Making my first D-bit

[Brian Rupnow]

While making the stand-off for the pivot arms for my opposed piston engine, the fickle finger of fate dove down and goosed me while I was building the second stand off. The 1/2" shaft was supposed to be turned to an exact 0.312" diameter. I calculated very carefully, and then turned the damned thing down to 0.305" diameter. I'm never quite sure how I do that, but I do. I do not want to build it over again. There is a fabricated brass bushing which is supposed to have a 0.375" o.d to fit in the pivot arm i.d and a .312 bore to fit the turned down end of the stand off. I have read numerous reports of people building D-bits, which seem like a perfect solution to make any size hole, dead nuts on spec. It seems that today would be a good day to learn something, and I do a short piece of drill rod (01) which I will attempt to make into a D-bit to drill a perfect 0.304" hole into the bushing. Follow along, and watch me either create another amazing shop tool "first" or else make a complete fool of myself!!!

 

[kspringer80]

I've successfully made a few of these and would love to see how you set up to accomplish the task. I learn a lot from seeing how others accomplish the same thing as I have with a whole different set-up... Hoping it leads to one of those "why didn't I think of that" moments!

 

[goldstar31]

Brian,
There must 1001 reasons for not getting something to size- despite ALL the careful precautions made. One is that the tool is not quite on centre, another is that it is not quite sharp enough to remove that small amount and another is that there is wear on the feed screw or its bearing creating 'slop' I recall you saying that your lathe had some tailstock wear. Obviously, there could be more reasons but one old way is to sharpen the cutting tool- AGAIN- after the roughing cut.

As for the D Bit thing, my thoughts- for what they are worth- are two fold. You make the D bit using traditional removing of 0.002" less that the diameter but - and this is the bit that people miss is- to be certain that your D bit is held in a chuck which is accurate. Many, are not Precision chucks- and if they are, that they are held dead in line with a properly aligned tailstock.

Meantime, my regards for bringing an important question to the surface.

Norman

 

[Dave Sohlstrom]

Brian

D bits are great tools. I have made both straight and tapered D bits. I did learn one day that a D bit, aluminum and to much speed make a perfect friction weld. When I make one I polish to diameter and then harden. Once hard I do not anneal. The bit then goes into the surface grinder and gets ground down to just 1 or 2 thou greater than half the diameter.

If you do not have a surface grinder then into the mill and machine down and polish to just 1 or 2 thou greater than half the diameter, then harden and sharpen the front cutting edge.

Dave

 

[Brian Rupnow]

This is a scrap piece of 0.375" 01 drill rod. The outer end is turned to a very precise 0.25". The step diameter is 0.305"diameter and longer than the length of the bushing that has to be bored.

 

[goldstar31]

As a sort of follow up on Dave's comments, I used to make Northumbrian small bagpipes which necessitated doing 13" long bores of less than 3mm at times- and perfectly parallel. It involves a lot of -swear words to get anywhere. I used to use silver steel( is this drill rod?) but I never ground more than an inch flat. Consequently, it needed lots of clearing of blackwood before the recess was clogged- otherwise it seized, got red hot, split the wood or came out in the wrong place( or all of them)

I used to temper the end- by sticking it red hot into a potato. One of my mates- long gone- was a clock maker and he used to dip the red hot silver steel in Kasenit casehardening powder before honing.

Probably someone will mention gunbarrel reamers- tools out of my experience.

 

[Brian Rupnow]

The great book of Knowledge of all Things than says to machine 1/2 of the diameter +.001 away on one side. I will probably get to the +.001 in my next step.

 

[Brian Rupnow]

The next steps weren't 100% clear. It seems to say that you should heat treat first, then clean up the newly milled side on an emery stone. Since the tracks left by my endmill were pretty course, I decided to clean it up on the emery stone first to get rid of the worst machining marks, then heat treat it, then dress it a final time on the emery stone. The heat treat consists of heating the business end cherry red, then quenching by dipping vertically into a container of oil or water. 01 is a water hardening steel, and apparently some water quench steels won't harden if you use oil (according to my supplier). After initial quench, then heat again to medium straw color and quench again. --I believe this second quench "draws it back" a little so it is not so brittle and apt to fracture.

 

[Brian Rupnow]

First off, I made myself a "dummy shaft" exactly 0.305" diameter.

 

[Brian Rupnow]

Then I set a piece of 1/2" brass hex rod up in the lathe and drilled and reamed a 1/4" hole. 1 1/4" deep.

 

[Brian Rupnow]

Then I took my new weapon and used it to open up the hole to 0.305---Had to back out often, as the chips build up quickly and there are no spiral flutes to carry them away.

 

[Brian Rupnow]

And the .305" test shaft is a perfect fit!!! Hooray!!! I learned something new today!!!

 

[Brian Rupnow]

I just spoke to my steel supplier---He had it wrong.--- 01 IS an oil hardening steel. It didn't affect the outcome of my experiment at any rate.---Brian

 

[Swifty]

I think that the O in O1 means oil hardening, and the W in W1 means water hardening. I always buy water hardening so there is no mix up as to what type I have.

Paul.

 

[BaronJ]

Hi Brian,

The angle of the cut back looks a little steep. When I've made mine I aim for about 10 degrees. I was once told "think lathe tool" when grinding the front angles. Its also an easy way to get an almost flat bottom hole without resorting to a milling cutter.

But it looks like you did a pretty good job of it.

 

[barnesrickw]

i'm currently working on a tap from W1, and just made my first boring bar. I think I like making the tools for my lathe more than projects.

 

[BaronJ]

i'm currently working on a tap from W1, and just made my first boring bar. I think I like making the tools for my lathe more than projects.

I agree ! I have yet to make an engine, but seem to be making tools. When I think about it, way back you couldn't just go out and buy what you needed, you had to make the tool to do the job.

Making a "D" bit is a good example.

 

[barnesrickw]

In woodworking making your own planes, etc. is considered part of the higher understanding of the craft. I'm guessing it's the same for metal.

 

[BaronJ]

In woodworking making your own planes, etc. is considered part of the higher understanding of the craft. I'm guessing it's the same for metal.

Yes I would think so.