Undersized taps?

[lethal375]

I have been researching undersized taps, particularly 10-32 spiral for blind hole tapping on a CNC lathe

The hole I am tapping is for a male threaded part that is made (not by me) to the low/small side of the spec but I need a tighter fit so when assembled they are more parallel, straighter, less slop

The way I understand, H1 thru H4 are .0005 per
H1 is .0005 over and H4 is .002 oversize
"H" means Ground Higher or Larger

"L" means Ground Lower or Smaller

When searching L1 thru L4 I am finding nothing?

L2/.001 under would be great

Any guidance or info is greatly appreciated

 

[guythatbrews]

Are you gaging the thread currently? You might find the sf tap is cutting way oversize. Hi spiral sf taps meant for soft metals will do that in steel. Had to cut lots of 3/8-24 lh in 8620 and the readily available 45 degree helix sf just wouldn't cut to size. 15 degree helix worked great.

Seems u/s taps may be specials. Sucks if you just need one.

 

[lethal375]

Are you gaging the thread currently? You might find the sf tap is cutting way oversize. Hi spiral sf taps meant for soft metals will do that in steel. Had to cut lots of 3/8-24 lh in 8620 and the readily available 45 degree helix sf just wouldn't cut to size. 15 degree helix worked great.

Seems u/s taps may be specials. Sucks if you just need one.

I am tapping brass

Tried several different setups to see if alignment is causing oversize and seems about the same

 

[guythatbrews]

I am tapping brass

Tried several different setups to see if alignment is causing oversize and seems about the same

Are you checking the tapped hole with a thread plug gage? Using tension/compression tap holder or sync tapping?

Might try tapping one by hand to see if it's any tighter. If it is something may be wrong with your setup.

 

[PeteM]

Is it possible to have mating features so parallelism is maintained and use a thread locker? As an example, a loosely threaded lathe chuck isn't designed so the threads provide alignment, but rather the mating surfaces. If possible, this will be a better design than trying to control thread fit.

If a better design isn't possible, it might be that your taps have too high a positive rake and are excavating a bit more of the thread than desired? Might try blunting the tap's cutting edge a bit by hand before resorting to a custom tap.

 

[guythatbrews]

So I re-read your initial post and I'm not sure if you want just a closer fit or line-to-line or maybe even a slight interference fit. The latter two sound like you will need u/s tap. I don't know if anyone stocks them but I suspect not. You don't mention the quantity or thread depth. Is single pointing an option? I've never done a 10-32 but may be doable in brass. PH horn will probably have a tool in stock.

 

[EPAIII]

Just a wild idea, but we sharpen files with acid. Why not dip a tap in an acid bath for a few minutes to make it smaller? Not the easiest thing to control, but you could try a minute or two and then tap a test hole. Continue until you get the fit you want or the tap no longer has any teeth.

 

[lethal375]

Is it possible to have mating features so parallelism is maintained and use a thread locker? As an example, a loosely threaded lathe chuck isn't designed so the threads provide alignment, but rather the mating surfaces. If possible, this will be a better design than trying to control thread fit.

If a better design isn't possible, it might be that your taps have too high a positive rake and are excavating a bit more of the thread than desired? Might try blunting the tap's cutting edge a bit by hand before resorting to a custom tap.

The threaded pin needs to be adjustable so thread locker is not an option.
Again I do not make the male threaded pins so I cant control their size but they do seem consistent size.

Female thread is only .375-.500 deep and the pin is about 2.5 long so excessive clearance makes the pin flop allover out at end

Using floating tapping head and I am sure there is a slight vertical misalignment until started.

Does not seem to effect size much, because tapping in the same material using any other means produces the same result

What about thread forming taps?
Never used them before

 

[TeachMePlease]

What kind of machine do you have? If you used a threadmill you can make whatever size thread you like.

 

[spaeth]

Lethal375
Try a smaller drill, for 10-32 thread you have a couple choices. #20 is.1610, #21 is.1590, 4mm is .1575 and #22 is .1570. Also for brass a straight flute tap will work, 2 or 3 flute. Take a honing stone and stroke the flutes a few times to give a slight negative rake on the face of the cutting edges. Also make sure your hole is straight to start with and not oversized. Try cutting oil, dry and wax see which gives you the best result. There are taps for brass only and they come with a zero or negative rake.
Better quality taps make better quality threads and last longer.
spaeth

 

[cyanidekid]

Lethal375
Try a smaller drill, for 10-32 thread you have a couple choices. #20 is.1610, #21 is.1590, 4mm is .1575 and #22 is .1570. Also for brass a straight flute tap will work, 2 or 3 flute. Take a honing stone and stroke the flutes a few times to give a slight negative rake on the face of the cutting edges. Also make sure your hole is straight to start with and not oversized. Try cutting oil, dry and wax see which gives you the best result. There are taps for brass only and they come with a zero or negative rake.
Better quality taps make better quality threads and last longer.
spaeth

5/32 is .1562, and sometimes you can get better choice and prices going with a standard fractional sizes as well. I usually use that for 10-32, except in tough material to tap, like 316 stainless. 10-32 is pretty easy to tap, I haven't had a problem with 5/32 drills.

 

[Fasturn]

As the other poster said, Nitric acid for a few seconds. Did this with Taps And Reamers.