Drilling. Bores oversized when done?

[SoloWorx]

Hello fundamental question about drilling. I've noticed when drilling some wood blanks but mostly acrylic blanks when I'm done the hole is larger than the diameter of the drillbit. This is usually larger where the drillbit enters and smaller where it exits the black . I've noticed that during drilling I get vibration at times and when I extract the bit large drill shavings come with the bit. These are built up in the grooves of the bit. I take several sequences drilling into the blank and don't do it in a single plunge. Quite frankly I go back and forth many times in and out. I clean the drillbit off after every extraction from the blank while running I use a brush. This mostly occurs on the larger sized drillbits say larger than 3/8".

Is this due to the build up on the drillbit while drilling? I do not appear to have run out on the bit or lash in my drillpress. Should I drill up to the final Bore diameter in sequence? Or should I drill the final size one time and be done?

This causes me when gluing to build up additional layers of glue between the Tube and the blank.

 

[SteveG]

Some of the usual causes for your problem are drill bits that are not sharp or that are unevenly sharpened. New bits are not necessarily sharp, unless you have purchased premium bits. Do you notice any excess heat build up? The "large shavings" may actually be some melted plastic building up in the flutes, which contributes even more heat from the added friction. Then you get the enlarged hole. Vibration may be caused if the bit has uneven cutting edges. You can also try decreasing the RPMs and or feed rate. Some will use water in the hole to reduce the heat build up. These are a few tings to consider. Hopefully you will resolve the issue with one of these suggestions.

 

[Tom T]

I use very low feed rate with very little pressure. That has helped me. Very little pressure on the drill press is key. Also making sure that I retract the bit and keep it clean. No shaving build up. Also the whole comes out strighter where it enters and exits.

 

[Dai Sensei]

What are you drilling with? Apart from the replies above, if using a lathe where the centre is slightly out, you will end up with larger holes.

 

[BradG]

As Neil said, if thats the case, put a dead centre in your head & tail stock and move them together so they touch. do the tips touch or is one off to the side? if so, adjust your tailstock so they align.

 

[SoloWorx]

The drill press is at a pretty high speed. So I will move the belt. I plunge often and clean buildup often. I usually plunge and drill until I see shavings exit the blank out the top of the blank. The bits may be key.

I notice this more often when drilling with the brad point bits. Regular bits not so much.

But what of drilling large diameter blank bores say 9/16. Should this be drilled in steps? How many. Differ for wood and plastic?

 

[PenMan1]

My first suggestion is actually two-fold and is relevant to both lathe and drill press drilling.

First, try to drill at the slowest speed that will work without bogging down the lathe or drill press. I've found that +\- 500 rpms works nicely for most of my applications.

Secondly, try starting the hole with a properly sized centering bit. A "catch all" set of centering bits is around $5. You may not be able to "see" it happen, but a regular bit will wander a bit looking for the "path of least resistance". A hole started with a center bit minimizes this "bit walking".

As others have suggested there are MULTIPLE causes that could be happening here, but the two things I've suggested will eliminate many of these problems.

 

[Phillikl]

Having a cheaper (none of them are truly cheap) drill press, I cranked the chuck all the way to the deepest setting and noticed quite a bit of wobble. Having the same issues as you, what I do now for square blanks, is drill half way down, mark the blank (have severe OCD & ADD, so I don't loose my true edge), flip it over and drill the other half. Haven't had any blow outs and the "wobble" holes seem to be much rounder.

Merry Christmas!
~Kendall

 

[CharlesJohnson]

Kendall:When my tapered driven chuck comes out o the spindle I knok it in with a block and hammer.Some times it dosn't seat straight.I have to keep trying.It is worth the efort.