Tapping question ??...

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56type

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Having a bit of trouble tapping and was hoping I could get some clarification on where I'm going wrong...this is my first machining project...

OK, making a muzzle nut which has an internal thread of 14 X 1.0mm LH. Got the hole drilled on the lathe (Atlas 10100) with a 0.50 cobalt drill bit, which by my math comes up to 12.7mm when converted to metric. I have a metric tap in the needed 14 X1.0mm LH size and aligned it using the lathe tailstock so it would start straight & square to the hole.

Problem is the tap doesn't want to start ?? I'm NOT trying to tap using the lathe's power...It's a small Atlas with a screw-on chuck, so no running it in reverse for left-hand thread. Also at this time I have not bought any tooling for threading, so I won't be threading it on the lathe. To top it off the drill chuck I'm using to hold the tap keeps wanting to unscrew itself from the MT shank that mounts it in the tailstock, even doing the tapping by hand (lathe unplugged, turning 4-jaw chuck in reverse by hand while holding drill chuck with other hand to keep it from turning).

Am I correct in thinking the hole is possibly to small at 0.50in. (12.7mm) ?? I checked & re-checked the measurements and it does measure at 12.7mm, so I was thinking maybe I'll have to bore it to 13mm to get the tap to start ?? Guess what I need to know is what size hole for 14 X 1.0mm LH tap...Thanks
 
What type of tap are you using? Taper, Plug or Bottoming? Ideally you want to make sure you are using a Taper Tap as it will be the most easy to get started. A plug tap will work but it has less of a taper, so it will be harder to start. A bottoming tap will be next to impossible to start.

Yes, at 12.7mm you are smaller than the recommended tap drill of 13mm.

Here's a Metric Tap Chart. http://www.kasthurimmc.com/tap-drill-chart.html
 
It may in fact be a bottoming tap since it doesn't have very much of a taper, maybe one or two threads & shallow taper...

Guess I'll pick up a 13mm drill bit and try again with the larger hole size. Thanks.
 
brino said:
I'll add that a chamfer at the start of the hole is also a good idea.
-brino
Click to expand...

The tap did accomplish that quite nicely !! Wouldn't start but did chamfer the the edge of the hole. Hoping that will help some when I go up to 13mm and try the tap again.
 
I should think that a 33/64" drill would be good enough (13mm = 0.512"; 33/64 = 0.516"). Personally, I would go a step bigger and use a 17/32" drill to be sure I can run the tap in steel.

As Brino says, chamfer the hole before starting with a taper or plug tap and it should go okay.
 
mikey said:
I should think that a 33/64" drill would be good enough (13mm = 0.512"; 33/64 = 0.516"). Personally, I would go a step bigger and use a 17/32" drill to be sure I can run the tap in steel.

As Brino says, chamfer the hole before starting with a taper or plug tap and it should go okay.
Click to expand...

My drill bit inventory is embarassingly small & consists of one 25pc. cobalt set up to 1/2in. and a Dewalt set with pilot tips up to 1/2in. Just ordered a 25pc. HSS metric set from amazon a few minutes ago though that includes the 13mm bit.
 
Are you using the lathe tailstock to push the tap into hole or just by hand? I would think your half inch hole would work, but might produce a more than 75% thread.

Sent from my SM-G930P using Tapatalk
 
56type said:
The tap did accomplish that quite nicely !! Wouldn't start but did chamfer the the edge of the hole. Hoping that will help some when I go up to 13mm and try the tap again.
Click to expand...

56type, If it is a bottoming tap, you can easily taper 3 or 4 of the first threads on the tap with a fine wheel on a bench grinder. I've taken many taps that broke up where the full threads are and tapered the first few threads and they worked fine. Good luck, JR49
 
carlquib said:
Are you using the lathe tailstock to push the tap into hole or just by hand? I would think your half inch hole would work, but might produce a more than 75% thread.

Sent from my SM-G930P using Tapatalk
Click to expand...

That was the initial plan but the tap refuses to engage... Even advancing the tailstock into the workpiece to maintain pressure and keep the tap straight & square to the hole didn't help, it just chamfered the edge of the hole but wouldn't advance any further. Since it is LH thread when pressure was increased from the tailstock it just started to unscrew the drill chuck from the MT2 shaft. I wouldn't have thought the difference between 12.7mm and 13mm would have made that much of a difference as far as starting the tap, however the tap seems to notice it enough to refuse to start.

I'm thinking of using the high-strength (red) Loctite to lock the drill chuck to the MT2 shaft in order to use it for starting LH threading with taps. Since starting a LH tap causes the drill chuck to unscrew from the shaft.
 
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