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I'm looking for a good 5/16 and other sized drill bits that will leave a flat mortise bottom, if that makes sense.
What would be a good choice, a brad point or end mill bit? Or is there something better?

I use split-point bits and prefer a 118 degree tip. That's flat enough for my purposes. I suppose if you wanted to get a perfectly flat end in the mortise, you could go with a chucking reamer.

For what it's worth, I think the quality of Norseman or Viking (I think they're two lines from the same manufacturer) drill bits is unmatched. They come razor-sharp and perfectly true right out of the package. Amazon sells the whole set from 1/16 to 1/2" in 1/16" increments. I've used nearly all of them for various tasks and would consider it one of my better tooling investments.

Using a milling cutter will end up with an oversize hole. 118deg split point and an reamer to finish will give a pretty good flat hole. Buy a stub/machine length drill bit, much stiffer and consequently more accurate.

sandahlpipe wrote:I suppose if you wanted to get a perfectly flat end in the mortise, you could go with a chucking reamer.

Nyet. Chucking reamers have a small 45-degree facet on the outside corner of each flute. After using one you still don't have a square-ended, flat-bottom hole.

caskwith wrote:Using a milling cutter will end up with an oversize hole. 118deg split point and an reamer to finish will give a pretty good flat hole. Buy a stub/machine length drill bit, much stiffer and consequently more accurate.

Thank you Chris, I checked into those at stub length and found there fairly inexpensive and will make a flat enough mortise base for me without the reamer I believe.
And thank you to all who replied.

LatakiaLover wrote:

sandahlpipe wrote:I suppose if you wanted to get a perfectly flat end in the mortise, you could go with a chucking reamer.

Nyet. Chucking reamers have a small 45-degree facet on the outside corner of each flute. After using one you still don't have a square-ended, flat-bottom hole.

I use a reamer and just ground the end flat.

wdteipen wrote:

LatakiaLover wrote:

sandahlpipe wrote:I suppose if you wanted to get a perfectly flat end in the mortise, you could go with a chucking reamer.

Nyet. Chucking reamers have a small 45-degree facet on the outside corner of each flute. After using one you still don't have a square-ended, flat-bottom hole.

I use a reamer and just ground the end flat.

Certainly an option. Good ones are spendy buggers, though.

If tool modification is on the table, the cheapest solution would simply be two identical drill bits, one left stock and the other ground flat on the cutting end. You couldn't drill a straight hole with it, but it would cut the bottom of an existing hole (in wood, anyway) flat with light pressure.

Probably the best way to get it uniformly flat would be spinning in a hand drill while holding against a grinding wheel or belt sander. (Wear a stout glove on the bit-holding hand, and dip frequently in water to cool)

wdteipen wrote:

LatakiaLover wrote:

sandahlpipe wrote:I suppose if you wanted to get a perfectly flat end in the mortise, you could go with a chucking reamer.

Nyet. Chucking reamers have a small 45-degree facet on the outside corner of each flute. After using one you still don't have a square-ended, flat-bottom hole.

I use a reamer and just ground the end flat.

Thank you Wayne, I'll see what it looks like with just the drill bit and see if I should go further and use a reamer.
I'd hate to give myself more then one chance to screw something up, if ya know what I mean.

LatakiaLover wrote: Certainly an option. Good ones are spendy buggers, though.

If tool modification is on the table, the cheapest solution would simply be two identical drill bits, one left stock and the other ground flat on the cutting end. You couldn't drill a straight hole with it, but it would cut the bottom of an existing hole (in wood, anyway) flat with light pressure.

Probably the best way to get it uniformly flat would be spinning in a hand drill while holding against a grinding wheel or belt sander. (Wear a stout glove on the bit-holding hand, and dip frequently in water to cool)

Flattening the end of the reamer , for me at least would be easier to just use my disk sander table making sure it's set level for the cut.
But, thank for the advice George.

