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

[woodchucker]

I would single thread it in the lathe.
I have tried to tap big threads by hand. And one of two things happen.
The part has so much torque from the tap handle, that it slips in the vise or chuck jaws and tears the outside of the part up.
I get so fearful of the slip I overtighten, and deform the part.
I should mention, even in collets I get slippage.
Single point is less taxing. Even if you don't finish single point, and decide to finish with the tap, you will have reduced the stress on you and the part.

 

[WobblyHand]

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.

Well, I'd try it. I've cut 4 TPI and 5 TPI in steel on a 10x22 G0602/G0752Z using AXA tool holders. I used an Aloris AXA-8 HSS threading tool for the Vee threads, and a hand ground acme form in an AXA-1 holder. 7 TPI ought to be a lot less difficult. I'd use modified flank (angle the compound) to reduce cutting force and have a go at it. For myself, I'd have to figure out the numbers for 7 TPI and then add it to my thread table.

I've also done 8 TPI internal threads in cast iron with a NEL3 3/4" bar. The internal threads were done in reverse, but I could have easily done them forward, since it was a through hole. I was a little intimidated by doing the back plate, and shouldn't have been.

There's really nothing to be afraid of, in my opinion. This assumes the lathe is capable (gearing or ELS) to achieve 7 TPI. Remember, threading is a just little cut at a time. If you take 0.005" cuts, tapering down to 0.001", you can thread a whole lot of things, as long as the material is reasonable. Hogging out cuts when single pointing might be useful in production, but for a hobbyist it's just a little cut at a time. I've done M8x1 internal threads, and M12x1 just a couple of days ago. Did a couple acme's in the past couple of weeks. It's just set up and do it...

I'd first practice on a test piece, with a similar material, kind of a dry run, and then do the real thing.

 

[metalmaster1]

will your lathe do 7tpi ?

 

[markba633csi]

Multiply my answer by 3:
Single point

 

[Charles scozzari]

Real Machinists will scoff at this but as a home shop machine guy you have to improvise. This is what I use for large tapping in the mill spindle and the lathe. I turn it with two adjustable wrenches. It gets it done for me.

 

[WobblyHand]

As long as you get it done and no one is hurt, just about anything goes..

 

[B2]

Hi Guys,

Lots of very good comments. I am glad I ask. This is great. Thank you.

I too am/was VERY concerned with getting a tap that big started straight. Also, I had not even thought about using my CNC Mill, although I am a bit hesitant as the backlash is poor along the y axis so getting things exactly round is a problem. While my lathe will handle this, I am not so sure my tail stock chuck is big enough or strong enough to hold a tap this big. Anyway, thanks for the link to the $29 tap. Running it through after the single point turning (whether on the lathe or the mill) is probably a very good idea. Yes, it is a through hole. Yes, I will practice. I am thinking something softer than steel the first time. Al or maybe even Nylon. I did do some heavy steel facing on my Mill at one point. When I did this I set up a flood cooling arrangement. What a mess. I had to have to go back to that but might. Also, I needed better ventilation as I had purchase expensive DuPont coolant and its fumes were probably toxic.

Wow! I was not thinking of using my mill to turn the tap! I doubt that it is strong enough for that! Also, I am getting kind of old, so not for sure I could turn those crescent wrenches without standing on the lathe!


I did my own lathe VFD conversion and so the lathe will turn very slowly if need be. It jogs at about 7 RPM but obviously the power would be limited.

VFD conversion using solid state electronic components.

As far at the 7TPI this is no problem. If you folks have not seen it I built a spread sheet that will calculate all of the possible TPI values basically for any lathe. There are Macros that make it automated. The lathe sellers never list all of the possible gear combinations, of external gears plus gear box lever positions, so that you have a list of all of the possible TPI values that your lathe will make. For my PM1440GT with just the external gears that come standard with it there are 21200 possible combinations. Turns out that there are about a dozen gear and lever combinations that yield 7TPI. There are are almost 200 combinations that yield 7TPI to withing 1%.

Maybe the spread sheet is useful for your too. I posted an updated version last year and more recently I have been rewriting the macro that generates the table of values to make it more accurate. It works correctly, but has a feature that allows you to cause it to skip duplicates. This has a bug in it, so just run it in a complete table format. There are multiple spread sheets in the workbook for different lathe models. If yours is not there you just copy a sheet that is similar to your lathe and then modify it a bit. For example if you have a different set of external gears you can just change the values to your set. Likewise, if your gear box has different knobs or ratios this is pretty easy to fix. Anyway, there is Readme file to explain a lot of it. And if that does not work, then ask me.

