How Much Contact Surface for an R8 Taper ?

A buddy of mine called me yesterday saying he was having some troubles
seating tools in his new bench top mill. I stopped by to see if I could
help and I found there appeared to be only a thin ring of contact
surface very near the large end of the taper on every tool and collet.
So little that tools tightened down HARD just fell right out when the
draw bar was loosened. Its supposed to be an R8 spindle and came with a
few R8 tools.
So how much contact surface should I expect to see between and R8
spindle and an R8 shank tool?
I have had one mill with an R8 spindle and if I had tightened a collet
into it as hard as I had to to tighten tools into his mill it would have
taken several hammer blows on the draw bar to knock them loose.
This is my buddy's second mill with an R8 spindle and tools he was
struggling with on this one seated easily and worked just fine on his
old one.
I wondered if maybe he got a spindle with an MT taper by mistake but my
uneducated finger says it feels like the wide taper angle of an R8, and
not the shallow taper angle of an MT. I do have some misc sizes of MT
taper tools for my lathe. I guess I could head over and see if they are
a better fit just to be safe.
Reply to
Bob La Londe
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That sounds a lot like the wrong taper fit to inexperienced me, too.
I just found out that there are self-holding and self-releasing classes of tapers. It seems that I have a lot to learn.
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. According to that, Morse tapers are the self-holding style, so it certainly seems that he has mismatched tooling/spindle tapers.
Excellent idea. But remember that there are many different tapers. If you have inside calipers, you could measure it and get a good idea. The chart on the wiki page should guide you.
Perhaps your buddy can get the seller (dealer?) to replace the tooling if you find that it's mismatched.
Reply to
Larry Jaques
I'd check how and where the collet was pulling tight. Perhaps the drawbar threads are too short, or a washer is missing? -jsw
Reply to
Jim Wilkins
I actually lengthened the threads for my buddy on both of the draw bars he had. I think they might have been right at the edge of hitting the shoulder, but the problem remained that tightening up the tool in the spindle (we tried both hard tools and collets) was difficult and they only engaged on a very narrow ring near the large end of the taper. I checked one tool by marking it up with a sharpie marker and tightening it, but if you looked at the light scuffs on the various tools they were all the same.
When loosening the draw bar I am used to having to give the draw bar a light tap with a mallet to release the tool. On his machine if you loosening the draw bar 1/2-3/4 of a turn it released the tool with no tap. It just fell out. That was after putting the muscle on wrench against spline wrench to tighten it. I didn't kill it, but I tightened a 1/2 turn further than I normally would have thought was good enough.
There was another issue I couldn't understand. With both of his drawbars there was a little binding as you tightened them up. Enough to cause that annoying wrench/no-wrench affect I hate. I cleaned the threads on the draw bars (obviously since I lengthened them), and cleaned the threads in the tools. Out of the machine they spun together freely. It just made the whole experience of tightening a tool up in the spindle that much more maddening.
In order to get a tool holder or collet to tighten up (his dog pin was removed) I had to hold the tool up into the spindle with enough force that my fingers were almost trembling. On other R8 machines I've never had to do that. Just holding the tool in place normally was more than enough.
The importer (normally very good which is why I have not named them) has been doing the "find one thing in your description of the problem and picking that apart" thing rather than trying to actually address the problem. Either that or saying he doesn't understand the problem. The day before yesterday we shot a video of tightening and loosening some tools and showed the minimal contact ring. I don't know if my buddy has sent the video to them or not yet. Its a brand new machine.
It was sold and marketed and documented as an R8 machine.
Just for the heck of it I took an empty collet, held it in place with a block of wood and smacked the block of wood with a mallet. Several times and as hard as I could swinging upward. Several attempts produced the same result. The collet just fell out. THAT surprised me. Even with a self releasing design that collet should have stuck.
My buddy had done most of this already. (Except lengthening the threads on the draw bars.) I think he just wanted me to try it out and see if I came to the same conclusions he had.
Reply to
Bob La Londe
A spacing washer under the head effectively lengthens the drawbar if the threads weren't cut far enough. I think I remember a similar RF-31 collet problem when the drawbar spacing washer was missing.
You could see how far dividers or inside spring calipers set to the OD of the collet shank will go in. Perhaps the spindle wasn't bored deep enough and the rear of the collet is bottoming out.
Pressing in crumpled aluminum foil with the quill feed will give you a rough "casting" of the recess. Pound it out from the drawbar end so it doesn't stretch. -jsw
Reply to
Jim Wilkins
Since several tools and collets show the same symptom, it seems that the spindle taper was not cut correctly. If this is a cheap Chinese machine, I am not really surprised.
I get just about full transfer of spotting dye on my machine (and the tool holders DON'T fall out).
Reply to
Jon Elson
Greetings Bob, The machine spindle is not an R8 taper. The contact length should be about .625" long for an R8 taper. It could be a #4 Morse taper. The major diameter of a #4MT is almost exactly the same as an R8 taper while the minor diameter is larger than the straight part of an R8 taper shank. So an R8 taper shank will fit into a #4MT hole. When an R8 taper collet is put into the spindle does the small end rattle around? It will in a #4MT. The spindle may also have some taper in it that doesn't match any known taper. In any case your friend needs to have the machine, or at least the quill assembly, replaced. Push a clay plug into the spindle and use it to measure the angle of the taper. Or use an indicator to measure the taper. Eric
Reply to
Yeah, something is hinky there alright.
That sounds like one piece going sideways. Was the spindle portion of the taper burred or rusted, maybe? Pulled the spindle and look for a sledge hammer mark in the middle? ;)
Maybe an R7 got through.
You certainly did.
Reply to
Larry Jaques

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