(Wear a stout glove on the bit-holding hand, and dip frequently in water to cool). George, why would You put a glove on and then soak it in water? Norm.

wdteipen wrote:

LatakiaLover wrote:

sandahlpipe wrote:I suppose if you wanted to get a perfectly flat end in the mortise, you could go with a chucking reamer.

Nyet. Chucking reamers have a small 45-degree facet on the outside corner of each flute. After using one you still don't have a square-ended, flat-bottom hole.

I use a reamer and just ground the end flat.

Me too.

If you don't spin it while you grind the tip you'll get uneveness... which is bad.

End mills are perfect for flat bottom holes, no need to grind drill bits or reamers.
An oversize hole would be a problem only if you use delrin tenons. [ still, I guess between metric and imperial sizes you should be able to find an end mill that would give you a good fit for the delrin...]

W.Pastuch wrote:End mills are perfect for flat bottom holes, no need to grind drill bits or reamers.
An oversize hole would be a problem only if you use delrin tenons. [ still, I guess between metric and imperial sizes you should be able to find an end mill that would give you a good fit for the delrin...]

And I plan on using Delrin tenons, trying to figure out what size of end mill will give me a proper fit is just too much.
I'll use a split point drill for my mortise and if not happy with the base of the hole we'll figure out one of two ways to make it right.
I believe simplicity sometimes is better in certain situations then to try an over complicate something that doesn't need it.
Thank you all for your in put I certainly do appreciate it.

andrew wrote:If you don't spin it while you grind the tip you'll get uneveness... which is bad.

Yes sir!

It will be hard to fit an end mill that will fit. Also because they cut on the side of the flutes, any slight big of misalignment will put it off course and make a much bigger mortice than you want. Drill bits are less prone to this and only really cut with the tip, you also have a much wider series of sizes. To dial in the perfect size for my delrin I have 4 drills bits and 3 reamers, this allows me to make minute adjustments depending on material, mortice angle and delrin variation.

I'll second the concerns about the end mill. I mistakenly went down that road probably three years ago. I now have a few nice end mills collecting dust....

Hey, Here's a crazy idea, and I say that because I haven't a clue. Why not drill the mortise with a 5/16" Forster bit to make the base of the mortise flat?
I really haven't any idea if the mortise really needs to be flat at the base, it just seemed like a good idea instead of having that recessed gap from a conventional drill bit.
Although I've heard you should have a little gap between the base of the mortise and tenon for expansion due to heat produced in the chamber.

RDPowell wrote:Hey, Here's a crazy idea, and I say that because I haven't a clue. Why not drill the mortise with a 5/16" Forster bit to make the base of the mortise flat?
I really haven't any idea if the mortise really needs to be flat at the base, it just seemed like a good idea instead of having that recessed gap from a conventional drill bit.
Although I've heard you should have a little gap between the base of the mortise and tenon for expansion due to heat produced in the chamber.

Forstner bits are not particularly accurately made in my experience, at least not to the same kind of tolerances as twist drills. Probably because they are designed for woodworking.

caskwith wrote:

RDPowell wrote:Hey, Here's a crazy idea, and I say that because I haven't a clue. Why not drill the mortise with a 5/16" Forstner bit to make the base of the mortise flat?
I really haven't any idea if the mortise really needs to be flat at the base, it just seemed like a good idea instead of having that recessed gap from a conventional drill bit.
Although I've heard you should have a little gap between the base of the mortise and tenon for expansion due to heat produced in the chamber.

Forstner bits are not particularly accurately made in my experience, at least not to the same kind of tolerances as twist drills. Probably because they are designed for woodworking.

Some how I figured that be the reason, thank you Chris.

The other kicker with forstners is heat. They tend to make a lot of it. If you follow it with a reamer it would be fine, but I gave up using them because their tolerances tend to be looser than twist bits (just like Chris said).

I have quite a few of them left from woodworking. I use them for woodworking only. Not pipes.