The Home Machinist!

I've been working on a backplate to use with what is going to be a dedicated soft jaw chuck (Harold's made me a convert).

I read all the prior posts on machining a backplate and wrote out all the steps I needed to to. Really helps to have a plan.

Harold's been giving me some help on this, and I've been taking pictures along the way, so he suggested that I share it with the group.

The backplate, which I bought from www.littlemachineshop.com for $30, comes machined and threaded. The rear surface is machined, but I'm not going to rely that it is perpendicular to the bore.

The existing 4-jaw and 3-jaw are about 7 thou larger than the spindle nose, and therefore, they are using the threads for location. We will shoot for having the bore locate the backplate.

The spindle is threaded 1-1/2x8 tpi and is threaded only part way down. The rest of it is available to register the backplate (about 0.2"). The spindle has a shoulder, but it is almost the same size as the rear of the backplate, so I will not use that to locate the backplate to the spindle. The rear of the backplate needs to be bored out to sufficient depth to fit over the unthreaded portion (including the portion where the thread gradually ends). The 4-jaw is unthreaded for about 3/8".

I made a plug gauge the exact diameter of the spindle. I did not make a complete clone of the spindle, although it is a good thing to have and I probably should at some point.

The backplate is deeper than I need, so to reduce overhang, I decides to machine off 1/4" off the rear. Prior to removing this material, I practiced machining the bore and getting it to the size of the plug gauge. Once I was comfortable with getting the fit right, I marked off 1/4" using calipers to mark it and took the off 1/4" off the rear.

To bore the back, I need to set the backplate out from the spindle, so I would need some kind of precision washer or spacer to hold it accurately. I bought a 1-1/2" milling machine arbor spacer. I got one 1/2" thick, as I needed some extra room to machine off the 1/4" from the back. I checked the spacer and found that there was a high spot, but the spindle had a high spot as well (no, I don't have the nerve to machine the spindle at this point). The amounts we are taling about are 1 thou or less. I mounted the adaptor to the spindle in the location that cancelled these out as much as possible.

To machine the inside of the bore, I used a small, round nose HSS boring tool. To face off the rear, I used a roughing tool with no rake, but a chip breaker. It gave a somewhat rough (fuzzy) finish.

I used a large radius round-nose tool with zero rake and power cross feed to finish and got a really good surface finish. I will use this tool to do the final work on the backplate. I do not have any C2 carbide, so we will be sticking to HSS.

Once I was done, I used a file to take off the rough corner on the outside and a 45 degree tool to chamfer the inside edge.

I've included a picture of some of the tools I used.

I took it off and reversed it and installed it on the spindle and the fit was snug, but by moving it back and forth, it fit easily. There was probably some surface roughness in the bore. I probably should have hit it with some emery cloth first.

I put some dykem on the backplate and threaded it on. Note that I used dykem layout fluid, which is not the proper thing to use. The correct one is a non-drying one, which I did not have on hand. It showed that contact was not perfect, and tended to be on the inside edge. I took it off and used the end of a file and scraped the high spots (I do not have a real scraper, but I think I'm going to find a worn-out file and make one). I got a better contact, but it was mostly on one side. I repeated this four or five times and got good contact around the spindle. Not full contact everywhere, but almost all of the way around, and not mostly on one side or the other. Having a full contact will mean that wear that will have less of a chance of changing how the chuck repeats when installed. If there are high spots, they will wear first and move the chuck not only in relation to the centerline of the spindle, but rotationally as well, and that can cause runout where there was none before.

The contact on the spindle looks pretty good. Note that not all of the contact points transfered the dye, as this is not the proper blue for doing scraping.

To test how close the bore was relative to the spindle, I installed the backplate, then turned it slightly loose and put a dial indicator on the top. Then I wobbled it and saw 2 thou of movement at the top, which is 2-1/2" from the edge of the spindle, so the amount that the bore is oversized is probably measured in 10ths.

I snugged the backplate up and checked runout, and there was only one thou at the full 6" diameter. This will be removed when I face, which is the next step (hopefully tonight).

I started rough machining the face by facing the outside edge of the boss to a diameter slightly larger than the opening in the chuck. The boss diameter will be fairly small, as this chuck has a small recess in the back. It is actually not a plain back chuck, as the hole in the back is meant to mount ot a spindle (but, hey I got it cheap enough, so I figured it will do the job).

The roughing tool did the job fairly quickly, but the roughed out face is again fairly fuzzy, but that's why it's called roughing

I then turned the boss down to a straight shoulder (still larger than the chuck opening)

I used a round nose tool to get a better finish on the face. This was not needed, as I was going to be facing it to final size later, but I figured I would use this as a test. Note that the toolpost is turned 90 degrees counter-clockwise, as the carriage did not have enough travel to reach the outside. I will need to figure out what to do to machine the outside diameter.

Ever wonder about that tailstock drilling pad? You probably don't use it for drilling, but you can use it to quickly set your toolpost square to the bed (see photo).

