Attempt to fix, or re-purpose, or just scrap?

[Bill PhinnParticipant @billphinn90025]

I recently bought off Amazon an R8-MT1 adaptor to enable me to use my increasing stock of taper shank drills in my milling machine.

Unfortunately, unlike an R8-MT2 adaptor I have that I bought from a UK-based supplier before they ceased trading, which had only a few microns of runout up the taper, this MT1 adaptor has about 0.1mm runout up the taper and is never going to let me do good drilling with it.

Question is, how feasible is it to re-bore the internal MT1 taper in a way that improves concentricity? I can hold the top section of the adaptor in a 3- or 4-jaw chuck but what would I indicate off to keep the 7/16-20 threaded section held in the chuck jaws as perfectly in line as possible with the bore I want to create anew? The black central section of the adaptor isn’t straight-sided but has a very slight and uneven taper on it, and the male taper doesn’t look like a very good reference surface either, especially as it may be contributing to the overall eccentricity of drills placed in the adaptor, which typically show considerably more than the 0.1mm runout measured in the bore.

As an alternative I could make it into a side-lock holder for end mills, but there isn’t really anywhere suitable to drill for a set screw as, unlike on dedicated end-mill holders, there is very little in the way of a straight-sided section down below the male taper.

After receiving a partial refund, the adaptor cost me less than £5, so whatever I do with it it won’t be a great loss.

[Trevor Drabble 1Participant @trevordrabble1]

Rotagrip have them on offer at £32. Maybe worth a phone call to check the accuracy of them before going any further ?

[bernard towersParticipant @bernardtowers37738]

Why did you take a partial refund?

[Bill PhinnParticipant @billphinn90025]

Thank you for the replies.

Trevor, I see their £32 one is branded Omega. I’m confident that neither that one nor the one they have for nearly £20 less improbably branded Bison is likely to have excessive runout, and if either did I’m sure a return would not be a problem.

Bernard, I suppose I could have pushed for a full refund, but the sales description made no claims as to the maximum allowable runout and the seller was unwilling to pay for a return to China, the cost of which would have comfortably exceeded the purchase price. I felt there was room for mercy.

[Clive FosterParticipant @clivefoster55965]

Your reference is the external taper of the R8 shank.

The options are either :-

1) get that running true in the lathe and correct the MT taper by boring

2) make some sort of MT1 shank accurately drilled for a centre in the end so it can be supported in the tailstock then trim the R8 taper so tit runs true to the internal bore. May need to add some clearance at the parallel end to ensure it fits in the spindle.

Classic can be done but life is too short jobs.

Clive

[Howard LewisParticipant @howardlewis46836]

FWIW, I would set the adaptor in the 4 jaw and clock the 1MT taper until running true, and then skim the R8 taper, as little as possible to clesan it up. (With tapers, a little goes a long way!)

Since the taper is likely to be hardened, you will need to use a carbide tipped tool.

Boring or reaming the 1MT taper with the R8 taper clocked to run true is likely to be more difficult.

Howard

[noel shelleyParticipant @noelshelley55608]

I’m with Clives last words – Lifes to short !  Noel.

[Michael GilliganParticipant @michaelgilligan61133]

Mount it on  suitable base and use it as a holder for your favourite Pen/ Pencil / Scriber … it will serve as a daily reminder !

MichaelG.

[Graham MeekParticipant @grahammeek88282]

Is the Adaptor Hard? If so boring a 1MT will be near impossible, it would require grinding. Even then this could be fraught with problems. Especially if the MT is “drunk” with regards tot he R8 Taper.

It would be far easier to get the R8 Taper to run true to the 1MT, but if it is Hard then this might be a problem. While it would be easy to just clock the front of the 1MT hole and hope for the best. There would be no guarantee the MT was not running out at the small end, (drunk). This would really need a 1MT Test Bar to set-it up properly.

Trueing the R8 Taper is something that could be easily done with a Tool Post Grinder.

Lastly I do not see in the photograph a Slot to accept a Drift to remove the MT, is there one?

Regards

Gray,

 

 

[OldironParticipant @oldiron]

Looks like slot here.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Gray, click on Bills photo, and you can just see it in the shadows where Oldiorn has arrowed.

Regards Nick.

