chuck backplate

[jcs0001]

My lathe (10 x 22 import) has a 1 3/4 in. x 8 tpi spindle.  It came with a three jaw chuck and a 6 in. 4 jaw chuck seen here:
chuck backplate

Note that the backplates have threaded holes to match with two locking devices that can be used to prevent the chuck from screwing off the spindle.  It also came with a faceplate - not shown.

I also have a new 5 in. 4 jaw chuck that requires a backplate.  This chuck is lighter and more useful for smaller jobs than the 6 in. chuck so I decided recently to get at making a back plate.

Spindle:
chuck backplate

Last week I picked up a piece of ductile iron - 6 in. in diameter and about 1.5 in. thick.  I also had on hand a pieces of 2.5 in. diameter hot rolled steel rod.
chuck backplate

A couple of days ago I started to make a fake spindle with the threads and shoulder.  The existing spindle was measured for diameter and length.

I cleaned off the outside of the hot rolled steel with a steel brush and got it running as true as I could in the 3 jaw chuck.  The outboard end was faced and drilled with a centre drill so that a live centre would give it support.

chuck backplate
(not sure if this is proper procedure but it seems to work).


The diameter of the shoulder is a bit greater than the threaded portion so I turned the full length to that diameter first.  The threaded portion was then turned down just under 1.75 in. and a cut taken at the back of the threads with a cut off tool - to provide and "ending" on each threading pass.

A 60 deg. hss threading tool was set up and I threaded the majority of the thread depth under power.  It is a bit tricky with this lathe as the slowest speed is about 150 rpm.  Once it was nearing proper depth I first used a thread guage then used the faceplate and 4 jaw chuck to test the fit.  Nearing the end my "threading mandrel" was installed and I completed the threading turning the spindle by hand.

At one point I thought I had the threads correct and dismantled my threading setup.  The threading was found to be too shallow and I had to pick up the thread again.  Being that the threads were 8 per inch it wasn't too difficult.

The final result can be seen below:
chuck backplate

Today I installed the piece of ductile iron close to centre in the 6 in. four jaw and faced both sides.  It had been cut on an industrial bandsaw but was still out some so it took a while and worked the lathe pretty hard.

Further to come although it may be a while as I will take my time with this.

John.

[Divided he ad]

Hi John,

I've read through this and have a couple of questions....

I'm trying to figure out what the fake spindle is to be used for? I don't think you say? Thread checking?

Where you chuck up the rusted stock and try to get it as true as possible would it not have been better to have skimmed one end and then turned it round and trued up the other and continued to machine that?

Just thinking about the problems you might have when you cut this off and the clean bit of bar is then clamped. It might be quite far out of register? (using personal experiences here!) This only matters if you're going to turn on it I suppose... Answer one might nullify this?

Still... You've made a huge thread and that's far more than I've done on my lathe 



Watching with interest 




Ralph.

[sparky961]

Something I've picked up over the years, is that when the part/operation is "finished", LEAVE IT BE and WALK AWAY for a minute.  It's very easy to grab the chuck key and triumphantly rip the part out of the jaws of the chuck, only to realize that there was something you forgot to do in that same setup!

If you can check the part while still on the machine, it will enable you to take another cut or put another hole in that you missed very easily.  If you have to set it up again though it can take a lot longer and give less accurate results.

Speaking of accuracy, I would have to agree with Ralph's concern.  The hot rolled surface is going to be far from round.

-Sparky

[jcs0001]

Ralph and Sparky:

Thanks for the reply.  The "fake or dummy spindle" is used to check the fit of the shoulder and threads when I make the back plate.  The alternative is to repeatedly take the partially finished back plate and chuck from the spindle, turn it around and check the fit.  Having said that I did screw up with the shoulder dimension on my fake spindle - it is undersized quite a bit  .  However the threads are a very good fit and I'm happy with them.

As to cleaning up the steel before chucking - I had thought of that however my 3 jaw chuck is not a paragon of accuracy - it's out about 7 thou. - and I did find that the steel spun with decent accuracy when I chucked it up.  I didn't plan to leave the piece full length and the actual finished part is as shown below (it should all be concentric as it was all turned in one go).  You are correct however - taking a skim off the surface of one end and chucking that end would be a better way to go - thoughts for the future.

chuck backplate

Note the black mark - this is the shoulder that I undersized.

