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How to fix overground nmtb50 arbor?

Cannonmn

Stainless
Joined
Jun 25, 2016
This arbor is threaded for a Criterion 206 boring head, has 2 1/4-10 thds. When received the taper was too beat up to put in the spindle so I stupidly decided to regrind it smooth on the old B&S universal grinder. The taper was at or just past the design max taper of the grinder, and the operator was inexperienced. Result was taking too much metal off, although taper angle is correct. So using blue contact paste showed almost no contact with spindle even when drawbar tightened. I figured the arbor is about 0.035 inch too thin on radius. A plating guy told me he could only add about 0.015 in. on surface. So another method is needed. What should I try? This is a backup to the head/arbor we use so repair isn’t urgent but want to get it done.
 

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John Garner

Titanium
Joined
Sep 1, 2004
Location
south SF Bay area, California
From a purely-geometric perspective, taking material off the taper side of the flange should take care of the problem.

Assuming a 7:24 taper, 0.070 on the diameter requires nearly a quarter inch of the flange to go away. That's a whole bunch, so you might want to add metal to the taper so you need to take less off the flange.
 

Cannonmn

Stainless
Joined
Jun 25, 2016
Thx, last time I asked OEM to price that arbor it was $600. +s. I’ll continue to look for a used one but that thread is scarce. Maybe somebody’s done a workable home brew solution, wonder how thick that aluminum duct tape is? : c ). Thx for calculating the flange mod method, it crossed my mind but I hadn’t done the math.
 

dgfoster

Diamond
Joined
Jun 14, 2008
Location
Bellingham, WA
A common Cat5 arbor is relatively cheap and easy to find. This may not be so cheap or easy to find.

1) A boring head places fairly light loads on such an arbor.
2) To function well, the taper really only needs to make a good fit at the far end and near end of the maybe 5" long (did not look it up). In many cases tapers are relieved intentionally for about 3/4 of their lengths leaving jsut the near and far 1/8 of their lengths intact and properly ground.
3 You have a grinder and can weld.
4) If this is a rather expensive and rare arbor, I'd be tempted to lay on a band of weld at the near and far ends and grind it to fit properly. I would not bother to weld it and grind it for the entire length of the taper. It would work fine just bearing for 1/2 inch or so at the near and far ends. Welding it up and grinding it might trake less time than locating a replacement.

Denis
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
Electroless nickle plate can be built up to any desired thickness.I suggest you find a plater to do it........a possible other option would be plasma spray of two bands on the taper.
 

Cannonmn

Stainless
Joined
Jun 25, 2016
Thx, sounds like welding or ENP would work then, and certainly the two bands top and bottom only are helpful. I guess while welding, I’d screw it back into boring head for heat-sink, don’t want it getting annealed.
 

Cannonmn

Stainless
Joined
Jun 25, 2016
Thx, already have some with enuf diameter to do that, had considered but “ass-u-med” they’d be hardened to much to thread. Guess I should pick one and check hardness of the extension.
 

David_M

Hot Rolled
Joined
Sep 30, 2014
Location
Midway, GA, USA
I'll bet carbide could thread it especially if you have to machine it down to get to size. If not, use a tool-post grinder (with a dressed to 60 degree included angle wheel) and grind the threads.
 

dgfoster

Diamond
Joined
Jun 14, 2008
Location
Bellingham, WA
I have ”customized” a 50 arbor. It was an old USA (good quality one) and was not particularly hard. It required no special tooling or methods and machined nicely. I also have an import 50 holder recently purchased that is the equal of the old one.

You asked about fixing the existing one and so I suggested the ring welds. I’d also not hesitate to give it a whirl. Preheating the part to 300 or 400 won’t hurt it’s temper. Fully annealing it also wouldn’t hurt it. It’s for a boring head. Perfect concentricity is usually not a factor when running a boring head!

Denis
 
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Cannonmn

Stainless
Joined
Jun 25, 2016
I have ”customized” a 50 arbor. It was an old USA (good quality one) and was not particularly hard. It required no special tooling or methods and machined nicely. I also have an import 50 holder recently purchased that is the equal of the old one.

You asked about fixing the existing one and so I suggested the ring welds. I’d also not hesitate to give it a whirl. Preheating the part to 300 or 400 won’t hurt it’s temper. Fully sneaking it also wouldn’t hurt it. It’s for a boring head Perfect concentric it’s is usually not a factor when running a boring head!

Denis
Thx Denis, did u mean “Fully sneaking it….” Or did spellcheck try to help u? If not, I don’t know what sneaking means in the shop context.
 

dgfoster

Diamond
Joined
Jun 14, 2008
Location
Bellingham, WA
Wow, autocorrect took some version of “annealing” helped me by changing it to “speaking.” Sorry! Spellcheck drives me crazy at times. I’ll fix my post. Thank you.

Denis
 

Cannonmn

Stainless
Joined
Jun 25, 2016
Thx, I will use one of my otherwise useless 50 arbors with enuf meat, and cut the threads. If I get around to modding the one with the F.U. taper, I’m only looking at cold or warm methods, nothing hot that’d mess with the hardness.
 

dgfoster

Diamond
Joined
Jun 14, 2008
Location
Bellingham, WA
A soft taper (even prehard is soft relative to the usual ~50+RC) can be annoying when it dings and gives erratic seating in the spindle taper. Or picks up and embeds a chip, for the same reasons.

If I was the OP, I'd go with the suggestion of modding any available holder that's got a diameter big enough to cut a new thread. Seems like the best solution to me.

I agree that a moderately hard taper would be ideal. But, I would also say that most of us use precision surfaces frequently that are not hardened. The table on my mill is an example. It DOES get dings from time to time—-small ones hopefully. Those are routinely dealt with by simply stoning them off followed by a sweep of the hand to make sure none large enough o feel remain followed by a visual scan to make sure that any shiny spots caused by stoning look like they taper off smoothly. The same works well on tapers. They will, hard or not, pick up the occasional ding when in general use. But a few swipes with a stone, visual check and hand sweep is all that is needed. This is particularly true for the use outlined by the OP.

For me, it would be easier to modify an existing holder, too. But very satisfactory results could be had repairing the one he has in hand. And that is what he is asking about.

Denis
 

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
What is hitting to prevent it from fully seating?

I have some funky NMTB50 holders. Some look shop made. The only thing I have had a problem with is the length of the drawbar socket. If it is too long it will bottom out on the shoulder of the drawbar in my mill. shorten them up in the lathe and good to go.

Holders are not very hard. I have cut up all kinds of mill toolholders to make special tools and the hardest I have found are probably RC45ish.
 

Dan from Oakland

Titanium
Joined
Sep 15, 2005
Location
Oakland, CA
If you are going to rework an old arbor and the rework area is hardened, take the arbor to a heat treater and have them induction anneal just the area you need to machine, that should preserve the remaining hardness on the taper.
 

David_M

Hot Rolled
Joined
Sep 30, 2014
Location
Midway, GA, USA
The one I posted a link to (post #9) started life as a blank bar that is machinable on the end to be threaded.

172472197.png
 

Nmbmxer

Hot Rolled
Joined
Jun 22, 2008
Location
VA
I’d find a set screw type endmill holder with an OD large enough to turn the register & thread for your boring head. I’ve made KS200 ER collet holders by boring out and threading the OD of end mill holders.
 








 
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