Drilling acrylic suggestions

[knowltoh]

Today i drilled an acrylic blank for a slimline and used a 7mm Dewalt pilot point bit. The hole is quite large for the tube, especially at the exit end. The hole appears perfectly centered, but larger than 7mm. I don't turn a lot of acrylic, but don't recall having this issue before. Any suggestions?
Thanks

 

[Akula]

does this only happen on acrylics?

is the drill running true?

do you clear the bit while drilling, acrylics will heat up and just twist around in the hole throwing things off

do you put a scrap piece of wood under the blank?

 

[knowltoh]

I am drilling on the lathe. The same bit works fine with wood. I do clear the bit often, but maybe not often enough. The blank did not feel hot to touch.

 

[SteveG]

Even when drilling on the lathe, you can get an enlarged hole if the bit does not start exactly on center, especially with smaller diameter bits. Using a center drill at the start will help prevent this from happening. You mentioned only one drilling event with this problem, so hopefully this was just a "One time bad deal".

 

[drgoretex]

This is a common problem for acrylic, especially on the lathe. Of course, it can be a problem with wood too.

Here's a few things to try if you are not already doing them:

-ensure the blank is very well secured in the chuck. If you can round it first and secure it in a collet chuck, even better.
-Try punching a small hole in the centre of the end of the blank with...well, with whatever. I use the centre (sorry, 'center' :biggrin tip of a Brad-point drill bit. Ensure your mark is centred well.
-Start the hole with a very small size drill bit, and work up to the size you want using regular tip drill bits. This allows for less debris to exit around the bit.
-Drill only 5-10mm at a time, withdraw, and cool the bit with a wet rag. This avoid excessive heat buildup, which melts the acrylic, and causes the drill to start 'bouncing around', enlarging the hole.

OK, there's my 2 cents worth of experience.

Cheers,

Ken

 

[Richard Van Hulle]

Heat can be a problem with acrylics. Go slow, clear the drill often, pulling it completely out of the hole and brush any chips away. If some of the acrylic material accumilates on the drill flues the hole will be enlarged. I use a spray of denatured alcohol. Keep in mind that alcohol is flamable. After use wipe the drill clean and put a coat of WD40 on it.

Richard Van Hulle, Aiken Pen & Pencil

 

[JCochrun]

I was having the same problem when using my drill press. I changed the drill speed to the slowest setting and cleared the bit alot.

Jim

 

[hewunch]

Have you tried stopping short and cutting off the last bit? This also helps with blow out.

 

[Shappy]

Barry Gross has an excellent DVD on the techniques used for making acrylic pens. It is a great source of information and I learned a lot from watching it. It is important to back out the drill bit periodically with acrylics to avoid overheating and damaging the blank. I picked up a pack of small bristle brushes from Woodcraft Supply that I use to clear out the material from the flutes in the the drill bit while it is running. I suppose A stiff bristle toothbrush might work just as well. I have a piece of scrap wood under my pen blank vise. Hope this helps

 

[Live2Dive]

I've always had an issue with my 7mm drilling of acrylic. What I have gathered, is the shavings come up the drill bit and get hot, and "wedge" between the bit and the inside of the blank, "stretching" the blank to a bit larger dimension. It does not do it with wood.

My solution is to drill the blank with a 1/4" bit first, then switch to the 7mm and do a quick in and out. Perfect every time now.

 

[moke]

IMHO drgoretex has hit it on the head. If you have a Harbor Freight close by you can get a set of five "center drill pack" for less than $10...maybe $5.00. Those are a must to start the drilling process. I also might try rolling your bit on your table saw(TS) It might be a distinct possiblity that the bit is out of round.

I also shoot air down the tube while drilling and then pulling the bit out every inch or so. It takes me at least 10 to 15 minutes to drill a blank. Don't be in a hurry.

Try those two things in addition to the things already mentioned and I think you should correct your problem.

 

[76winger]

Is there any chance the tailstock has gotten out of alignment with the headstock? Of course this would cause equal problems on wood and acrylic, but if it's just happened, you might be able to fix it before any others get drilled out too big.

 

[moke]

I failed to mention that when a hole in the blank gets too big, it works well to use gorilla glue. I get a 4 oz bottle at HD and it even comes in a style that dryes white. I usually use epoxy but occasionally I have one with a "too big" issue and this method allows it to be turned safely because it expands. Fill the tube with plumbers putty and wet the tube slightly before using the Gorilla glue.