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How much nmtb arbor/spindle contact is enough?

gregormarwick

Diamond
Joined
Feb 7, 2007
Location
Aberdeen, UK
“I'm sure for your purposes it could be salvaged after correcting the taper by grinding the face and the drawbar boss back to the correct distance from the gauge line on the taper“

Where is the gauge line on the taper? Top of flange? At the “break” where the taper begins above the flange? That’s what I still don’t know.

As much as I hate to suggest it, that information can be found in the Machinery's Handbook.

Alternatively a google image search should throw up plenty of results.

For reference, the gauge line on a steep taper holder is some short distance from the top of the flange. There is a specific diameter at the gauge line on the taper, and all other geometry is referenced from there.
 

jz79

Stainless
Joined
Mar 21, 2017
it should be 69.85mm for 50 taper, or 2.75", diameter at the gauge line that is
 

gregormarwick

Diamond
Joined
Feb 7, 2007
Location
Aberdeen, UK
Ok thx, had been to google but “gauge line” without “taper” goes nowhere. But this is the best image I found, in case any other newbies drop by.

Flickr: Page Not Found

Better one here, this is for DIN 2080 which is to all intents and purposes the same as NMTB (metric drawbar thread is the only real difference)

26GLmBL.png
 

Cannonmn

Stainless
Joined
Jun 25, 2016
you might want to relieve the taper in the middle to make things easier.


Thx but I’m trying to figure out the benefit there. The top, small end taper, is undersized, and wide end is over-diameter. Looking at my crude hi-spot blue test, I’m not sure what grinding more off the middle would do. Seems I should set up another grinding session where the wide bottom end contacts the wheel first and ideally when I first see sparks All along the taper, I’d stop.
 

gregormarwick

Diamond
Joined
Feb 7, 2007
Location
Aberdeen, UK
Thx but I’m trying to figure out the benefit there. The top, small end taper, is undersized, and wide end is over-diameter. Looking at my crude hi-spot blue test, I’m not sure what grinding more off the middle would do. Seems I should set up another grinding session where the wide bottom end contacts the wheel first and ideally when I first see sparks All along the taper, I’d stop.

You need to set up the angle on your grinder by indicating the good arbour. Use a tenths indicator and try and get as close to zero indicator movement as you can along the length of the taper. Then swap the bad arbour in and start grinding, then blue and tweak as required.

Here are photos of dye-coated spindle and printing of same on a new arbor.

John Morris has shared 2 photos with you! | Flickr

It's probably good, but I'm afraid that's still too much blue to really tell anything.

Remove the drawbar, get everything spotlessly clean using whatever safe no-residue solvent you can lay your hands on, wash your hands, and apply a very small amount of blue using your finger and thoroughly smear it around for a uniform thin film all over the surface of the spindle taper only.

By hand only, no drawbar, push the holder up into the spindle with a little force, don't twist it, and remove. Then examine the contact pattern.
 

Cannonmn

Stainless
Joined
Jun 25, 2016
“ You need to set up the angle on your grinder by indicating the good arbour. Use a tenths indicator and try and get as close to zero indicator movement as you can along the length of the taper. Then swap the bad arbour in and start grinding, then blue and tweak as required.”

Thx, sounds like a winner! The old manual not surprisingly gives you the impression that you set the shiny scale on the end of the upper table and Voila, yer almost done. Maybe that was possible with a brand-new grinder fresh from factory tests, but not my well-worn beast.
 








 
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