[Graham MeekParticipant @grahammeek88282]

Thanks Oldiron and Nick,

Your eyes are much better than mine. I can just make out the slot now you have shown me where it is. There is a brief moment when the image is reloading when it is much clearer.

It is a good job I see the Optician later this month,

Regards

Gray,

[BazyleParticipant @bazyle]

Maybe re-purpose it by gluing in a MT stub to make an R8 stub for eg slitting saw.

[Bill PhinnParticipant @billphinn90025]

Thanks a lot for everyone’s replies. Very useful.

It looks like the consensus is that fixing the problem may not be worth the candle, and that a salutary re-purposing as suggested by Michael or a stub mandrel as suggested by Bazyle may be the most useful thing I can make out of it.

In answer to Graham’s query, I don’t think the item is of the “hardened and ground all over” variety because filing off the heavy burr left by whatever had been used to tap the 7/16” thread was very easy and the metal decidedly chewy.

I’m still not sure that’s enough to persuade me, for the foreseeable future at any rate, to spend a chunk of  my limited workshop time trying to make a silk purse out of this particular sow’s ear.

In the meantime, before I buy a more decent replacement, I’ve discovered an unopened MT1-3/4” straight socket adaptor in a box. Since I also have a 3/4” direct R8 collet, these two should double up for now as a more viable alternative to the sow’s ear for holding MT1 drills in my mill.

[John PurdyParticipant @johnpurdy78347]

Any possibility of mounting it in the mill, inclining the head, and boring it in situ? Not an option if it is hardened of course unless you use a small grinding point. That would ensure concentricity. I have done something similar with a slitting saw arbor.

John

[Graham MeekParticipant @grahammeek88282]

Hi Bill,

Some of the adaptors are soft at the thread end, but I guess given the source you are probably right about this one.

As you already have a 2MT version, why not use a 2 to 1 reduction sleeve?

These are a fraction of the R8 to MT1 adaptors.

Regards

Gray,

[Bill Phinn]

On 3 August 2026 at 19:26 Bill Phinn Said:

As an alternative I could make it into a side-lock holder for end mills

You could make a side lock holder, but for something that does not need the gripping power that a cutting tool would require. The parallel OD of the holder would accommodate a smaller screw.

For example, an edge finder, a coaxial indicator, a spring loaded tapping guide.

A soft plug loctited into the taper and then bored insitu could sidestep any issue with the holder being hardened.

Bison is a long-established (1948) European manufacturer, so it is only improbable that they are the American species.

[HollowpointParticipant @hollowpoint]

This is interesting because I have a box full of r8 (16 total) drill chuck arbors that where given to me because the tapers have been incorrectly ground. I’ve been unsure of what to do with them. But I was gonna make one into a spring loaded tapping aid. A nice big one with plenty of travel. Maybe an idea for yourself.

[Bill PhinnParticipant @billphinn90025]

Thanks a lot for the further replies.

John, something vaguely similar occurred to me. I think it would have to wait until I need to incline the head for a more essential reason. I’m not averse to these sorts of jobs, though, because I think you can learn quite a lot from them.

Graham, I know I have an MT2-1 reducer so I will do as you suggest and see which of the two options achieves the best runout.

I’m fairly confident the thing isn’t hardened, or at least not significantly, mainly because of the price and provenance but also because I’ve machined three other R8 arbors, and none of them was particularly hard.

Putting flats on a flatless ER25 collet chuck did seem tough going, though, with an HSS end mill and so I switched to carbide. In contrast, an MT2 ER32 collet chuck I bought a while ago was very easy to machine with HSS (its flats were 35.3mm from the factory, hence my machining them to 36mm).

DC31K, all good suggestions, thanks, and the soft plug idea would allow re-machining of the bore to start from scratch, which may be my best chance as long as I use the right type of retaining compound.

Hollowpoint, it appears my mis-machined R8 adaptor has many brothers and sisters out there. Worrying to think how much highly processed steel in the world probably gets wasted.

[HollowpointParticipant @hollowpoint]

Unfortunately Bill, I think they (the faulty stuff) gets sold to unexpexting buyers. I also have a couple more r8 arbors that are out of spec. Both a very tight fit in the socket  I’ve also had them in the past with wild runout too! I remember one of them having over 0.1mm! Which for a drill chuck arbor is quite terrible.

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