To Tap or to cut 1.25" x 7TPI internal threads?

[B2]

I turn to the considerable experience of the folks at HM.

My questions is, should I cut and thread big holes using my lathe tools or should I just buy a 1.25 x 7TPI tap? Can I even turn a tap this big?

@WobblyHand I see that you have cut threads this size using an ELS modified lathe. So maybe you have an opinion?

I have not cut heavy threads in steel so I thought I would ask for advice. I have never had the need to cut such large large threads but I may need to do so in the near future. I want to have some threaded holes in 0.75 inch thick (TBD) construction grade steel plate (A500?) to receive 1.25" by 7TPI threaded rod. These are for some special steel post I would like to make which would have two threaded rods inserted in one end and one threaded rod at the other end of the post. (Three holes per post.) I may have the tooling to cut such deep threads as I have a 5/8" boring and turning set from PM. I have the standard PM1440GT tool post holder plus a BXA Tool Post etc.

I have a PM1440GT lathe and a PM940M CNC mill, but most of my projects have been much smaller parts than this one. I will need to make two holes in one plate that fits into a steel post and one hole in the plate at the opposite end of the post. Each "pair" of holes are offset from the center. The single hole would be centered. The plates will probably about 4"x 3" or 4" diameter depending upon which tubes/post are readily available. I will need to repeat these two plate builds 4 times (12 threaded holes).

Dave L.

 

[DavidR8]

I turn to the considerable experience of the folks at HM.

My questions is, should I cut and thread big holes using my lathe tools or should I just buy a 1.25 x 7TPI tap? Can I even turn a tap this big?

@WobblyHand I see that you have cut threads this size using an ELS modified lathe. So maybe you have an opinion?

I have not cut heavy threads in steel so I thought I would ask for advice. I have never had the need to cut such large large threads but I may need to do so in the near future. I want to have some threaded holes in 0.75 inch thick (TBD) construction grade steel plate (A500?) to receive 1.25" by 7TPI threaded rod. These are for some special steel post I would like to make which would have two threaded rods inserted in one end and one threaded rod at the other end of the post. (Three holes per post.) I may have the tooling to cut such deep threads as I have a 5/8" boring and turning set from PM. I have the standard PM1440GT tool post holder plus a BXA Tool Post etc.

I have a PM1440GT lathe and a PM940M CNC mill, but most of my projects have been much smaller parts than this one. I will need to make two holes in one plate that fits into a steel post and one hole in the plate at the opposite end of the post. Each "pair" of holes are offset from the center. The single hole would be centered. The plates will probably about 4"x 3" or 4" diameter depending upon which tubes/post are readily available. I will need to repeat these two plate builds 4 times (12 threaded holes).

Dave L.

If it were me and I had access to a CNC mill I'd threadmill the holes.
Hand tapping a 1.25x7 TPI thread is not going to be easy. I had to tap a 1/2-10 TPI Acme thread and while I could do it, it was not easy.

 

[RaisedByWolves]

If it’s relatively thin plate and you have a big enough tap handle you could use a tap easy or even power tap in the lathe.

However it would be considerably cheaper to single point these vs buying a one time use tap.

You could even probably buy a decent quality threading tool with inserts.

 

[silverhawk]

I tried to use a 1-1/2"-8 tap making a back plate for one of my lathes, and that was a bear to turn. It essentially took a crescent wrench with a pipe to turn that more than a few turns in.

Since you are using 0.75" plate steel, that (most likely) means that your holes will be completely through the material, which would negate the potential for incomplete threads (blind holes can't be tapped all the way in), so a tap becomes feasible. But those taps could be quite expensive, around $400-$600.

If the plates are small enough to fit on the lathe for each hole, you could go that way, but again, depending on where those holes are, it might not fit.

If you can mill the threads on that CNC mill (like @DavidR8 mentioned), that would be the best option - the part (your plates) don't move, and you don't have to break a heavy effort trying to spin a large tap in a hole. Set up may take longer overall, but once you have a repeatable set up, you should be able to crank through those rather quickly with tool changes being the long efforts.

 

[GeneT45]

I'd single-point them. The tap is bound to be expensive (especially if you get a quality one), 12 holes, particularly once you're set up shouldn't take that long. I'd grind a threading tool for a boring bar if I didn't have a threading tool large enough to do 7 TPI. Make sure your lathe can go to 7 TPI as well.

I wouldn't spring for that tap (and you'd probably need a new tap handle (buy / make) as well) unless I couldn't do it on the lathe.

GsT

 

[RaisedByWolves]

I can’t edit the above post as the board is buggy, but I meant to say you could easily buy a threading set with inserts for the cost if that size tap and handle.

 

[mmcmdl]

PM940M CNC mill,

Thread mill them ! Wrenching a 1 1/4" tap is not fun .

 

[Ulma Doctor]

For about $40 USD , tap it

1 1/4 7 NC GH-4 HSS PLUG TAP 4 STRAIGHT FLUTE 1.25 MADE IN USA THREADING | eBay

<p dir="ltr" style="margin-top:0; margin-bottom:0;">This high-quality plug tap is perfect for your threading needs. Made by M & J Tooling LLC, it features a 1.25 cutting diameter and 4 straight flutes for efficient and accurate threading. The HSS material ensures durability and reliability...

www.ebay.com

 

[Jake M]

My questions is, should I cut and thread big holes using my lathe tools

Yes. For so many reasons, it's easier to put the right thread in the right place that way,

or should I just buy a 1.25 x 7TPI tap? Can I even turn a tap this big?

Absolutely. Fast and efficient, although it's not a cheap tap (and if it is, good luck with one that size), and it's gonna take a three foot snipe on each end of your tap wrench to get it through...

I have not cut heavy threads in steel so I thought I would ask for advice.....

...0.75 inch thick (TBD) construction grade steel plate (A500?) to receive 1.25" by 7TPI threaded rod...

I'd suggest then, stick the stuff in the lathe, and single point. That's the lowest barrier to entry, and really should get the job done. HOWEVER... If you start the thread in the lathe, it's a big cut, you'll need very rigid tooling, and generally to be on your A game. It might go well, or it might go otherwise... But if you have a thread that's half or two thirds done, and it's arguing with you, that "unfinished" thread that you cut with the lathe- The tap will go a WHOLE LOT easier through a partial thread than it will through no thread at all, and you're pretty much guaranteed to get it started straight.
Nothing wrong with hedging your bets on any given operation you're not familiar with. If you do it once, pretty good chance it goes well, whether the lathe does well, or the lathe "attempt" makes the tapping go well. (This is a serious tap, doing that by hand is a real commitment....). If you do it a whole lot, you'll settle in on what works for you, and get more efficient at it.

 

[GeneT45]

For about $40 USD , tap it

1 1/4 7 NC GH-4 HSS PLUG TAP 4 STRAIGHT FLUTE 1.25 MADE IN USA THREADING | eBay

<p dir="ltr" style="margin-top:0; margin-bottom:0;">This high-quality plug tap is perfect for your threading needs. Made by M & J Tooling LLC, it features a 1.25 cutting diameter and 4 straight flutes for efficient and accurate threading. The HSS material ensures durability and reliability...

www.ebay.com

"...while the absence of coating allows for maximum performance."

Now *that's* a sales guy!

GsT