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whats wrong with the thread I taped? Answered

I drilled a 3/8 size hole, taped it with a 10 x 0.5 mm plug tap. Than die taped a 10 mm size rod with a 10 x 0.5 die. I did it all on the drill press so it is at a 90 degree angle.

The problem I am having is:
The Rod is a little loose in the threaded hole, It won't slip out it is just loose. So why is this?



Whilst I agree about the JPL issue, we DO mix units in our stuff all the time !

M10 x 0.5 is a pretty fine thread, so its susceptible to small errors,

Taps come in many varieties. I would never use a plug tap in a fine pitch thread

If the tap isn't VERY sharp, it will cut over size anyway, in aluminium, so we would generally drill 0.1mm below the recommended tapping size

3/8 = 9.525mm, and M10 x 0.5 tapping is going to be 9.458, so you are 67micron out to begin with, thats nearly 12% of your thread depth.


What kind of tap would you recommend to use?
The plug is the only one that i could find that was 10 x 0.5 mm.

The tap is brand new so it should be sharp, I don't really know how to gauge the sharpness of it.

So I need to drill the hole with a 9.5 mm size drill bit?

You should definitely use a taper tap, for preference then.

A machinist answer is that you can't drill an accurate hole, you have to drill undersized and ream to exact size : O If the center of your drill bit is a little off or the tip wandered, that could do it.

Is the rod 10 mm? Just because that's the nominal size doesn't mean that it's accurate.

Which type of tap and die set are you using? There are several different types that will give more or less space to allow for dirt, rust, plating, ect.

Take a look at the threads. Do they have a flat spot on the top? If so, the rod or hole was the wrong size-the bigger the flat area, the less the threads mesh (but only 50% of the TOTAL thread needs to mesh for 80-some percent of the total strength of the connection-that's 3/4 cut on each side).

The chances are your die was too tight in the die holder, if so it would easily cut the rod too small for the threaded hole I have done it myself many times when I first started out in engineering.
Try setting your die in the holder just tight enough to hold but not enough to squeeze, it is really a matter of practice & of course getting used to your tools, I have found that even dies from the same maker can differ slightly in how you set them so new tools can be a real pain at times.
Another possibility could be insufficient coolant especially if you are cutting under power, the heat can cause tearing of the thread peaks & as a result cause slackness in the threads, always make sure you have plenty of lubrication on you tool & workpiece.

I did not think that having the die stock to tight would affect the cut. I thought I should tighten it down as tight as it would go, to prevent it from slipping. It was also in a vice so that would have made it even tighter.
What type of lubrication do you recommend to use? I read on a form that it is ok to use dish washing soap for cutting, so I have been using that.

Tapping fluid is dependent on the material. I think steel uses a sulphur based oil...tapping fluid for aluminum seems more like a soap, but I can't recall what it is exactly. I often use 3-in-1 oil because it's handy and since my tool bag was ripped off in a car break in a few years ago, I don't have any anymore, and I can't really afford to replace all the stuff they got. (About $2500 worth of hand tools, a brand new portable Makita drill kit, fluids, tapes, etc.). Anything to lube the hole is better than dry. I was taught to go slow in aluminum and brass, back it out every few turns to remove debris, because the debris can act as a grinding agent, compromising the threads in these soft metals.

The stuff we use is kerosene for aluminium. Brass works best dry. We use something called Rocol RTD on everything else, and sometimes on ali.


The die should be fastened by the split, tightly into the holder, then the outer screws tightened down. The tap should be its MAXIMUM size, not minimum as you start. If the thread is tight, you adjust the holder.


I think your 3/8 bit was just a tiny bit too large. That's what the chart I check showed. And your hole might have been slightly over 3/8 so the threads in the hole are not fully formed leaving the rod a little loose.

Best Answer. Mixing U.S. and metric is always a recipe for disaster. Just ask JPL :-/

lol... (said with a frown and furled brows). I ran into that all the time when I was tat NASA. I am a lover of the imperial measurements I think in feet and pounds and so forth for every day stuff, wood working, cooking, etc, but I was raised to do all true science in metric. Time and time again I'd encounter people at NASA who talked in imperial (ex: 32 ft/sec2 instead of 9.8m/sec2). Our facility barely had any metric tools, and the old school techs for the most part refused to use them. Funny thing is, they seemed to rule the roost and the scientists would almost inevitably acquiesce to their constant balking...

There are three basic types of taps. Close, normal and loose fitting. (although they have actual names I can't recall) The incorrect selection of the tap will cause behavior unexpected.

Taps require EXACTLY the right drill bit prior to tapping.

Proper tapping requires lubricant, to avoid both freezing of the tap in the hole, but also to prevent excess material from being removed.

Tapping a hole requries a steady hand. Movement off axis will cause deformation of the thread.

Aluminum is highly suceptible to poorly tapped holes.

Any of the above can lead to the situation in which you find yourself.

I'd suggest that you get some locktight.

.  To summarize what others have already said: everything has a tolerance. Your hole is not exactly 0.375"; the rod is not exactly 10mm; the tap is not exactly 10mm; the die is not exactly 10mm.
.  To do precision work, you need really good tools. Many DIYers have tools that are just good.


7 years ago

I will assume that the tap and die are from the same set and that the sizes were accurately milled and identified. That leads me to the conclusion the the diameter of the rod is slightly undersized. Take a close look at the threads on the bolt. The threads should have a nice sharp edge. If they appear flattened, then the rod was slightly undersized and will result in a loose fit.
The only other alternative that comes to mind is that the tapped hole is too large.