39/64” drill bit for rail inserts

HaiNhi said:

Good morning all.

I got the inserts (3/8-16) for hardwood from McMaster for my rails, and a drill bit from Champion Cutting Tool Corp (USA made). The bit looks great, but is a pointy regular drill bit since I can’t seem to find any forstner bit with the same size.

So, when i drill press the hole, it needs to go into the rail 1/4” deeper to compensate for the point head (which is not end of the world - I just don’t like the unclean look). Most importantly, since the bit spiral is big - I think - it vibrates which in turn makes the hole at least 1/64” bigger. Now i can use my 5/8” forstner bit to put inside the hole fitting perfectly.

I will use glue with the inserts. But the questions are:
- can/should I just use the 5/8” forstner bit then?
- if not, what bit/brand/method you’re using?

Thanks much

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https://www.rayjoncarbidetools.com/...47o-Eu9RXNTIWCBVcglJ633e1VW8x1CMaAsqxEALw_wcB

Try a brad point drill.

Trying to glue them into an oversized hole will only give you (or the next guy) trouble.

Good luck.

That's a "silver & Deming" (refers to reduced shank) "jobbers length" drill for deep-ish holes in metal started by a center drill as the first step. As you have discovered, it is a poor choice for shallow holes in wood.

You require a 39/64" or 15.5mm diameter hole.
For a shallow hole, the bit should be short to minimize deflection.
For wood, the tip should be a grind that is referred to as a "sheet metal" or "brad point" tip, which has a short center pilot, and spurs at the OD to prevent tear-out so long as the bit is fed in a drill press at correct speed and moderate feed rate.

With the existing bit, you can try a couple things (in SCRAP wood first). 1.) simply use a center drill, and drill a starter hole for the big bit to follow. For best results, the small end of the CD should be about the diameter of the web thickness on your bigger drill. It is not necessary (or desirable) to make the countersink lips cut much deeper (or wider) than a small shallow depression, before following with the bigger drill. IOW, don't try to make the tapered section as big as the next drill, it is mostly the small dia hole that matters.



2.) you can re-grind the tip on your existing drill. The idea here is to leave a center section of the bit projecting well above the cutting edges with spur. This digs into the wood before the spurs start cutting, and "pilots" them to minimize or even prevent deflection. Of course the center has to be fairly close to actual drill center, or the drill will cut smoother than what you have but the wider wing will still cause a larger dia hole.

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In some respects, the easiest thing to do is put a 5/8" forstner in a low cost import spindex like cue makers often use for indexing, and spin the forstner down on a surface or tool and cutter grinder to .610" diameter. It is best to leave about a degree or two of back taper. The rim will be thin, but most forstners will accept a 1/32" off the diameter - you only need .007 - .008 off the radius.

If your drill press spindle or chuck are loose or have run-out, it will be difficult to get a good hole with a bit like you show, but trying it with a properly sized pilot hole can make a big difference. The other grinds mentioned will cut quite well in an accurate rigid machine, but may still cut slightly oversize in one with run-out. If properly ground, they should still cut smoothly and very close to size.

smt

PS, sorry about oversize pix snagged off web, don't know how to reduce

You might have misunderstood me - I'm saying re-grind (or have it re-ground) a 5/8" forstner down to the correct size - .610" diameter.

But that is easy for me to say as a sometimes toolmaker.

Something else that may give you an "aha" moment if you have any dexterity at all: Buy a cheap 5/8" paddle bit for wood at the big box store. Hopefully it will be one you can file, because that can be easier than starting with a grinder for some people. Unfortunately if it is induction hardened, you will have to use the grinder.

Cut the shank off so little more than the flat end sticks out of your chuck. Put it in the DP and turn it around by hand, noting the long side. File that off with some back taper as well a clearance, and keep going around until both sides are equal and the Bit has been reduced to .610" across. Hopefully the center pip will still be ("mostly") centered. Try it in some scrap wood. Let the results inform whether to use it, file it smaller, or go get another and try again. :grin:

smt

Well most peoples' eyes glaze over but from previous posts it seems like you have an actual curiosity and interest in process. Or call it tinkering to start. Correct me if i'm wrong. Anyway, control the tools and you control the process, and an awful lot of consumable woodwhacking tools are not terribly difficult to modify. It boils down to control dimension and control geometry. For wood the geometry is often loose enough (clearance angles, rake, and such) that precise, workable tools are attainable with hand methods.
So long as we are not getting into carbide. (just requires diamonds & better machines).

Where are you located (state is close enough)

smt

HaiNhi said:

Fits like a glove.

I’m happy like a clam!!!

Thanks all.

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Good news! Which bit did you end up using?

I've had good luck getting doing the holes with a plunge bit/router clamped to the rail. Flat bottom, exact size and no wobble.

How do you drive in the inserts? I use a drill press, threaded rod, nuts and wrenches to get them in straight, but it's slow and a pain in the rear.

HaiNhi said:

I’m actually doing just that - trying to modify things with what I have (and buying more tools in the process if need be). I’m a tool hogger, haha.

In this case, since I don’t want to wait for new bit to come, im trying to sand the bit down. I’m spinning a 5/8” forstner bit on the DP and using sand paper. But it’s pretty slow if not anything. I’ll try a little more to see what’s going on.

One note though: I measure the body of the insert less its teeth. It is bigger than 5/8”!!! So when I drive it in the 5/8” hole by the forstner bit, it actually tearing the wood up to go in. Is that what’s it supposed to be doing?

If that’s so, wouldn’t 5/8” still be ok given my earlier torque test result (it can widthstand a 50 ft/lb in plywood - probably much more in hardwood)?

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What in the hell are you doing tightening rail bolts down to 50ft pounds? 15ft pounds is actually more than enough!!!

If you can clamp the rails before you drill them the holes will come out better.