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M5x.5 tap drill size?
- Thread starter Erstwhile
- Start date
- Replies 25
- Views 7,210
BugRobotics
Stainless
- Joined
- Jun 22, 2015
- Location
- Denver, CO
Good rule of thumb that works pretty well for metric taps is take the tap size and subtract the pitch to get the tap drill size. So a 4.5mm drill in your case.
tobydbaker74
Aluminum
- Joined
- Sep 8, 2018
- Location
- Columbus
For a guide you can ...let's use a 1/4-20 for example. Take .250 and subtract depth per revolution which in this case is .05 (1÷20) . Take .250 - .050 gives you a drill size of .200
Do the math on any tap size and will get you within .001 of what it calls for.
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tobydbaker74
Aluminum
- Joined
- Sep 8, 2018
- Location
- Columbus
tobydbaker74
Aluminum
- Joined
- Sep 8, 2018
- Location
- Columbus
Just curious... for just say a 1/4 20 2A you would use less than a .190?? I feel like I would be breaking alot of taps??
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I would hope not, considering the minor dia for a 1/4-20(which is a 2b not a 2a btw) is max.207- min.196
where did you come up with that number?
oh forget it, you looked at the external thread data on a 1/4-20 thread and came up with your number which is a .1894 Max minor Dia.
FYI there is a huge difference between external data on thread and internal data on a thread. This is why you have to pay attention to what your looking at,
Street
Hot Rolled
- Joined
- Apr 16, 2015
- Location
- Melbourne Victoria Australia
according to machinerys handbook
M5 coarse ...drill 4.2mm recommended gives 81 percent engagement
or 4.3 giving 71 percent engagement
does not state the pitch though only coarse thread.
if its fine metric fine then it no applicable to that.
i just checked the chart i have P and N chart
lists coarse as 5 x 0.8 so no good to you in regard to the above information
need to keep looking, ring up the tap supplier to see maybe the best bet
M5 coarse ...drill 4.2mm recommended gives 81 percent engagement
or 4.3 giving 71 percent engagement
does not state the pitch though only coarse thread.
if its fine metric fine then it no applicable to that.
i just checked the chart i have P and N chart
lists coarse as 5 x 0.8 so no good to you in regard to the above information
need to keep looking, ring up the tap supplier to see maybe the best bet
tobydbaker74
Aluminum
- Joined
- Sep 8, 2018
- Location
- Columbus
Yes you are correct I'm getting old and looked at the wrong chart.
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DMF_TomB
Diamond
- Joined
- Dec 13, 2008
- Location
- Rochester, NY, USA
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i believe thats for 75% thread and yes its commonly done. just look at tap drill chart, usually tap drill size is close to that rule of thumb. drill bits can easily drill oversize especially if badly sharpened. just saying 4.5mm drill "might" drill bigger than 4.5mm. fine threads got to watch for that even more
Screwmachine
Titanium
- Joined
- Mar 8, 2001
- Location
- Switzerland
BugRobotics got it, see image from DC Swiss site- but I will second what Tom said about accurate drill size, you can lose a lot of thread if your drill goes oversize just a little at a fine pitch like that.
Peter from Holland
Diamond
- Joined
- Jan 15, 2005
- Location
- The Netherlands
Nibleswick
Aluminum
- Joined
- Dec 31, 2019
I'm assuming this is for cut taps. How would this rule of thumb look for roll or form taps?
tonymor
Aluminum
- Joined
- Feb 9, 2017
- Location
- Chambersburg PA USA
The wonderful world of metric threads. It doesn't just work for taps. Boring for an M80x2 thread? Ø78 will do just fine
Vancbiker
Diamond
- Joined
- Jan 5, 2014
- Location
- Vancouver, WA. USA
Same rule works for any 60 degree thread, metric or imperial. I haven't used a tap drill chart in 40+ years since learning this little tidbit.
DMF_TomB
Diamond
- Joined
- Dec 13, 2008
- Location
- Rochester, NY, USA
not only a badly sharpened drill but a drill bit that hit a hard spot of slag can get damaged and drill oversized holes.
.
i have even seen carbide drill damaged drill holes .040" oversize before. usually its best to inspect parts and look at threads and see if internal threaded holes look ok or that look like a oversized drill was used. worst case is to have damaged drill with 60 minutes tool time left, drill hundreds if not 1000's of oversized holes on many many parts.
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most blow out holes and what better time than to look at threaded holes right after ? I never assume drilled holes is same size as drill bit from decades of experience.
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i have even seen carbide drill damaged drill holes .040" oversize before. usually its best to inspect parts and look at threads and see if internal threaded holes look ok or that look like a oversized drill was used. worst case is to have damaged drill with 60 minutes tool time left, drill hundreds if not 1000's of oversized holes on many many parts.
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most blow out holes and what better time than to look at threaded holes right after ? I never assume drilled holes is same size as drill bit from decades of experience.
Bobw
Diamond
- Joined
- Feb 8, 2005
- Location
- Hatch, NM Chile capital of the WORLD
Pitch diameter plus a few thou.
Take the tap size and subtract 1/2 the pitch.
Check out the tap charts. The ones I checked all followed that formula
