View Full Version : Getting Morse taper precise to bore ?

David S Newman
08-16-2009, 03:07 PM
I am making a fixture that needs a 2 MT precise to the OD. If I set the shaft running true in a 4 jaw chuck and then use a fixed steady and bore it would this be best way ? I can't get the shaft through the lathe mandrel so have about 8 inches to support from chuck to end of shaft. Don't want to make a mess up as I have only this one piece of this PGMS.
Any advice please. David

Bruce Griffing
08-16-2009, 03:31 PM
If the MT has to be precise to the OD, why not make the MT and then trim the OD to the MT?

David S Newman
08-16-2009, 03:56 PM
If the MT has to be precise to the OD, why not make the MT and then trim the OD to the MT?

See your point but using ground stock. David

08-16-2009, 04:02 PM
Using appropriate care in setting up, that method will work just fine. The only problem will be in getting a good taper in the bore.

MT#2 is pretty small ID and a borlng bar that long will flex a great deal. It would be a good idea to look into the purchase of an MT#2 reamer. Step bore to two or three rough diameters and finish with the reamer.

08-16-2009, 05:55 PM
I'm with JC on buying a #2 MT reamer. You will play hell getting the taper exact.

08-16-2009, 06:58 PM
Same here .But would bore the taper close to size the finish with a reamer . A bored hole will be on center a drilled hole will not be.

08-16-2009, 07:32 PM
I agree with the drill-bore-ream method not only for the reasons given previously.

The other reason - which is rarely mentioned - is that the top-slide seems to always be assumed to be perfect - as it needs to be to cut a good taper - internal or external.

That's a pretty wild assumption unless you've checked it.

I hear all the chatter about how perfect lathe beds and their guides/ways have to be, and how cross-slides have to be very good and "square". Tails-stocks too. Etc. etc.

But no mention of the top-slide.

If the top-slide is worn it will not cut straight. If it won't cut straight you won't cut a straight taper. If the wear is bad the gib will be set at a compromise for the varying wear in the slide. Loose in parts, OK in some, and " a bit too tight" in others.

Now for the tool which it is said must "absolutely be very accurately set to centre-height or else the taper will be wrong".

Possibly true - but almost not as much a problem as some assume.

For that "really good" centre height to be maintained (I have reservations about this too) - in theory - the top-slide should be very accurately vertically parallel to the lathe head-stock spindle axis. If it isn't the the tool will "dip" or "rise" with the top slide and the centre-height will be "out". Wear in the cross-slide is an extra source of top-slide "dip" as well.

If the MT is for location purposes only (ie not as a "driver" as with drilling in the tail-stock) then all that is needed is the 1/4 taper length (or less) at each end and the middle 1/2 (or more) length of the taper just cut away/relieved.

That way you are cutting two independent short tapers.

First rough both out, then cut one (either end) to finish and then cut the second (end) to match the first (use "blue" - or anything that suits -on the "mating part").

If the ends are all that matter, why bother about the middle bit that can be done without?

So far it can all be done for an MT2 taper without a reamer.


A reamer is definitely the way to finish-off.

A MT reamer is just another "taper reamer" - with the same problems. The longer the taper face it has to cut, the harder it is to get it to cut - removing the middle 1/2 (or more) taper length will assist the reamer to work a lot better too.

I'd use cutting/tapping oil for finish boring and reaming - just as I would going very slowly as if I were tapping that hole - and for the same reasons.

J Tiers
08-16-2009, 07:40 PM
Of all the parts of the lathe, the top slide (compound slide) probably gets the least use, and consequently the least wear. (one might argue it gets the least oil also, but that's another matter).

So obsessing about it's wear and the possible effects that *might* have is probably over-blown, even if the technical basis of the comment is valid.

Let's not over-complicate the matter. Fundamentally, it isn't complex. The biggest hassle is that a bored taper of that size is just hard to inspect. While you can easily determine that there IS an error, it is harder to look at the inside and see the problem, particularly if you (as you should) still have it in the chuck.

I would bore the taper, and finish ream it. The boring needs a clean-up in most cases if not power-feeding, and the reamer is not that expensive. Follow the reamer with a ball bearing center in the T/S, to keep it aligned.

The reamer will, however, follow the errors of the existing hole, so boring prior to reaming makes a lot of sense, as opposed to just drilling steps and reaming them flat.

if you want to obsess, don't forget that the reamer itself has errors, and the less you pay for it the bigger the errors are likely to be. So even avoiding the taper error that *could* be due to the "topslide", there is another *possible* error due to a cheaper reamer...................

08-16-2009, 08:10 PM
I did not suggest step drilling, but step boring. A MT#2 taper is rather short as tapers go, boring to the small diameter, and step boring to something around the diameter at the center will remove most of the material and provide a straight, centered bore for the reamer to follow without the angst of setting the taper and cranking on the compound to feed it.

Use whatever method you prefer, but definitely bore the rough hole.

When reaming a rifle chamber, there are a couple of methods of controlling the reamer while allowing it to float and follow the bore. Using a center in the tailstock to support the reamer will force it out of line unless the tailstock is perfect, and you can pretty much count on it not being perfect. The same will hold true here.

There are floating holders available, but they are pricey for one time use. A simple method uses a flat pusher with a sleeve slightly larger than the reamer OD to control position. A flat is ground on the reamer shank away from the area of the sleeve to hold the reamer with a tap handle.

The subject will cause as much discussion on a gunsmithing board as way oil vs. bar oil will here, especially if benchrest smiths are involved. Thus, I will go no farther.