.

Peter.

 

[Jim]

Many a good advice here Mark, but i will also throw in the use of Colt drills on acrylic blanks, if you can get the size you need they go through plastic like butter. They are expensive, but i have had a 7 an 10mm for almost five years now and they still cut well at speeds of 800rpm.. Also drilling on the lathe gives better results for me also ..

 

[Bigblackdog]

i actually have colts for the kits that I have, so all is well there Jim.

 

[Walter]

I would certainly agree that drilling before turning is the way to go. The chances of getting a 100% accurate bore parallel to the turning afterwards are zero so you will then end up turning an off centre blank back to parallel with the bore, plus turning removes material and weakens the blank. No brainer IMHO.