# Chuck To Backplate Bolt Thread, Nc Or Nf?



## MikeMc (Jul 10, 2015)

I'm ready to drill the backplate for my new 8" 4-jaw chuck. It came with metric bolts only 0.010" larger than 3/8". Since I already have SAE taps it seems a waste to spend $10+ for a metric tap and drill that I'll likely only use once then have cluttering up the toolbox for the rest of my life.
  Should the chuck be mounted to the backplate with course or fine threads?
Thanks,
MikeMc


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## RJSakowski (Jul 10, 2015)

MikeMc said:


> I'm ready to drill the backplate for my new 8" 4-jaw chuck. It came with metric bolts only 0.010" larger than 3/8". Since I already have SAE taps it seems a waste to spend $10+ for a metric tap and drill that I'll likely only use once then have cluttering up the toolbox for the rest of my life.
> Should the chuck be mounted to the backplate with course or fine threads?
> Thanks,
> MikeMc



I think either will work.  If I recall correctly, fine threads have a greater tensile strength due to the larger root diameter and greater clamping force due to the finer pitch.  Fine threads have more of a locking tendency due to the shallower pitch angle.  Coarse threads are more resistant to stripping  though.

Since you are retapping anyway, you have a fair amount of latitude in selection of your thread.


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## kd4gij (Jul 10, 2015)

I would buy the tap, Never pass up an opportunity to buy more tools.  Metric taps are a must anyway, every things use them these days. But if you must! I would use corse.


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## MikeMc (Jul 11, 2015)

I'll go with coarse then. 
Thanks for the feedback.
MikeMc


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## wa5cab (Jul 11, 2015)

I don't understand why you would have to tap anything.  Although it's true that the ISO metric threaded fastener system is the poorest design ever on the planet, if you buy a Chinese chuck, that's what you are going to get.  So all that you need to buy is one proper size drill bit for the holes in the back plate, plus the screws.  There are no holes that you need to tap because they are all already tapped..

Also, if the screws are about 10 thou larger than 3/8". they must be M10's.  If you do try to convert the chuck to UN threads, you can't use 7/16-14 UNC because the cleaned up hole will already be too large.  You might just barely get away with 7/16-20 UNF.  The holes are likely to be too shallow for either 1/2-13 UNC or 1/2-20 UNF.  Minimum recommended hole depth is generally 1.5 D full thread, and 2D would be much safer.  I certainly would not risk drilling the holes any deeper.  No telling what you might drill into.


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## MikeMc (Jul 11, 2015)

Well it turns out this Chinese chuck is front mount so the backplate must be drilled and tapped.
Thanks for sharing your thoughts.
MikeMc


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## wa5cab (Jul 12, 2015)

OK.  In that case, I would use 3/8"-24.  The screws don't position the chuck relative to the back plate.  That is usually done by the spigot on the back plate fitting into the counter bore on the back of the chuck.  Or the gender may be reversed.  Unless the chuck is a Buck (Set True, or several other names) style, but most if not all of those come with a back plate already attached..


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## chevydyl (Jul 19, 2015)

i just finished an adapter to adapt a Bison direct mount D1-6 3jaw to my 12" atlas threaded spindle, i started with raw cast iron round stock 7-5/8 diameter, i drilled into the chuck and tapped it 1/2-13, i chose coarse threads mainly because thats what home depot had in the cap screws, otherwise i woulda went fine for the high clamping force although my adapter plate i made had the 7deg taper turned onto it that locks into the chuck perfectly.....


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## 4GSR (Jul 27, 2015)

All of my Buck Adjust A True chucks are mounted with 3/8-16 SOC HD CAP Screws, so is the chuck on my super spacer and a couple more chucks I have.  So yes, coarse threads are fine for fasteners to secure chucks to back plates!  I've mounted large massive 4-jaw hollow spindle chucks on A-20 spindle mounts using 1"-8 UNC socket head cap screws, too!


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