New PM-1236T Lathe has arrived

[skcncx]

It is probably alright for most lathe work. But I have a milling vise that I want to use on the lathe as a backup milling option. I can't do accurate milling with a vise that runs downhill with respect to the spindle. I have reached out to P.M. for a solution. They are contacting the factory.

I would think a cross slide that runs down or up hill to the spindle is an issue for turning as well... if you have a larger diameter that you are turning down a bit on the outside you could be above center line and below the center line as you get closer to the center and vice versa.... maybe not enough to make a big difference. I have no expertise!

What milling vise do you have? I've wondered if one is worth it when I get my PM-1236T for the simple things.... though, I'm actually considering their PM728VT as a starter mill when I pick up my lathe in a couple months. They have a lot of these in stock.

 

[riveter]

I would think a cross slide that runs down or up hill to the spindle is an issue for turning as well... if you have a larger diameter that you are turning down a bit on the outside you could be above center line and below the center line as you get closer to the center and vice versa.... maybe not enough to make a big difference. I have no expertise!

What milling vise do you have? I've wondered if one is worth it when I get my PM-1236T for the simple things.... though, I'm actually considering their PM728VT as a starter mill when I pick up my lathe in a couple months. They have a lot of these in stock.

There is a cosine error introduced by a downhill running cross slide. It would affect the accuracy of diameter turning. It is minor, but my aim is to reduce sources of error. The milling vise is a Palmgren. It looks like it will require an adapter to fit it to the lathe.

 

[tq60]

Your ways ma be at different heights reference to center

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[mksj]

I do not follow what the issue is, as long as the distance from the center of the spindle to the cross slide remains the same when measured at 90 degrees from the cross slide to the center of the spindle along its travel. The carriage ways are separate from the tailstock ways, so using 123 blocks as shown is measuring the flat of the tailstock way to the flat of the carriage way. As I mentioned the the bed flat surfaces height varies relative to the cross slide level, all that matters is there is no bed twist and no change to the cutter height/spindle center through the cross slide travel.

 

[riveter]

I do not follow what the issue is, as long as the distance from the center of the spindle to the cross slide remains the same when measured at 90 degrees from the cross slide to the center of the spindle along its travel. The carriage ways are separate from the tailstock ways, so using 123 blocks as shown is measuring the flat of the tailstock way to the flat of the carriage way. As I mentioned the the bed flat surfaces height varies relative to the cross slide level, all that matters is there is no bed twist and no change to the cutter height/spindle center through the cross slide travel.

The two flat ways appear to be in the same plane. And the PM-12136-T manual says:

Check and adjust level front-to-back across the bed using a matched pair of spacer blocks to clear the Vee tenons on the bed ways.

So using the flat ways is how the bed is leveled. I did that and then placed my level on the cross slide and found it to be way out of level. It took a 0.030 inch shim to bring the level back into level on the cross slide. I then placed a ground straightedge on the 123 blocks, mounted an indicator in the tool holder and fed the cross slide across the bed indicating off the straightedge. The indicator showed the tool holder traveling down hill 0.015 inch in about 7 inches. So if I mounted a milling vise on the cross slide and took a cut with an endmill, the cut would not be parallel to the fixed jaw of the vise. I posted a video which should make it clear.

 

[mksj]

The manual indicates you use one or the other, they are not the same. On my 1340GT the difference was the same as you show, on my ERL-1340 there is also a difference in level between the two surfaces. As I mentioned, the only thing that is relevant is that is that the surface that is moving cross slide remains at the same perpendicular distance (height to center i.e. parallel) accross the cutting plane. You could attach a dial gauge or test indicator to the center of the chuck down vertically and then sweep the top of the cross slide and measure the relative change in height, it should remain the same (+/- 0.001"). Level is relative to determine bed twist, as long as you use the same measuring points along the bed, but I have yet to see a lathe that the flats are level with the cross slide surface or the V ways.

I prefer to use the 123 blocks on the flat parts of the way to determine level, others will use the peaks of the ways or the carriage. On a lathe with worn ways, it is a bit different.

 

[riveter]

The manual indicates you use one or the other, they are not the same. On my 1340GT the difference was the same as you show, on my ERL-1340 there is also a difference in level between the two surfaces. As I mentioned, the only thing that is relevant is that is that the surface that is moving cross slide remains at the same perpendicular distance (height to center i.e. parallel) accross the cutting plane. You could attach a dial gauge or test indicator to the center of the chuck down vertically and then sweep the top of the cross slide and measure the relative change in height, it should remain the same (+/- 0.001"). Level is relative to determine bed twist, as long as you use the same measuring points along the bed, but I have yet to see a lathe that the flats are level with the cross slide surface or the V ways.

I prefer to use the 123 blocks on the flat parts of the way to determine level, others will use the peaks of the ways or the carriage. On a lathe with worn ways, it is a bit different.
View attachment 418107

You could attach a dial gauge or test indicator to the center of the chuck down vertically and then sweep the top of the cross slide and measure the relative change in height, it should remain the same (+/- 0.001")

I will do this test tomorrow.

 

[qualitymachinetools]

Mkjs got it. The cross slide is not going to be in plane with the 2 flat ways. Carriage only uses one flat way in the back, and then the V in the front. The other flat is for the tailstock. They are parallel to each other, but not in relation to the carriage.
The flats are one way to level the bed to make sure no twist. I have never seen a lathe that was dead on or even within a few thousandths of the flat ways compared to the cross slide.

Its hard to explain, but think slant bed CNC Lathe. Your tool is not moving .030 up and down with the difference there.

 

[mksj]

Below is the measurement in height sweeping my cross slide about 5" (I did not want to remove my QCTP), the dial indicator lines are 0.0001". If you recall my cross slide appeared tilted relative to the flats, but essentially the perpendicular height from the cross slide to the center of the chuck is unchanged while sweeping the top of the cross slide with the indicator attached to the chuck. Pretty tight tolerances when you consider all the surfaces that need to be spot on when machined/scraped.

 

[riveter]

I indicated my cross slide with respect to the spindle and it looks good. I think this is the correct way to test what I was looking for. Thanks everyone for helping me with this. Now I can get on with making nice things.