How do I ream a 15mm hole on a Unimat 3 ?

This topic has 8 replies, 6 voices, and was last updated 27 August 2026 at 11:11 by JasonB.

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27 August 2026 at 09:37 #862344
paul1956
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@paul1956
As first proper project I’ve bought a Beng Danni stationary engine kit.

From what I read before purchase everything ought to be able to be machined on my Unimat 3. Now the kit has arrived I see that the cylinder will need to be 15mmH7 finished in the brass bar stock.
I think I can work up through drill sizes with what I have available to ½” (12.7mm) and then use a boring bar to get close to 15mm. That leaves the final reaming, but a 15mm reamer won’t fit in either my chuck or ER16 collet set.
Do mount a 15mm reamer dialled into a 4 jaw chuck mounted on the tailstock ? Or is there another method that won’t involve great expense ?

Thanks in advance for any suggestions.
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27 August 2026 at 09:48 #862350
ega
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@ega
Do you really need to ream? A bored hole can be as good as a reamed one.
27 August 2026 at 09:53 #862351
Dusty
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@dusty
Paul

Do not use a reamer, the chances are the bore will not be round. Bore to finished size. Your bore will be round and not threepenny bit shaped (slight exagaration) Reamers are sizing tools not boring tools.
27 August 2026 at 10:06 #862358
JasonB
Moderator
@jasonb
Even if you could hold the reamer chances are it will be too long. But for future reference I used to hold large drills etc in the 3jaw lathe chuck either on the tailstock or milling attachment.

As others have said if you are set up to bore it out to reaming size you may as well just bore to final size. Use the piston to gauge the fit rather than working to actual measurements.
27 August 2026 at 10:10 #862360
noel shelley
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@noelshelley55608
Assuming the reamer has a square shank and is centre drilled then using a large tap wrench and align with a tailstock centre ream out as needed. Rotate the work by hand in the chuck. Noel.
27 August 2026 at 10:25 #862361
Michael Gilligan
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@michaelgilligan61133
I don’t know the kit, so this may not be practical … But

The job seems to cry-out for a between centres boring bar, and the component clamped firmly to the cross-slide

MichaelG.
27 August 2026 at 11:05 #862365
paul1956
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@paul1956
Thanks everyone for such quick helpful replies.

Rather nice that you think I should be able to bore to size without reaming. I’ll have to practice that before committing to the kit part.

On 27 August 2026 at 10:06 JasonB Said:

Use the piston to gauge the fit rather than working to actual measurements.

My reading so far has suggested to do it the other way, get a good parallel cylinder bore then machine the piston to fit. (There’s also enough material supplied to make two pistons, so room for one mistake).
27 August 2026 at 11:08 #862366
ega
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@ega
MichaelG:

Agreed, but the method may be more difficult for a beginner – OP says “first project”.
27 August 2026 at 11:11 #862367
JasonB
Moderator
@jasonb
I did not know what material they supply for the piston. if it is nomimal 15mm stock then you would have to live with that.

But yes generally it is better to do a hole first as it is harder to adjust than an external diameter.

Tailstock ctr and a spanner won’t work on a U3, as I said there is not enough bed length on a little U3
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