Zeitlow Designs DRO2

You’ve mastered the basics of using your minilathe and are sure you are in this hobby for the long run. You’ve done all the standard mods and a few of the more exotic ones.

Then you start thinking about some really cool stuff you could do - like adding a digital readout (DRO).

A DRO uses sliding scales - very similar to a digital caliper - to translate the X or Y position of the lathe carriage to a direct readout to the nearest thousandth of an inch (.001").

Besides the obvious convenience of being able to know the actual position of the cutting tool relative to the work at all times, a digital readout can be especially helpful to those of us in the “over-50 crowd” whose eyesight is not as good as it once was.

Additionally, you can switch freely between metric or inch units and work with the confidence that “what you see is what you get” with respect to measurements since, once zeroed to the work, the DRO is reading the actual position of the tool.

Zietlow Design manufactures a really nice readout that’s just the right size for the minilathe. About 2 1/2 x 4 1/2 x 3/4", the housing is available in either black or light gray color.

Large bright red (or green - your choice) LED readouts with digits about 3/8" x 1/2" in size are plenty big to read from several feet away and bright enough to be visible in just about any lighting conditions you are likely to encounter in a shop.

The photos in this article don’t really do justice to the true appearance of the digits - they are actually a deep red color with excellent contrast. The photos on the Zietlow web site do a better job of capturing their appearance.

Features#

On the left side of the housing are three jacks:

• Top - input for top display
• Middle - input from power supply
• Bottom - input for bottom display

The unit is powered by a plug-in power cube like a wall-wart on Slim-Fast. Unlike typical plug-in power supplies, the thin design of this one does not block access to the adjacent power outlets.

Since you can never have too many outlets in the shop, this is a nice feature.

On the front face of the unit are the two 6-digit displays and three control buttons with the following functions:

• Top - set top display to zero
• Middle - Toggle between two programmable display modes
• Bottom - set bottom display to zero

The three buttons are also used to program the various selectable operating modes.

There’s more to this DRO than you might guess until you read the thorough and well-written user manual. There’s a lot to describe, so rather than reinvent the wheel, I have reproduced below a segment of the DRO-2 user manual:

- Dual Display Functions -#

DRO-2 has two separate display settings. The user toggles between the two by pressing the center (mode) button. The two settings are freely programmable, to display any of the different modes.

- Zero Buttons -#

Of course, each scale has its own zero button located to the left of each display. Pressing the button will zero the display. It should be noted, that since the scales’ own LCDs cannot be zeroed in high speed mode their reading will not change.

Not a problem, because you will never have to look at them again…

- Inch / MM modes -#

The displays can be programmed to convert the scale readings to either inch (imperial) or mm (metric). The range in mm mode is -999.99 to +999.99 with a resolution of 0.

01 mm. In inch mode the range is –99.9995 to +99.9995 inches with a resolution of 0.0005 inches. The half mil is displayed by the decimal point to the right of the least significant digit.

- Sign Selection -#

The direction of the scale readout can be programmed. This allows the scale to be mounted in a way that is easiest from a mechanical viewpoint without having to worry about the direction of the readout.

This is programmed once during setup and retained in non-volatile memory.

- Diameter Mode –#

Each display can be programmed to read either 1x (radius) or 2x (diameter). 1x is the normal mode as it should be used on a milling machine for example.

The 2x mode is useful for lathe cross slides when the user is more likely to be interested in the diameter of a part being turned. Just make sure to turn it back to 1x when using a milling attachment on your lathe.

- RPM Mode -#

DRO-2 has an input for an optional photo-interrupter sensor on the upper scale connector to detect the rotation of a spindle and display its RPM. The range is 30 to just under 100,000 RPM.

When the range is exceeded either –SLO- or –HI- will be displayed. The RPM number is updated once per spindle rotation, but no more than 4 times a second to facilitate easy readout.

- SFM Mode -#

DRO-2, when equipped with an RPM sensor, also calculates cutting speeds in surface feet per minute for lathes. This works by taking the reading from the upper scale, which on a lathe should be connected to the cross slide, and calculating the cutting speed using the following formula:

SFM=RPM * 2 * Pi * cross slide reading in inches / 12

The cross slide has to be set to zero when the cutting edge of the lathe tool bit is at the center for SFM mode to work correctly. The SFM reading will be calculated properly regardless of the readout mode of the cross slide scale (1x or 2x)

The display range is 0 to 100,000. Just as in RPM mode when the range is exceeded either –SLO- or –HI- will be displayed. Just don’t try cutting anything at 100,000 SFM…

- Limit Stops -#

When pressing the zero button while holding down the center button the current position is stored as an electronic limit. Whenever the scale is moved across this position a pin on the lower scale connector changes logic states.

This can be used by some external circuit to stop a power feed and thereby limiting a cut to not go beyond the set position. The two scales have separate limit memories, but act on the same pin.

This shouldn’t be a limitation even if a lathe or mill has two power feeds as they wouldn’t be used at the same time. Just make sure to set the limit of the unused axis at a position outside the work envelope of the part you are machining so you don’t accidentally trigger the limit bit.

If there is enough interest, we will design a control box to hook up to this. If you are interested in building your own circuit please contact us, so we can help you get started.

- Display Off -#

Removing power from the DRO-2 box will erase position information and the scales will be reset to zero on power-up. When you want to turn off the display without losing position, just press the center button for 3 seconds.

