# Tap drill size for 1/2 -10 Acme



## DavidR8 (Apr 20, 2021)

I'm planning a floating vise build and I need a hole sized for a 1/2-10 Acme tap.
This chart  tells me the tap drill size is .400.
This page says it's .405
.400 is impossible to find it seems. 
.405 is findable.
What is the correct size?


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## FOMOGO (Apr 20, 2021)

You could just drill it slightly under-size, and bore it to your .400. I would also think a bit must be available. Mike


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## matthewsx (Apr 20, 2021)

I can't imagine that .006" is going to matter for your application. Perhaps that's the margin that keeps the tap from breaking?


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## DavidR8 (Apr 20, 2021)

FOMOGO said:


> You could just drill it slightly under-size, and bore it to your .400. I would also think a bit must be available. Mike


It would seem like if .400 is the size then finding one should be no problem right?


matthewsx said:


> I can't imagine that .006" is going to matter for your application. Perhaps that's the margin that keeps the tap from breaking?


Perhaps. The tap I have coming is a tandem tap which is a new thing for me. Seems as though it taps in two stages, a rough and a finish.


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## FOMOGO (Apr 20, 2021)

Here is some info that puts it at .405. Mike


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Tap drill size for Acme Taps
 
Tap drill size for acme taps​ 
this page is for information only​ 
link to Acme Tap page
​
*tap drill size for acme taps*

Based on a 100% of thread as per ANSI
to calculate tap drill size for acme taps.

the magic formula
step 1
1 Divided by TPI ( threads per inch)
step 2
subtract this amount from the OD.
for pitches 10 and greater always add .005" for the drill size.
for pitches less than 10 TPI take the amount from step 1 and multiply x.05 ( 5 %) and add this to the drill size.
This is for 95% to 100% threads.


eg 1/2-10
1/10 = .100 ( step 1 )
.500 ( the OD ) minus .100 ( from step 1) = .400 drill size.
since the pitch is 10 or greater add .005" so the drill size is .405"


eg 3/4-6

1/6= .167"
.750 - .167 = .583" ( 100% )
.167 X .05 = .008" ( to obtain 5% )
.583 + .008 = .591" ( 95%)


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## DavidR8 (Apr 20, 2021)

Thanks Mike, looks like a 13/32" is .406". Surely .001 wouldn't make a significant difference would it?


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## mikey (Apr 20, 2021)

David, just curious. What class fit did you order your leadscrew stock in - General or Centralizing? What fit is the tap you ordered? This impacts on the inside diameter you choose so I was just wondering ...


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## John O (Apr 20, 2021)

My drills usually make a larger hole so maybe a .400 drill will give the .405


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## WCraig (Apr 20, 2021)

You have a set of letter drills, right?  An X drill is nominally 0.397 inches, Y is 0.404 and 13/32 is 0.4063.  The actual size they create is likely to be a little larger, as @John O says.  Why not drill and tap a few test holes in some scrap and see how you like the resulting fit with your acme rod?

Craig


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## DavidR8 (Apr 20, 2021)

mikey said:


> David, just curious. What class fit did you order your leadscrew stock in - General or Centralizing? What fit is the tap you ordered? This impacts on the inside diameter you choose so I was just wondering ...



Both are class 2. 
What are your thoughts on tap drill size?


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## DavidR8 (Apr 20, 2021)

WCraig said:


> You have a set of letter drills, right? An X drill is nominally 0.397 inches, Y is 0.404 and 13/32 is 0.4063. The actual size they create is likely to be a little larger, as @John O says. Why not drill and tap a few test holes in some scrap and see how you like the resulting fit with your acme rod?
> 
> Craig



Yes, I have number drills. 
Your suggestion is a good one. I’m definitely going to make some test holes and see how they go.


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## MrWhoopee (Apr 20, 2021)

Considering that your are building a vise, I would be FAR more concerned with breaking the tap than getting the maximum percentage of the thread. Speaking from experience.


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## tkalxx (Apr 20, 2021)

Machinery's Handbook states the tolerance for a 1/2-10 Acme class 2G internal thread minor diameter to be 0.4000-0.4050.

