Brad point bit speed problem?

[John Schroeder]

Hi all,I'm finishing up a roubo bench and had some nasty tear out front and back when drilling the 3/8" holes in hard maple on the parallel guide for my leg vise. I'm using a 3/8 brad point bit from lee valley in a new floor mount jet drill press, initially at 500 rpm. I bored all the dog holes with a 3/4 forstner bit no problem, but these smaller holes turned out pretty rough. I checked the chart for speeds and then tried uping the speed to 700 and 900 rpm and it was marginally better, but still not great. I got much better results with test cuts in the same maple with a corded drill and a regular old 3/8" twist bit. FYI i did use a backing board and went pretty slow, so i am really at a loss. Is the speed just way too slow? It seems to be okay from the charts i have seen. Any thoughts on what i am missing would be greatly appreciated. Thanks!

[Kevin Presutti]

Did you test a 7/16" to see if it wasn't just the drill bit at those speeds? Just a thought, it could be the bit

[mike holden]

John,
if the backside of a hole is important, then you should back it up with a piece of scrap when drilling. This will eliminate most blowout - I hesitate to say "all" because I am sure that someone, somewhere has blown out the backside using a backer.
Mike

[pat warner]

Drop the drill on the drilling end of the tool and you get what you're getting.
Inspect with 5 or 10x lens.
Spurs & point look good? Then it could be a bad grind.
Speed & feed important alright but I suspect a bad drill.
Enter at some cocked-eyed angle and you can tear out the entry.
One key to a sharp brad point is its exit wound.
Round and clean or torn and broken out? Broken: Then the drill could be snookered but
a poor/fast feed rate can make the same kind of mess.

[ian maybury]

I can't claim to be an expert, but a good brad point normally enters pretty cleanly unless it's started too fast.

My instincts and experience suggest (a) carefully check the drill as above for both sharpness and correct angles/dimensions, (b) check the chucked drill for run out/wobble at the tip (it could be a chucking/machine issue), (c) use plenty of cutting speed, (d) start the hole and break through at a very slow feed rate, and (d) as above use a backing board. (it's possible depending on workpiece layout/ability to locate the hole to at a push to use a backer on the entry side too)

ian

[John Schroeder]

So I had a look at the drill tip under magnification and sure enough the brad point and one cutting wing are off, particularly when compared to the other bits. Not sure what happened, especially as it's a new set, from LV if I recall correctly. I've got bit on order to replace it. Thanks for the ideas, as it just didn't occur to me that it might be the bit itself. Of course, my close up vision has really started to go in the last year or so, as I'd have thought I would have caught that one. At least that's my story and i'm sticking with it.