Big Tap

Baithog

Baithog

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I am planning a spindle mounted ER40 chuck for my G0602. The thread on that spindle is 1 3/4-8. My little internal threading tools won't cut deep enough to do 8tpi, so I was looking at threading bars and inserts. Well, that frizzed up the short hair. IR16 tool holders that I found won't fit in my tool post, even if I could justify the cost of the bar and insert for a one-off.

I discovered the world of special taps last year. A quick Ebay search and I found a 1 3/4-8 NS tap for half of what the bar alone would have cost. Using a tap should also eliminate making a spindle copy to use as a thread gauge during single pointing.

So has anybody used taps this big? The plan is to use my spring center in the tail stock to get it started straight. But how much force is it going to take once the tap is fully engaged? Hopefully it won't spin in the chuck or need 2 feet of pipe on the wrench to turn it.
 
I WOULD NOT subject locking the back gears or the gear head to the twisting force needed to propel a large tap. You will VERY LIKELY break off some gear teeth. I TRIED that back when I had threaded spindle lathes,like my Jet 10x24". It had decent size back gears,too. FORGEDABOUDIT!

I have a large tap wrench about 4 feet long,if that gives you some idea of the torque needed to twist a big tap like that. It would really harm your lathe to subject it to that much force. Remember,regardless of how big your gears are,only 1 or 2 teeth are engaged. Even a locking pin would not stand the force.

If I were you,I'd still single point thread the holes. Maybe just use the tap to clean out the last few THOUSANDTHS from the threaded hole. After REMOVING the piece from the lathe. But,I'd be afraid I'd lose some concentricity even at that.

I will also assure you,that even if you could use the tap with the tailstock center,the threaded hole WILL NOT be perfectly concentric in the hole. Much experience with doing that with smaller taps has long ago taught me that. The only way to get the hole threaded concentric is to single point thread it.

You'll never be threading a large number of back plates. Just take the time to single point them. I will warn you that it took my friend Jon TWO YEARS to get parts for his quick change gear box from Grizzly. And,I can assure you,the best metals are NOT USED in Grizzly machines.
 
First of all, it will take a Gorilla to twist that tap to create the thread you are wanting to cut.

Next, the tap is made with a "limit fit" so the thread it creates will be tight or sloppy with the mating thread it will screw into, depending on its class of fit.

Without knowing what your spindle thread was cut to, pitch diameter wise, you could have a very tight thread or a very loose thread. You hardly get lucky and get it in the middle where it is a "just right" fit.

What's wrong with grinding a piece of HSS to thread with? That s how most of the old threaded back plates and chucks were threaded with.

You have to single point the thread to near finish size before running the tap to take out that last few thousandths to get it right and not require a six foot cheater to twist the tap with.

The reason for single pointing the thread is getting the alignment right with the face of the backplate so the chuck will not wobble with runout.

EDIT : George beat me to it!
 
Thanks to the 2 of you.

George, I don't have a back gear on this lathe. I wish I did. I lock the spindle with a 5/16 drill rod in a hole in the spindle nose. That isn't going to work.

If I was smart about this I would go ahead and get the internal threading bar and turn or mill down the shank, But I'm trying to be frugal in my retirement. My wife says I'm stupid cheap, as opposed to being just cheap. I have a 1/2" bar around here some place. It takes a square HSS tool bit. I think I'll grind a thread tool for it. That will kill a little time. My metals order is back ordered. It was supposed to be 3 days, but its 5 days late at this point. All of my projects are on hold for this or that piece of metal. I even went out and pulled weeds out of the flowers yesterday.

I'm sure glad you guys straightened me out, or I'd have to hear about how dumb it was to by a big tap.
 
I did the same thing for my G0602 except I bought a threaded backplate from Grizzly. It is a replacement part for the 4-jaw chuck that came with the machine. Most of the machine work is done and it fits the spindle well.

P0602054-2 for $31.25. In stock at Grizzly. Freight was about 10 bucks.


CORRECTION! That backplate wasn't in stock at the time I needed it. I found another one that was larger but it fit my spindle. I had to turn it down which was a trick, but I did it.

http://www.tools4cheap.net/proddetail.php?prod=134back8in
 
Last edited:
NightWing said:
I did the same thing for my G0602 except I bought a threaded backplate from Grizzly. It is a replacement part for the 4-jaw chuck that came with the machine. Most of the machine work is done and it fits the spindle well.

P0602054-2 for $31.25. In stock at Grizzly. Freight was about 10 bucks.
Click to expand...

That is a "No Brainer!"

I've bought a couple back plates from LMS that have 1-1/2-8 thread for my SBL, perfect fit! I can't buy the cast iron for what they get for them semi-finished ready to fit to your chuck.
 
4gsr said:
That is a "No Brainer!"

I've bought a couple back plates from LMS that have 1-1/2-8 thread for my SBL, perfect fit! I can't buy the cast iron for what they get for them semi-finished ready to fit to your chuck.
Click to expand...

I edited and revised my original post. See above. It cost more but it did the job.
 
Hi, Larry,

You can put a tick for me in the George and Ken column. I'd go with the boring bar and single-point solution.

Jim
 
You will the your lathe over before you get the tap in.
Make or buy a very stout boring bar. Put in a threading tool. When you get close, take off your Chuck with the part still in it, and try the part on your spindle.
It worked for me.
Steve in Santa Barbara
 
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