I would use a Y or 10.2 mm twist drill. If none of those are available to you, a 13/32 would be the next best option. The fit of the thread will most largely depend on the pitch diameter of your tap (i.e. class 2G) and not the size of the pilot drill, unless you drill way undersized or way oversized.


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## DavidR8 (Apr 20, 2021)

MrWhoopee said:


> Considering that your are building a vise, I would be FAR more concerned with breaking the tap than getting the maximum percentage of the thread. Speaking from experience.





tkalxx said:


> Machinery's Handbook states the tolerance for a 1/2-10 Acme class 2G internal thread minor diameter to be 0.4000-0.4050.
> 
> I would use a Y or 10.2 mm twist drill. If none of those are available to you, a 13/32 would be the next best option. The fit of the thread will most largely depend on the pitch diameter of your tap (i.e. class 2G) and not the size of the pilot drill, unless you drill way undersized or way oversized.



Breaking the tap is definitely my concern too. I think that I'll start on the larger end  of the scale and see how it goes.


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## markba633csi (Apr 20, 2021)

For a tap that size a few thousandths won't make any significant difference, especially for a vise project
-M


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## SLK001 (Apr 20, 2021)

Most drills will drill oversize, so I would use an "X" lettered drill (0.397").  If this is too small of a diameter, it is close enough that your tap will correct for it.


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## DavidR8 (Apr 20, 2021)

The tap should be here in a day or two so I'll be able to test it out.


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## mikey (Apr 20, 2021)

DavidR8 said:


> Both are class 2.
> What are your thoughts on tap drill size?



My concern was that your tap and leadscrew match in terms of fit class. If your leadscrew is a general and the tap is centralizing then you are not going to be happy. Given the expense of an Acme tap, just wondered if you made sure of your fits when ordering your stuff. As for tap drills, I would consult with the tap manufacturer and ask them for a recommendation. If you drill it to the size they recommend and your tap snaps you may have some recourse. 

If you haven't use an Acme tap before, I think you will be surprised at how much force it takes to cut that thread. Even the dual taps that rough and finish require a LOT of torque. Expect that the stock may spin in the chuck.


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## DavidR8 (Apr 20, 2021)

mikey said:


> My concern was that your tap and leadscrew match in terms of fit class. If your leadscrew is a general and the tap is centralizing then you are not going to be happy. Given the expense of an Acme tap, just wondered if you made sure of your fits when ordering your stuff. As for tap drills, I would consult with the tap manufacturer and ask them for a recommendation. If you drill it to the size they recommend and your tap snaps you may have some recourse.
> 
> If you haven't use an Acme tap before, I think you will be surprised at how much force it takes to cut that thread. Even the dual taps that rough and finish require a LOT of torque. Expect that the stock may spin in the chuck.


Fortunately (?) the stock is rectangular so it will be tapped in a vise. I plan to try different size holes to find the  best combination of fit and least 
tap-snapping potential


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## mikey (Apr 20, 2021)

DavidR8 said:


> Fortunately (?) the stock is rectangular so it will be tapped in a vise. I plan to try different size holes to find the  best combination of fit and least
> tap-snapping potential



The area of contact with an Acme tap is comparatively huge. Having broken them before, and having to pay for the original and the replacement, I just wanted to suggest that you ask the maker for suggestions. Yes, I know how to look stuff up but I've done this stuff before and paid real money for the mistakes. It will take just a little time to ask, which may save you time and bucks.


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## DavidR8 (Apr 20, 2021)

mikey said:


> The area of contact with an Acme tap is comparatively huge. Having broken them before, and having to pay for the original and the replacement, I just wanted to suggest that you ask the maker for suggestions. Yes, I know how to look stuff up but I've done this stuff before and paid real money for the mistakes. It will take just a little time to ask, which may save you time and bucks.


Absolutely hear you there. I've contacted the manufacturer and will let you know what they say.


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## devils4ever (Apr 20, 2021)

I used a tandem tap recently and I thought the first portion was going to snap before I got to the second section. The first section was _really _tough. Get a LARGE tap wrench! Once the first section got through, the second section was easy.


