View Full Version : Going CRAZY!!!!

01-28-2008, 03:52 PM
I am being driven crazy by a problem. I am making a back plate to fit a new a chuck for my lathe. Below are sketches of the different parts. I am trying to get the newly made back plate to fit the 2¼”-8 threads on the lathe shaft. To aid in the machining of the new back plate, I made plug that screws into the newly made back plate. As test, I used an existing back plate from a chuck that currently fit the lathe shaft as a check. The plug I made freely screws into BOTH the existing chuck back plate AND the newly made back plate. However, if I try to screw the newly made back plate on to the lathe shaft, it makes a half rotation and then locks up. Below are the dimensions I have measured of the parts and there shouldn’t be any interference.

One would think that since both back plates fit on the plug, both would fit on the lathe shaft, but only one fits. I can not think of any reason why the newly made back plate would “lockup.” There no chips in the threads, and I have tried three times to cut the threads deeper but it still does the same “lockup” routine.

I’m at wits end. Any suggestions would be greatly appreciated.




01-28-2008, 04:09 PM
There has to be something wrong with the thread in the new backplate.All things being equal,it should have fitted.If it is the correct pitch,perhaps you could relieve the peaks of the thread a little.That's about all i can think of.

01-28-2008, 04:14 PM
Threads mate on their pitch diameter, assuming identical thread angles. The PD of a thread doesn't have a lot to do with the major diameter on an outside thread or with the minor diameter of an inside thread (ignoring the case where the PD of the thread is not in the thread cut - in that case you simply have an oddly shaped piece of metal).

To match a spindle you're want to start by making an identical thread on a plug using some way of measuring the pitch diameter. Measuring pitch diameter is often done using precision wires or triangular gages or, if your finances allow, thread micrometers. When you have 2 threaded plugs with the same measured pitch diameter they'll both fit anything one fits. If one doesn't fit then it's time to start looking at thread crests and valleys.

So I suspect that you didn't replicate the thread pitch diameter for your test plug and are working with inside threads with different PDs.

01-28-2008, 04:27 PM
I can see the problem, the dimentions don't match. You have to make the plug shaft the same OD and then use the wire system to match the pitch dia. on both the lathe shaft AND the plug gage.

01-28-2008, 05:42 PM
You don't mention thread wires, so I imagine you don't have any. The proper size for 8 tpi is .072" (according to my set). If it's a sharp thread 1/16 is too small, but I expect neither the nose or your your plug are a sharp top thread, so some welding wire, or three same size drill bits work in a pinch. You don't need to actually know the pitch diameter, just check to make sure they are the same.

Other things that come to mind, improper angles on the threads. I don't know if you used store bought threading tools or not. Next is different shapes in the thread root of the nose vs the plug and your back plate.

01-28-2008, 06:41 PM
2.25" x 8 tpi was a giant hurdle for me just a short time ago. I spent more on material for that 1st backplate than I could have bought one for already threaded!

Thread wires 1st. If you don't know the diametral pitch (DP) of your spindle you're only hoping you get the stub right. They only cost $20 or so and a set will do from 4 to 40 tpi. They're a bit fiddly but as was pointed out, the major and minor dia's aren't what you need to get right. The DP is where the rubber meets the road.

The other thing is having your bit on center. Raising it slightly helps with negative rake but it also effectively widens the bit angle.

An accurate bit ain't easy either. Probably overkill but I wound up lighting up a piece of white paper on some glass placed on 4x4 blocks with my drop lamp under the whole mess to SEE that damn angle! LOL! That helped.

A fish gauge for grinding an accurate angle and aligning the bit to the workpiece worked a lot better for me too.

Hope that helps in some way. Believe me, I been there ....


J Tiers
01-28-2008, 10:55 PM
Small details make a difference.

A sharp V on a "standard" thread will NOT go even if they are both correct pitch diameter, because the crests interfere.

I have a Logan with a 1 1/2-8 thread spindle. The faceplate that came with it is a Southbend with a 1 1/2-8 thread in it.

They did NOT fit together, the faceplate would lock up in a turn or two (obviously the P.O. didn't use it, if he even knew how).

I had to "scrape" the threads of the faceplate a bit to allow it to screw on, even though it was nominally the same size as the nose.

01-29-2008, 06:46 AM
Thanks for the tips. No, I have neither thread wires nor a pitch micrometer but I guess that is somenthing I will have to look into. The thread wires will work on the outside pitch diameters but what is used for internal threads to determine pitch diameter? Is a special micrometer made for that?

