Oct 272013
 

This article is still “Under Construction”. (I know – a web term from the archaic past ;^)

Note – You can click on any of the photos for a much larger view….

Image of Bridgeport Knee Mill with DRO

A recently completed Bridgeport – Added a Sino DRO and X Powerfeed.

This article will present an overview of the installation of a 2 axis Sino Brand DRO on a Bridgeport Mill. We have currently settled on the Sino DRO as a “best value”. It offers quite good performance and accuracy, as well as a comprehensive set of features for a very reasonable price. It is also relatively easy to install. While a Newall DRO may be our choice for maximum performance, the Sino provides a lot for a much lower cost.




    But, first things first – Where to start…

Image of a Sino DRO Mounting Arm on a Bridgeport Mill

This is the Mounting arm for a Sino DRO installed on a Bridgeport

So many “bits & pieces” what to do first? I like to start with the mounting arm for the DRO Readout. On a Bridgeport, I have decided the most accessable, and least “intrusive” location is on the right side of the ram. In this picture, you can see the arm mounted about 1/4″ above the bottom of the ram. Using two 1/4-20 bolts, drill and tap the two mounting holes, and then tighten it down while using the  supplied jackscrews to level it.




Image of transfer punches and center drills

Examples of Transfer Punches (Left) and Center Drills (Right).

Before we go on however,  a little advice on those – or any – mounting screws and how to ensure they show up in exactly the right spot. When mounting anything, locate the item where you want it, then use a transfer punch of the proper size to mark JUST ONE of the holes. Then remove the item/bracket from your way. BY doing one hole at a time, you ensure things are level and true, and that all the holes will line up. And, never, ever, try to drill and tap the hole THROUGH whatever you are attaching. Mark the location, and make sure to use a center drill to start the hole before following up with the proper size tap drill (a number 7 in the case of 1/4-20). Tap the hole, then bolt your bracket back on and mark the next hole. – Rinse & Repeat. While this seems like extra work, it will pay off handsomely in improved accuracy.

OK. We have the arm mounted. Now that the tough stuff is out of the way, the next thing we will tackle is the crossfeed, or Y, axis. Actually, this is normally the most “challenging” in a two axis DRO. If you are installing a third (Z knee) – that will likely tax your creativity even a little more (stay tuned). But, we are just doing the two axis for now.

Image of a Sino Y Axis scale and read head mounted on a Bridgeport.

The Y Axis scale, reader head and bracketry as installed on a (real) Bridgeport Knee Mill.

Normally, the Y axis can be Mounted on the right side neatly tucked up under the saddle. Many of the newer (a relative term) mills have a rather flat side to mount the scale – as this one does. In the Sino, the scale itself is mounted on a mounting strip which is actually attached to the mill. This allows us to make the strip true before mounting the scale itself. As for the read head, the normal place to mount that is on a bracket bolted to the right side of the saddle itself. The SINO kit supplies a few brackets which will make this easier. We will figure out, and mount the bracket for the read head first, then locate the mounting strip for the scale. In the photo at right, you can see the finished installation (minus the cover over the scale)…

Image of a Sino DRO Y-Axis scale mounting bracket

Another view of the completed mounting bracket on our Bridgeport Mill…

In this case, we will be using one of the existing 1/4-20 holes in the saddle to mount the “L” bracket. The first thing we will do, is mark, drill, and tap the saddle for a second 1/4-20 mounting screw (the one on the right) to hold the “L” bracket. Be very careful to get this hole positioned correctly, or you will have trouble down the line getting things aligned. The end of the saddle describes an arc, so you must also be careful to get the hole centered up & down in the surface. Speaking of surfaces, if there is a buildup of dirt, grime, paint, or crud in this location, now is the time to clean it off!


Image of Sino scale mounting brackets

The two supplied Sino scale mounting brackets we will be using.

For the second half of this bracket, we will use a supplied flat plate. This plate has two slots which will bolt to the L bracket, and two (currently threaded) holes through which we will mount the read head. You will have to slightly drill out the two holes to get the screws for the read head through them. To attach the two brackets together, we are using two 1/4-20 cap screws with lockwashers and nuts.

At the right you can see a photo of the two brackets we are using. These are both included with the SINO DRO installation kit.

Another digression here to talk about read head positioning. The SINO scales – like most DRO’s – come with the read head clamped to the scale itself with small plastic brackets – the red pieces in the photos that are on either end of the head. First – DO NOT remove these until you are done with the install!!!! Second – never, ever, EVER, try to remove the read head from the scale, put it under any kind of tension, twist, bend, fold, spindle, mutilate, or otherwise distress this relationship between the head and the scale. You will likely immediately ruin your scale. These holders position the read head exactly where we want it to stay in the finished setup. The specs call for this relationship to stay true plus or minus .004 inches for the entire travel. We will shortly show you how to ensure this happens. But for now, just be keenly aware that where the read head is held by these red brackets is where it MUST stay – forever and a day (well, plus or minus four thousandths of an inch) . Also be aware these clamps are not very strong. Throughout the handling of the scale, you will have to “reseat” them occasionally – make sure you do this! And, and, and one more thing (but wait – there’s more!). The cable attached to the head will attempt to ruin this relationship at every twist and turn. Be constantly aware of the cable, where it is, and ensure it does not put strain on the attached head… OK, now we are done with the lecture.