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## DavidR8 (Apr 20, 2021)

Ok, I'm very nervous, bordering on terrified about doing this now!


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## mikey (Apr 20, 2021)

DavidR8 said:


> Ok, I'm very nervous, bordering on terrified about doing this now!



This is one reason why I will screw cut my next Acme nut. It will allow me to cut to the fit I require on the lead screw.


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## DavidR8 (Apr 20, 2021)

mikey said:


> This is one reason why I will screw cut my next Acme nut. It will allow me to cut to the fit I require on the lead screw.


Screw cut as in internal single-point?


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## mikey (Apr 20, 2021)

Yup.


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## DavidR8 (Apr 20, 2021)

I should have looked at the McMasterCarr site for a bit of info on tap drill size. It clearly says 13/32"








						McMaster-Carr
					

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.




					www.mcmaster.com


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## John O (Apr 20, 2021)

From the description
"Because these sets of threads have different shapes, reversing the tap for removal may cause damage to the workpiece and is not recommended"
would that mean after threading you need to continue turning right and remove tap from bottom of part(putting shank thru work)?


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## DavidR8 (Apr 20, 2021)

John O said:


> From the description
> "Because these sets of threads have different shapes, reversing the tap for removal may cause damage to the workpiece and is not recommended"
> would that mean after threading you need to continue turning right and remove tap from bottom of part(putting shank thru work)?


Yup, that's what that means.


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## devils4ever (Apr 21, 2021)

DavidR8 said:


> I should have looked at the McMasterCarr site for a bit of info on tap drill size. It clearly says 13/32"
> 
> 
> 
> ...



For reference, I used a Z drill, 0.4130" on my project. In my case, a little bigger is better.


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## SLK001 (Apr 21, 2021)

I would mount the item to be tapped in a 4-jaw and single point the thread to ~90%, then tap the rest of the way.  Depending on the material, the force required to tap it may be too much for the tap.


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## DavidR8 (Apr 21, 2021)

SLK001 said:


> I would mount the item to be tapped in a 4-jaw and single point the thread to ~90%, then tap the rest of the way. Depending on the material, the force required to tap it may be too much for the tap.



It’s just mild steel but your idea is a good one.


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## mattthemuppet2 (Apr 21, 2021)

drill/ bore it oversize. There'll be plenty enough thread depth in steel for what you want to do and minor diameter will have next to no effect on fit of the screw to the nut. I did a 1/2-10 acme nut in bronze recently (tail stock mod) and bored it to 0.410". Tapping it was VERY hard work, using a single pass (very long taper) tap. It went easier once the full thread profile part of the tap was in the work. I wouldn't even consider 0.400" in steel.


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## MrWhoopee (Apr 21, 2021)

DavidR8 said:


> Ok, I'm very nervous, bordering on terrified about doing this now!


Now you're getting it.
"If you ain't scared man, you ain't right."

I think I was tapping bronze when I broke a tap of about the same size.

@SLK001's suggestion to single-point rough it is excellent, even with a crudely ground tool. Just remove the bulk of the material.


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## DavidR8 (Apr 21, 2021)

MrWhoopee said:


> Now you're getting it.
> "If you ain't scared man, you ain't right."
> 
> I think I was tapping bronze when I broke a tap of about the same size.
> ...



I have a 60 deg internal threading insert bar. Would that work?


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## ErichKeane (Apr 21, 2021)

DavidR8 said:


> I have a 60 deg internal threading insert bar. Would that work?


That would remove most of it.  and would make the tapping easier.  The biggest issue is going to be making sure you start the ACME tap in the right place, but if you end up getting 'most' of the material out (to the point where it is nearly square near the root) it should be enough to engage the first few teeth of the starter tap.


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## MrWhoopee (Apr 21, 2021)

DavidR8 said:


> I have a 60 deg internal threading insert bar. Would that work?


Yes, but my first inclination is that it might be possible to go too deep, removing material that should be left.


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## ErichKeane (Apr 21, 2021)

MrWhoopee said:


> Yes, but my first inclination is that it might be possible to go too deep, removing material that should be left.


