Making you own test bars

[MontanaLon]

So I have been looking at test bars for a while and just can't afford one. So I decided to make my own. This will be a short story of my adventure this far.

I grew tired of the nearly endless process of adjustment to get my tail stock aligned within a reasonable degree to my head stock. 1939 lathe that has some wear and no real way of measuring the distance the tail stock moves when the adjustment screws are moved. While the play of the tailstock on the bed is somewhat "generous" the fit between the top and bottom have of the tailstock are tight with no perceptible movement at all in the Z axis even with the adjusters loosened and the bed clamp released. But once I started adjusting the adjustment screws things went to hell fast. I pondered buying an alignment bar, but as I know many of you here are not doing this for work and expenditures are hard to justify for a hobby.

So I set my brain to work and my hands into the scrap pile to find some workable pieces that would fit the purpose. I came up with a .750 bar about a foot long that I had picked up somewhere for a little bit of nothing and the thought that it may come in useful. And it did. But it took a while to figure out the best use and how to make it work for me with the least amount of trouble. And I read many different explanations on how to make a test bar and watched some videos and found what seem to me to be a better way.

First of all, the videos always said to turn down the middle and have ends like a barbell. I found that is completely unnecessary even though any chance to make chips is a good time, turning down the center section of the bar doesn't really make a difference and is just that much more chips to sweep up.

So here is how I do it. Chuck up the bar in the 3 jaw chuck and drill a center into it on both ends. Then pull a length out of the chuck and use a live center in the tail stock to support it. Turn down just the end, an inch is plenty to a size as closely as you can measure. Now reverse the piece and turn the other end to the exact same size. Get it within 0.0001 if you can. It is important to use the tail stock as that will make the ends concentric to the center in each end. If you just turn it in the 3 jaw it will probably still work but why use probably when you can be sure?

That is it, your test bar is done. Now to put it to use. Put a center in the head stock and put the bar between centers. Using an indicator starting at the head end take a reading and zero the indicator. Now move to the tailstock end and take another reading. Adjust as needed to get alignment. You can go as tight of a tolerance as you want but 0.001 is close enough for me. It is easy and it will be true, but when you take your first cut on the workpiece look for taper. Normally I can get it down to 0.002 over the length of the part being turned.

There are some issues with the test bar and the lathe or rather the wear in the lathe. If you get it dialed in and then move the tailstock at all you may end up with taper. Just the nature of working with old iron. So use the headstock quill only to move the center. If you slide the whole tailstock things can change quite a bit. So what do you do when the desired part is shorter or longer than the test bar? Make another test bar of the right size of course. I have a bunch of them shorter than a foot as I rarely do any work longer than that. But if I do ever want to do longer work, I can just make a test bar of the appropriate length and run with it. Is it dead nuts accurate like a factory test bar would be? Probably not, but it is certainly accurate enough and you can make a whole bunch for way less than the cost of the factory model.

I used to spend a lot of time getting rid of taper in long parts, Now I can do it in no time at all and get on with the project that much faster.

 

[graham-xrf]

First of all, the videos always said to turn down the middle and have ends like a barbell. I found that is completely unnecessary even though any chance to make chips is a good time, turning down the center section of the bar doesn't really make a difference and is just that much more chips to sweep up.

I have two purchased test bars, and I have one homemade re-cuttable barbell (dumbel shape??) bar. It is 20mm most of the way, and what started out as 25mm on the ends, now shaved down by a few finishing cuts. I will need to be doing all this again for my own kit soon, but I understand the whole purpose of using the barbell shape quite differently. Some folk will go to considerable trouble to construct a deflection-proof test bar, plugging the ends to have centres, and adding the larger diameter ends.

The whole point is to be able to have the carriage take a fine finish cut near the headstock end, and have it travel all the way to to the tailstock end without touching any other adjustment at all ! Everything in the stiffness path through cross slide and compound (if used), up to tool must not move in Y axis. Without the barbell ends, this can't reasonably be done without also cutting along the length. The whole operation is a tightly controlled calibration to get at micrometer diameter measurements My first tries were not careful enough. All of this was to establish what taper, if any, was there, and blame tailstock Y-axis offset if it was seen (it was!) .

This use of a test bar is not quite the same as then running it with a dial indicator on it. Once one gets it to the point it turns the same diameter on both ends, one can then use it with a dial indicator in other ways. It sounds as if you already got well into doing that.

I used to spend a lot of time getting rid of taper in long parts, Now I can do it in no time at all and get on with the project that much faster.

Yes - I agree what you have explained could improve the taper issue. I went at it a whole lot more hardball in discovering exactly what wear there was, where it was, and how much it affected the outcome. Others replied to me in this forum saying that one can get good results, even with pretty worn ways, if only because the tool is never displaced (from where one starts the cut) by as much as the wear. It starts at a (worn) place, and usually ends up at a (similarly worn) place, the error being only the difference in wear between the two locations. Also, vertical wear on prism ways does not directly translate into Y-axis tool displacement. The error can be tiny.

If I understand it right, your experience of moving the tailstock messes up the taper, but moving the quill does not. (I think you meant to say "tailstock quill"). Is this because there is enough curve in the ways, from wear, to mess up the tailsock Y-position somewhere in the space in the middle? I would look hard to establish this. If the tailstock adjusters are locked such that there is no Y-axis play at the position by the end of your test bar, but are able to slop sideways when the tailstock is moved forward into the suspected worn region, then you have found the cause.

Regardless, if you have fixed it up to the point you can turn to better than 0.001" even over a part the length of your rod, you must be doing OK