The scales are still being read, so it will keep track of position. Pressing any button will turn the display back on. This is a handy feature to have if your machine shop also happens to be your bedroom (!?!) and you can’t sleep under the bright light of our high efficiency LEDs…

- Data Output -#

For use with future products all readings are also sent out over a serial data link on two unused pins of the power connector. We currently have no concrete plans for this, but thought it would be nice to have…

Currently (06/03) the DRO-2 is priced at $169 - an excellent value and well within the reach of many home shop enthusiasts. The only other DRO units that I’m aware of in this price range use LCD displays that are much less visible.

Included are the cables for the two displays and the power supply. For another $10 you get an optical sensor assembly that drives the RPM and SFM readout modes.

Digital Scales#

If you intend to outfit your lathe with digital readout, you will need to invest about another $100-150 for the digital scales that plug into the DRO-2. These are available from a number of vendors on the Internet in lengths from 4" to 30" and priced from around $40 for a 4" scale up to $170 for the longer units.

eBay is a good place to see offerings from several vendors and compare prices; search on “DRO Quill kit”.

You will need a 4" scale for the cross-slide and a 10" scale for the carriage travel (8" for a 7x10). You actually get about an extra inch of motion beyond the nominal size of the scale, and the carriage motion on the 7x12 is limited to about 11" even with the threading dial removed. Mounting the scales is a whole engineering exercise in itself which I won’t get into here, but I plan to publish a Premium Content article with detailed plans by Fall of 2003.

Meanwhile, Zietlow Design provides some suggestions for mounting the scales in the DRO-2 user manual.

Performance#

Once you get the scales mounted and the cables connected and tied down, you’re ready to go. As an exercise, let’s turn down the end of a 3/4" diameter rod to .

625 for a length of .500.

Selecting a tool with a fairly sharp radius, say 1/64", we face the end of the workpiece. With the carriage still locked and without moving the compound, we zero the DRO.

This establishes the reference for the carriage travel along the length of the workpiece.

We have set the DRO to read directly in diameter mode. Next we take a light cut to smooth up the diameter of the workpiece and establish a reference surface for the diameter. Without moving the cross feed or compound, we zero the DRO. Using a dial caliper we carefully measure the diameter of the workpiece.

At this juncture, we have two choices. One option is simply to calculate what the reading will be when we get to the desired diameter of .625. For example, if the diameter reading is now .

738 then our target DRO reading will be .738-.625 = .113, so we take successive cuts until the DRO reads -.113.

The alternative choice is move the tool back past the end of the workpiece, then advance the cross feed until the DRO reads -.738. At this point the tip of the cutting tool is at the centerline of the lathe.

Now we zero the DRO again and back the tool out. The DRO is now reading directly the actual diameter of the cut, so we just take successive cuts until the DRO reads .625.

This approach is very handy if we need to turn several different diameters on the same workpiece. As long as we don’t change the angle of the toolpost or change tools, we can read the successive diameters directly on the DRO.

Regardless of which approach we choose, we benefit from having a continual, exact readout of the tool’s position. We can cut with confidence, eliminating the need to stop the lathe and recheck the diameter as we approach the final desired size.

This is a big timesaver and is sure to reduce the likelihood of cutting too far and ruining a workpiece.

RPM and SFM Modes#

I’ve have stated elsewhere on mini-lathe.com that for hobbyists, knowing the exact RPM’s or surface feet per minute (SFM) is not all that important. I’ll continue to say that, but having the ability to measure those parameters easily and inexpensively can only improve your capabilities.

Using the optional ($10) optical sensor and a shop-made slotted disk (details provided in DRO-2 manual) mounted on the back end of the lathe spindle, the DRO-2 can directly read either spindle RPM or SFM of the work.

For each type of material there is a range of cutting speeds that is more or less optimal depending on the hardness and machinining characteristics of the material and the properties of the cutting tool.

Typically, harder materials such as stainless steel are machined at lower speeds than softer materials such as aluminum or brass, while harder cutting tools, such as carbide, permit higher cutting speeds than high speed steel (HSS) tools.

SFM is a measure of how fast the material is moving past the cutting tool at the cutting surface. It depends both on the RPM of the spindle and the diameter of the workpiece.

For a given RPM setting, the larger the diameter of the workpiece, the faster it will move past the cutting tool. Thus, as a workpiece is reduced in diameter, the SFM is reduced if the RPM remains constant.

When facing, the SFM approaches zero as the cutting tool nears the center of the workpiece. You can find tables of recommended cutting speeds for various materials and tool types in reference books such as Machinerys Handbook and on the web by searching for “lathe cutting speeds” or “surface feed per minute”.

Again, if you are a beginner, don’t be overly concerned by all this - you’ll do just fine by experimenting, starting with lower speeds and lighter cuts, but for the more advance user or those who simply enjoy the technicalities of such things, the DRO-2 will open up a whole new range of capabilities.

Conclusion#

I’ve had my DRO-2 for several months now and it has worked without fail every time I’ve powered up the lathe. I’ve come to depend on it as I would a reliable assistant.

For quite some time before I got my DRO-2 I had my 7x12 rigged up with dial indicators on both axes. These were a great help but lacked the convenience of instantly setting the zero point, and had a limited 2" range in the Z-axis direction and, of course, no metric capability.

The DRO-2 overcomes all of these limitations while providing a highly visible display.

While the DRO-2 would not be the first accessory I would recommend for a new lathe owner, it is certainly one I would recommend for the more experienced user who is looking to save time and improve accuracy.

Of course, if you happen to be flush with cash - don’t hold back! If you’re into machining in a serious way, this is an accessory you’ll definitely want.

I’m anxiously awaiting the introduction of the 3-axis DRO-3 for my mini-mill.