# PM1236 4-jaw back plate question



## Lynrob (Nov 9, 2014)

Today I thought I'd try out the 4-jaw chuck that came with my PM1236 lathe.  I tried to mount the chuck on the back plate on the bench and not surprisingly, I found that it wouldn't quite fit.  I measured and found that the boss on the back plate was roughly 0.003" too big.  I then mounted the back plate on the lathe and measured axial run out of 0.005" and radial run out of 0.003".  So far so good.  I was counting on having to true up the back plate.  To my surprise, I was unable to get the carriage close enough to the head stock to all turning the mounted back plate.  I looks to me as though the spiral leadscrew cover is bottoming out and preventing the carriage from moving the last inch and a half I need to be in position to face the back plate.

Since I am new to this I thought I should ask if others have run into this issue?  Also, is there a workaround other than pulling the leadscrew off of the lathe and removing the cover?  Am I right in thinking that truing the back plate is a normal requirement when mounting the 4-jaw for the first time?  Finally, do any of you have a recommended process to ensure a good fit?  I have watched several videos on you tube on the subject and I think I have a pretty good idea of how to proceed, but any pointers are welcome.  Thanks!


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## tmarks11 (Nov 9, 2014)

Lynrob said:


> To my surprise, I was unable to get the carriage close enough to the head stock to all turning the mounted back plate.  I looks to me as though the spiral leadscrew cover is bottoming out and preventing the carriage from moving the last inch and a half I need to be in position to face the back plate.


Turn the compound slide so it is pointed longitudinally (z-axis) along the lathe, and extend the compound slide the distance you need to get the tool on the workpiece.  I suspect this would give you the 1.5" you are missing.



Lynrob said:


> Am I right in thinking that truing the back plate is a normal requirement when mounting the 4-jaw for the first time?


You are correct.  The back plate "boss" (also called SPIGOT or chuck register) is oversize on purpose, since you really want to true it on the lathe so it registers the chuck accurately to reduce run-out. 

When you do turn the spigot down, don't get enthusiastic.  You want it to be a very tight fit on the chuck.  The leading edge of the spigot needs a small chamfer, and needs to be deburred (you don't want a burr tricking you into taking another 0.001" off).

Some manufacturers recommend heating the chuck to about 100F and freezing the backplate prior to assembly, with a light interference fit (back plate 0.001" over recess diameter). 

Look at the assembly directions here starting on page 1:
http://cdn0.grizzly.com/manuals/sb1211_m.pdf

Also note that the spigot surface should be clear of the inside surface of the chuck by 0.010-0.015" to ensure that the back surface of the outer rim of the chuck contacts the front surface[CHARGE][/CHARGE] of the outer rim of the back plate firmly.

Clear as mud?

Here is a picture that I shamelessly lifted (blue is the interference fit, and red shows the part where the front face of the back plate contacts the back face of the chuck, clearance between flat front surface of spigot and back flat surface of chuck recess exaggerated slightly):



(and the source of my plagiarism: http://bbs.homeshopmachinist.net/threads/34734-Fitting-a-chuck-to-a-back-plate?p=441623#post441623)

You might think that this precision assembly is really only required for mounting scroll (self centering) three and six jaw chucks, rather than independent four jaw chucks.  Perhaps, but relying on friction and bolting force to hold the chuck in position against that back plate could serve you poorly, as the centrifugal force and heavy weight of the chuck could cause it to shift while spinning, inducing runout that you had corrected for while centering the part.


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## Lynrob (Nov 9, 2014)

tmarks11 said:


> Turn the compound slide so it is pointed longitudinally (z-axis) along the lathe, and extend the compound slide the distance you need to get the tool on the workpiece.  I suspect this would give you the 1.5" you are missing.



Thank you very much!  Unfortunately, I am and inch and a half short, even with the compound fully extended.  I should have mentioned that in the OP.  I think my only choice is to pull off the spiral cover on the lead screw.  I got one of the lathes that was accidentally sent out as metric instead of imperial as ordered so I am familiar with changing out the lead screw.  I was just hoping to avoid having to do it again.

