Shopsmith Forums

What size tap do I need?

NPT == National Pipe Thread Taper? Is this is different than my NC tap? Do I need to go find a plumber?

Jim

Wow,

Good point on the threads inside the miter-bar. On looking I see my oldest one is pretty butchered inside.... I just ordered a new bar and the OEM tapered screws as well.

thanks,

Bump - to see if anyone can share the size/thread count for the NPT tap.

Jim

I have a slight different problem. I can turn the screw really tight, but the miter bar still moves - it never locks in tight. Anyone experience this before?

Jim,

I'm not a machinist but I will give some input and hope others will add to it (or correct me) as needed.

NPT is a tapered thread used primarily for joining pipe and fittings. It is (as you noted) a tapered thread meaning the overall diameter of the thread changes with the depth of the thread. This feature, along with a bit of sealing compound, helps lock the pipe and fittings together to form a pressure-tite seal (not our application). In the case of a 1/4" NPT the thread has, I believe, 18 threads per inch (TPI).

A drill size of 7/16" diameter, or there about, would be used to drill the hole that would accept the 1/4" NPT tap. Remember we are dealing with 1/4" pipe (i.e., an OD of .54")

NC is a straight thread meaning the diameter of the thread remains the same over its entire length. For a 1/4" NC thread you have 20 TPI (i.e., our old common 1/4-20 bolt). For a 1/4" NF you have 28 TPI.

A #3 drill (about 13/64") would be used to drill the hole to accept a 1/4" NC tap.

As you can see 1/4" NPT in addition to being tapered, is about twice the diameter of a 1/4" NC.

A plumber would certainly own a 1/4" NPT tap, but they are also included in many tap and die sets, so you may have a friend that does a lot of car or other machinery work that might have one in his set.

Last but not least you should be able to buy one individually at a good hardware store.

I will now let others take over while I ponder our miter bar application.

Dennis

jmoore65 wrote:Bump - to see if anyone can share the size/thread count for the NPT tap.

Jim

That size should be on your exploded parts listing.

Hi Jim,

To this point nobody has jumped in to correct me but it appears to me upon further investigation that we should be looking at a 1/8 NPT thread. This thread has a TPI of 27. I was able to easily screw the 1/8 NPT tap into my miter bar about half way as shown in the photos below. I hope someone else can confirm this.

Just remember that since it is a tapered thread, it will matter which way you insert the tap (there is a top and bottom to the miter bar).

I think that my other comments about the NPT tap being much larger than the same fractional size in NC tap stills stands.

Dennis

perryobear wrote:Hi Jim,

To this point nobody has jumped in to correct me but it appears to me upon further investigation that we should be looking at a 1/8 NPT thread. This thread has a TPI of 27. I was able to easily screw the 1/8 NPT tap into my miter bar about half way as shown in the photos below. I hope someone else can confirm this.

Dennis

Dennis,
Looks like you have lined the information up well. I would now chase the threads, make sure the tapered screw SS #501401 is correct and usable then dab just a small amount of antiseize grease McMaster-Carr #1279K31 on the taper screw threads with a Q-tip. I have found that a nickles worth of pm on that little taper screw has saved me a lot of grief.
ldh

Thanks for the followup Dennis!

Jim

timster68 wrote:I have a slight different problem. I can turn the screw really tight, but the miter bar still moves - it never locks in tight. Anyone experience this before?

Timster68
I was always taught by Shopsmith to tear a stripe of paper and wrap it around my mitre bar in the area of the screw to shim the bar into place. I thought everyone did this.
Ed