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Is there such a thing as an 'external reamer'?

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Robin Graham18/08/2021 00:33:01
874 forum posts
261 photos

I want to make some parts which require turning thin discs with 10mm 'spigots' - I don't know if that's the right word, but to give an idea:


Ignore the 'features' on the spigot which are due to lapses of attention . At the moment it's 11mm so I can turn them out. I was listening to a jolly interesting murder mystery at the time, and sudden plot twists lead to involuntary muscle movements. That's my excuse anyway.

My plan is to turn the spigot to size (I'm looking for an easy push fit in an H7 hole), part off a slice, then do another on the same bar and so on. As I have to do a few I was wondering if there is a tailstock tool that could form the 'spigot' accurately in one go if I rough turned to say 10.5 mm.

To give an idea of what I have in mind there are woodworking plug cutters like this:


Does anyone know if metalworking tools are available for this purpose? I suppose I could make something myself, but I'm reluctant to believe that this wheel hasn't already been invented.



Edited By Robin Graham on 18/08/2021 00:33:59

Emgee18/08/2021 01:17:10
2156 forum posts
265 photos

You may find a Rotabroach cutter with the desired bore, if not make one to suit from silver steel and harden.

But if you can turn accurately to 10.5mm why not 10mm ?


Robin Graham18/08/2021 01:46:27
874 forum posts
261 photos
Posted by Emgee on 18/08/2021 01:17:10:

You may find a Rotabroach cutter with the desired bore, if not make one to suit from silver steel and harden.

But if you can turn accurately to 10.5mm why not 10mm ?


Yes, I could make a cutter but I'm a lazy b*st*rd, and if I could buy off the shelf it would be easier. Also I wanted to know if if such things exist for self-education. Strange as it must seem to those who have spent a lifetime in engineering, I didn't even know how screw threads were made for the first fifty years of my life. I'm an old dog trying to learn new tricks.

I can make an internal bore to H7 with a boring bar, but for small diameters it's easier/quicker to drill and ream. I can turn 10mm externally accurately sure enough, but I was wondering if there was tool, analogous to the internal bore reamer, which would make the process faster and less prone to error.


Edited By Robin Graham on 18/08/2021 01:57:56

Paul Lousick18/08/2021 01:50:36
1855 forum posts
661 photos

M ount a boring head in the tailstock and set the cutter to the required radius for the finish cut.

boring head.jpg

Robin Graham18/08/2021 02:49:22
874 forum posts
261 photos

Thanks Paul. That should work , didn't think of it. Compartmentalised thinking on my part - boring head is a milling tool... blush


Speedy Builder518/08/2021 06:43:48
2407 forum posts
191 photos

Boring head - You would have to be certain that there was NO play in the tailstock quill for a boring head to be accurate.

DiogenesII18/08/2021 06:50:57
308 forum posts
153 photos

Time to attend to that cross-slide stop.. ..doesn't have to be anything elaborate, but any kind of repetition work becomes so much easier..

Edited By DiogenesII on 18/08/2021 06:52:38

JasonB18/08/2021 07:15:07
21435 forum posts
2448 photos
1 articles

You may well just end up with the same problem - what is to stop you over feeding the tailstock and marking the surface of the disc?

May as well just note the handwheel reading on the cross slide or as said above work out some form of adjustable stop.

If you do still want to use a tailstock then look up "box tools" often used on capstan and repetition lathes to cut to a set diameter

Edited By JasonB on 18/08/2021 07:17:02

Clive Foster18/08/2021 07:57:09
2835 forum posts
103 photos

+1 to what Jason says about capstan and repetition lathe tooling.

That problem was solved many, many years ago to make "end-on" manufacture of accurate components possible. Since then the number of components produced on capstan and repetition lathes is way beyond convenient, or even inconvenient, counting.

Time to start digging in the historical records to find something simple to make, easy to set up and effective. Modern box tooling tends to be a bit overkill and somewhat specialised for small ranges of sizes.

Easiest would be a simple disk with a slot in it to carry a short length of HSS as the tool bit. Slot needs to be angled at the back so the side of the tool bit cannot come into contact with the job and mar the face. If working brass you can probably get away with no rake as the cut is shallow. Tool setting becomes much easier.