Now we will let the casting cool down before machining to final dimensions.

Looks like I have something to do this weekend. Nothing on TV these days anyway

Steve,

Harold is certainly a good one to go to for anything like this and I'm sure he will have already told you much of what I'm going to say, but here goes.

I did the same thing you are doing for Don when he got his 13 inch South Bend which came with no chucks. When I do this, I like to start with a blank backplate so I can make sure everything is as concentric as possible, threads, locating surfaces, etc. What I do differently is to take the blank backplate and chuck it in a 4 jaw & true it up before I start machining. Holding it in the chuck means I can remove the chuck from the headstock and turn it around to check the fit without losing the registry. You will wind up doing this check a number of times in the course of fitting the backplate to the spindle. The fact that we both had the same lathe made things easier, but you are going to want to have more than one chuck so you will wind up doing the same thing I did.

The important points are as follows:

First, as you know, the threads are NOT to be used for registry, as a matter of fact, they should be sloppy so the backplate can shift to locate properly on the shoulder, both on the spindle diameter and the shoulder face. The fit on the diameter should be a close slide fit and obviously, the backplate locating faces must be clean and smooth before doing each check.

Second, once the backplate fits the spindle to your satisfaction, mount it on the spindle and machine it to match the back of the chuck. Good chucks will have a tapped bolt circle (3, 4, 5 or more bolts) which is used to hold the backplate on the chuck, but NOT for location. The corresponding holes in the backplate should be oversize so the backplate can move a bit.

Third, good chucks will also have a small step (up or down) in the back face and that is what the backplate locates on. What you want to do is to machine a boss or recess (whichever fits) in the backplate that matches this step as precisely as you can. Make sure to machine the depth (or height) of this feature so that the outer part of the backplate seats on the part of the chuck where the bolt circle is. If you are really finicky, you can make the part of the backplate that matches this step on a slight taper so that the bolts will pull the backplate up tight on the taper. That's what I did with the backplate I made for Don. By doing things this way, you will wind up with a back plate that can be removed from the chuck and put back on and still be true.
You mention that the chuck you have is threaded. Perhaps it already has a backplate bolted on?

Hope some of this helps.

Richard Trounce.

I'm not yet at the point that I can cut internal threads with a single-point tool, which is why I went with the pre-threaded backplate, but I agree that if you can, that is the way to go.



That's a very interesting strategy. That eliminates having to maek a spindle clone. Why use a clone when you can use the real thing.




Mine has 6 holes. They actually hold two halves of the chuck together, but I plan to fill the recess that the screw head fits into with a sleeve (so that the screw is not unsupported) and use longer bolts to go through the backplate and both halves of the chuck (see photos).

I'm going to use the lathe to scribe the bolt hole circle (with the diameter measured by the cross slide from a center in the spindle), then use the indexing pin on the bull gear to locate the holes (there is a pin on the headstock and 60 holes in the headstock that can be used for indexing). I can use a pointed tool in the tool post to scribe the circle, then slide the cross-slide to scratch a line horizontally where the holes should be located. Just have to be sure the scribe is at center height.





The rear is not threaded, but the hole in the back is straight for some depth, then tapers a bit and has a locating pin. I can use the straight portion of this to locate it to the backplate. The part of the chuck that appears to be a larger area for registration does not have enough depth for that. This chuck was inadvertantly sold as a "plain back" chuck. The register will be a smaller diameter than would be on a real plain back chuck, but I do not see any reason why it would not work. I got the chuck for $60. It is a 6" Hardinge with 2-pc jaws and in very good condition with no slop in the jaw masters, but it had no tops. For that price, plust the $30 for the backplate and some work to make the jaws (got the specs from Hardinge's website), I'm getting off pretty cheap.



Very much so.

Steve,

Cutting threads, both internal and external is much easier if you "cheat" a bit & cut a groove for the threading tool to run out into. That way you don't have to be exact in backing out the tool at the end of the cut. Neither I nor my nerves are good enough to use the "backing out" method. Besides, with the locating faces on the spindle nose, that groove comes in just the right place on the backplate to make things easier.

Something else you might think about is that when cutting internal threads, if you set up the tool so you cut on the far side of the bore, run the lathe backwards and start with the tool in the "runout" groove at the bottom, its much easier. You are starting from a "known" position, and because you are cutting on the far side of the bore, you can see what you are doing much better. Because single point threading cuts are generally light, you shouldn't have to worry about the chuck unscrewing. I don't do this very often, but it does have its place.

Just something else to think about.

Richard Trounce.

Add some Dykem to grease and you'll (almost) have non-drying.

Another trick I've used a lot when I did not have proper spotting blue or black (the non-drying Dykem blue kind of stuff), is to use an old lipstick (preferably a shade your wife/girlfriend doesn't like anymore!!) - it has a grease/oil base, is slow-drying and comes in an easily-applied form. Usually cheap or free, too!

Mixing a little oil-based black or dark-blue paint into some cup grease works, too, in a pinch, but it's a lot messier to use.