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chucketn
01-06-2013, 02:04 PM
I need to make a 'D bit' to ream a bearing for a connecting rod. I purchased 1/4" O1 drill rod for this purpose. This is my first "D bit", as well as my first reamed hole.
I cut a 3" piece, chucked it in the lathe and faced the ends. Next, I placed the blank in my mill vise on a paralell and attempted to mill to 1/2 the diameter. I touched the mill cutter to the top, zeroed the Z axis DRO, and lowered the Z axis to .125, locked it, and milled the blank for approximately 1/2". The bearing is 1/4" thick brass. When I removed the blank from the mill vise it measures .118 on the "D" part. Is this going to work? How critical is this thickness to the accuracy of the reamed hole? Do I need to try again?

Chuck

camdigger
01-06-2013, 02:15 PM
Have a look at the D-bit reamers in the top of this image. One was done from drill rod (the 3/8" one), one was done freehand grinding a hardened shaft on a bench grinder, and the smallest was done from a prehard drill blank. All cut freely and to size.

http://i766.photobucket.com/albums/xx301/camdigger/Shop%20made%20tools/PB291375.jpg

chucketn
01-06-2013, 02:27 PM
Hard to see details. Does the bevel on the top two go all the way across the diameter? Obviously, that would be easier to make.

Chuck

loose nut
01-06-2013, 03:30 PM
Have a look at the D-bit reamers in the top of this image. One was done from drill rod (the 3/8" one), one was done freehand grinding a hardened shaft on a bench grinder, and the smallest was done from a prehard drill blank. All cut freely and to size.

http://i766.photobucket.com/albums/xx301/camdigger/Shop%20made%20tools/PB291375.jpg

If you want to be accurate, they are reamers of various types but not D-bit reamers. A D-bit is a specific type of reamer which can act as a drill as well as a reamer, most of the ones in the picture are just reamers.

I don't have a picture of a proper D-bit at hand.

gizmo2
01-06-2013, 04:36 PM
I don't think it'll work for you. Try this; mill as before, but deliberately leave it fat and measure it with a mic as you come down to .126". Then you can shape the nose with grinder or file. Then harden. Then stone/hone down another half thou'. Should be a nice sharp edge at .1255" and should cut properly. This is the best luck I've had making d-reamers; 1 thou over half diameter and then honing after hardening.

darryl
01-06-2013, 07:29 PM
I'm thinking that you're so close to half that it will work for you. If it's over half it may not want to begin cutting in the hole. Less than half and you have some rake angle, but also the possibility that it will cut the hole slightly small. If the tool is exactly on center it will probably cut exactly right- if it's above center by a couple thou it may cut the hole small, and if it's above center by more than about 6 or 7 thou it might cut the hole large. The whole idea of a D bit is that the hole it's making guides the bit.

If you imagine milling away more than half the diameter by a large amount, it would be seen that the tab left is narrower than the diameter of the shank. But the cutting edge is still on the OD of the shank, so if the bottom of the tab maintained contact with the bottom of the hole that it's opening up, the hole diameter would still be correct. The force of the cutting action acts to keep the bottom of the tool in contact with the bottom of the hole. In theory then, it will do the job. If the tool is being held at a height other than dead-on axis, or if swarf tends to raise the bottom of the tool, then you have problems.

Forestgnome
01-06-2013, 08:38 PM
Have a look at the D-bit reamers in the top of this image. One was done from drill rod (the 3/8" one), one was done freehand grinding a hardened shaft on a bench grinder, and the smallest was done from a prehard drill blank. All cut freely and to size.

http://i766.photobucket.com/albums/xx301/camdigger/Shop%20made%20tools/PB291375.jpg
Those are usually called toolmaker's reamers. They're not D reamers.

Mcgyver
01-06-2013, 08:42 PM
I'll leave it to the D bit experts as to whether it will work, haven't used them much myself, but in general you'll have better luck milling if you take a cut close to where you want to end up then measure how more is to come off and take a final finish cut rather than rely on picking up a surface and doing it all in one cut

TGTool
01-06-2013, 10:25 PM
I've made a few and they've worked fine when I've needed them for odd and precise sizes. I think I've seen a percent figure for size over one half diameter but can't lay a hand on it right at the moment. George Thomas's practice is to allow 3 or 4 thousandths for small bits up to 10 thousandths max for 1/2". The flat has to be over half diameter to get it to cut on size. The end can be ground for clearance all the way across, or it can be ground straight, then cleared a few degrees half way across on the non-cutting side to get a flat bottom hole if you need it.

For the OP, if you need a .250 hole I think you'll need to measure something like .128 over the stock you have left. IIRC you also mention coming down from the top when you milled it. That's one approach, but if you come from the side you'll leave a rounded fillet behind from your endmill that will be stronger than the sharp corner.

Do you need a picture?

camdigger
01-07-2013, 01:52 AM
Those are usually called toolmaker's reamers. They're not D reamers.

What ever you call èm, they work. They will not drill a new hole, but they will definitely open up an unersized hole to a good tolerance with lube and the other usual necessities....