I'm not sure I'd be too concerned about 'depth' so much as 'width'.  If you cut too deep (in the bottom of the thread) you are just hitting the 'relief' part of the thread.  If you cut too wide, you are taking out the force bearing-surfaces.


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## DavidR8 (Apr 21, 2021)

MrWhoopee said:


> Yes, but my first inclination is that it might be possible to go too deep, removing material that should be left.





ErichKeane said:


> That would remove most of it.  and would make the tapping easier.  The biggest issue is going to be making sure you start the ACME tap in the right place, but if you end up getting 'most' of the material out (to the point where it is nearly square near the root) it should be enough to engage the first few teeth of the starter tap.


The tandem tap should be here today or tomorrow. I have some 1" stock I to experiment with to see what works the best. I'm most concerned with the initial 'roughing' part of the tap.

1) drill with a 13/32 and try tapping. 
2) drill with a 13/32 and try internal threading with a 60 deg tool for 2/3/4/? passes


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## ErichKeane (Apr 21, 2021)

DavidR8 said:


> The tandem tap should be here today or tomorrow. I have some 1" stock I to experiment with to see what works the best. I'm most concerned with the initial 'roughing' part of the tap.
> 
> 1) drill with a 13/32 and try tapping.
> 2) drill with a 13/32 and try internal threading with a 60 deg tool for 2/3/4/? passes


Could you just rough-grind an HSS tool for this?  You could basically just 'undersize' a cutter and depth and let the ACME tools cut as little as possible.

Using the 60 degree tool you have to make sure you don't cut them 'too wide'.


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## DavidR8 (Apr 21, 2021)

ErichKeane said:


> Could you just rough-grind an HSS tool for this?  You could basically just 'undersize' a cutter and depth and let the ACME tools cut as little as possible.
> 
> Using the 60 degree tool you have to make sure you don't cut them 'too wide'.


I can try though I doubt my capabilities to grind an internal threading tool


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## ErichKeane (Apr 21, 2021)

DavidR8 said:


> I can try though I doubt my capabilities to grind an internal threading tool


The nice part is it doesn't have to be 'good', since it is just a roughing tool.


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## devils4ever (Apr 21, 2021)

You can buy carbide Acme internal threading tools. I didn't think my mini-lathe was up to the task although I may try it in the future.

Check these out.


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## MrWhoopee (Apr 21, 2021)

ErichKeane said:


> not sure I'd be too concerned about 'depth' so much as 'width'.  If you cut too deep (in the bottom of the thread) you are just hitting the 'relief' part of the thread.  If you cut too wide, you are taking out the force bearing-surfaces.


That is actually what I was thinking of, cutting to correct depth for a 60° thread would be too wide for an Acme.


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## DavidR8 (Apr 21, 2021)

MrWhoopee said:


> That is actually what I was thinking of, cutting to correct depth for a 60° thread would be too wide for an Acme.


I would likely only make a couple of passes to remove a bit of material.


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## SLK001 (Apr 21, 2021)

I recommend that you DON'T use a 60 degree threading bit, but instead grind your own ACME profile on a piece of HSS.  You'll want to go deep enough to remove most of the material (that you DON'T want) and a 60 degree bit will remove material that you DO want.  The ACME bit doesn't have to be perfect, because your tap will clean it up to your final spec.


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## DavidR8 (Apr 21, 2021)

The tap arrived today. 
I drill a hole in the mild steel stock using a Z letter drill. 
The tap wouldn’t even begin to bite no matter how much pressure I put on it. 
So either the tap is junk or it needs a bigger clearance hole.


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## nnam (Apr 22, 2021)

How about try a larger hole and put a bevel to it?


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## DavidR8 (Apr 22, 2021)

nnam said:


> How about try a larger hole and put a bevel to it?



Yeah, I’m going to increment up, bevel the start and see what happens. 
Started with .413, I think I’ll try a 27/64 or 7/16 and see how that goes. 
I measured the tap and the bottom of the thread is dead on .406.