I appreciate all of the tips.  I'll work slowly and hopefully I won't over shoot!


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## Lynrob (Nov 9, 2014)

The radial run out I measured on the spigot was 0.003" and the spigot is nominally 0.003" oversize.  The instructions in the link tmarks11 provided say the spigot should be 1 to 2 thousandths oversize for a press fit.  It seems that by the time I get the spigot concentric with my spindle, it might be too small for a press fit.  I will try to install the back plate in the two remaining positions on the D1-4 chuck to see if that improves the run out.  Any other thoughts?  I really want to have a plan before I start taking off metal.  I'd prefer not to mess up the back plate.


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## tmarks11 (Nov 9, 2014)

hmm, really seems based on the dimensions that this isn't a "semi finished" back plate... seems almost finished. Seems like you run a skim coat across the face to get rid of the axial wobble, barely hit the spigot edge (if at all) and you are ready to mount your chuck.

Consider this: you are mounting a 4 jaw chuck.  As long as you fix the axial run-out, and the spigot fits your chuck tightly, 0.002-0.003" of radial runout isn't going to be a big deal.  It is an independent 4 jaw chuck, so you adjust out the run-out every time you mount the work piece in the chuck.  

The important thing is that axial run-out, since that would cause a work piece to have different run-outs as you go down its length.

Also make sure you clean the spindle nose and the backplate where it mounts to the spindle, and make sure there are no burs on it before you start.

If you mess it up in the end, a D1-4 back plate of this quality is only $65-85.

Corvallis, that is where I went to school (ages ago)...


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## Morgan RedHawk (Nov 10, 2014)

When I did mine, I removed the cover without removing the lead screw.  First I moved the carriage as far as possible towards the tailstock.  Next I pulled the cover back from the carriage and carefully separated the end and moved it to the other side of the leadscrew.  Then I simply, and carefully "unscrewed" the cover from the lead screw.  It wrapped around itself and assumed the form of a long tube.  Once I got my backplate trued up (and I did my faceplate while I was at it), I "screwed" the cover back on to the lead screw and re-sat it in the socket on the carriage.  It was pretty easy.  I was very careful not to let the cover spring loose once it was off, and I wore gloves and a face shield just in case it got away from me while working with it.  Hope this helps!


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## Lynrob (Nov 10, 2014)

tmarks11 said:


> Consider this: you are mounting a 4 jaw chuck.  As long as you fix the axial run-out, and the spigot fits your chuck tightly, 0.002-0.003" of radial runout isn't going to be a big deal.  It is an independent 4 jaw chuck, so you adjust out the run-out every time you mount the work piece in the chuck.
> 
> The important thing is that axial run-out, since that would cause a work piece to have different run-outs as you go down its length.
> 
> ...



Thanks!  I feel much better about what needs to be done, now.  I appreciate your advice.

Lyn


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## Lynrob (Nov 10, 2014)

Morgan RedHawk said:


> When I did mine, I removed the cover without removing the lead screw.



Thanks, Morgan!  That is good to know!  I will do it your way for sure.  Not that removing the lead screw is that big of a deal, but to get it out I have to dismantle part of the wall in my shop or move the lathe, both of which are painful.  

Lyn


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## tmarks11 (Nov 10, 2014)

Lynrob said:


> TNot that removing the lead screw is that big of a deal, but to get it out I have to dismantle part of the wall in my shop or move the lathe, both of which are painful.


Strategic hole with a  3" hole saw?


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## Lynrob (Nov 16, 2014)

I finally got some time to get back to the lathe.  I was able to follow Morgan's advice and remove the lead screw guard without incident.  I was then able to true up the back plate axial run out and worked on the concentricity of the spigot.  I was able to get it within half a thousandth but the fit to the chuck is not a press fit.  It goes on fairly easily but once on, I am not able to detect any movement so I think I might be ok.

