New Project. Chuck Back Plate.

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Hukshawn

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So, I bought a 6" 4 jaw chuck. Quite happy with that. Issue... the store does not supply a back plate. Even a blank back plate. Crazy... and my other avenues in which I purchase equipment only have a 5"... looks like I'm making my own.
Fun situation... I have a 5" 3 jaw chuck and zero way to grab my stock. I bought a 7"x7"X1" slice of hot rolled plate steel. I did some reading just now and they say cast is better.... but, this is what I have. I asked the guy at the metal supply his opinion and this is what he came up with. And they are a full out machining shop.

So here's my plan, please critique it.

I have a 1" thick piece. My lathe spindle is 1 1/2" 8tpi. The shoulder to the tip is just less than 1". So I think I'm fine on thickness. Would be happier thicker, but again, I'm using what was avialable and affordable.
I am going to centre drill on the drill press to the largest hole I am capable of, (which I believe is 3/4"). Make a jig to mount this between centers on the lathe and carefully hog out about 1/2" and form a rough collar where the threads will be. Just taking enough material off to allow me to then grab that collar with the chuck jaws. Then bore out the spindle hole and thread it. Mount it on the spindle and from there basically machine the final profile of the back plate to my satisfaction... i feel doing the final machining with the plate mounted on the spindle gives me the best hope of being true with a minimal runout.
in my relatively inexperienced mind, (aside from the safety issues of slinging this chunk of steel across the garage...) i think this works...

Thoughts!?

I'll deal with the fact that the mounting bolt holes are not thru holes so I can't use a drill hole punch, later. I have to measure and scribe and hope for the best or make the holes big enough to... essentially... throw the hot dog down the hallway... and do the final truing with the chuck mounted with a dial gauge and tighten once satisfied.
 
I have a similar plan but I think I will be able to hold my blank in my 4 jaw (which is larger than my existing 3 jaw) to bore and thread it, but then would mount it on the spindle to do the majority of the machining similar to your plan.
So I say go for it and let me know if it works . But seriously I would also so be interested in the the professionals opionon of this approach and any pitfalls to watch out for.
 
That is precisely why I bought the larger 4 jaw. I plan on building a milling jig. So need the 4 jaw to hold erregilar stock. Also bought the angle plate today. The Christmas break should be fun. Several good projects... pending my wife gives me the time...
 
I'd say you have a good plan... I just machined one for a 9 inch chuck this morning on my old South Bend.

I did the final machining with the plate mounted on the spindle also.

-Bear
 
When I did mine, I bored out a piece of round stock for the threads (Boss) and then counter-sunk it into the plate and welded it. Allowed for more threads in the round bar stock; say 2 inches. Make sure everything is as true as you can get it when you weld and then true up the rest on the lathe.
 
Hukshawn said:
i feel doing the final machining with the plate mounted on the spindle gives me the best hope of being true with a minimal runout.
Click to expand...

Agreed!

One small issue is how square with the plate your first drilled hole is. Once you put it on an arbor or mandrel I think you will notice some "wobble" of the face of the plate. However, when you complete the (possibly interrupted) facing cut, you can find the new square plate within. Just be aware that you lose some thickness.

Once bored and threaded you can mount it on the spindle and take a final facing cut to make the big face square.
One key is to get the shoulder that registers against the lathe spindle to be square to the threaded bore.

It can be tough finding an appropriate lathe speed for facing. You're really aiming for a particular sfpm (surface feet per minute) based on the cutter and material.
However since the diameter changes so drastically from the centre to the outside rim of the plate the sfpm changes too.

-brino
 
brino said:
Agreed!

One small issue is how square with the plate your first drilled hole is. Once you put it on an arbor or mandrel I think you will notice some "wobble" of the face of the plate. However, when you complete the (possibly interrupted) facing cut, you can find the new square plate within. Just be aware that you lose some thickness.

Once bored and threaded you can mount it on the spindle and take a final facing cut to make the big face square.
One key is to get the shoulder that registers against the lathe spindle to be square to the threaded bore.

It can be tough finding an appropriate lathe speed for facing. You're really aiming for a particular sfpm (surface feet per minute) based on the cutter and material.
However since the diameter changes so drastically from the centre to the outside rim of the plate the sfpm changes too.

-brino
Click to expand...

Yeah exactly, I wasn't going to rely on turning between centers to get me anywhere near true it was merely to get enough material off to allow the Chuck Jaws to grab the collar. That would allow me to thread it and then I can mount it on the spindle and do all the final truing.
I should be able to face both the front and back this way, just take the back plate off and spin it around and screw back on, and face that side.

Do you think hot rolled was a good choice?
 
Im not an expert on threaded spindles, guess the one on my indexing head has one though. You said you measured the thread as just under 1 inch, but you need to machine a tapered recess for the shoulder just behind the thread on the spindle. The chuck centres on that shoulder, the thread only holds it there. Im thinking your plate isn't thick enough for the full thread and shoulder.
I'd try using you're three jaw as a face plate. Remove the jaws and grind bolt heads to fit into the slots, same idea as t-slots. Then drill 3 holes in your blank to bolt it on. Drill, bore, thread and cut the bore for the shoulder with it there. You can face it from the new bolt circle to the centre and clean up the rest after its mounted on the spindle. With the bore done mount it on the spindle and do your machining for the chuck mount.

Greg
 
f350ca said:
Im not an expert on threaded spindles, guess the one on my indexing head has one though. You said you measured the thread as just under 1 inch, but you need to machine a tapered recess for the shoulder just behind the thread on the spindle. The chuck centres on that shoulder, the thread only holds it there. Im thinking your plate isn't thick enough for the full thread and shoulder.
I'd try using you're three jaw as a face plate. Remove the jaws and grind bolt heads to fit into the slots, same idea as t-slots. Then drill 3 holes in your blank to bolt it on. Drill, bore, thread and cut the bore for the shoulder with it there. You can face it from the new bolt circle to the centre and clean up the rest after its mounted on the spindle. With the bore done mount it on the spindle and do your machining for the chuck mount.

Greg
Click to expand...

I did account foe the shoulder. From the shoulder to the end of the spindle is just less than 1". Even still, there is room beyond the back plate inside the chuck if the thread sticks out a bit. Shouldn't cause any issues, you're right, as long as the shoulders seat fully to align.

I do like the mounting to the 3 jaw chuck idea tho, at least to bore and thread. Best case scenario to get a good straight thread. Straighter than trying to turn between centers then trying to grab the collar with the chucks. And since I don't have parallels, seating true would be tricky.
 
I use HSS tool blanks for small parallels all the time. You have to check the Chinese ones for size, they vary.

Greg
 
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