Any susgestions on how to salvage what I have or do I just have "scrap metal" and need to get a new blank and start over?


01-29-2008, 08:09 AM
I was never able to salvage any of my 1st attempts at a backplate. ENCO has 6"x1" rounds of cast iron for $15 .....


01-29-2008, 09:58 AM
I don't know of any measuring instrument to measure internal pitch diameter but there may be one.

The accepted way is to make a plug gage with the EXACT dimentions of the threaded shaft that you are making an internal thread for.

I always look up the thread size in Machinery Handbook to get the correct OD of the thread because the shaft you are measuring the thread from may not be the right OD. Then I measure the pitch dia. of the thread I am copying.

I then cut the plug shaft with the correct OD and pitch dia. and then try an existing internal thread on the new plug gage before removing it from the lathe if posible. If it don't fit I try filing some off the OD of the plug and test the fit again. Sometimes I use a thread file to clean the threads. Although I can remove the plug and rechuck the plug and set up on it I don't like to have to do it. It takes a lot of time to get it right to recut the thread.

Now you can set up the work to cut the internal thread and everything should work fine. Please understand that if you are making a class fit thread it is very important that all dimentions are correct and to standard specs. listed in the Machinery Handbook.

Paul Alciatore
01-29-2008, 10:52 AM
The first thing I would do is use a thread gauge to check the pitch. If the pitch is different, no amount of cutting will make it fit.

The above comments about making the plug to exact dimensions are right on. Pitch diameter or the equivalent measurement with thread wires is the first thing. You don't need the exact diameter wires, just something close,but good quality stock with a consistent diameter. Use them on the spindle threads and on your plug until the plug is exactly the same measure or even just a bit (like 0.001") bigger to be on the safe side. Then debur it and check the OD. Again, the same size or just a thousanth bigger. It looks like your plug is already too small so you need to toss it and start again on that. Also, when threading the plug, make sure the tool leaves the same flat in the valley between the threads. If you use a sharp Vee tool, there is no flat and it could give deceptive results. Again a larger flat would be on the side of the angles here while a smaller one would allow for failure.

Deburring is also important: depending on material, I ususlly use a wire brush (steel, brass, or?) to debur any threads I have cut. On fine threads a bit of Scotch Bright or the industrial version thereof works well. I have three different grades of this stuff (McMaster). You would be surprised how much trouble an almost invisible burr can be in fitting threads. Debur first, then check.

01-29-2008, 11:42 AM
From your description of the problem, I am thinking that although your test plug may screw into both the old and new adapters, I bet one has a looser fit than the other. Your dimensions verify that as well. The one you are making and trying to fit stills needs some reduction in diameter.
Most likely not much, but some. Do it carefully or else it is suddenly going to screw on easily and be to loose a fit. I have had this same problem before myself and it is always solved by a careful final approach of several final turnings of as little as .001 removed.
I do not own or use the wires with my mic either, nor am I concerned with pitch diameter when I am trying to match an existing thread. All you have to do is thread a test plug that fits both pieces with the same class of fitment. I set the compound at 29.5*and only advance the compound screw. I also use a formed thread insert...........pg

01-29-2008, 02:03 PM
i would take your spindle plug gage (the one you made) and lay it on top of the actual spindle threads so the threads interlock and see if they mate. i'm not sure how to explain what i'm talking about, but i think you know what i mean. my guess is you will see right away where your error is and MAYBE you'll be able to fix it using the pieces you already have. if not, you'll be cutting some more threads. :)

andy b.

01-29-2008, 06:46 PM
You need to make a new plug gage as I described and then your going to have to rechuck the new face plate and set up on the thread and recut it. I don't think you are qualified to do that so you will have to make a new face plate.

01-29-2008, 07:45 PM
I recently did the same job and posted some photos here. I had the same problem. The plug fit but the plate wouldn't screw on my lathe. After carefully inspecting the internal threads on my new plate I decided it was the sharp crests causing the problem. I rechucked it in the lathe and took a small cut to remove the crests. It threaded on perfectly.From your sketches it looks like you made your plug too small.I threaded my plug until it JUST fit into my original plate. If you make it smaller(in your case you made it .012 smaller) the internal threads on your new plate will be smaller. You may need to rechuck it in your lathe and cut a few more thou out.Rechucking zeroing and lining up your threading tool isn't a big deal. I'm sure you can accomplish that. If not there are lots of people here who will be eager to help.