D bit query

[MikeKParticipant @mikek40713]

HI guys,

So I need to create a flat bottomed 0.374" hole.

I know that drill bits create neither round nor accurately diametered holes and my smallest boring bar is too big for this task. I have made one D bit in the past, but don't remember if it created an accurately sized hole or not.

Should I make a 0.374" D bit, or should I make it a bit smaller? Or should I spend the time grinding a wee boring bar?

Thanks,

Mike

[MikeKParticipant @mikek40713]

[Paul LousickParticipant @paullousick59116]

Have you considered using a flat bottom end mill ?

[MikeKParticipant @mikek40713]

Don’t have a 0.374″ end mill.

[Boiler BriParticipant @boilerbri]

How deep will the hole be? You could drill it to depth and undersized then use a small boring bar made from hss if it is not too deep.

What is fitting in the hole? 

Edited By Boiler Bri on 16/09/2015 06:31:52

[JasonBModerator @jasonb]

If its not too deep you can use a smaller dia slot drill held in the toolpost as a boring bar, should be able to creap up on your 0.374" with that as even a D bit may cut a little oversize or follow the drille dpilot hole.

J

[john carruthersParticipant @johncarruthers46255]

how about grinding a 9.5mm twist drill off flat?

0.374" =9.4996mm

I had to do just that yesterday for a flat bottom 8mm hole.

Edited By john carruthers on 16/09/2015 08:37:30

Edited By john carruthers on 16/09/2015 08:38:17

[mechman48Participant @mechman48]

+1 for Jason & John suggestions; I have a couple of flat faced drills / slot mills in my tools collection, plus a 1/4" broken tap that I have ground as a boring bar somewhere in with that lot… or is it 3/8"

George.

[MikeKParticipant @mikek40713]

Sorry, I should have given more information. The hole will be 0.250" deep and will receive a pressed-in magnet.

I've done this before with other magnets, but with those I made the hole deeper and used a 0.374" reamer. With this one I need the magnet seated firmly at the bottom of the 0.250" hole (and without thread locking compound).

I have this, but it's the wrong diameter and don't remember if it worked well. If I make one of the right diameter will it give me an accurate hole?

Edited By MikeK on 16/09/2015 13:07:19

[chris stephensParticipant @chrisstephens63393]

Hi Mike, that D bit looks about right but you really should hone the cutting edge after heat treatment as the heating tends to blunt sharp corners. Oh and it wants to be slightly thicker than half thickness by a few %.

[JasonBModerator @jasonb]

Without doing a test hole its a bit hard to say how accurate the hole will be. If you can pilot drill then use say a 8mm or 5/16" slot drill to open it up that will ture any error if the drill wandered, then try your newly made D bit. If the hole is too small you can heat up and alter the bits size and try again.

I may not have been clear on my suggestion of a slot drill a as boring bar, don't need to grind up an old one just hold a good one in the toolpost with cutting edge nearest you on ctr height and use like a boring bar.

[MikeKParticipant @mikek40713]

Thanks for all the replies.

chris: Yes, the cutting edge for that bit was honed with a diamond hone. It may look dull, but it's actually quite sharp. I didn't know about milling to greater than 0.5 x diameter. Thanks.

paul, jason: Ohhhhhh! I didn't think to use end mill as a boring bar. Great idea! I need to try this!

[Neil WyattModerator @neilwyatt]

If you want a dead size hole, don't machine/grind all the way to the half-way point, stop just short. This will ensure the d-bit only cuts on the end. leave the end flat across to half way., then angle it. This reduces the tendency to cut oversize and reduces the width of the chip. A very shallow (1mm) drill hole helps get a d-bit started accurately.

[MikeKParticipant @mikek40713]

Like this? I wasn't sure if the cutting edge should be relieved in two planes so the top view or the side view may be wrong.

[Neil WyattModerator @neilwyatt]

Not quite, the green line should be straight – you need a slightly larger than half-circle to make sure a D-bit cannot cut on the edges.

Neil

[MikeKParticipant @mikek40713]

Okay, like this. What's a good angle to relieve half of the cutting edge?

[chris stephensParticipant @chrisstephens63393]

Hi Mike, I see you have got the shape now but reading your reply to my note, I would have put it as mill to less than half diameter not greater.

I think you will find the idea of having greater than half Dia is to stop the tool bit chattering, or otherwise moving about, and therefore cutting a larger hole than itself. If for instance the flat is less than half thickness it could move about as the cutting edge is less than its diameter. If the D is greater than half, the cutting edge is also less than Dia but it now has support.

The better shape would be to have only a quarter or third section removed, which would give far better support, and this is indeed one of the commercial shapes for gun drill and the like.

[MikeKParticipant @mikek40713]

Thanks, Chris.

And I finally read the link Neil posted. It links to another page on how to make a D-bit. So I'll see what I can accomplish tonight. I'll post pics of the silly thing I'm using it for.

[MikeKParticipant @mikek40713]

I decided to take the advice to use an end mill as a boring bar. Since I didn't have a way to hold the end mill securely, I made this tonight:

[HopperParticipant @hopper]

Why not drill 3/8" and glue the magnet in place?

[JasonBModerator @jasonb]

"I've done this before with other magnets, but with those I made the hole deeper and used a 0.374" reamer. With this one I need the magnet seated firmly at the bottom of the 0.250" hole (and without thread locking compound). "

[Anthony KendallParticipant @anthonykendall53479]

What George Thomas says about D Bits here….

**LINK**

[MikeKParticipant @mikek40713]

I don't want to use glue, because I want the magnet to be in complete contact with the steel at the bottom of the hole.

Thanks for that link, Anthony.

[Ian PParticipant @ianp]

Posted by MikeK on 18/09/2015 14:18:25:

I don't want to use glue, because I want the magnet to be in complete contact with the steel at the bottom of the hole.

Thanks for that link, Anthony.

Mike

I have been following this thread and have been wondering why the hole had to be precisely 0.374" diameter (I still dont know) but now we know that this magnet is going to be sat in a flat bottomed hole in steel (a ferrous and magnetic material!).

If your magnets is a cylindrical rare earth button type then once its in the hole (flush with the top of the hole?) it will be nicely 'shorted out' thus reducing its effective strength.

With or without glue, Loctite or whatever, it will be very difficult to ever get the magnet out of the hole. Any adhesive would be squeezed out from under the magnet anyway and probably not affect its installed depth.

Going back to the hole diameter. Why cant you use a 3/8" (0.375&quot cutter. I fail to see how a one thou difference in diameter can matter.

Ian P

[Ian PParticipant @ianp]

Oh, I forgot another question.

Why do you want the magnet to be in 'complete contact' with the bottom of the hole?

Ian P

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