Machsupport Forum
General CNC Chat => Show"N"Tell ( Your Machines) => Topic started by: N4NV on March 03, 2009, 04:25:29 PM
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I know, I have too many projects already, but in case I am off work for any length of time, I am trying to gather the items I will need to convert my Levin watch lathe to CNC. The attached picture shows the basic layout of my Z axis. Besides the mess of my watch bench, in the picture you can see a Taig cross slide I made to fit the lathe. I had do some special machining to get it low enough to fit under the spindle. There is also I micrometer XY slide I put together. I have two ball slides that are going to be the Z axis. In the bottom of the picture you can see a spare lathe base that I will use for this conversion so I don't have to drill holes in my chromed Levin bed. I am pretty good at hand scraping now and will make sure it is better than .0001" flat.
In between the ball slides you can see the precision ACME screw I got from NOOK. It was cheaper than a ball screw but has better tolerance than most rolled screws. NOOK guarantees better than .0001" per foot. It is 1/4x20 thread. The little stepper at the end of the screw is 200 steps per revolution. This will give me an accuracy of .00025" per step. With micro stepping it will be ever better. I purchased 4 of these little steppers from eBay for $25 total with the encoders! The guy was selling the encoders and the steppers were something he threw in because he did not want to take the encoders off.
I also got some plastic ACME nuts from NOOK for the screw. I plan on cutting them in half and installing a spring between the two halves so I can get zero backlash. The loads on this lathe will be so light that it won't take much preload. The X cross slide will look similar. I have one more ball slide that I will cut in half to get me the X direction. The total X movement will be about 2", it looks like I will get almost 5" on the Z though 2" would be more than enough.
I hope to be able to make custom balance staffs when it is all completed. A balance stall might be 3mm long, 1mm at it widest, and have pivots that are .12mm.
Vince
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Looks like a fun one, Vince, I look forward to seeing it progress-
EDIT: by the way, your digital calipers are on, don't forget to turn it off - ;)
Dave
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EDIT: by the way, your digital calipers are on, don't forget to turn it off - ;)
Dave
The calipers turn off automatically after 5 minutes of no use.
Vince
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Well excuuuuuuuuuuse me, Mr. Fancy pants...... :D
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Vince,
It should be neat project. Here's a suggestion before you cut the nuts in half. Take a look at Sherlines anti backlash
system. Since your screw is not an odd ball you can make an adjustable and replacable antibacklash system for both axises and incorporate it into the design. Worth looking into.
Looks like your getting ready for those addictiive small model engines.
RICH
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Rich, the only thing I could find on the Sherline anti backlash system was a locking lever that would lock the Z axis once it is in place. It was not meant to use on a moving axis. Do you know of something else?
Vince
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Vince,
Originaly just a lock for the mill, but then they found out that it worked for taking backlash out.
I put it on my Sherline and works great. It's also adjustable. Say's it right in the instructions.
The only reason i couldn't make it for the Sherline was the goffy thread they use for the Z and you can't find a left hand tap for that size no where or at least not cheaply and they know that.
RICH
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Vince
get 2 steel nuts.mount on a piece of shaft tighten them together so you can machine outside.machine outside of nuts for 20 tpi . on the end of each nut drill two holes for for adjusting. bore a sleeve for
the 20 tpi nuts to mount in.and use the sleeve to mount to the carriage. slice the sleeve so you can
lock the nuts after adjustment.the slot in the sleeve also gives you a way to oil the nuts.
bill
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This will be one of the slowest builds. I have several other projects I am working on at the same time. I scraped the watch lathe bed today. I was lucky enough to find a Biax scraper on eBay a while ago so it went pretty fast, less than an hour. The process of scraping is pretty simple to say but takes a lot of skill to do. You just put some Prussian blue or high spot paint on a surface plate, put the surface of the object you want to scrape face down on the surface plate, move it around a little, then just remove the blue by scraping. The blue will be the high spots. I will not go into scraping very deeply, that is a subject for a book, namely "Machine tool reconditioning" by Connelly. There are a few quick tips. Make sure you don't have any big burs on the surface you want to scrape or you could gouge your surface plate. After each session of scraping, run a file lightly across the surface to remove any burs that are brought up by scraping. Make sure you put even pressure on the surface that you are marking. When done properly, the surface you scrape will be just as accurate as the surface plate you used to mark. With the three plate method you can easily get an accuracy of 50 millionth of an inch. This was done in the 1800's.
I have 4 pictures in this post and two more in the next post (I can only load 4 pictures per post). The pictures were taken during various steps of scraping.