As the cutting tip has to be on centre any top rake makes life more complicated. Back in the day tool setting was a job for a specialist.

I'd just set up stops and have at it "side on" in the normal manner with an old fashioned wide parting tool. Of which I have several in stock which mostly get used for this sort of thing. But my lathes are much more industrial than yours.

The modern professional uses a diamond shape insert.


not done it yet18/08/2021 08:20:38
6322 forum posts
20 photos

A DRO on the cross slide is the most obvious way to go on a manual machine?

Or a single cut on consecutive parts which have previously been roughed to the same size might well suffice, with the cross slide locked.

With the auto long-travel trip on my lathe, I expect I could make some fairly close tolerance parts using that and the cross slide DRO - but I simply would not want to go to the trouble unless making a lot of similar components.

I do know someone who used to make batches of same sized parts, on a manual lathe, for his sole-trader business.. Once set up reproducibility was good enough for his needs, but I don’t know how close his tolerances actually were. His output was regarded as high quality products.

larry phelan 118/08/2021 08:22:10
1088 forum posts
14 photos

Some of you may remember my seeking advice about a similar problem some time ago.

I had made an adjustable stop for my cross slide, which worked, sort of, but was just too light.

The advice I received was to beef it up, and Hey Presto it worked !

I still use it from time to time and once set, it is a pleasure to use, since it has three different settings.

Easy enough to make.

Speedy Builder518/08/2021 08:55:28
2407 forum posts
191 photos

Don't worry about the "fit" turn to size (use a stop or DRO for positioning) then light knurl ??

Rod Renshaw18/08/2021 09:48:37
325 forum posts
2 photos

Watchmakers use "rose bits" for this kind of thing, but on a much smaller scale!

Tubal Cain ( the original one) describes how to make them in his book "Simple Workshop Devices"


Rob McSweeney18/08/2021 09:55:16
59 forum posts

At least one of the old lime lathe books (possibly Mason, have checked the index of Sparey and it isn't listed) describes the making of 'running down cutters', which are very close to the woodworking plug cutters in the O.P. I made a few years ago to make some BA screws in brass, and they worked well enough.

Andrew Johnston18/08/2021 10:42:07
6264 forum posts
677 photos
Posted by Rob McSweeney on 18/08/2021 09:55:16:

...making of 'running down cutters', which are very close to the woodworking plug cutters..............

That's what I did when I needed to machine an enclosed spigot:

valve body and tool.jpg

I'm not convinced a roller box would be suitable. They're designed to machine long parts to diameter, not short spigots. Herbert did sell one indended to machine to a shoulder, but I expect they're in rocking horse territory. If I was making the part on the repetition lathe I'd use a knife tool and just set stops for diameter and depth. For a lot of parts I'd machine in two stages, a plunge cut to rough out and then a finishing cut on diameter and shoulder.


Tony Pratt 118/08/2021 12:53:42
1692 forum posts
8 photos

The OP is after a 'easy push fit' so some degree of precision is required, I would suggest a DRO or cross slide stop or even use the dials providedwink I can't see that an external reamer would do the job even if one existed.


Neil Wyatt18/08/2021 13:38:45
18776 forum posts
733 photos
80 articles

A box tool might do the job.


Mick B118/08/2021 15:18:59
2018 forum posts
116 photos

On roller boxes the tool is generally ahead of the rollers, making it at best marginal on such a short spigot. A vee box might be better, but either would risk scoring on the broad face of the OD. I'd think Andrew Johnston's pretty much on the money.

Robert Butler18/08/2021 17:39:24
291 forum posts
6 photos

I needed to create a 8mm diameter spigot within a cavity, an engineer pal opened up the internal bore of the rotabroach cutter to suit. To create a depth stop i inserted an 8mm plug of the required length to bottom out on the toolholder held in the tailstock turret. Repeatability to required accuracy and the rotabroach assisted with centering.

Robert Butler

John Reese18/08/2021 22:27:48
986 forum posts

Hollow mills are available. Here s an example:


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