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Making Reamers particularly tapered reamers

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martin perman05/06/2012 09:43:27
1688 forum posts
70 photos


A friend of mine is making a new arm for a flick magneto and he has to put a tapered hole in its centre to match the tapered shaft on the magneto, would it be possible to make a hand reamer to cut the taper after he has step drilled the hole.

Thanks in anticipation,

Martin P

Swarf, Mostly!05/06/2012 10:46:56
498 forum posts
41 photos

Hi there, Martin,

I suggest that you research tapered D-bits.

Best regards,

Swarf, Mostly!

Ady105/06/2012 11:09:13
3463 forum posts
513 photos

I think it was Sparey who had some home made d-bit reamers in his book

Cut a silver steel bar to the required size then harden

PekkaNF05/06/2012 11:15:53
94 forum posts
9 photos

Would a google with "D-bit reamer", "D taper reamer" or "D bit chambering reamer" help?

I ocassionally stumble upon on D-bit type taper reamer instructions, but can't remember bookmarkking any right away.

I have made several too from silversteel/dil rod and they seem to work reasonably well, require quite a bit effort to use though.


There is one rather compilicated, but good method:



martin perman05/06/2012 11:46:12
1688 forum posts
70 photos


Thank you very much for your help, I shall find a piece of silver steel of the correct size and make the reamer for him.

Martin P

Ramon Wilson05/06/2012 12:49:15
676 forum posts
72 photos

Martin, I have made many tapered 'D bit' reamers over the years all of which have produced good holes and surface finish mainly in steel but also brass and aluminium.

The big problem with tapers is that to be effective they have to match the corresponding part exactly and this is usually achieved by making sure the hole is done at the same setting as the cutter without moving the topslide.

When this can't be done ie in your case - if its possible you need the magneto shaft as you do it. Turn a tapered hole in a piece of scrap (I usually use brass) to match the shaft as best you can and then check the shaft in this using engineers blue to see where it fits - slightly moving the topslide and recutting if required to improve the fit until it's satisfactory and mating its full length. It's surprising how little it has to be moved to make quite a big difference in fit. Once you have the taper right you can then proceed to make the cutter reasonably confident that it will match the shaft

Hope this helps - Ramon

martin perman05/06/2012 13:01:43
1688 forum posts
70 photos


Thanks for that, I have a similar magneto that I'm going to use as my template.

Martin P

Gordon W05/06/2012 13:19:23
2011 forum posts

If you can set the shaft between centers you can then set the topslide with a dial gauge, should be spot on. Difficulty I have is cutting down to exact center without a mill.

Richard Parsons05/06/2012 21:28:57
645 forum posts
33 photos

Actually I would measure the taper and then do a bit of research in the books/Google. I will bet you a single knob of dog biscuit to a pinch of snuff that the thing is a known standard. I do not know what the size is but manufacturers always to use ‘standard tooling’ if they can.

There is another way. Set your compound (or taper attachment if you have one) as Gordon W describes. Turn a plain taper next put a small end mill in your tool post milling attachment (mine is the headstock from my first lathe a Unimat SL) I was planning to ft an overhead shaft system but the builder fitted a cavity ceiling when he was told not to. He did so because he said a plain concrete ceiling would not look nice.

With the mill make 6 passes down the work piece so that you have now a 6 sides broach. Harden and lap in the usual way.



Gordon W06/06/2012 09:46:41
2011 forum posts

From memory:- the taper will be 1 in 6 or 1 in 7.

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