Drill press & straight holes

I've ruined two WD-409 parts. I'm one to practice drilling a new hole vertically thru the mani body. I've gotten the part about finding the center line and clamping figured out but now when I try to drill through the bottom the holes wander all over.

The hole with the black mark next to it illustrates the issue. The top is dead on the center line and good. Drilling through to the bottom gies me whacky holes all off the center line.

How can I maintain the drill along a vertical line while drilling through the tube?

Spot drill

Have you tried a spot drill bit to mark the hole?

And I might be wrong, but I would think drilling thru is not the right approach. I would think I would flip the part over and drill the other side,,, but maybe someone knows better than me.

Wrap a piece of paper around the tube with tape so you can mark accurate holes on both sides. Center punch the holes and drill #30 or so. Then though drill the hole a little larger. I always drill undersize and ream to final size for the best bolt fit.

Ed Holyoke

Drill tube

Bicyclops said:

Wrap a piece of paper around the tube with tape so you can mark accurate holes on both sides. Center punch the holes and drill #30 or so. Then though drill the hole a little larger. I always drill undersize and ream to final size for the best bolt fit.

Ed Holyoke

Click to expand...

That works. I also use a V-block. Pretty handy device for all sorts of drilling. I use it a lot. The block can be flipped around.

I used this.

https://www.aircraftspruce.com/search/search.php


works great.

Drill-guide V-block

https://www.browntool.com/Listview/tabid/344/ProductID/3635/Default.aspx?txtSearch=drill+guide

I used this when match-drilling aileron push tubes, and it worked out great. I clamped the piece(s) to be drilled to the bench, clamped the drill guide on the piece with the hole centred then drilled.

If you're drilling steel, make sure your drill speeds are correct (usually slower than for aluminium) and use lube on the bit.

i would drill undersize holes on each side. start with a small bit and go up and pay attention to drilling your indent mark as accurate as possible. then finally drill to full size , going in one side and out the other in one pass.

Drilled Tail Spring

So I was able to successfully drill the tail spring holder with surprising results.

First step was to order the appropriate bits, a letter D size bit which is just under 1/4".

I was going to with Bob Noff's suggestion about drilling undersized holes and gradually expanding them on both sides. But I wasn't confident in my ability to exactly line up the holes. I also couldn't get a drill guide to fit onto the piece and I couldn't obtain a Drill-Rite guide. Turns out this was fortunate.

So I started by measuring the hole location. Leveled the piece in a press vice using an electronic level. Located the center of the piece using an old machinist trick - use the drill press to hold a thin ruler against the part and tap the vice with a tiny hammer until the ruler is level. Then you have the center. Another trick to drawing straight lines on the tube is to use a piece of angle. Held against the tube it allows you to draw straight lines along the length. Finally I used a piece of paper cut to exact length to determine the point 180 degrees from the marked hole.

I started drilling with a #30 bit, then removed everything an remeasured to make sure it was centered. Then a #20, then a #12, and finally the D bit. I now had a hole exactly centered in the top of the tube.

Next, I took the actual tail spring, oiled it, and inserted it into the holder. I lined up the holes in the mount and the spring to use as a drill guide. Here's the first interesting thing. When drilling on the old part to practice, I had to drill to 0.25 to get the AN4 bolt to fit. On this part, it fit smoothly with the D bit (0.246"). So I used the actual spring as a drill guide to go through the tube. Drilled it out, cleaned it and fit the bolt neatly through. Perfect.

Now the interesting part. I took everything apart and cleaned up. My wife wanted to see so I put the spring back in the mount, threaded the bolt in and it didn't fit. Huh? I took it apart, rotated the spring 180 degrees, reinserted it and voila it fit.

As far as I can tell, the spring hole is not perfectly centered. It's a good thing I didn't use a drill guide to get exactly offset holes as the hole would have ended up 1/64" off.

I thought it might be the hole in the spring being enlarged by the drilling but I carefully measured it and inserted a new AN-4 bolt. The hole was snug enough to basically strip the conversion coating off the bolt so it hasn't been oversized. The only thing I can figure is that they were drilled as a set at Van's and are ever so slightly off center.

Wierd result I didn't expect. But alls well that ends well.

Attached are pictures of the process.