Having trouble power tapping

I ran into something lately that is strange. After years of power tapping in a lathe, all of a sudden my tapped holes are stripped.

I use a tap handle or a crescent wrench against the carriage and put a live center in the end of the tap. Slow (65 rpm) and it looks good, plus the tap is tight coming out. These rods that are giving me problems are 1" to 2" diameter x 2 ft long. Could it be that the end that is up inside the spindle is moving and toggling around? These are chrome cylinder rods and I have sandpaper between them and the chuck jaws to prevent scratching. Maybe, the back end is flopping around. They aren't spinning or slipping in the sandpaper, I have that marked.

When you take the tap out and look the hole is tapered from the start of it and the threads are tapered. In about 1" deep the threads look ok. I measured after drilling and the hole is onsize.

I will figure this out and reply,

But, has anyone else had this happen before?

Let me know,
JAckal

What size are you tapping?

Are you tapping in the jaws?

Is it possible your chuck pressure is collapsing the hole a little?

This could result in a tapered hole when you took the part out of the chuck.

Is it possible your live center is not so centered after all?

If everything's in line on the machine I'd look at the tap,........ can you try another tap?

I second Limy - taps can look sharp to the naked eye, although a lens will often show them to be 'anything but' sharp

Your comment about sandpaper to avoid scratching chrome - I take it the abrasive side is inwards? I did a double take at first but I can sortof see how that would work to prevent gross marking, but does a lens show 'frosting' where the jaws clamped?

I've always just used plain paper, and I normally single-point thread in chrome situation, but for extra grip I have idly wondered about using thin rubber, perhaps from inner tubes ... bicycle might be better than car or truck, but if it's too thin I imagine the jaws would cut right through

taps

First a decent chamfer.A good (new ) tap ,taper and I'd go to a drill chuck in the tail stock.An old tap plus the possible side pressure of the wrench could be causing the problem .Tail stock tight.
jim

Of course you don't say what size the tap is, but if it is a necked down sized tap, but if it is a three flute necked down style - like 1/13 - you can lave sorta loose in the three jaw drill chuck and drive off the flutes. Once halfway in jump to a sqr socket/ratchet to finish/extract.

Option 2 is to put the sqr socket in the drill chuck and drive the tap. This offers plenty of float in all directions.


(If this was just covered by someone - you need to understand I started this post a few hrs ago. )


----------

Think Snow Eh!
Ox

KilrB, you got it.

I have never checked the height of the tailstock before. This lathe was bought by me 2 years ago, and did ok on everything until now.
It's a 1945 Monarch C.

With a coarse tap it works ok, but with fine thread like 3/4-16 or 7/8-14, or 1"-14 it goes to hell. It first started ( was noticed ) about 3 weeks ago on a 7/8-14 NEW tap. I bought another tap, different supplier, different brand, and it still was there. In the past the shafts were long & large enough they had to be used in a steady rest and that took out the mis-alignment and made the tailstock seem inline.< (just my guess).

I chucked a piece for PSQ 2 inch material and indicated it in. Then put an indicator on the newest live center I have. The center is ok.

Then I mounted an indicol on the live center & swept the 2 inch piece on the chuck.

It is out about .041.

The tailstock is .0205 low. Looking under the tailstock it isn't worn that bad, or the ways either. My suspicion is that the tailstock came off of another lathe. Knowing the people that this lathe came from & it was treated rough in a conveyor fabricating facility.

I really don't like the idea of shimming up the tailstock between the ways & the tailstock. My thought is put some shims between the base of the tailstock and the top part that offsets for taper .

This really is a cheap lathe and does ok for most stuff, but I don't have the time for a teardown right now. I have several cylinder rods, elec motor & pump shafts to make. It does ok on this. No taper, and yes I know the height isn't right , but I have ran a lot worse in the past.

So, what do you guys think about the shimming idea? For a temporary fix?

It looks like around June I may have another lathe, and then I can use this one for the roughing stuff, metal spray, sleeves, etc.

Thanks for the quick responses of help and suggestions,

You guys are great,

I appreciate it,

JAckal

PS. When I was in college, we had a 5/8-11 tap. It looked perfect. But, when you tapped a hole it was smooth from one side and appeared stripped from the other side.

THe instructor put it in an optical comparator and you could see it was messed up on one angle. He called someone he knew that used to grind taps. The guy told him the wheel dresser for prepping the grinding wheels had either lost a diamond, or was messsed up.

jackal said:

.. My thought is put some shims between the base of the tailstock and the top part that offsets for taper .

...
So, what do you guys think about the shimming idea? For a temporary fix?

...

Click to expand...

That's the fix. Hey, it's why there made in two pieces; isn't it? Shim it a little high ( 0.001"- 0.002") so the cutting forces bring it down.