I have a Taig lathe for some time. One of the things which annoys me is that the cross slide turning dial (as well as dials for the other axes) has a lot of backlash which means that it is tedious if not difficult to move the tool to a precise position and make an accurate cut. To get around this problem I have decided to install a direct digital readout (DRO) at least onto the cross slide initially. However DRO's are expensive so I decided to make an economical yet reliable and reasonably accurate one.
There are a lot of cheap Chinese digital calipers which you can find on the popular e-purchase sites. I have chosen a miminal 100mm one for the cost of a few dollars and build my DRO around it.
In addition to the calipers the following parts are needed.
1. M3 6mm brass hex standoff for electronic boards with two matching M3 screws and washer;
2. Small piece of brass some 8 x 30 mm
3. A little epoxy resin; and
4. A small piece of double sided adhesive tape.20 x 20 mm.
No special tools are needed. There is no need to drill any holes or modify the cross slide, and the assembled DRO can be removed at any time without any damage to any part of the lathe.
Step 1: Modifying the Digital Calipers
The digital calipers are carefully disassembled by peeling off the backing sticker and removing 4 screws. Carefully cut away all the jaws from the sliding arm and the main body. This type of callipers is made of plastic (or carbon fibre as claimed which is doubtful) and cutting is very easy. I used a dremel with a plastic cutting disk. You can simply use a small coping saw, jig-saw or whatever saw conveniently in hand.
The stripped arm is dressed clean at the cuts.
A small piece of brass about 8 x 30 mm is bent into an L bracket bent at about 8 mm on one side and 20 on the other, and a 3 mm hole is drilled into the centre of the shorter length. I happen to have a small piece of junk brass bracket of roughtly the right size but with a slightly different configuration which I reshaped for use. The long end is then attached to the '0' end of the arm with epoxy glue as show in the photo above.
Reassemble the calipers and attached a 20 x 20 mm piece of double sided adhesive tape to the back, in the space between the two rows of screws.
The device is now ready to be installed.
Step 2: Installing the DRO
In order for the DRO to work properly, the modified digital calipers need to be attached at two points. The measuring end or '0' end of the arm is attached to the corner at the front of the movable cross side, and the body of the calipers is attached to the base.
The small M3 screw just about slips into the side slot of the movable slide. If it does not, just file away a little bit from two sides of the screw head. Slide the screw head into the slot and fix it with a hex M3 standoff. Exact positioning is not important. Attach the sliding arm of the calipers onto the standoff with the second M3 screw, using a small plastic or metal washer to get a good grip.
Peeling off the protecting sheet from the double sided adhesive tape, push down and attach the body of the calipers onto the far side of the cross slide bed. Again the exact position is not critical.
The DRO is now installed and ready for use.
Step 3: Using the DRO
As the cross side is moved in and out, the reading on the digital calipers changes with its new position. Because of the way the DRO is mounted, the device gives correct positive readouts.
On first use, the DRO is caliberated. This is simply done by securing a centre point in the jaws of the chuck and moving the cross slide in until the tip of the mounted tool touches the centre tip. Zero the readout. As the slide is moved away, the display will show a true reading of how far it has moved. Choice of metric or imperial units is on the calipers.
The calipers use a single 1.5v button battery but with very little drain in use it is expected to last a long time before it needs to be replaced.
This installation is a complete success and it only costs me a few dollars and maybe half an hour of work. I commend it as a worthwhile project to undertake.
Step 4: Pros and Cons
Having installed this DRO I have tested it with a number of sample turns. There are some pros and cons of this cheap contraption.
This DRO is simple, requiring no setting up and little or no maintenance except re-caliberating every time you change to a new tool bit in the tool holder.
It allows quick and easy repeat of a dimensioned cut.
Although the display is clearly visible it is not in a very convenient position. One has to lean forward over the lathe to see it clearly. However this is only a minor irritation.
The cheap calipers have only a resolution of 0.1 mm which is good enough for rough work but not quite adequate for accurate work.
I am getting a new set of calipers with 0.01 mm resolution which may cost up to $20. Modification and instalation should be just as simple. This more expensive model also has a serial interface which will allow me to make a stand-alone display which can be placed in a more convenient spot. I am looking forward to these improvements a little later.
Improvement Complete (20140423):
Another set of calipers with higher resolution which I have ordered earlier has just arrived, and I have now installed it on my Taig. This set is made of stainless steel so it takes a little more effort and care to cut off the jaws. Otherwise the modification process is the same.
I have also found an alternative and more accurate method of caliberating the DRO. First, chuck in a cylinder of any uniform radius. Use a separate set of calipers and measure its diameter. Divide this by two to get its radius (r). Now, move the tool bit to just touch the edge of the cylinder. Zero the DRO display. Remove the cylinder and move in the cross slide until the display shows the negative value of r. This is now at the rotational axis of the lathe. Zero the DRO again and the caliberation is done.
The result of this improvement, I must say, is as good as expected. I can now repeat the same cut from both ends of a rough cylinder and meet smoothly in the middle. The cost of a few extra dollars is completely worthwhile.
Photos of this improved version are posted on top.