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## devils4ever (Apr 22, 2021)

DavidR8 said:


> The tap arrived today.
> I drill a hole in the mild steel stock using a Z letter drill.
> The tap wouldn’t even begin to bite no matter how much pressure I put on it.
> So either the tap is junk or it needs a bigger clearance hole.



Where did you get the tap? I think I bought mine from McMaster-Carr or MSC.


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## DavidR8 (Apr 22, 2021)

devils4ever said:


> Where did you get the tap? I think I bought mine from McMaster-Carr or MSC.



I bought it from MPI tools.


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## devils4ever (Apr 22, 2021)

I looked and I got mine from McMaster-Carr. Not cheap, but quality stuff.


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## sdelivery (Apr 22, 2021)

DavidR8 said:


> The tap arrived today.
> I drill a hole in the mild steel stock using a Z letter drill.
> The tap wouldn’t even begin to bite no matter how much pressure I put on it.
> So either the tap is junk or it needs a bigger clearance hole.


It will take alot of force to get this tap started,  it doesn't have the pull of a regular tap.
If using a lathe set up the feed to help you push it.
If you are going to rough in the threads by single pointing then you might as well just finish them that way.
Don't fear your equipment,  respect it and think the process through,  beware of the what ifs but dont drown in them.


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## SLK001 (Apr 22, 2021)

DavidR8 said:


> The tap wouldn’t even begin to bite no matter how much pressure I put on it.


These taps require a LOT of force to start.  Another reason to "start" the tapping with a single point tool on the lathe.


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## WCraig (Apr 22, 2021)

DavidR8 said:


> The tap arrived today.
> ...


Can we see a picture or two?

Craig


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## MrWhoopee (Apr 22, 2021)

DavidR8 said:


> The tap arrived today.
> I drill a hole in the mild steel stock using a Z letter drill.
> The tap wouldn’t even begin to bite no matter how much pressure I put on it.
> So either the tap is junk or it needs a bigger clearance hole.


You are beginning to understand why I just bought a nut, turned a shoulder on it and pressed it in.


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## Papa Charlie (Apr 22, 2021)

If you make the part a little longer than you need and create a tappered beginning to get the tap to start, that would help. You can see how much force it will take to drive the tap. I would be concerned that I might apply a little too much side force accidentally and snap the tap if I was using a vice and free handing it. I would recommend leaving the part in the lathe and do your tapping there after you bore and taper the entrance, where you can turn the part using the chuck key and have the rigidity of the lathe to use to your advantage.


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## DavidR8 (Apr 22, 2021)

Thanks so much for all of the excellent advice. I'll have another go at this tonight I hope.


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## DavidR8 (Apr 22, 2021)

WCraig said:


> Can we see a picture or two?
> 
> Craig


Here's the tap.


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## ErichKeane (Apr 22, 2021)

Woah, that is such an interesting profile!  It looks like a normal 60 degree tap until the 1/2 way point with squared-off tops, then 'squares' up the holes in the 2nd half.


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## DavidR8 (Apr 22, 2021)

ErichKeane said:


> Woah, that is such an interesting profile!  It looks like a normal 60 degree tap until the 1/2 way point with squared-off tops, then 'squares' up the holes in the 2nd half.


Yup, it looks pretty much exactly like a 60 deg tap for the first part. If there was such a thing as a 1/2-10 60 deg tap I'd buy it.


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## mattthemuppet2 (Apr 22, 2021)

that looks about right for a tandem tap. The V-shaped 1st part isn't a 60deg thread form, it's a narrow acme thread form. The idea is for the 1st part of the tap to remove the bulk of the material and the 2nd part generate the correct thread form.


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## nnam (Apr 22, 2021)

Although I didn't do ACME thread before and I know it is harder, my experience shows that a good tap and lubrication and doing it correctly, it goes very easy and smooth, while bad tap is very hard and breaks.  So translate from that, I would be very cautious to put excessive force just because it is an ACME thread.   But how much is excessive would be up to your determination.


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## ErichKeane (Apr 22, 2021)

I was just thinking... is there any overlap in size with a 7/16-10 acme thread (https://www.amazon.com/Acme-Speed-Steel-Choice-Sizes/dp/B018A3WR3Q?th=1)?  You could start by tapping 7/16-10 with a massively-oversized start-hole (since 7/16's drill size is 40 thou under what you're cutting).

