Anyone unscrewed arbor from large Criterion boring head?

Cannonmn · Oct 10, 2020

I need to regrinding the nmtb 50 taper on this boring head. My B&S ni. 1 univ. grinder won’t take the head plus arbor so I need to unscrew the arbor. Have tried “the usual” but no luck yet. Any suggestions?

John Morris has shared 5 photos with you! | Flickr

Lakeside53 · Oct 10, 2020

lol... They can be tough. Some ... a good whack with a hammer longitudinally, but one I had one I had to machine off to release the threads. It's not "loctite" but thread tension against the flat (ground) flange.

Grab the head in a chuck, run a thin parting blade in near the junction. Careful to not hit the threads.

Hardplates · Oct 10, 2020

Try keeping the boring head submerged in water while heating the taper. The goal is to get the male threads to grow but not the female.

I would also use a large rosebud. The faster you can get the heat in the better.

Lakeside53 · Oct 10, 2020

I think you'll find the taper has the male threads.

Cannonmn · Oct 10, 2020

Thanks, yes, arbor male threads are 2 1/4-10 RH per the maker. Think I’ll call them soon and ask if they have any secrets, they must rebuild them for customers. Suggestions so far sound good, might just cut that groove in it.

bikebuilder · Oct 10, 2020

I just unthreaded mine last week. Shank in vise with aluminum jaws, big ass c clamp on square body..twist off.

dian · Oct 11, 2020

i think heating is no good here, whatever you do (apply coolng spray to shank?), the male thread will stay at the temperature of the head.

Cannonmn · Oct 11, 2020

dian said:
i think heating is no good here, whatever you do (apply coolng spray to shank?), the male thread will stay at the temperature of the head.

Thx, I use heating often and don’t rely on one part moving away from another. Heating both parts equally can help by expanding everything slightly then allowing contraction, especiallly if penetrants are introduced during contraction. My theory anyway, and I’ve freed up many otherwise frozen joints that way. With this one, I’ve begun using impact/vibration. I put a Hard bolt shank in the extended transverse hole in the body-the in unthreaded end of slide advance screw threads channel-and whack it with 3 lb. hammer in direction the head needs to unscrew. If it won’t fix it in the somewhat loosely-goosey vise fixture I’m using now, I’ll complete Frankenfixture to hold it. Start of said holding fixture pictured. I’ll also scribe a short index mark across the joint So I can tell when things start moving. In the past I’ve found that’s important so I won’t give up On the current method if movement has begun.

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Or I might cut a groove as described above, or drill a hole axially to allow penetrant to enter the threaded area, assuming none has reached there yet. I think the easier hole to drill is from the ram-screw cavity “upward” to bottom end of arbor threads.

dian · Oct 11, 2020

yes, heat helps, epecially if rusted. i was more thinking about the water bath.

Cannonmn · Oct 11, 2020

Right. If one is interested in having a temperature difference, the head portion could be dunked, while hot, in water just deep enough to cover the head, not touching the arbor. These are easy to handle by screwing a 1-8 bolt/capscrew that’s long enough into the arbor. Socket-heads are obviously easier on the hand than hex-head, but if you wear gloves it won’t matter.

One question I still have: What’s the highest temperature to which I can heat this, assuming air cooling, without changing the physical properties of the steel? I’ve limited my heating to about 375 deg. F., monitoring with a non-contact laser-aimed thermometer.

Lakeside53 · Oct 11, 2020

Cannonmn said:
Or I might cut a groove as described above, or drill a hole axially to allow penetrant to enter the threaded area, assuming none has reached there yet. I think the easier hole to drill is from the ram-screw cavity “upward” to bottom end of arbor threads.

The issue isn't penetrant onto the threads. The threaded portion has been over-tightened by years of interrupted milling making for the likes of an impact driver. It's pulled the two ground shoulder surfaces together hard - no penetrant is getting in there. Just release that tension with a parting blade very close to the head shoulder and it will literally unscrew with fingers (barring rust or other on the threads!).

As for heat - it's the differential that matters, but with two mating ground surfaces, not only it is difficult to get a differential, if you do... the expansion of one against the other make it worse.