Vince
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I think I did about 10 marking and scraping steps even though there are only 6 pictures. You can see the dark blue on the ends of the lathe bed at the start. The blue get lighter and lighter after each step, not because there is less blue left on the surface plate (I actually added more about half way through), it is lighter because the surface is getting flatter and flatter and there is less room for the marking blue. At the last picture, the lathe bed is flat within the limits of my surface plate +-.00005". I have a black granite surface plate. The marking blue will permanently stain your surface plate. You can barely tell on my black plate, but I would think twice about using a pink surface plate.
Vince
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Nice work Vince.
Tweakie.
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Viince,
Was the bed hardened?
How's the elbow's feel after doing the scraping? ;)
RICH
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I do not believe the bed was hardened. The Biax scraper I was using is electric. http://www.dapra.com/biax/scrapers/
I learned how to scrape by reading the Connelly book and re-scraping a Southbend lathe. That was all done by hand with a carbide scraper.
Vince
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I found some anti-backlash nuts on the web that have the same 1/4-20 acme thread as my screws. The manufacturer claims they are backlash free up to 6 pounds. Since I will be operating in the oz range, they should work fine. In the picture they are the red part, the round disk is the mounting flange that I will have to machine smaller to fit.
Vince
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Vince,
Should work for what your doing and time will tell. Remember that another source of backlash will be
at the bearing so take a look at how Sherline does their screw to motor mounting. Just a few thoughts.
RICH
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With the loads so light and the travel so small, I am not going to use a bearing. The lead screw will attach directly to the stepper motor. The stepper motor has a thrust washer installed and my test shows it takes 5.2 pounds to get this washer to deflect and as a result cause backlash. So at the few ounces of thrust I will be using the lathe should stay zero backlash.
Vince
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I have been busy with other projects. I just about finished my Miser stirling engine. You can see that here:
http://www.youtube.com/watch?v=vdBYzzhe_To
I laid out, drilled and tapped the holes in the lathe bed for the Z axis linear rail. I made lots of measurements with my Mill prior to drilling the bed and found that it was straight and parallel to at least .0005" (my mill has about .0002" repeatability). After drilling and tapping the holes I put the bed back on my surface plate and marked it with high spot to check to see if any of the holes pulled up some high areas. I had to do a little touch up scraping around three of the holes, but other than that it stayed flat to at least .0001".
I have to find some special metric screws to proceed so I can attached the cross slide to the "trucks".
Vince
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Pretty nifty, Vince, did you dust this on a grinder to get it within a tenth?
Thanks,
Dave
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Pretty nifty, Vince, did you dust this on a grinder to get it within a tenth?
Thanks,
Dave
A grinder is not accurate enough. I hand scraped it and used a surface plate that was good to .00005". You can see part of the process at this thread:
http://www.cnczone.com/forums/showthread.php?p=587073#post587073
Vince
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??? a grinder is not accurate enough?
Um, ok, I'll just say you're doing a great job and stay out of this one - ;)
Thanks,
Dave
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When finishing very accurate machine tools, grinding is usually the step before scraping. If you ever get a chance look at Connelly's book Machine Tool Reconditioning.
Vince
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I loosely mounted the Z rails until I install the X platform. I will be working on that next. I have about 20 holes to drill in it to mount it so the Z axis and also to mount the X rails. I installed longer screws on the ends of the rails to prevent the trucks from running off and loosing all those ball bearings.
Vince
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I started on the X axis cross slide. I ordered some precision ground flat stock. It was +-.0001". After I cut it, it curled up to where the ends were now .0004" low and the middle was .0004" high. It took me almost 4 hours of scraping to get it back to flat +-.0001". I cut a groove down the center (before I started scraping) with a 5/8" ball end mill to made room for the Acme nut. It did not warp any after the groove which surprised me.
Vince
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I cut the ball slides for the X axis. It turned out they were hardened and the only way I could cut them was with a peanut grinder and an abrasive wheel. I mounted them on the carriage to see how they would fit. I found when trying to adjust the Z axis ball slides parallel, even .0002" would cause the carriage to bind a little. It took a lot of adjusting to get is just right. Of course I will have to do it all over again since I have to take it all apart to get the motor mount supports attached to the lathe bed.
Vince
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:) All ready for York by now I suppose. :)
Thanks,
RC 8)
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Thought you were done with the lathe? Keep posting your progress.
RICH
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Thought you were done with the lathe? Keep posting your progress.
RICH
When I was working on the lathe earlier this year I was laid off. I am back working now and very busy. I need to get back to it but I have had several projects that are taking president. I still plan on completing it, and when I work on it I will post.
Vince