Drive dogs, what's their intended purpose?

[Paul Alciatore]

You missed the point. I was not talking about a FULL WIDTH round pin. I was talking about a center pin with a diameter about equal to the width of the tangs.

So this does not hold water.

I will try to post a photo.

MT tang:

The slot in the spindle for removal wedge weakens the spindle. A narrow slot and tang weakens the spindle less than a larger full round would. Also, the angled edges of the tang provide better contact with the removal wedge.

Just a guess. I don't know.

[Paul Alciatore]

"... will keep it cutting repeatably at the same diameter."

I had not thought of that and it is a good point. But it does not work for a collet, just for a milling cutter with the taper built in.
And there are not many of those around.

Well, it would keep the collet in the same orientation. Ditto for a boring head with a dedicated taper adapter.

It should be a given. And after long experience in the manufacturing industry: there are many "pros" who don't know all that much. Take what most say with a grain of salt. Look up the information yourself. Think. Would the drive keys be there if they weren't intended to serve a purpose?

As already mentioned multiple times here, when using a tiny little cutting tool this might be fine. When using a tool of any size or using a lot of horsepower, use the drive keys. It's a very good idea to use the drive keys for any type of tool that needs to be indexed for accuracy as well - like Stefan's boring head for instance. Just like on the lathe where we match mark our spindle mounted workholding devices to ensure minimal runout, on the mill keeping that boring head indexed with the same orientation to the spindle every time it's removed and inserted will keep it cutting repeatably at the same diameter. Randomly orienting will cause bore size to deviate. Same goes for endmills cutting to the right size at the same dial or DRO numbers if you don't have zero runout and a perfect spindle taper. Repeatability is the name of the game if you're making multiple parts.

[Paul Alciatore]

You say the tang would twist off before the tool. The standard tang has a very generous fillet where it connects to the tapered section of the adopter. A tool mounted in a holder or collet has no such fillet so the stress at the point where it enters the holder of collet must be a high stress area. Is there any actual evidence for your statement or is it just a wag (wild ass guess)?

Yes it is odd, isn't it? Even in most working shops. Just goes to show how little care most "pros" give these pieces of precision equipment. Seating shanks with burrs, raised dings and even chips/grit on them. Leaving the taper socket open to debris when the machine isn't in use (or even when it is!), Then seating a tool without cleaning the socket, etc. I've seen it all.

A Morse taper is intended to be a self-holding taper - but there is a prerequisite to that working out: the tapers (male and female) must both be in good enough shape to ensure solid contact and full seating. The tang will never hold up to as much torque as a fully seated taper shank. If the taper unseats, the tang will twist off before the tool also in almost every case since it's soft.

[Paul Alciatore]

..........NO!

Could you please elaborate further?

[J Tiers]

You say the tang would twist off before the tool. The standard tang has a very generous fillet where it connects to the tapered section of the adopter. A tool mounted in a holder or collet has no such fillet so the stress at the point where it enters the holder of collet must be a high stress area. Is there any actual evidence for your statement or is it just a wag (wild ass guess)?

I have MT tools with the tang twisted off. I did not do it, so I cannot give report on exactly what happed, and what machine it was on.

I also have one or two which have a factory-made round "tang". They work as well as any of the others.

I also have several shop-made MT3 mill tools. They were turned and filed on the lathe to fit. They work fine, despite the fact that I did not make the taper full length, because the Logan hasn't got enough compound travel.

[eKretz]

You say the tang would twist off before the tool. The standard tang has a very generous fillet where it connects to the tapered section of the adopter. A tool mounted in a holder or collet has no such fillet so the stress at the point where it enters the holder of collet must be a high stress area. Is there any actual evidence for your statement or is it just a wag (wild ass guess)?

Experience dictates just about all of my responses. I have seen it many times.

[SVS]

Cleveland Forge 4mt 1 7/8”

Somewhere else in my collection I have a bit that I cut the tang off, seated it solidly in a 4-5mt sleeve and it’s forevermore a 5mt shank.

[MrWhoopee]

Hey, MrWhoopee, I know that the R8 taper is close to the critical angle for self locking. I understand it was picked on purpose for that. So MOST of the time a little upward pressure would be enough to let the drawbar get a hold and set the locking pressure. But did you find that now and then one of the arbors would not self grab all that well?

Can't say that I've noticed. I make sure that the threads in the collets/tool shanks and on the drawbar turn freely enough that holding the collet or tool up whilst giving the drawbar a spin pulls it up and seats the taper nicely. Never encountered one that wouldn't. Maybe I've just been lucky.

[Tundra Twin Track]

Correct. The tang on those Morse taper tools that actually do have a tang only "engages" with the removal wedge, nothing else. I have cut off the tang on an MT3 drill chuck adapter so that the lathe tail stock could read "0" rather than starting at about 5/8in. All the holding on MT is provided by its shallow taper, and in fact in many cases you could shorten the taper by a fair bit and still have plenty of holding power.

Some MT with no tang are not compatible with some holders,the taper is the same but near impossible to remove after seated.My Mt 4 Skoda Live Center fits fine in a Dormer MT extension but is short not extending into the slot so no meat for drift key to contact.

[MattiJ]

Also remember - mills used to/might still have have gear boxes. The K&T's low gear is 32:1... So with our measly 5hp spindle motor at 15 ft-lbs at 1800rpm - that is 480 ft-lbs at 56rpm.

50 taper holds quite a bit of torque even on its on but not 480 ft/lbs
Assuming 5000lbs drawbar force as per recommendations https://tacrockford.com/technical/ma...amping-forces/
--> 50 taper max torque is about 250 ft/lbs

Note also that Morse taper relies heavily on feed pressure to transmit torque.

[JRouche]

I saw a video recently...

So now I am wondering if that's true. I am always sceptical of opinions from a single person on the net, I try and find what the aggregate opinion is.

We all do that in one scale or another right. Just a part of every day life. Some say. JR

P.S. Your problem is fixable. JR

[old mart]

The R8 taper is very close to the BT30, 40 and 50 taper angle. So close that I have some BT30 taper cleaners to use on the Tom Senior. The cleaner only works because the pin was never installed when I did the R8 mod, the spindle had a location hole partially drilled, but not reaching the bore. The only tooling that can slip during installation are the R8 collets which have next to nothing to hold onto. I make sure the drawbar threads have had a tap put through them and are clean and lubricated with moly grease.
I would like to have drive dogs in the R8 design, but so far with 2 mills we have never had a slippage. R8 is an old design best used on smaller home shop mills of 2hp or less. I like it for the easy changing of tools, no jamming Morse tapers and the huge variety of tooling still available.
The drive dogs on industrial mills mostly using ISO/BT40 are insurance against misshaps as dozens of toolchanges are made during many CNC operations.

[psomero]

R-8 should really be used with a drive key also but they are notorious for breaking and causing all sorts of damage.

R8 drive key? huh?

[psomero]

all of this "R8 key" talk is nonsense. It's a friggin' dog point set screw or a set screw with two parallel flats ground on the nose.

Can someone provide an example of a machine in which it's not a set screw?

I have never seen a "key" in an R8 spindle. "Pin" would be more apt

[eKretz]

all of this "R8 key" talk is nonsense. It's a friggin' dog point set screw or a set screw with two parallel flats ground on the nose.

Can someone provide an example of a machine in which it's not a set screw?

I have never seen a "key" in an R8 spindle. "Pin" would be more apt

The nonsense is that they ever fitted such a thing to the machine in the first place. Awful design. Should have been updated.