618 Headstock Alignment

[marchyman]

Background: My lathe is a Craftsman/Atlas 618 (sleeve bearings) probably made in the '40s. It had been sitting for 15 to 30 years before it was given to me as mostly a pile of dust and rust. I took it apart, cleaned it, removed the rust, slapped on some primer, and with the help of the 618 Yahoo group was able to replace most broken or missing parts. The result is a mostly running (if well worn) lathe.

The issue is this. The headstock is lower than the tailstock. A center placed in the tailstock points above a center in the headstock. My choices are to mill the tailstock or shim the headstock. I've not the equipment or experience to hold the tailstock square and level for milling. With shims I can safely make mistakes.

I want to measure the difference in heights to figure out what size shims I need. I was thinking of mounting some drill rod in the four jaw and adjusting until it runs true then measure the height with a test indicator in the tool post. Next I'd put the same drill rod in the tail stock. Measure again. Calculate the difference. That should be the thickness of the needed shim.

Will that work? Is there an easier way?

 

[RJSakowski]

It seems strange that the headstock would be lower than the tailstock. There is no opportunity for wear of either the bed or the headstock. I would suspect that wear at some other point on the lathe ways is creating the misalignment. It may be that when you bring the tailstock up to check the center alignment, the back end of the tailstock is in an area of wear on the ways which would tilt the center upwards. You could check this by measuring the misalignment of the centers with the tailstock quill retracted and extended. If there is a tilt, you will see a difference.

You can tilt the tailstock center downwards by placing a shim between the tailstock and the base at the rear of the tailstock.

 

[marchyman]

I measured the height of the quill extended and retracted. That's only 1 and a half inches. I used a bore gauge (because it's what I have) between the quill and a parallel across the ways and made measurements with the tail stock at different locations. My ways are worn so I expected differences. Surprisingly the tailstock was mostly parallel to the ways. Measurements were within 0.002" and it didn't make much difference if the quill was extended or not.

I don't like the idea of shimming the rear of the tail stock since it is mostly parallel to the ways.

But I don't know if my spindle is parallel. Hmmmm....

 

[RJSakowski]

As has wisely been stated in other threads, you need to identify the source of the misalignment before attempting any corrections. Corrections made in the wrong places will create other problems, oftentimes more serious. It isn't too likely that the lathe came from the factory with the headstock set lower than the tailstock but it is possible that the previous owner made some changes. At this point, nothing should be ruled out though.

Dirt or a burr in the Morse taper sockets can throw the center off quite a bit. In the headstock, it would manifest itself as runout which can be checked by rotating the spindle. It is harder to detect in the tailstock though.

You can evaluate wear in the ways with a straightedge and the assumption that the bed nearest the headstock and the far right end have experienced little wear. A known good straight edge and some feeler gauges should give you some information. I have a 6" Craftsman, albeit a decade or so newer than yours and there is a small strip running down the middle of each way where neither the carriage nor the tailstock travel which should be a fairly pristine surface.

It is also possible that the bed is twisted.. The lathe has only two mounting points on either end but it has a fairly broad foot. If the lathe were mounted to a less than flat surface, tightening the mounting bolts could twist the bed. Improper shimming could also create a twist problem. If the bed hasn't been permanently sprung, loosening the mounting bolts should improve the situation.

Wear in your spindle bushings could cause the spindle to droop. My lathe has bearings and is tight but bushings require some clearance to function properly. The clearance will increase with use. If the spindle is tight, then you need to check the spindle axis alignment. A two collar test or Rollie's Dad's method will tell you if the spindle axis is parallel to the ways.

With the spindle running true to the ways, you would move on to the tailstock. There are a number of ways to check for tailstock alignment. Edge Technology (http://www.edgetechnologyproducts.com/lathe/) makes a tailstock alignment bar. An adaptation of Rollie's Dad's method will also do the job. Have you checked your tailstock for shims? It is possible that the tailstock was replaced with one from a different year which may have had different dimensions. There are others on this forum who are far more knowledgeable than I am regarding Craftsman lathes who could offer some insight into that.

A few ideas. A systematic approach to diagnosing the problem should determine where the problem(s) lie. Good luck!

