"out of round" drilling

[Freds]

I am having a problem drilling a 10mm hole in acrylic materials.

I end up with an oblong hole. I have had this problem with two different drill bits. I laid the bits on a flat surface and looked to see if the bit was bent. It wasn't.

I do not have this problem when using my 7 mm bit on wood, corian, or acrylic.

Any ideas


Thanks

 

[Randy_]

We could probably do a better job of helping if you would give us a few more details about your drilling set-up.

 

[bitshird]

Could well be the flutes of the drill are not even, that can cause a drill toi drill oval holes or wander off center, happens often on Chinese drill bits.
Ken Ferrell

 

[Woodlvr]

When I started drilling the larger holes I had the same problem and in my case I did not have the wood low enough in the vise. I was told to put the top of the blank even with the top of the vise. I hope that this makes sense and helps.

Mike

 

[karlkuehn]

Watch the heat on acrylics. The offal can melt and build up on the bit, which in turn melts the surrounding hole and can cause what you describe. Also, be careful about cramming your bit all the way up into the chuck, that's got me a time or two. Shove it up there and then back it off a 1/4".

I use a cheapo Harbor Freight drill press, and there's this weird slop in the first 1/2" of downward cranking that throws the bit off center a little bit. Not sure if that happens on all presses, but I've learned to situate my blanks about 1/2" below the bit before drilling, and that's helped some. If it's any help, I've only seen the out of round problem with 10mm bits, as well. Bigger and smaller, no problems. Maybe we got our bits from the same place!

 

[iamfredweb]

You need to slow your roll. 200 rpm or less. Stopping is a good thing to do. There are times when I will take ten minutes to drill half a blank. Heat kills.

 

[Rifleman1776]

Like Randy said, we need more info to help. Could be any one of a dozen things. What kind of drill press, vice, etc. Does this happed to you when drilling wood? If not, that's a good hint to look to your own technique.

 

[Freds]

Thanks for the comments. I will take them to mind and try again.

 

[workinforwood]

I used to have this problem, then someone told me to use water. Drill 1/3 way in at first, no problem, bit is cold, but heat builds fast. drop some water in hole then start drilling a bit more. add more drops of water in hole and drill some more. Keep this going and the problem goes away. The first 1/8" might still have a bit of oblong but the rest is perfect and so cut blank extra 1/8" long. You can hear/see the sizzle when bit hits water in hole. Thanks to however it was that told me that!

 

[Mikey]

A vice being too tight will give you an oblong hole every time.

I don't buy the other arguements simply because the bit spins in a circle. (material build up on bit, or bit that wasn't perfectly straight) If something was wrong with the bit, it would make an oversized hole. DAMHIKT[]

I had/have a set of cheap HF bits ($29 set) and was always getting holes slightly oversized. I was given a high quality bit one day for a Baron and when I compared holes drilled with that bit to the one drilled with the HF bit, there was no comparison. The HF bit drilled a hole at least 1/64th larger. I now use only high quality bits when I can and have no problems paying $10 a bit if I have to.

Anyway, clamping a vice too tight will give you an oblong hole. Won't hurt at first, but as you drill the hole, there is less material to support the force that the vise puts on the blank.

 

[Rifleman1776]

Originally posted by workinforwood
<br />I used to have this problem, then someone told me to use water. Drill 1/3 way in at first, no problem, bit is cold, but heat builds fast. drop some water in hole then start drilling a bit more. add more drops of water in hole and drill some more. Keep this going and the problem goes away. The first 1/8" might still have a bit of oblong but the rest is perfect and so cut blank extra 1/8" long. You can hear/see the sizzle when bit hits water in hole. Thanks to however it was that told me that!

I'm sure that is a useful tip. Not sure if it will solve the oblong problem but it could well reduce, or eliminate, the blowing up during drilling. I do so few acrylics that I really don't have a technique. But, I do have a syringe on my shelf that I remember buying just for this purpose. Very handy to add a drop or two at a time. BTW, if keep it a secret [] and don't tell the druggies, syringes can be purchased a farm or vet supply stores.