Enlarged hole from countersinking

[Bill Cannon]

I am dumbfounded by this. I am countersinking my spars on the HS (10). I am not going too deep but the bit seems to be cutting the hole larger. The countersink is fine but the hole is coming out oblong on too many. I am using the right bit and the depth is good. I am holding it as steady as I can. Could the tip of the bit be damaged (doesn't look bad and can't think of how it could be damaged)?

Any ideas?

Thanks,

Bill

[PJSeipel]

If the material isn't very thick, then you'll stretch the aluminum as the bottom edge of the hole becomes thinner. Use another piece of aluminum with a hole in it cleco'd to your part to back up the hole. Or, what I do is align the hole in the part with a #40 hole in my table and then countersink. The hole in the table helps keep the bit from wandering.

PJ Seipel
RV-10 #40032

[Bill Cannon]

Thanks. I will try that and let you know. I will use a scrap piece of aluminum since it has to rest on the inside of the flange.

Bill

[Bill Cannon]

Made a backer with an aileron spar from a training project. It's about 18" long so I can do a bunch of holes before moving it. Works great. Thanks again for a quick and very helpful response. I will do all my spars like this going forward.

Bill