 

[RJSakowski]

Another quick and easy way to check headstock/tailstock center alignment is to trap a piece of shim stock between the two centers. The shim stock will align in a plane perpendicular to a line between the two points. The shim should be fairly thin as the angle developed is the arcsin of the offset/shim thickness so a thin shim is more sensitive to offset. I used piece of utility knife blade which I ground to about .8" x 1.5". As I recall, the thickness is around .035"

 

[marchyman]

My spindle bushings are new. Spindle axis alignment is unknown and needs to be checked. The Rollie's Dad method was unknown to me -- looks easy enough. Thanks for that pointer. Bed twist is a real possibility. I have almost zero space in the garage and the lathe is on top of a movable cart/table. I have to move the cart to use the lathe. It is not the most stable of work surfaces. I should see about trying to stiffen up the base before doing anything else.

The MT2 taper in the spindle was badly chewed when I got the lathe. I lightly ground the taper such that I can get repeatable readings, but I still don't trust it. If use of a center is needed I re cut the center taper each time it is installed in the lathe. Or I cut a center held in the chuck.

My vertical offset can easily be seen with the naked eye -- maybe .020". I need to get it a lot closer before using a shim between centers to check alignment.

Thanks for your comments. I've plenty to do to address this issue and get the best results possible from the old lathe.

 

[David S]

The tail stock body is actually two parts. The lower part that travels in the ways and the upper part that can be adjusted to offset it for taper turning. Have you checked to make sure the two sections are seating securely?

David

 

[marchyman]

Yes, there is no discernible gap between the two sections. Given the large offset between headstock and tailstock it would be easy to see if a gap in the tailstock was the cause.

And I just remembered this:

I found that crack after removing the rust from the bed. It is at the headstock end of the lathe. It does not go all the way through the bed but it could contribute to misaligned spindle, e.g. the spindle might be pointing down enough to make me think there is a height error. I'll be checking if the spindle axis is parallel to the ways later this afternoon.

 

[David S]

Oh that does not look good at all.

David

 

[Round in circles]

I checked the alignment of my headstock chuck & the tail stock chuck using two large sharp darning needles that I'd cut the eyes off .

With every thing closed up , the trailstock ram inboard there was barely a discernable difference between the needle points , four inched back with the tailstock & fully extend the ram , a bit more showed , reduced to nothing when I gently applied the ram clamp a bit but not so tensioned as to lock the ram . .


In truth when you think it through , it matters not much whether the center line is up or down a few thou , it is across the lathe bed that alignment that matters .
That's why you should use the gibs on & the adjusting screw at the rear lower edge of the tailstock to set it true to the centre line or for an offset as the need arises .

It took me a while for the penny to drop wrt alignment till I purchased an old second hand book of lathe practices . It showed how to make test bars & said why you needed them .
So may I suggest that you look up turning a test bar between centers ?

Make a 4 " one & then without doing any adjustments try & make a 14 " between centers . No doubt you'll be very surprised to find that on most lathes you can't do it with any great degree of accuracy . So to do the 14 " test bar between centers you'll have to sort out /check the alignment again .

If you make up an accurate set of say a 4 , 8 ,12 & 14 inch long test bars ( treat them like delicate eggs & protect them in individual packages ) it is a matter of just a few minutes work to change the alignment for the different distances & to compensate for any wear between the bed surface inner edge faces . all you do is pop the centers back in the head stock & tail stock then use a rounded blunt tool or a roller bearing on an angled bar securely held in the tool post clamp to lightly touch the bar at both ends ..... adjusting as required at the tailstock adjuster .

It may sound a total PIA but after a while when you get used to doing it with accuracy you'll speed up & start to get some precision in your turnings over varying lengths.

Scrub all the above I've just seen the " Very Interesting crack " after I posted this .

A pal had recently repaired a heavy casting off a big wood cutting cast iron bed bench saw for me that was broken in two at a point that was 3 inches long x 1/2 inch thick cast iron .It's holding a big 24 inch dia bench saw blade spindle to the saw bench .

Perhaps someone with cast iron arc brazing knowledge for an autisinal ( SP?) weld can give you some better advice than me .

For I'd say grind the faces clean of all paint , rust & crystallized broken cast iron . Then grind it out with a 2 mm thick disc into an equal Vee at the edges & cut right through the crack as far into the metal web as you dare .
Set it to be level over blocks of steel , using heavy weights or big clamps to ensure the bed is flat & true . Then arc braze it from both sides of the cleaned out crack . Leaving a decent ridge of braze standing proud across & above the crack on both sides .