Improving Accuracy of Mini-Lathe Tail-stock

Introduction

I am still learning how to use my MicroMark 7x14 mini-lathe and MicroMark milling machine. However, one of the things that has been causing me a lot of grief is how inaccurate the tail-stock on the mini-lathe is. Because I am a complete novice, and because the equipment is so expensive (at least for me). I have been very hesitant to modify it. However, the inaccuracy of the tail-stock was becoming a show stopper and had to be corrected.

Before I begin explaining what I did I want to make a few very important points.

• This might not work for you. If you repeat what I did then you do so at your own risk. Like I said, I am a novice and I might have simply gotten lucky. You could destroy your equipment by repeating what I did. If you do then don't complain to me.
• It is your responsibility to keep yourself safe. I do not discuss how to safely use this equipment. I have removed chip guards and other safety features to make the pictures more clear. This equipment can cut through metal - it will cut through you a lot more easily.
• I didn't come up with any of this - I am standing on the shoulders of better machinists than I. (More below)

Great Sites on Tuning Tail-stocks

• Paul Hayward's Mini-Lathe Page. This is where I got most of the idea for this from.
• GadgetBuilder's MiniLathe Alignment Page
• Indicating Tailstock Adjustment Surfaces
  

Copyright

Copyright 2010 by Brian K. Alexander, Jr. All rights are reserved.

No one has permission to copy the information contained here. Please feel free to link to this page but do copy its contents or embed it into your site.

Terminolgy

I am a complete novice - I am going to use the following terms but they might not be the correct ones. If someone can tell me the correct terms I will update this article.

Horizontal Angular Alignment

The angle of the the tail-stock taper relative to top of the bed of the lathe. I want this to be completely parallel.

Vertical Angular Alignment

The angle of the the tail-stock taper relative to front edge of the bed of the lathe. I want this to be completely parallel.[/list]

Finished Product

I think it will be easier to understand what I am doing if you can see the finished product.

If you compare this to a standard MicroMark tail-stock you will notice that the two black screws in the bottom of the left edge of the tail-stock have been added. These will allow me to adjust the vertical angular alignment. The shiny nut and bolt just above them, next to the the cam-lock, is also new. That was the result of replacing the cap screw. This allows me to securely tighten the top of the tail-stock to the base without releasing the cam-lock (and knocking everything out of adjustment).

Cap-screw Replacement

The first problem I had is that the upper and lower halves of the tail stock are held together by a bolt that is only accessible from the bottom. This means that you have to slide the tail-stock off of the mini-lathe to tighten the bolt. I quickly learned that I could not remove the tail-stock without knocking things out of adjustment.

Fortunately, I could use the existing threaded hole as a guide. I disassembed the tail-stock and clamped the top half, up-side-down, on my mini-mill. The mini-mill was overkill for this but it was sitting right next to my lathe. Besides, its the best drill I have.

I used the mini-mill's table to position the existing threaded hole for the cap-screw so a 5/32" drill was slide right down the middle of the threads. I then drill the 5/32" hole right through the bottom of the tail-stock piece. I then widened the hole up to 5/16. This would allow a 1/4"-20 hex head bolt to fit through the hole with enough of a gap to allow the tail-stock to be adjusted. I found that a 1/4" hex head bolt would fit in the existing tail stock base without rotating but loose enough to slide in the channel for adjusting the tail-stock.

This modification is enough to adjust the horizontal angular alignment of the tail-stock. (See below).

Adding Adjustment for Vertical Angular Alignment

While measuring how out of tune my tail-stock was I found that its vertical angular alignment. I use Paul Hayward's idea for using two screws in the front face of the tail-stock base to adjust this. These screws provide stops when tightening the existing set screw in the back side of the tail-stock. Instead of the existing milling for the tail-stock base providing the angle the screws do.

The biggest problem with doing this is tapping the holes. The taps I have are too small to go all the way through the part of the tail-stock I needed to drill through. For this reason I had to drill the #25 holes entirely through the tail-stock for the 8-32 tap. I then had to drill about half way through the tail-stock with a 15/64" drill so that the tap could get entirely through.

The process was pretty simple.

1. I slid the head, with chuck installed to just above the tail-stock and set the stop for the head.
   
2. I drilled the #25 hole entirely through the front half of the tail-stock. I drilled these holes roughly halfway up the grove for the tail-stock base and about a 5/16" from the front and back faces of the tail-stock.
   
3. I drilled a 15/64 hole about half way through. This will allow the tap to get through the piece.
   
4. I started to thread the hole with an 8-32 tap in the mill's chuck. I did this by hand. The point is to ensure that the threads are strait.
   
5. Finished threading by hand. If you look closely at the second picture you can see that the shaft of the tap needs to descend into the hole for the threaded tap to come out the bottom.
   

Adjusting Horizontal Angular Alignment

There are already some great articles on this. In short, I mounted a dial indicator to the carriage of the mini-lathe and ran it down the top edge of the extended tail-stock. I added folded bits of aluminum foil until the indicator barely moved while using the carriage to move it along the tail stock.

Adjusting Vertical Angular Alignment

Once again, threre are already some great articles on this. In short, I mounted a dial test indicator to the carriage of the mini-lathe and ran it down the back edge of the extended tail-stock. Each time I needed to adjust it I

1. released the cam lock
2. loosed the original factory set screw in the right hand side of the tail stock
3. adjusted one of the screws I added in the instructions above. Tightening the front screw turned the tail stock clockwise. Tightening the back screw turned the tail-stock counter-clockwise. Note: You should only need one or the other.
4. tighten the factory set screw
5. lock the cam lock
6. remeasure and repeat as needed

Results

If you read the articles I referenced that you know that the real die-hard guys also shim the head of the mini-lathe. I am still nervous about messing with that so I tested with just the tail stock improvements. The picture below is before (left) and after (right) results for drilling and threading an 8-32 thread through the center of a PVC dowel. The dowel is about 1.25" long.

As you can clearly see the left hand hole was very far from center even after only 1.25". Even without messing with the head of the mini-lathe the accuracy is vastly improved.