Setting-up the PM 1236-T Lathe

[Titanium Knurler]

Knurler, I really like your idea of a solid plate to mount your lathe to... I'm always looking for a better way to do something!

When I built my lathe stand 38 odd years ago, I couldn't afford a big plate, nor did I have the equipment to move or lift it... So I built a stand with two small 3/4" thick plates around 10" X 12", with 3" X 3/8 angle iron connecting them, The whole thing has to be leveled together - not much of a burden, and has worked well all these years.

Dabbler, sounds like a lightweight version of a solid plate to me or what the airplane and boat guys would probably say was a plate with a lightening hole. Sounds like it has served you well for many years.

 

[Dabbler]

well it has, but it isn't nearly as rigid as your plate. I was in a hurry to get my lathe off the concrete, so I went with the money I had at the time.

My legs were also 3/8 3" angle splaying forward and back - the diagonals were 3/4 X 3/16 angle. Not pretty but later I found that all that space under would work for storage:

here's a pic or 2:

 

[Firstgear]

Firstgear, are you about something like equipment dollies with casters instead of wheels under each column of your stand?

View attachment 285878

If so, that will certainly allow you to move the lathe but they will not keep the stand from twisting and affecting the lathe adjustment. You would need to check and re-adjust with each move. I hope that helps.

Hmmmm, I know I am missing something, but what makes that different than yours? You have your ”leveled” lathe sitting on a base that moves. I would have a leveled lathe sitting on a base that moves. If yours comes back to level when you put it back why wouldn’t this come back to level when put back to the starting point on the floor where the lathe was initially leveled? What am I missing?

 

[Firstgear]

I basically want to easily move the lathe from the wall to get behind it, cleaning, into the electrical box, into the area where the coolant is added etc.

 

[mikey]

@Firstgear, that is an 1800# lathe. 4 casters might work but 6 would probably work better. I should think that 4 under the headstock end and 2 at the tailstock end would work well, and you could attach them directly to the cabinet instead of a plate. This would simplify things, keep costs down and give you the mobility for moving and solidity when the wheels are raised. Many a lathe is configured this way and it is known to work well.

 

[Titanium Knurler]

well it has, but it isn't nearly as rigid as your plate. I was in a hurry to get my lathe off the concrete, so I went with the money I had at the time.

My legs were also 3/8 3" angle splaying forward and back - the diagonals were 3/4 X 3/16 angle. Not pretty but later I found that all that space under would work for storage:

here's a pic or 2:

Ah, yes, I think I see the angled leg just under your chuck wrenches. It looks like you have probably had many happy hours at your lathe. Your chuck wrench holder reminded me of a slightly different design for my plate. It incorporates a couple areas for tools. If I had it to do over I would do it this way; always nice to have plenty of space for tools.

 

[Titanium Knurler]

Hmmmm, I know I am missing something, but what makes that different than yours? You have your ”leveled” lathe sitting on a base that moves. I would have a leveled lathe sitting on a base that moves. If yours comes back to level when you put it back why wouldn’t this come back to level when put back to the starting point on the floor where the lathe was initially leveled? What am I missing?

I think I may not be fully understanding your set-up. It seems to me that you plan on making TWO 1” thick plates that will fit under the headstock AND tailstock columns of your cabinet. If that is the case, the difference between your set-up and mine is that the when you move your lathe the two independent columns can move, well, independent of one another and will transmit that movement to the lathe bed. In my set-up that independent movement is eliminated or at least dampened by the equipment mounts so the SINGLE LONG PLATE ,that the entire lathe sits on, is not distorted.

I agree with Mikey, if you are going to rarely move the lathe put some casters with leveling feet directly on your cast iron columns or make some outriggers to attach them to and re-level the base and re-adjust after moving the lathe. If you want to try to eliminate twist with movement you would need to have a plate that runs the whole length of the lathe. I hope this makes sense. To look at Carrymaster casters go to Zambia.com for their catalogue. I hope this helps and good luck, you have a beautiful lathe!

