First chuck backplate turning/setup

[GunsOfNavarone]

I’ll get there with the 4 jaw, but why spend 20 minutes indicating in round bar...that seems like something better suited for a salt centering jaw. I purchased the back plate...2 of them if I’m honest, but one of them I need to open up from a 1” to a 1.75” and thread it, the other plate I just “need” to take a skim cut
off off of...I’m just Thuringia to learn WHY?

 

[Chipper5783]

The bigger 3 jaw chuck will be handy. You can make the transfer screws from a cut off bolt (for a "one off" it doesn't need to be hardened) - just cut off a bolt and point the end nicely using your existing lathe set up. Then file two flats at the point so you can turn it with needle nose pliers (you only want it to stick up slightly). You can do it one hole at a time: mark with transfer screw, center punch, drill, assemble with one bolt and mark the next. To begin, you want to make the holes size on - but once you have all the holes, increase the size (i.e. 1/64 to 1/32" over to ease assembly). Don't fuss too much about the positioning - it does not need to be within 0.001" - the bolts do not locate.

As rgray says - learn to use the 4 jaw chuck. It is not a big deal and work really well.

 

[DavidR8]

Sorry, yes. Register. I fell asleep about 10 minutes after typing that...12 hour days killing me. I’m just perplexed that the register that is already cut into backplate, would be different if I did a light skim cut. Isn’t centered...centered?

If the register on the backplate is .009 oversize I don’t think that it’s going to fit the chuck.

I was in your situation a while back. New backplate for my 4-jaw. Didn’t want to mess it up.

I took careful measurements of the register on the backplate and the corresponding hole in the chuck.
I needed to remove about .005.

I didn’t trust my metrology skills to just go by the numbers so I took it very slowly and after every cut I tried fitting the chuck until it went on.

When I got the register right then I took a facing cut of the mating surfaces.

I know that it’s a 4-jaw so any runout can be addressed but I still wanted it as good as I could manage.


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[GunsOfNavarone]

Can anyone answer that question? If for all intents/purposes, lets say the back plate is made perfect, same for the chuck. How does the skim cut on mating surfaces and the register cut (besides allowing the chuck to seat properly) adjust for my setup and the error inherent in the lathe? I'm trying to picture the spindle not being dead on parallel and how a skim cut on the actual lathe the chuck/plate will be used on will address this. Being it rotates, you can't make any cuts that can offset these issues. None of the answers will change my need, I'd just like to learn something here.

 

[GunsOfNavarone]

Alright, went to turn the back plate. The perpendicular position....won't reach, parallel position...still an inch or more shy from reaching back plate. It seems against whats trying to be achieved if I mount the back plate in a chuck...no? The carriage is all the way in at this point.

 

[Tozguy]

Why the skim cut....
The important reference point is the axis of rotation of your spindle. The axis of rotation is different from the center of mass by an unknown small amount. Also referred to as runout which in many cases is too small for us hobby types to measure. So the only way to get a back plate 'perfect' is to turn it on the spindle it will be used on.

 

[mikey]

I have a few comments:

• You now have a backplate that threads onto your spindle and you want to accurately mount a chuck to it. The plate has a female taper in the back of it that mates to the male register on the spindle, right? So, assuming those tapers fit well, you now you want to mount the chuck to the plate as accurately as possible. Your backplate is not yet machined and the snout in the center of the backplate has to be machined to fit the hole in the back of the chuck. Is this an accurate summary?
• Assuming the above is accurate, what needs to happen next is to take a skim cut on the face of the backplate to true it to the spindle. In the pic above, you cannot even reach the backplate with your tool. I suggest you try turning your compound so it parallels the ways and put the tool holder on the side of the tool holder closest to the center of the lathe. If this is not enough to reach the back plate with your tool, extend the compound out a little bit and lock it down before taking cuts. If you still cannot reach it, you might try a boring bar.
• The cut on the face of the backplate is just a light truing cut to make the face of the plate perpendicular to the spindle centerline. Take light cuts until the tool is able to face the entire surface, then back out. Now dial in a tiny bit more depth of cut, maybe 0.005", increase your speed and take a final finishing pass to achieve a nice surface.
• The next step is to trim the center snout to fit the hole in the back of the chuck. Measure the hole and then measure the snout. There are two schools of thought on how to best do this. One contends that you should make this a light press fit, on the order of 0.0005 - 0.001" would do it. The other way is to make the snout about 0.002-0.003" smaller than the hole. This allows you to install the chuck and knock it into alignment before tightening the chuck mounting bolts. Samchully, maker of some of the best chucks in the world, suggests the latter approach. Our very own @darkzero has also done it this way and calls it the "tap-tru" method of chuck alignment. Other guys have done this by cutting too much off by mistake; they call it the tap-tru method, too, but don't mention the mistake part. Whichever way you go, try to do it on purpose and then just trim the snout to your desired size.
• Then mount the chuck and mark the holes. If the screws pass through the chuck body, use transfer punches. If the screws pass through the plate from the back, take the plate of and use transfer punches from the back.
• Drill and tap and you're done.

Before you do any of this, I strongly suggest you plug up the spindle bore of your lathe with a rag or some other stuffing to prevent chips from getting into your spindle bore. These chips will eventually make it back to your gearing in back of the headstock and wear them down. I also suggest you cover the entire lathe with plastic painter's tarps or some other thing of your choice; leave only the backing plate and cutting tool exposed. When the job is done, brush any loose particles off the exposed surfaces onto the plastic and wipe those surfaces clean, then remove the plastic. Then clean the hell out of the lathe until there are no particles anywhere. Trust me; please do this.

See, not much to it, eh?

 

[DavidR8]

Yup, what @mikey said.



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[GunsOfNavarone]

Thanks guys & @mikey I will be sure to cover everything up. I said I would never turn cast iron..THOUGHT I’d never need to. My Kathe isn’t ridged enough to turn the compound perpendicular, but it’s just a skim cut..
I noticed the chuck has plugs in the backside (I assume for balance) this brings up the question, The new back plate is drilled for 3 holes already, I need to clock it about 20 degrees & drill 3 more holes. The hole distribution (therefore balance) will be off. I could fill the old holes with lead or cut off bolts but the balance will no longer be “perfect”. Do I bother with this issue?

 

[derf]

Why would the balance be off? The chuck is already balanced, and if the back plate is solid, it probably close enough. I'm assuming the original 3 holes are equally spaced, so put the other 3 holes between them.