Blazemaster · Oct 11, 2020

I had a problem just like this with a big flynn boring head on a 50 taper shank. I used copper jaws on a 6 inch vise with a 24" crescent wrench and cheater pipe. I did have to use a large hammer to break it loose. I clamped the adapter in the vise over the 50 taper notches, the boring head had a square feature that the big wrench was just able to grip onto.

Cannonmn · Oct 11, 2020

Lakeside53 said:
The issue isn't penetrant onto the threads. The threaded portion has been over-tightened by years of interrupted milling making for the likes of an impact driver. It's pulled the two ground shoulder surfaces together hard - no penetrant is getting in there. Just release that tension with a parting blade very close to the head shoulder and it will literally unscrew with fingers (barring rust or other on the threads!).

As for heat - it's the differential that matters, but with two mating ground surfaces, not only it is difficult to get a differential, if you do... the expansion of one against the other make it worse.

Thx, the cutoff tool method is definitely on the table if the other methods don’t start movement, I’m very reserved about altering the original configuration of tools/tooling.

Cannonmn · Oct 11, 2020

I’m sure now that Lakeside is correct. I tried again today with scribe mark in place, which can detect a few minutes’ or so of angular displacement. I applied torque then impact with sledge hammer after mounting the thing very solidly, and got no movement whatsoever. More force would damage the tool, so the arbor-cutting should happen this week.

For anyone else having to cut the arbor and stop short of the threads, Criterion heads With nmtb50 arbors normally used 2 1/4-10 male threads. Arbor big end of taper is 2.75 in. Dia, thread major dia. 2.25 in., so diameter difference is 0.5 in., meaning you can only cut in 0.25 in. On the radius before hitting threads, and slightly less if the arbor has been re-ground.

4GSR · Oct 12, 2020

Something else to think about. The threaded connection between the body of the head and the shank is probably fluid locked, which is almost impossible to break loose. May want to drill a thru hole in the body of the boring head to the shank. This would let off any trapped fluid and maybe let it release easier. This may help and may not change anything. Also, while you are at it drill it out to around 2" and drill out the arbor to around 1-3/4" ID. This may allow the pin thread to collapse just enough to break it loose. Just thinking.

I had a 203 boring head with a R-8 shank that took a 36" pipe wrench and 6 foot cheater to break it loose. The head was pretty much junk before doing this task, later rebuilt the head and gave it away. Ken

Cannonmn · Oct 12, 2020

Thx. I’ve put enuf torque on it to cavitate most fluids, and see some slight movement, but there was none. Whether that’s accurate or not, the circumcision described above would resolve the possibilities you mention as well as the ground-surfaces-lockup condition.

TGTool · Oct 12, 2020

Cannonmn said:
Thx. I’ve put enuf torque on it to cavitate most fluids, and see some slight movement, but there was none. Whether that’s accurate or not, the circumcision described above would resolve the possibilities you mention as well as the ground-surfaces-lockup condition.

Maybe you need priest to come in and do the exorcism, or perhaps it's a job for the rabbi.

Cannonmn · Oct 12, 2020

TGTool said:
Maybe you need priest to come in and do the exorcism, or perhaps it's a job for the rabbi.

The specialists are known as mohel. I learned that from a Seinfeld episode.

Dan from Oakland · Oct 13, 2020

This works on stubborn and almost stripped head socket head cap screws very well- so. Take a VERY big brass hammer and pound the small end of the shank toward the boring head. The idea is to very slightly crush the face of the tool holder into the top of the boring head, releasing some of the compressive load between the tool holder flange and the boring head body. Obviously support the boring head in a way so you don't damage it. Good luck.

Cannonmn · Oct 14, 2020

BANG! With all usual techniques having failed, I cut the arbor to release what must have been incredible tension. When enough of the steel was cut away there was a Big Bang when the arbor thread tension was released. The arbor snapped back toward the head, snapping off the parting blade with horizontal impact, and capturing the broken piece. My WAG at the reduction in length of the threaded section at that time is at least 0.050 in. That’s about the gap I left in order to avoid cutting the boring head. Note that no corrosion is evident inside the threaded area. I was able to unscrew the head from the arbor by hand after knocking the broken blade out of the gap.

John Morris has shared 11 photos with you! | Flickr

Anyone unscrewed arbor from large Criterion boring head?

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