3/4-32 minor diameter? And other fine thread questions.

[toolholder]

I need to tap a hole so that it has a .72 or larger diameter, also it needs to have a 25-35 TPI. I was thinking the 3/4-32 might work if I cut the crests down slightly, I can't find a chart anywhere that lists the minor for this.

Also where should I try sourcing oddball taps & dies like this? I have ENCO's catalog but it doesn't have 13/16's in the TPI I am looking for. I am getting tired of looking hew:

 

[blacksmithden]

Sometimes, a manufacturer requires an odd ball size/pitch, and will have the tooling custom made for them. Not all sizes are available as over the counter products. For a one off job, the costs to have custom tooling made are cost prohibitive. Even finding specific major/minor specs are going to be a little tough. I would be getting my boring bars out with a very finely sharpened 60 degree tool bit...fire up the lathe....start cutting and test fitting the male piece after every pass until it fits. This is of course assuming your work piece will fit in your lathe chuck. If not.....well.....maybe you want to consider making the threads a more standardized size that you can get a tap for.

 

[toolholder]

My machine doesn't thread and I have 100 holes to make and 100 inserts to thread into them, custom made tap and die is out of the question. I will probably go with the 3/4-32 if all else fails.

 

[John Hasler]

Sometimes, a manufacturer requires an odd ball size/pitch, and will have the tooling custom made for them. Not all sizes are available as over the counter products. For a one off job, the costs to have custom tooling made are cost prohibitive. Even finding specific major/minor specs are going to be a little tough. I would be getting my boring bars out with a very finely sharpened 60 degree tool bit...fire up the lathe....start cutting and test fitting the male piece after every pass until it fits. This is of course assuming your work piece will fit in your lathe chuck. If not.....well.....maybe you want to consider making the threads a more standardized size that you can get a tap for.

Choose a pitch diameter, TPI, and class of fit and you can calculate the major and minor diameters for the internal and external threads. However, while 3/4-32 may not be stock at Fastenal it is a standard size. Here's a link: http://www.efunda.com/designstandards/screws/unified.cfm?start=148&finish=227

 

[JimDawson]

Try these 13/16-32

http://drillsandcutters.com/dwtsr112-131632roundadjustablespecial.aspx


http://drillsandcutters.com/qualtechdwtst1316-32b1316-32hssbottomingtapspecial.aspx

 

[OldMachinist]

3/4"-32 should work, Machinery's Handbook lists the minor diameter as .716"-.724" for a class 2B fit and .716"-.7219" for 3B. So your .720" would be fine. On these extra fine tapped holes I wouldn't trust a drill to make the holes to size. They should be finished either with a reamer or boring head.

 

[toolholder]

Try these 13/16-32

http://drillsandcutters.com/dwtsr112-131632roundadjustablespecial.aspx


http://drillsandcutters.com/qualtechdwtst1316-32b1316-32hssbottomingtapspecial.aspx

Bingo! That is exactly what I need, thanks.

- - - Updated - - -

3/4"-32 should work, Machinery's Handbook lists the minor diameter as .716"-.724" for a class 2B fit and .716"-.7219" for 3B. So your .720" would be fine. On these extra fine tapped holes I wouldn't trust a drill to make the holes to size. They should be finished either with a reamer or boring head.

13/16 will work so I don't have to worry with the super tight tolerances. My thinking was I would gear (it has a belt drive) the lathe head down to around 60 RPM and insert the tap or die in it. Then I would make some sort of sliding jig to put the aluminum material in and let it self feed. I am threading about .25" so I'll probably have to cut down the end of the tap.

 

[OldMachinist]

You'll still have make sure the holes are the right size even for the 13/16" tap. The minor diameter for that size is .779"-.786" which only leaves around .013"-.016" per side of material to be threaded.

 

[toolholder]

You'll still have make sure the holes are the right size even for the 13/16" tap. The minor diameter for that size is .779"-.786" which only leaves around .013"-.016" per side of material to be threaded.

What would you recommend? It is 6016 aluminum tube. I am using 1.25" OD and then going to turn it down, maybe make a collet to make sure it doesn't spread from tapping.

 

[OldMachinist]

I'm trying visualize what you're saying so let me see if this is what you're doing.
You have some aluminum tubing with a .720 inside diameter and 1.25 outside diameter
You need to thread the ID and turn down the OD to ?
I'm not sure what you mean about spreading when tapping.
Maybe a sketch of the finished part would help.