Nov. 6 2023 Excel workbook file name: TPI_ManyLathesRev1 NB06_0054.xlsm

Thanks,
Dave L

 

[WobblyHand]

Hi Guys,

Lots of very good comments. I am glad I ask. This is great. Thank you.

I too am/was VERY concerned with getting a tap that big started straight. Also, I had not even thought about using my CNC Mill, although I am a bit hesitant as the backlash is poor along the y axis so getting things exactly round is a problem. While my lathe will handle this, I am not so sure my tail stock chuck is big enough or strong enough to hold a tap this big. Anyway, thanks for the link to the $29 tap. Running it through after the single point turning (whether on the lathe or the mill) is probably a very good idea. Yes, it is a through hole. Yes, I will practice. I am thinking something softer than steel the first time. Al or maybe even Nylon. I did do some heavy steel facing on my Mill at one point. When I did this I set up a flood cooling arrangement. What a mess. I had to have to go back to that but might. Also, I needed better ventilation as I had purchase expensive DuPont coolant and its fumes were probably toxic.

Wow! I was not thinking of using my mill to turn the tap! I doubt that it is strong enough for that! Also, I am getting kind of old, so not for sure I could turn those crescent wrenches without standing on the lathe!


I did my own lathe VFD conversion and so the lathe will turn very slowly if need be. It jogs at about 7 RPM but obviously the power would be limited.



As far at the 7TPI this is no problem. If you folks have not seen it I built a spread sheet that will calculate all of the possible TPI values basically for any lathe. There are Macros that make it automated. The lathe sellers never list all of the possible gear combinations, of external gears plus gear box lever positions, so that you have a list of all of the possible TPI values that your lathe will make. For my PM1440GT with just the external gears that come standard with it there are 21200 possible combinations. Turns out that there are about a dozen gear and lever combinations that yield 7TPI. There are are almost 200 combinations that yield 7TPI to withing 1%.

Maybe the spread sheet is useful for your too. I posted an updated version last year and more recently I have been rewriting the macro that generates the table of values to make it more accurate. It works correctly, but has a feature that allows you to cause it to skip duplicates. This has a bug in it, so just run it in a complete table format. There are multiple spread sheets in the workbook for different lathe models. If yours is not there you just copy a sheet that is similar to your lathe and then modify it a bit. For example if you have a different set of external gears you can just change the values to your set. Likewise, if your gear box has different knobs or ratios this is pretty easy to fix. Anyway, there is Readme file to explain a lot of it. And if that does not work, then ask me.


Thanks,
Dave L

Based on your lathe, and your ability to gear it to 7 TPI, I'd give single pointing a go. Can't see a reason you wouldn't be successful. Your lathe is far more capable than mine. Shoot, I did 5 TPI at 100 RPM, (my minimum speed on my VFD, at my lowest pulley ratio) against a shoulder, you can do 7 TPI on yours! Especially since this is a through hole, rather than a blind hole. Go Dave, go!

Post results, we want to cheer you on!

 

[projectnut]

I would be a little suspicious of a 1 1/4" tap that has an asking price of $29.95. Over the years I've purchased dozens of taps online and never seen a quality brand of that size with a price like that.

A good source for quality taps online is Zeitlin Industrial Supply on eBay. They currently have a listing for a WIDA 1-1/4-7 hand bottoming tap listed for $86.99. That's about the least expensive quality tap I've seen of that size.

WIDIA 1-1/4"-7 HSS Hand Tap 4 Flute H4 Bottoming Tap USA Made | eBay

Chamfer ‎1P-2P. Cutting Edge Material HSS Series/List # 5303 | 2303. HIGH SPEED STEEL HAND TAP. Tool Type Hand Tap Thread Size ‎1-1/4″ - 7. Overall Length (OAL) ‎5-3/4″ Cutting Direction (HAND) Right Hand.

www.ebay.com

As a side note I have had to hand tap 1 1/4-7 holes in DOM tubing for tie rod ends on construction machinery. It was difficult to say the least.

 

[jwmay]

Cut them. Since it’s not me doing it, I recommend trying aluminum first. Or maybe even a piece of wood, then aluminum, then steel.