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Making a d bit reamer question...

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bogstandard:
I saw your post yesterday, so I waited until this morning, while I am still tidying up my shop, to show you how I make them.
Everything what the others have said, I do, except that now I grind the half diameter using my surface grinder. I tend to attempt to get the bit exactly half the diameter, only because I have the facilities to grind it very accurately, if I was grinding by hand, I would err on the high side, as suggested by the others.
To me a D-bit is a temporary tool, only used for maybe a few holes at most, so unlike the others, I rarely temper after hardening, unless I think the tool will be used again or reshaped by grinding in the future, or if it is going to be removing a lot more than a few thou that they are usually used for. I just polish up the surface and grind away to shape.

Only ever dress the cutting edge from the top flat face, and very gently, you don't want to be removing too much metal. I tend to stick with a fine Arkansas stone with thin oil as the lubricant, only because that I find diamond laps tend to clog up too quickly when used on steel, and also they have a tendency to remove too much metal.


This is my collection that I have saved over the years, a lot of the ones I make, I throw away after they have finished the job.




This shot shows the front rake that I use.




To me, one of the most important bits is to relieve the front cutting face on the right hand side.




I relieve away just over half the width, unless it is a round nose tool, then I try to get it spot on centre.




You can just make out the cutting face on this shot.




As Dave and Stew have said, there are many ways and methods to make D-bits, and everyone has their own favourite way to grind them up and heat treat them. My way is just one of many. You just have to find what works for you and stick with it.

Making tapered D-bits require a slightly different technique as you don't know when you have ground down to half diameter. If you need to know about that, just let us know.


Bogs

Pelallito:
John,
Please tell us about making a tapered D Reamer.  I will be trying my hand at that soon and could not pass up on your offer!
Thanks again.
Regards,
Fred

DavesWimshurst:
Sean,
A dremel tool mounted in the lathe may not be rigid enough to grind the d-bit shape, at least I never got one to make a decent finish when I tried it many years ago. 

You want some clearance in your bearing about one and a half to two thousandths or lubrication may fail and the surfaces may gall.

I wouldn't try lapping the rod because the abrasive may embed itself in the softer aluminum wearing out the crank pin.

I would first try just milling the d-bit shape and stoning the flat top surface after hardening.

Hope I haven't confused the issue.
Dave

seanol:
Gentlemen,

Mr. sbwhart:
Thanks. I will try that if I temper.

Mr. Bogstandard:
Thank you. I have G. Thomas' books and they agree exactly to what you have shown. It is hard to envision this without the pics you have provided. Your experience is invaluable and makes it seem less daunting. It is only a hole in aluminum but lessons learned here should make for fewer pieces to be recycled in the scrap bin! :bang:

DavesWimshurst:
No confusion. I am going to try to mill the flat a couple thousandths oversize, harden and slowly take off the rest with the dremel. If that doesn't work I can use the grinder to get the rest. When I try for the fit in the conrod I can always try polishing the crank pin with scotchbrite. This is how we used to do it on crankshafts to get the last couple of tenths for bearing clearance. The conrod rides directly on the crank pin so I am still wondering how I know I have the right fit. I figure if I ream the hole with a measured reamer and polish the crank pin to about .002 under reamer size I should be there?

I believe that one of the main reasons I got involved with this hobby was to learn something to pass along to my kids and enjoy working with them in the shop when they get older. I hope as my experience toolbox grows I can pass along the lessons you folks have so freely passed along here. You have definitely demystified the mysterious!
 :nrocks:

bogstandard:
Fred,

Cutting the taper wasn't the problem, that was done with the topslide set over. It was when it came to making it half thickness my problems started.

I struggled for ages trying to get a decent tapered D-bit, making sure everything was truly level when cutting it to half diameter, but it always came down to where you measure the half diameter along the length.

The normal method was to half the diameter of the original blank and measure that. Unfortunately, because of wear on chucks, concentricity could not be guaranteed, and you could be a few thou out, and you stood the chance that the cutting face would be below centre.

The solution was totally obvious and a thing that never occured to me (being a young smart arsed engineer who knew everything). Then a chappie in the model shop saw me struggling, clipped me round the ear and said 'use you brains lad, cut a spigot top or bottom'. It had never occured to me to cut a datum face to work to after the taper was finished either at the small, or the large end of the taper, depending which was the easiest to do. So the sketch shows how I do long tapers, for a short taper, I would cut a datum at the thick end and measure there.

I seems totally obvious to most people to do it that way, but I had a complete mental blockage on how to solve the problem. It was after that, while working in the model shop, that I started to absorb knowledge from the experienced ones, rather than gathering all my information from books. Books are good, but for working knowledge, experience is better.


Sean,

Dave has a very valid point about the Dremel as a grinding unit.

It would be OK for very small projects, but it might really overwhealm it trying to remove that much metal to make a D-bit. You really need something with a substantial bearing surface for toolpost grinding

I always turn mine to almost size, then gently emery it down to finished size. But the emery must be used with a rigid backing board, otherwise you will get undulations along the length of the bit.

A well made D-bit will perform just as well as a reamer if it is made well, but don't go running around trying to achieve tighter and tighter tolerances. In most circumstances, a running clearance is required for parts to work in unison. Make then too tight, and expansion will soon tighten a tight running fit into a siezed up solid lump.

Quoting Dave again, an oil film will take up on average about two thou clearance, and without that oil taking the pressure, the joint will soon gall and sieze up.

John

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