Working out cross slide travel

[savarin]

On my 9x20 asian lathe the cross slide divisions are 0.025mm per division.
So, if I have a 12mm dia rod in the chuck and wish to turn it down to 8mm I nudge the tool tip to the rod and zero the cross slide dial.
I need to remove 4mm of material, so, as the cross slide feed removes twice the amount of material that the feed states I divided 2mm by 0.025 to get 80 divisions of feed.
I then start turning using the fine auto feed advancing the cross slide 0.5mm at a time (I'm not comfy with heavy cuts yet) until I reach 80 divisions.
When I measure the new dia its never 8mm its usually something like 8.2 to 8.78.
I know its not a huge amount and I can cut a tad more to get to size but I'm wondering why it doesnt add up.
What am I doing wrong?
I use digital calipers to measure the dia.
Thanks

 

[Phils69]

Put a dial indicator where you can measure the travel as you move the handle in. Zero the handle and indicator then do a full 360 degree revolution of the handle and compare. This will show you any error that you might have in the screw. Let us know what you find. Good luck

 

[Pacer]

Thats not uncommon - especially on a light lathe. Tool spring and type of tool, differences in metal, looseness in your cross/compound, etc, etc.

But, of course do as Phil says and determine just how your dial compares to an indicator as he described.

I think most of us experience this and just learn to compensate - I almost always finish that last little bit with a file or emery. You will also get the occasional cut where it will sometimes be 'laddered?' having .0005/.015 or so variations in diameter along the length - here again I break out the file or emery and clean it up.

 

[Bill Gruby]

I take it you still have the 8mm lead screw on the cross slide. The lead screw nut is cast, not billet. I have no clue what metal it is. I know mine failed because of heavy cutting. Yours could be worn beyond usable limits. I made a new one for my 9X20. It is now a 7/16-40 lead screw with a heavy Bronze nut. Backlash is now minimal. Problems like yours went away.

"Billy G"

 

[PurpLev]

as mentioned above this could be due to tool/post deflection, work holding, backlash in the x-slide screw to name a few.

While the replies and the question was about the 'why', I thought I'd throw the following in just in case - it's good practice to take initial measurement to get a rough idea of how much material to take off, then take a few rough(er) cuts to get you close but still UNDER the desired total cut depth, take a 2nd measurement to see how much more material should be taken off in the final finish(ing) cuts and take those finishing cuts which would have less deflection forces on the cutter/workpiece resulting in more precise cuts, and higher chance of meeting that total (4mm) cut requirement on the money.

remember:
1. initial measurement
2. rough cuts to get you within a finishing cut depth to the final size
3. 2nd measurement to affirm how much is left to take off
4. finishing light cut to meet requirement

 

[savarin]

Thanks guys,
I dont feel quite so bad now.

Pacer, I have seen the laddered effect on a 6" length and also felt a difference in the hand wheel when using the carriage feed, I wondered if this contributed to that effect.

Phils69, I will do this asap and report back.

Bill, I think its a zinc alloy of some kind but I dont trust my standard of work to make a new one as yet.

PurpLev, thats basically what I've been doing to get the correct size but I havnt got the experience to hit the nail on the head exactly yet.

I'm still trying to get my head round what is "good enough accuracy" I'm anal enough to want it spot on to the accuracy of my test gear but I'm not there yet.

 

[savarin]

Some numbers.
1 full revolution of the cross feed handle is 50 divisions and should read 1.25 mm of travel.
It was spot on, but,
2 full revolutions ended at 2.48mm of travel.

The dial gauge I used was 0.01mm per division and gives consistent readings.
There is 0.012mm backlash in the screw.
What would be a reasonable amount of backlash to live with? I would like none but believe thats not possible is it?

 

[PurpLev]

Some numbers.
1 full revolution of the cross feed handle is 50 divisions and should read 1.25 mm of travel.
It was spot on, but,
2 full revolutions ended at 2.48mm of travel.

The dial gauge I used was 0.01mm per division and gives consistent readings.
There is 0.012mm backlash in the screw.
What would be a reasonable amount of backlash to live with? I would like none but believe thats not possible is it?

Thats 0.02mm (or under 0.001") error for 2 full revolutions - I don't think it's that bad. you have to remember that you measure and focus your measurements on your work piece, not your machine (as long as your machine is dependible within reason which it seems like it is). for final finishing cuts you will not be taking 2 full revolutions per cut, so that error will be far diminished and as you stated, a single revolution did not produce any error at all - which also suggest there might be something on your lead screw at that rage of the 2nd revolution to push it out of reading.

the backlask also seems reasonable and with the nature of these nuts and screws you'll always have some amount of it present.