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Post by Deleted on Sept 6, 2008 14:39:09 GMT
Help please. I was never taught about D-bits in my youth: in fact I did not know about them until I started ME, and they are mentioned in the Brit Words and Music so many times that I have to make some. I am perplexed by the plethora of shapes and clearance angles; do I use virgin Silver Steel for the diameter or turn my own; is it 10 degrees return angle on the front face or is it perpendicular; will .010" above half thickness be OK or just .005 etc.....
Who knows the truth? Enlighten me please! Or should I just purchase the Chronos set.......
Many thanks.
JB
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Post by AndrewP on Sept 6, 2008 15:00:32 GMT
Geo H Thomas says 6 degrees clearance on the front face and size over half diameter varies from 3 or 4 thou for small bits (0.10in) up to 10 thou for 1/2in bits. He also makes the cutting edge square to the axis of the bit so it cuts a true flat bottomed hole and has 6 degrees back from square on the trailing edge.
If it's any consolation he couldn't find a definition of a D bit either.
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Post by Tel on Sept 6, 2008 21:29:22 GMT
My experience is that the angles ain't real critical, just go as close to 1/2 diameter as you can get. You can use either the natural surface of the steel, or turn 'em, or make 'em tapered or .......
Master making them - it'll open a whole new world for you.
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russell
Statesman
Chain driven
Posts: 762
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Post by russell on Sept 7, 2008 8:20:42 GMT
They make a good substitute for reamers - and much cheaper! Just make sure that they are a gnats over half diameter so that they cut only on the leading edge, not on the side.
Russell.
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Post by Deleted on Sept 7, 2008 9:07:22 GMT
I've just made a couple of tapered bits to form the choke of a small i.c. engine - about the only way I've found to do it satisfactorily. They were both made from the same piece of 5/16 in. diameter silver steel, brought to
the same gnat's over half diameter and finished on an oil stone after hardening. Interestingly, one cut like a dream but the other wasn't as good as it rubbed on the side over a short length. Could have been a variation in the aluminium casting material, I suppose.
Anyway, they are very useful tools and its worth mastering their manufacture.
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dva
Seasoned Member
Posts: 110
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Post by dva on Sept 7, 2008 11:55:14 GMT
May need to brush up your hardning skills. After a few abortive attempts I settled for leaving the D-bit fully hard and not bothering with the tempering. For the occasional use I find it works ok.
Dave
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Post by houstonceng on Sept 7, 2008 18:21:51 GMT
JB
I've always made my D-bits to the instructions in an ME article (Novices' Corner - Vol 103, No 2584, Nov 30 1950) which I obtained many moons ago in a "job-lot" from a club "bring and buy" and just happened to have to hand today.
The dimension given is one half diameter PLUS 0.001" (one thou) for every 1/10" (0.1") of diameter. So a half-inch diameter D-bit would have its flat 0.005 inches above centre-line. 3/4" D-bir has its flat 0.0075 above, etc.
Front and side clearances are usually 5 degrees each, ground carefully after hardening and tempering (H&T).
No reason you can't file the flat before H&T (leaving an allowance for final honing after hardening/tempering & grinding the clearances) - as suggested in the article - but measurement of the remaining portion - as also recommended - should be by micrometer or similar.
If the D-bit is less than one-half diameter it won't cut properly (usually chatters). Exactly half-diameter will just about cut with no chatter. Made to the above dimensions - perfeck !
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Post by Deleted on Sept 10, 2008 18:38:41 GMT
Thanks to all for the info: after downloading the USA tome on metal removal I am now getting interested in Gun Drills!
Thanks again
JB
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Post by reedx on Sept 11, 2008 8:18:10 GMT
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