Switching from 3 jaw to 4 jaw chuck

Usually, the backplate fit is not determined by the mounting bolts but instead, by a rim or shoulder that fits the plate to the chuck.
Therefore, the mounting bolt holes in the plate can be oversize or even slotted
Look for a pre-threaded blank backplate that fits your spindle then fit it to the chuck. Take another close look at your spindle so you know for sure what the thread is- 7 tpi sounds odd- could it actually be 8?
-Mark
PS good job on the wrench tool :cool:
 
Last edited:
The bolts only hold the adapter to the chuck. There should be what is sometimes referred to as a register on the chuck that the adapter fits into. This is a precise fit and is what keeps the chuck concentric with the spindle bore. Not the bolts. You don't need a mill or a precise drilling machine to drill the holes for the bolts. A drill press will work just fine. You might even be able to drill the holes with a hand drill using the chuck as a guide. To locate the holes for the bolts it would be nice to have a set of transfer punches. $10 or so from Harbor Freight. Transfer punches are really handy to have. You will use them often. The people who are really good at machining can transfer punch all 4 holes in one go. For us mere mortals I find it best to do one hole first. Bolt the adapter to the chuck and then transfer punch the remaining 3 holes.

If you haven't read it yet Mikey's very long thread on grinding tool bits will clue you in to everything you need to know on how to grind tool bits. https://www.hobby-machinist.com/threads/models-for-grinding-hss-lathe-tools.62111/ It is worth the time to read it. I have read it several times.

Post a picture of the back of the chuck. Even chucks/backing plates/adapters with threads have a register.

lathe adapters.jpg
 
I am turning down a piece of 1 1/2" square stock. One end will retain the square shape in order to fit a wrench to it. the other end will be bored out and fitted with a pin to connect it to a shaft. So I don't need to make a square shape.

When I removed the backing plate from the chuck, I realized the the plate was indexed (registered) to the chuck. I'm not totally sure about the thread count. I only laid a ruller on the threads, and counted the high points of the thread. Could be 8.

With all the work that I do in my shop, mostly automotive, I can't believe that I have never heard of transfer punches! Those things are greater than sliced bread! All these years I have been transfering holes using my brad point cobalt drill bits. Gotta get some of those!

My next dilema is finding some 5 1/4" diameter steel bar. I just called the local steel yard, who sells all their steel by the foot if you don't need an entire 20' piece. $123.00 for one ft.! Guess I'll be watching Craig's list and Facebook market place for steel, or just looking for a used adapter with the correct threads that I can modify the register to fit my chuck.


IMG_20200617_154718 (Custom).jpg

4 jaw is on the left

IMG_20200617_154731 (Custom).jpg
 
You might try contacting the machine shops in your area. They usually have scrap pieces left over from jobs. They might have what you need. I typically pay $0.50 per pound for scrap. I would also ask the steel yard who they may have sold some 5 1/4 bar to. That shop might have some scrap pieces. Speedy Metals sells by the inch. Shipping can be expensive. https://www.speedymetals.com/p-727-5-14-rd-cold-finished-1018.aspx

From looking at the picture of the chuck on the left it looks like it is threaded. Have you measured those threads and the ID? The picture you posted of the spindle still has the adapter on it that you have removed. Have you measured the threads on the spindle and the OD of the spindle? If you don't have thread gauges you should get both imperial and metric. The gauges are dirt cheap and you will need them.
 
Good idea! I didn't think of calling machine or fab shops to see if they have any small scrap pieces. I'll give that a try Monday. The picture of the 4 jaw chuck has the adapter that it came with setting on it. The chuck itself has no threads. I'm guessing that it is somewhere around 10 to 12tpi. Definitely not the threads on the spindle. The adapter that is on the spindle is for the 3 jaw. The adapter shown goes from 2 1/4 up to 2 3/4. I assume that I would want to make or buy a back plate that would thread to the spindle itself and not to another adapter.
 
You want to make a backing plate that screws onto the spindle and is a very good fit into the chuck. Basically identical to the existing backing plate but threaded to fit onto the spindle. You may get lucky here. First what is the OD of the register portion of the spindle and what is the OD and TPI on the threaded portion of the spindle. The OD of the threaded portion of the spindle may be smaller than the OD of the register portion of the spindle. I ran into this on the adapter that I made. Now if the threaded portion of the existing backing plate has a smaller ID than the OD of the spindle threads you may be able to bore out and thread the existing backing plate to fit. You will need to compare the major and minor diameters of the threads and the TPI on the spindle and the backing plate. It would be really nice if the spindle and the backing plate have the same TPI.
 
Last edited:
mriplaybass said:
OK, so I am just about to head out into uncharted territory, and start my first simple project. I thought that I had pretty well figured out what I needed to do, but all came to a screeching halt. I want to turn part of a 1" piece of bar stock down so that it will look like an old fashion square head bolt. I'm so proud of myself knowing that I needed to change over to a 4 jaw chuck to turn square stock. :cheerful: I removed the 3 jaw and prepared to install the 4 jaw when I discovered that the 3 jaw had a bigger threaded diameter than the 4 jaw. I looked among the many different parts that the previous owner included, and found the correct sized back plate. Problem is, the chuck has 4 mounting holes and the back plate only has 3! Upon further exploration, I see that there is an adapter on the head stock which I would assume if removed, would expose the correct diameter for the 4 jaw chuck. But I can't get it off!! :confused 3: There are 3 holes drilled around the shoulder of the adapter that look like some sort of spanner wrench with round pegs might fit into. Am I on the right track? and how do I know what type of wrench to get?
Click to expand...

Amazon , sells square head bolts If you want to buy them.

Jimsehr
 
Fast forward 8 months. I have been gone or otherwise unable to spend much time learning about my lathe. About 4 weeks ago, I decided to continue trying to learn what's what on the lathe before I forget what I have already learned.
After learning more about the lathe's controls, I decided to try to make some shock absorber studs that fit on the U bolt skid plates on my off road Jeep. I bent one of them pretty badly running the Rubicon Trail last September.
Not exactly rocket science, but I did manage to figure out how to face, turn a diameter, turn to a shoulder, and cut 2 different threads, and diameters.

PXL_20210204_185336547 (Custom).jpg

OK, enough of my self proclaimed atta boys.
As I stated above, I need to make a backing plate for my 4 jaw chuck, and I think that I am ready to try it. My actual question to all of you is, do I have to make it out of cast iron? or can I use cold rolled steel? I have not had much luck finding either at a reasonable price, so I hope to narrow the selection of material down so I don't have to waste time looking at the wrong thing.
 
neilkingent said:
go to Shars and order a back plate

Sent from my SM-N975U using Tapatalk
Click to expand...
Wow! Great place for tooling! Thanks! Unfortunately, nothing that will fit my lathe that I can see. The only back plates that I can find with 6TPI are ones with a 2 3/8" spindle diameter. Mine is 2 1/4" x 6TPI.
The 4 jaw chuck is 9", but the existing back plate is only 5.325" diameter with a 4.719" register. I guess that I could cut the diameter to whatever I need, but the 2 3/8" diameter for the spindle is too big.
I did find a guy on Facebook market place near me who will cut some 5 1/2" bar for me and sell it by the pound. The current back plate is 2 1/2" deep. So I figure that I will buy a piece somewhere in the 5" to 8" range. That way I can mess 1 or 2 of them up. He is offering 12L14, or 4140HT for the same price. Which one would be best for this project? I know that 4140HT is harder. I do have both HSS and carbide bits.
 
You must log in or register to reply here.
Back
Top