Thread issue with QCTP adapter on 10x22 lathe?

[Frank O]

Sometime back I bought a Little Machine Shop AXA quick-change tool post for my Grizzly 0516 10x22 lathe (Sieg C6 equiv).

Recently I picked up one of these adapters billed as a bolt-on solution to mount the new QCTP:

https://littlemachineshop.com/products/product_view.php?ProductID=3712

Basically it's a hollow bolt with M8-1.25 threading on the inside, so that it screws onto the stock Grizzly post, and 9/16-18 threading on the outside, as required by the new QCTP.

For what it's worth, a shot of the stock Grizzly post, with the handle that goes on the top:



So here's my dilemma:

-- The new hollow-bolt adapter does *not* thread smoothly onto the stock Grizzly post. It goes about 1 rev and binds.

-- The stock Grizzly handle (in photo above) does thread smoothly onto the stock Grizzly post.

-- LMS suggested picking up an M8-1.25 bolt at the store, and seeing if the hollow-bolt adapter will thread onto it. It does, quite smoothly. In addition, the stock Grizzly handle threads smoothly onto the M8-1.25 bolt from the hardware store.

So I'm scratching my head why all the parts play together well, except for the new hollow-bolt adapter and the Grizzly post.

I should also mention that I noticed after buying the adapter that the list of compatible lathes on the LMS web page (see link at top) does not include the G0516 or C6.

Be that as it may, why would the Grizzly handle thread smoothly onto both the Grizzly post and the M8-1.25 bolt, and the M8-1.25 bolt threads smoothly into the new adapter, but the Grizzly post is a no-go with the new adapter? Are there thread sizes so close to each other that they would explain the above? I'm going to assume this adapter isn't going to work on my lathe, without significant surgery on either the adapter or the Grizzly post.

 

[markba633csi]

I would measure the major dimension of the M8 bolt vs the Grizz bolt. Maybe a significant difference in size. I would also mesh the two threads together and see if they nest or if there is daylight showing at one end or the other
You might need to chase the post threads with an 8mm die. I've made a "poor man's die" out of a nut by making a hacksaw cut in one side, squeezing it a bit in the vise, then hardening it by heating and quenching. Sometimes that works well enough for a single use, even without the heat treatment
-Mark

 

[Ulma Doctor]

another way of dealing with it is to drill out the threads in the handle, press fit a slug of bronze or steel and drill and tap for the size of the post threads.

 

[Frank O]

I would measure the major dimension of the M8 bolt vs the Grizz bolt. Maybe a significant difference in size. I would also mesh the two threads together and see if they nest or if there is daylight showing at one end or the other

Thanks, Mark. The M8 and Grizz bolt threads appear to nest together. There's a very slight amount of light getting through at certain angles which might have to do with the thread contours. But it looks like the t.p.i. is the same.

Putting a mike on the M8 shows a diameter of .312", whereas the Grizz bolt is .305". Since the Grizz bolt's threads aren't wider diameter, maybe they're fatter?

At this point I'm hesitant to try to recut the Grizz post's threads until I'm absolutely sure I know what I'm doing.

Frank

 

[Frank O]

another way of dealing with it is to drill out the threads in the handle, press fit a slug of bronze or steel and drill and tap for the size of the post threads.

Thaks, Mike, that'd be a good solution for the handle. But not clear that it would solve the basic issue of getting the QCTP mounted on its rest. Unfortunately, there doesn't seem to be a scrap of instructions anywhere. I could really use a complete idiot's guide to mounting quick-change tool posts today.

Frank

 

[markba633csi]

Must be the thread profile- your call which way you want to tackle it. Probably a tolerance stacking issue between the male and female parts.
Couldn't hurt to run an 8mm tap into the female part and a die on the male. That would guarantee a fit.
Years ago I invested in 6, 8, and 10 mm taps. They get used pretty regularly

 

[Ulma Doctor]

Thaks, Mike, that'd be a good solution for the handle. But not clear that it would solve the basic issue of getting the QCTP mounted on its rest. Unfortunately, there doesn't seem to be a scrap of instructions anywhere. I could really use a complete idiot's guide to mounting quick-change tool posts today.

Frank

i'm sorry i guess i misunderstood the whole crux of the issue.


i would proceed with a M8x 1.25 tap , then a M8x 1.25 die on the male threads as Mark suggested .

there could be a misformed or otherwise damaged thread causing the greif

 

[Frank O]

Must be the thread profile- your call which way you want to tackle it. Probably a tolerance stacking issue between the male and female parts.
Couldn't hurt to run an 8mm tap into the female part and a die on the male. That would guarantee a fit.

Sounds doable. I'm also thinking about taking an entirely different approach. Instead of using the hollow bolt to adapt the post diameter, LMS also sells a sleeve that sits on top of the compound rest, and which forms the base for the QCTP and its post. It's said to specifically work for the G0516:

https://littlemachineshop.com/products/product_view.php?ProductID=3787

This apparently would mean replacing the Grizz post entirely by a new post that apparently comes with the QCTP.

The only downside is that it's a little more work than the hollow-bolt approach. Two holes have to be drilled and tapped in the end of the sleeve to accommodate the hold-down screws for the compound's feed screw. There are also a couple of other points I need to get sorted out on how it would all be attached.

 

[matthewsx]

If you aren't looking for "a little more work" I think you're in the wrong place

Folks around here will use $10k worth of equipment and many hours of labor to produce a $2 part

Do it right and you will be pleased with the result I'm sure.


John

 

[markba633csi]

Whichever is the stronger or more rigid approach, is usually the best