 

[Titanium Knurler]

Vertical Misalignment and Taper

This is a brief discussion of how taper can be induced by a vertically misaligned lathe as the cutting tool travels up or down relative to the center-of-rotation(COR) as one moves longitudinally along the workpiece. There is a bit of math, but not much.

I guess one advantage of being a newbie is that you question just about everything. So, when I read in the PM manual that vertical alignment was not “nearly as critical as Center-to-Center alignment” I wasn’t exactly sure what that meant. Was it something that was a little bit non-critical and should be evaluated and adjusted or a lot non-critical and could be ignored?

I wasn’t able to find any information on this but surely it should be easy to figure out. I made some sketches and I was pretty sure that using the Pythagorean Theorem would be the key to solving the problem. As some of you may recall the Pythagorean Theorem just says that if you have a triangle that has a right angle in it and you know the length of two of the sides you can calculate the length of the third. I knew two of the sides of the triangle(the drop of the tool was one, and the radius of the work piece the other) I should be able to find the third side(the new radius of the workpiece). This will make more sense in a little bit when you see some diagrams. It seemed to work, but eventually I realized that the change of the depth of the cut depended on the radius of what I was cutting. For example, if you are turning a piece that is 12” in diameter and the tool drops down an eighth of an inch as you travel along the workpiece that will have much less of an impact on the depth of cut than if the workpiece was 1” in diameter. As the tool travels down the lathe longitudinally and the tool descends, the 1” workpiece surface will “move away” from the tool faster and the depth of cut will be less because of its greater curvature. Makes sense, but how do I account for this in the calculations? Well, I needed someone smarter than myself so I consulted Dr Craig Jackson, the Chairman of the Dept. of Mathematics at Ohio Wesleyan University and told him what I was after. Fortunately, he worked as a machinist while in graduate school and got to work on it and solved it in no time. This is his note to me(I wouldn’t spend a lot of time trying to figure it out):



Here is my translation:



The bottom line is that the change in depth of the cut is equal to the distance the tool descends(or ascends) squared divided by two times the radius.

He also created a nomogram!



To use the nomogram go to the right side vertical axis and select the radius of the workpiece and follow the curved line down to where it intersects a line extended upward from the horizontal axis that represents the vertical error(wear), then follow that line over to the left vertical axis to get depth of cut error. For example, for a 1” radius piece with a vertical error of .03 gives you a depth of cut error of 0.0004. Pretty neat, thanks Dr Jackson. Now I know that the vertical error of my lathe, which is somewhere near 0.0005 is nothing to worry about. Will I use this often, I doubt it but now I can read the statement, ”vertical error is not nearly as critical as center-to-center alignment” with new understanding.

How does a descending or ascending tool effect taper? I think I will just attach a couple of drawings and let you think about it a bit. In the drawings the tool travels from a to b(the result is the same if you go from b to a), COR is the center of rotation and the “x” marks where the tool is at the level of the COG:





Well, that’s it for leveling.

 

[mikey]

I assume by vertical misalignment, you mean the vertical misalignment at the tailstock? If so, and given that most lathes that are in decent shape will have very small amounts of vertical tailstock variance , maybe 0.010" or less, then the impact of this on work pieces of a practical size that will fit in most hobby lathes is essentially nil, with nil meaning a tenth or two. If this is so, and also given that scraping a tailstock so it is even front to back while also taking it down to compensate for said 0.010" of misalignment would be difficult, then the point is that we shouldn't worry about it at this level of error, correct?

My Emco lathe has under 0.0002" of vertical tailstock misalignment. Haven't worried about it before and I'll worry about it even less now.

 

[Titanium Knurler]

Mikey, what I mean by vertical misalignment is what one would see if the lathe bed was not leveled or badly worn. As the carriage moves longitudinally along the workpiece it, and the tool holder, descend(or ascend depending on the direction of the twist in the lathe bed), changing the depth of cut. It is what I assumed we are trying to eliminate when we “level” a lathe.