The result would be a pretty solid start-thread that would significantly reduce the load on the 1/2-10 tap.


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## SLK001 (Apr 22, 2021)

DavidR8 said:


> Yup, it looks pretty much exactly like a 60 deg tap for the first part. If there was such a thing as a 1/2-10 60 deg tap I'd buy it.


It just looks like a 60º tap.  Because of the force required, only part of the tap is a complete ACME profile (ie, it cuts half the profile).  Later, another part cuts the other half.  Only a gorilla can tap a large ACME profile by hand.


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## DavidR8 (Apr 22, 2021)

Just checked and my internal threading tool is larger than .5"  So I need to make/buy a smaller boring bar.


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## mattthemuppet2 (Apr 22, 2021)

that's why I ended up using a tap. It's a fairly deep thread relative to the minor ID, so making/ grinding a tool that could fit in the hole and still be able to cut the profile was not trivial.


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## sdelivery (Apr 22, 2021)

Two step acme tap pictures.....
EVERYTHING is better with Pictures!


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## DavidR8 (Apr 22, 2021)

What size is that?


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## sdelivery (Apr 22, 2021)

3/4-10 RH,  but it should look like yours.
I have a couple other sizes but not with me.


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## ErichKeane (Apr 22, 2021)

DavidR8 said:


> What size is that?


Not sure you saw my suggestion above, but if you still have a hard time getting it started, 7/16-10 Acme taps are cheap and should be much easier to start, as they are cutting only about 1/2 their thread depth with a .405 drill.  You should then be able to chase it with your 1/2-10 tap with a lot less material to worry about, which would reduce the force.


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## sdelivery (Apr 22, 2021)

ErichKeane, that actually sounds like a good idea.


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## sdelivery (Apr 22, 2021)

I cant find a 7/16-10 acme tap only 7/16-12...


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## MrWhoopee (Apr 22, 2021)

sdelivery said:


> I cant find a 7/16-10 acme tap only 7/16-12...


Nevermind


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## ErichKeane (Apr 22, 2021)

sdelivery said:


> I cant find a 7/16-10 acme tap only 7/16-12...


I posted an amazon link to one above.


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## DavidR8 (Apr 22, 2021)

ErichKeane said:


> I posted an amazon link to one above.



Thanks Erich, I did see that. I bought the tap I have from the same company.
I'm going to try your suggestion.


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## DavidR8 (Apr 23, 2021)

Just spent some time in the shop.
Using a new piece of stock I drilled a hole with a 29/64 (.453) bit. 
I started the tap in the mill and went till it stalled. I left it in the stock and removed the tap from the chuck.
Put it in my bench vise and using my 12" Patience and Nicholson tap wrench I went at it. Definitely tough going for the first part of the tap. I could see the tap twisting and I feared it might snap. Lots of Tap Magic and I made it through!

So I think that starting with the 7/16-10 tap on a 13/32 hole ( the size spec’ed for the 1/2-10 tap) will give me a good chance of tapping the hole and achieving the full thread depth.


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## WCraig (Apr 23, 2021)

I see about 8 threads in your test piece.  I believe I read somewhere that a nut develops maximum strength in only 4 threads, give or take.  I'm pretty sure they were referring to 60 degree V threads but I can't imagine that acme threads would need more.  So you could likely clearance drill part way through your stock so you have less to thread.

Also, your test piece doesn't seem to have been countersunk.  If you are going to hand tap your vice jaw, the bevels may make it easier to get the tap started.  Or not; I don't have any hands on experience with acme taps.

Craig


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## DavidR8 (Apr 23, 2021)

Thanks Craig, this piece (and the eventual vise jaw) is 1" thick.  I thought about countersinking, the starting end of the tap is about .450" so slips right into the hole. Would countersinking ease the entry of the rest of the tap?


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## DavidR8 (May 3, 2021)

Picked up my Acme rod this afternoon. 
The test threads I tapped provide an excellent fit with minimal play.


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