Tap drill size for 1/2 -10 Acme

[DavidR8]

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?

 

[FOMOGO]

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

 

[matthewsx]

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

 

[DavidR8]

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?

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.

 

[FOMOGO]

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


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Tap drill size for acme taps​

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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.

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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"

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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]

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

 

[mikey]

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 ...

 

[John O]

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

 

[WCraig]

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

 

[DavidR8]

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?