One side of the 6 in. back plate blank.  I only need 5 in. in diameter so didn't completely face this side as it's getting quite close to the thickness that I require.
chuck backplate

My plan to work around the undersized shoulder is to turn a short cylinder to the exact size of the shoulder - it will be turned on the end of a short piece of suitably sized steel.  I can use this to test the corresponding portion (behind the threads) of the back plate as I bore it and can use my fake spindle to test the threads.

I realise that this is not a very elegant method of working however I doubt I'll use the fake spindle for more than two back plates - this one and perhaps another for a collet chuck (someday).

Sparky - you are correct in leaving things be for a bit.  It's easy to get in a hurry.  Since my mistake with the shoulder size I've been carefully planning the next steps to reduce the potential for further problems.  It's all a learning experience.

John.

[Divided he ad]

It's all a learning experience.

Indeed it is 

As said if it was a thread tester then the register was not a major concern.

Thoughts on the undersized spindle shoulder....
My solution would be make a ring out of the remaining part of the original stock and soft solder it over the shoulder you have marked.

Then I'd chuck it back up, check for concentricity and turn to correct size.  But that's just me 


It's a hell of a piece of iron your chopping there John   (I think I'd stand slightly to the right when cutting that one! )







Ralph.

[jcs0001]

Onward we go:

I roughly centred the 6 in. blank in my 4 jaw (since I only need 5 in. diameter eventually) and centre drilled it then step drilled up to a 1 in. hole right through. 

chuck backplate

I have nothing bigger than that in drills and besides my lathe wouldn't be happy pushing a bigger bit I expect.  So then it was off to the boring bar.  Only I have a hodge podge of boring bars and of course the bigger one was just a little too big and the smaller one just long enough to go through.  I have one other smaller one but it takes a 1/8 in. toolbit on either end and of course I can't get them locally.  Just ordered a couple of hss 1/8 in. blanks but it may be a week or more to get them.  Anyway I'd better get at making a couple of boring bars as I'm sadly lacking different sizes.

Using the smaller boring bar I was able to work the hole up to a size large enough for my bigger boring bar.  Now I need to bore it to size.  The shoulder is to be 1.811 in. in diameter and the bored portion for the threads will be "about" 1.583 in.  The reason I say about is that the measured distance across the internal threads on my faceplate is 1.583.  If I do the calculations for thread depth based on a 1.75 in. x 8 tpi spindle thread then the bored diameter before threading will be 1.615 in. and if I do the calculation based upon a 75% thread I get 1.635 in.

(formula for thread depth - 0.6134/8 tpi        formula for bore = 1.75 - (1.0825/8tpi)         formula for 75% thread = 1.75 - (2 x depth x 75%)/100)  I got these out of Screwcutting in the lathe by Martin Cleeve.

I am inclined to go with the smallest number - ie the measured one on the faceplate.  Any other suggestions would be appreciated.

Ralph:

Thanks for the suggestion about soldering a ring on and turning it down again.  I will keep it in mind.  Lots of ways to "skin a cat".  Yes it is a large chunk of steel - they showed some 5 in. in inventory but couldn't find it so I ended up with the 6 in. piece.  He had a great deal on a 6 in. piece that was just over double the thickness but I couldn't see splitting it with my equipment so I passed on it.

John.

[John Rudd]

Get some thread gauges...Least then you can tell if the thread is the correct depth on the male portion..Once that is correct you can use it to gauge the fit in the female part...( Even though you could try and use the existing backplate as a gauge for your male test piece, it gets you into the practise of precision rather than best fit....)

If you go the soft solder route make sure you use coolant or dont overdo the cutting...Generating heat will soften the solder and dislocate the ring...

Just my 2 cents...

[jcs0001]

John Rudd - thanks for mentioning the concern about soft solder and heat from turning.  I didn't end up using that method but will keep it in mind for future.  I did use a thread guage on the male portion and the threads were good.

After a number of hours of mostly turning, I have the new back plate made and mounted on the chuck.  The back portion needs to be thinned out but it is functional as is.  There are 4 bolts holding the back plate to the chuck and using a pointed transfer screw I was able to line the holes up perfectly with the threaded holes in the chuck.  No need to drill them slightly oversize - 

chuck backplate

It took a long time to turn the 6 in. plug of ductile iron down to approx. 5 in. in diameter and would have been easier had I been able to get a chunk of 5 in. diameter material.

John.

[John Rudd]

Very good...Glad you managed to resolve any issues you had...

I have two new CI blanks that need the same treatment...although my Chester has an M49*4 thread....