Adding a cheap DRO to a Proxxon XY-table

[Kris Avonts]

Hello All,

 

I use my Proxxon micro-mill a lot when it comes to precise shaping of wooden parts. The complete setup consists of a MB 200 drill stand, a MICRO compound table KT 70 and a 230V ISB/E drill/grinder.

 

 

One of my frustrations with this setup is keeping track of the position of the end mill relative to the workpiece. How many turns did I already gave in the X or Y direction? Of course each decent mill is equipped with a Digital ReadOut system (DRO).

 

 

But adding such hardware to the KT 70 is not obvious, especially because the KT 70 is a ‘MICRO’ xy-table. I also didn’t want to spend a lot of money on it, so I parked it in a wish list. But then I saw that digital calipers were sold at 7.99 euro in a supermarket. I couldn't resist and bought 2 calipers guided by ‘it’s now or never’.

 

 

So I will describe here the process of adding these calipers to the KT 70. It is certainly not the only way to do it but it is what worked for me. And maybe it can inspire other members. So lets start.

 

 

The first thing is to dismount the complete caliper. Then the metal parts need some cuts.

 

 

 

I used a small hacksaw but soon experienced that the metal was quite hard and I had to replace the saw-blade twice before finishing the job.

 

 

I was afraid that these cuts would disturb the measurement principle. I quick check gave me piece of mind.

 

 

Then you have to decide where you want to attach the ruler part of the caliper and where the readout unit. After some puzzling, I decided to position the readout units close to the respective hand-wheels. The ruler then needs to be attached to a moving part of the xy-table. For the Y-direction I will use a 3 mm dowel pin glued in a filed grove at the ruler end.

 

 

 

To mount that Y-ruler as low as possible, a small cut had to be made at the right front corner of the xy-table.

 

 

This way the Y-ruler can be positioned horizontal as the check on the next picture illustrates.

 

 

Next the Y-readout needs a support. I found a complex metal (zamac) part that could be used when all excess material was cut away.

 

 

 

It will be fixed in place using the right screw (original screw replaced by a longer one) in the lower cover.

That is enough work for today, have a nice evening and see you back soon.

 

Best regards,

 

Kris

[Kris Avonts]

Hello,

 

The next thing is to make a hole at the right of the cast aluminium block where the dowel pin will be inserted. I made a quite complex setup to drill that 3 mm hole as correct as possible. First job was the fixation of the xy-table to my work surface and checking its squareness (see the next picture).

 

 

Then a rather unusual setup for my drill press where the base was turned to the back and using a counterweight to stabilize the drill press.

 

 

That went well and the dowel pin fits with very little play.

 

 

As a last adaption for the Y-readout, I soldered some wires to the metal battery clips in order to power the readout from a power supply. That should avoid battery replacements in the future.

 

 

The X-DRO was made following the same principles but the ruler will be fixed to the mill table with a support instead of a pin. The next picture shows the readout already prepared with wiring, the ruler and both supports. You may notice that I cut away some plastic at the battery location to bring the readout as close as possible to the X-table side.

 

 

Then the supports need to be fixed. The ruler support is attached to the mill table.

 

 

 

 

With the threaded hole for the readout support I had bad luck.

 

 

The M3 tap broke off but there was just enough of it sticking out so that I could remove it with a good forceps.

 

 

The next picture shows the parts of the X-DRO ready for glueing to the mounted supports.

 

 

Then with some metal epoxy glue all parts are glued, compressed and left for 24 hours to allow the epoxy to cure to its final strength.

 

 

As a last addition I soldered the wiring to a small step down converter that will supply the 3V DC for both readouts.

 

 

At the back is now a connector that mates with an old 12V DC supply of a laptop. Here is the finished KT 70 with its added DRO.

 

 

Next time I will try to report on some test regarding the precision of this setup.

 

Best regards,

 

Kris

Edited April 5 by Kris Avonts
spelling correction

[Mike Y]

Very interesting mod!

 

Have you considered decoupling the screen from the rest of the measurement device? Since you have pretty much disassembled it anyway, and I assume the screen is just attached with some connector and can be soldered with a longer wires, creating a convenient "DRO panel" on the front of the mill.

Though it might be just integrated to the rest of the circuit entirely...

 

Is it actually possible to crank the Y axis wheel with the caliper being that close, or is it designed for a left handed user? From the pictures it looks to be right in the way. Pardon if I am just overthinking it   

[Kris Avonts]

Hi Mike,

I understand your wish to position the display at a more convenient location. The display is an LCD type and requires a large amount op signals to display the information. The power and signal connection to the pcb is done with a 'zebra strip'. That is a kind of foam with alternating layers that are isolators and layers that conduct current. It is compressed between PCB and display, so no connector is used. On some PCB's of digital calipers you can find 2 interface signals intended for connection to a remote interface. These are clock and data signals. In combination with a small microcontroller board (e.g. Arduino) you can then decipher the data and put meaningful values on an extra display. This type of caliper had no such interface. So I had to use the display as it is positioned over the ruler.

 

You are right, the Y-axis hand wheel is very close to the readout but it is possible to use it. I should have cut the extruding bulb before mounting. That gives about 5mm extra play. Now that it is already glued in place, I leave it as is.

 

I'm glad you like my report on this modification.

 

best regards,

   Kris

[Kris Avonts]

Hello,

 

It’s time for some evaluation of this modified proxxon KT 70. As a test I made a setup to use the xy-table over its whole range with fixed steps in both x and y direction. In x it was 8.9 mm step and in y 3.1 mm. That way the whole range is covert with 15 steps. I use the scale ring to make the steps and note the DRO display values. Here you see the setup with a camera positioned to show the readings.

 

 

This is how the table is stepped from bottom-right to top-left.

 

 

I also did the inverse movement from top-left to bottom-right and again noted all displayed values.

When entering the values in a spreadsheet and plotting things you get the following results.

 

 

You can see that the curves are not ‘flat’ and that moving up and down follow a different path.

Not flat means that the pitch of the xy-table is slightly off its nominal 1 mm. In the x direction we have about 0.3 mm error after 120 mm and that can be explained with a pitch being 0.9975 mm.

For the y direction it is about the same result, a pitch of 0.9980.

 

The different paths are a result from backlash, that is ‘play’ between nut and screw. For both x and y it is about 0.1 mm.

 

I assume the DRO is correct and has no play. To be sure I also checked a small movement (8 mm) in the x direction somewhat in the middle of its range with a dial indicator. The result is shown next.

 

 

The dial indicator and the readout give almost identical values. Ok, then I trust the DRO from now on. It is convenient because it keeps track of the millimetres already moved and it eliminates the backlash error.

 

That is it for now, hope you liked it.

 

Best regards,

Kris