Author Topic: Why doesn't my D bit cut?  (Read 5562 times)

Offline RotarySMP

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Why doesn't my D bit cut?
« on: June 15, 2016, 02:26:10 AM »
I need to make 3 collars, with 11mm bores. The material is stainless. I predrilled the bore to 10.2mm. Made a D bit out of silver steel (drill rod, 1.2210, 115CrV3). The bore end is 10.98mm, and the half thickness 5.48mm. Hardened and tempered back to deep straw colour.

The front cutting edge as ground on the Clarkson to 10 front cutting angle, and 7 front relief. Then honed on the top and front faces.

When I tried it on a collar, it deflected and made a mess.

Any suggestion for correcting the geometry would be appreciated.
Mark

Offline SwarfnStuff

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Re: Why doesn't my D bit cut?
« Reply #1 on: June 15, 2016, 02:37:40 AM »
Hi Mark,
       In my very limited experience with "D" bits the front end would be domed like looking down from above = D not as you have like a lathe tool.
       That way any deflection of the bit pushes against the opposite edge of the "D"
Other, more experienced members will be sure to help.
Have a look at the first bit of Clickspring's video on  them, where he shows various forms of them below.
   

Regards,
John B
Converting good metal into swarf sometimes ending up with something useful. ;-)

Offline RotarySMP

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Re: Why doesn't my D bit cut?
« Reply #2 on: June 15, 2016, 03:42:57 AM »
Clicksprings videos are very good. I looked at that video before making mine, otherwise I wouldn't have had the pretty scooped out look :)

His video is for making a 45 milling tool, but I screen shot his collection of bits, and there one similar to mine, so I guess I'll grind a flat on  the nose to see if that helps.

Mark

Offline mfletch

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Re: Why doesn't my D bit cut?
« Reply #3 on: June 15, 2016, 04:04:25 AM »
I would say its rubbing on the bottom of the tool or just down from the cutting edge

Offline PekkaNF

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Re: Why doesn't my D bit cut?
« Reply #4 on: June 15, 2016, 04:21:56 AM »
I need to make 3 collars, with 11mm bores. The material is stainless. I predrilled the bore to 10.2mm. Made a D bit out of silver steel (drill rod, 1.2210, 115CrV3). The bore end is 10.98mm, and the half thickness 5.48mm...

You didn't do relief under cutting edge (There should be no):
http://www.practicalmachinist.com/vb/attachments/f38/42457d1326104953-how-do-you-make-d-bit-d-bit.jpg

You need to hone the cuting edge just tad under 1/2 the thickness, or "front" will rub and it's pretty near impossible in cutting.

Many varieties of stainless are proverbial to cut with an obtuse (or near) cutting edge.

Pekka

Offline Joules

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Re: Why doesn't my D bit cut?
« Reply #5 on: June 15, 2016, 04:58:36 AM »
I'll second more back relief on the cutter, even to the point of the rest of the half circle and LOTS of cutting fluid/coolant to save the tip of your D cutter.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline RotarySMP

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Re: Why doesn't my D bit cut?
« Reply #6 on: June 15, 2016, 07:10:13 AM »
I thought D-bits only cut on the front face not on the side, therefore needing no side clearance. I kind of loath cutting stainless, but it was the right size bit of scrap.

I'll throw it back on the Clarkson, and grind some relief under that side edge.
Mark
« Last Edit: June 15, 2016, 09:31:03 AM by RotarySMP »

Offline Joules

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Re: Why doesn't my D bit cut?
« Reply #7 on: June 15, 2016, 08:06:34 AM »
A quick read shows the D bit is more a finishing tool (reamer in harder materials) and perhaps what you want with the amount of material to remove is a boring tool ?
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline RotarySMP

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Re: Why doesn't my D bit cut?
« Reply #8 on: June 15, 2016, 08:11:03 AM »
If it fails to work as a D-Bit, it will be easy to regrind this to a lathe bit. I was only opening up the bore hole from 10.2 to 11mm, so not a huge about of metal to be removed.
Mark

Offline PekkaNF

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Re: Why doesn't my D bit cut?
« Reply #9 on: June 15, 2016, 08:17:05 AM »
I thought D-bits only cut on the front face not on the side, therefore needing no side clearance. I kind of loath cutting stainless, but it was the right size bit of scrap.

I'll through it back on the Clarkson, and grind some relief under that side edge.
Mark

That is the idea, only on front. No clearance on side, but the cutting "point" should lie exacatly on the middle or just a tad below. Did I said it wrong?

Therefore there is other way of bodging D-bit: The front is cut at the angle and there will be an oblique surface that shaves: There is a point where this slant angle is at 1/2 diameter. I have used it on brass, but never anything stronger.

Pekka

Offline RotarySMP

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Re: Why doesn't my D bit cut?
« Reply #10 on: June 15, 2016, 09:30:34 AM »
I got the half height nearly perfect. 10.98mm diameter cut and honed to 5.48mm There is 7 of clearance angle below the cutting edge, I was running in Backgear at around 250RPM but it didn't want to cut, just pushed away. The hardening must be okay, as the corner is still sharp.

Mark

Offline DavidA

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Re: Why doesn't my D bit cut?
« Reply #11 on: June 15, 2016, 12:40:49 PM »
Mark, that tool you circled in the picture looks as it it is for forming a flat bottom to a blind hole, or maybe for forming a flat 'ledge', maybe to clean up a counter bore for use with capscrews. Not for reaming. It doesn't appear to have any sloping 'lead' to shave the metal at the diameter.

Dave.

Offline Baron

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Re: Why doesn't my D bit cut?
« Reply #12 on: June 15, 2016, 04:05:55 PM »
Hello Mark,

From the info you have given you are trying to cut too much material away.  A "D" bit is more like a reamer and 0.6 mm is far too much. A tenth of a mm (4 thou ) would be more reasonable.  Particularly in a harder material like stainless.
Best Regards:
                     Baron

Offline SwarfnStuff

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Re: Why doesn't my D bit cut?
« Reply #13 on: June 16, 2016, 04:08:58 AM »
Mark,
      I agree that you are possibly expecting a bit much of a "D" bit, however, here is one of the type I was thinking of doing much more.     
      It is being used to do the bore of a model canon 1/2" and 10" or so in depth. Took my friend ages at 2.5mm per peck, clean and go again - 2.5hr if I recall.
      He gave it a short starting hole of the correct diameter and the D bit did the rest. As you can see from the pic complete with chips it is much as I described with the off side keeping things from wandering. The barrel was brass, the bit of drill rod / silver steel and hardened after grinding and honing appropriate reliefs.
John B
« Last Edit: June 16, 2016, 04:33:58 AM by SwarfnStuff »
Converting good metal into swarf sometimes ending up with something useful. ;-)

Offline RotarySMP

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Re: Why doesn't my D bit cut?
« Reply #14 on: June 16, 2016, 05:12:42 AM »
I appreciate all your responses. Wasn't able to try fixing it yesterday, but should get back to it this weekend.
Mark