I had a problem at first when mounting the chuck to the back plate.  It turned our that the supplied cap screws were too long and were bottoming out before tightening the chuck.  I mounted and dismounted the chuck on the lathe several times before I figured out what was going on.  I replaced the cap screws with shorter ones and everything works great now.

Now I think I may have a different issue.  I put of 12" length of 3/4" drill rod into the chuck and centered it for zero run out.  I then moved the dial indicator out to the end of the rod (about 10") and checked for run out again.  I tapped the rod with a dead blow hammer and the best I could get was 0.0015" run out.  Next to the chuck run out is still zero.  I then ran the dial indicator which is attached to my tool post axially along the rod and I appear to have a difference of about 0.0045".  Can I assume that since the rod is centered in the chuck and has relatively little run out that the difference I am seeing from one end to the other is showing that the spindle is not parallel to the ways?  If that is the case, how do I adjust that?  My owner's manual has a page in it showing the results of factory tests and the the spindle was parallel to the ways within 0.2mm in 300mm  which is 0.007" in 11.8" so it looks like it might even be a little better now.  Is this a typical value?  It seems like it shouldn't be that far off to me but then again maybe it doesn't matter since I can't imagine turning something that long without the tailstock holding the far end.

What do you guys think?


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## tmarks11 (Nov 16, 2014)

Chuck up the drill rod in the three jaw chuck (or collet chuck) and see what kind of variation you have from end to end.

I wouldn't jump into aligning the spindle based upon a crummy chinese 4 jaw.  The problem could be jaws that are not parallel... been known to happen.


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## Lynrob (Nov 17, 2014)

Thanks Tmarks!  I had considered that the chuck might be contributing to the problem but I thought I had neutralized that contribution by getting close to zero run out at each end of the rod.  I'll have to think that through some more.  In the mean time, I'll try the 3-jaw and see how it compares.  Ironically, I put on the 4-jaw because I was unhappy with the run out I was getting with the 3-jaw!  Unfortunately I don't yet have a collet chuck so that will have to wait.  Again, thanks!


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## JimDawson (Nov 17, 2014)

Take a look at my 4-jaw adventure, it might be useful to you.   http://www.hobby-machinist.com/show...uck-Adventures?p=234042&viewfull=1#post234042


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## Lynrob (Nov 17, 2014)

Thanks, Jim!  I just noticed, we're practically neighbors!  A little colder where you live I'll bet.


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## JimDawson (Nov 17, 2014)

My Pleasure! Yeah, we're pretty close.  I actually grew up in Corvallis, but moved up here to this area in the late 70's.   It may be a bit colder here, but I'm only at about 800 Ft so not too bad.


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## tmarks11 (Nov 18, 2014)

JimDawson said:


> My Pleasure! Yeah, we're pretty close.  I actually grew up in Corvallis


Small world.  I went to school there, and my wife grew up there.


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## tmarks11 (Nov 18, 2014)

Lynrob said:


> Thanks Tmarks!  I had considered that the chuck might be contributing to the problem but I thought I had neutralized that contribution by getting close to zero run out at each end of the rod.  Ironically, I put on the 4-jaw because I was unhappy with the run out I was getting with the 3-jaw!



Three jaw will always have a bit of run out, but it doesn't matter once you take a cut.  The piece sticking out of the chuck will (should) run true.  You just can't take it out of the chuck and put it back end in such a way that the next cut will be concentric to the first (at least without an unholy amount of work).  Chuck up a piece of aluminum, and turn it down until it is cut all the way along it's length, and than measure the diameter at various points (to see if it is turning a cone shape), than run a dti on the criss slide down to length of it to measure axial variation.

But before you do that, this might just be a lathe bed leveling issue.  If you haven't leveled your lathe properly, the bed could be twisted, giving you all sorts of strange results.  Lots written on the topic, and you can find lots of you tube videos.  Goggle "two collar method lathe".


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