So, where were we. We have mounted the arm for the display box, we have drilled and tapped a second 1/4-20 hole to mount the “L” bracket to the end of the saddle, and we have drilled out (enlarged) the holes (opposite the slots) on the flat bracket plate to allow us to attach it to the read head. What, you forgot to do the last one – well do it now, then come on back.

A final prep item for this install. We need to cut off the end of the long side of the “L” bracket to allow it to clear the lead screw. Cut it off about 1/4″ PAST the slots as shown.

Not another lecture! – Never cut something “mid-slot” and leave an unterminated slot. If you do, when the mounting bolts are tightened down, the “arms” of the slot will tend to splay out. weakening the connection, and making it very hard to position things accurately. over time this type of connection will slip and loosen. This advice applies to anything, DRO’s alternator brackets and bridge trusses included.

We are now going to determine where to mount the mounting rail for the Y scale on the side of the knee. Yes, we are going to do a “mock up”. To do this, bolt the flat plate to your read head – loosely, and attach the L bracket to the end of the saddle with the two 1/4-20 bolts. Position this bracket mid-way, and snug the bolts. Then attach the mounting rail to the scale. In a bit of juggling hold the scale up in places, and loosely slip one of the 1/4-20 cap screws and nuts through the joint between the plates. You should now be able to get a good idea of where the mounting rail should be attached. Our goal is to make sure the read head mountings are in the middle of their adjustment range, everything will be able to line up flat & true, that the scale will clear the saddle and anything else in the way, and that the travel of the scale is greater than the travel of the saddle on both ends. On this install all the stars lined up, and it worked out to be fine about 1/4″ below the surface of the crossfeed ways. DO Remember there is a cover that also attaches to the rail, and adds about 3/16″ to it’s height. Once it looks to be in the proper position, mark the end of the rail (Both length and height) as to where it will be. Now reverse the process to disassemble the mockup. detach the scale from the rail, and lay the scale aside (in a safe place)…

To mount the mounting rail. You will notice there is a screw slot in the front that goes horizontally, and one in the rear that goes vertically. Making sure you get the rail positioned “right side up” (with the horizontal screw slot towards the front), hold the rail in position next to your marks, and mark the location of the front (only) screw. Put your mark in the middle of the slot. Drill and tap this hole for a 10-32 capscrew. Mount the rail with this screw, and tighten it up snugly. Position the rail so it looks level. We will now proceed to determine where the rear rail mounting screw (RRMS) goes.

Image of checking the y-axis mounting rail with a dial indicator

Tramming the y-axis mounting rail during DRO install

Finally, it’s time for your dial indicator. We are going to indicate (or tram) this mounting rail twice in the up & down direction. The purpose is to ensure the saddle bracket and attached read head will follow the exact same path as the mounting bracket and scale. In this first iteration, we are going to get it “close” so we can mark the second mounting hole. We will still have some leeway for adjustment when we subsequently bolt the rail on and finalize the positioning. For now, figure out how to mount your dial indicator so it follows the long up & down dimension of the rail – as seen in the photo. I am using a clamp on mount I use specifically for this – your mileage (and mount) may differ. Once all is setup, we will traverse the crossfeed throughout its travel while watching the dial indicator. By moving the rear (back) end of the rail, we will attempt to get it true within AT LEAST.004 inches end to end. While this is a rough adjustment at this stage – the more accurate you are, the easier the rest of the install will be – and the more accurate (and repeatable) your scales will end up being.

Once you have the mounting rail close to the final desired position, mark the rear screw location (in the middle of the up & down slot) with your transfer punch, remove the mounting rail, and drill and tap that mounting hole for a 10-32 capscrew…

Then, reinstall the mounting rail with both capscrews, and tighten them up just a little snug.

We are now going to indicate, or tram, the mounting rail into it’s final position. That position will will involve it being both equidistant from the read head mounting bracket (parallel to) throughout the travel of the Y-axis, as well as level with that bracket throughout the Y travel. This will require indicating in two different planes. I use two different dial indicators mounted to the read head bracket to indicate both directions simultaneously. You may have to move one between the axis.

Image of two dial indicators while used to check the scale mounting.

Two dial indicators used for indicating the Y-Axis scale mount. The one in front is to check the mount for parallel to the read head mounting bracket.

In this photo you will see both dial indicators, with the one for the parallel axis predominant. This is the axis we will check, and adjust, first. We will adjust this by adding shims under one or the other of the mounting screws. Many times the side of the machine is flat, and no shims (or maybe just one) will be required. After setting up your dial indicator, and ensuring it is in the middle of it’s travel (really), Start at one end, and move the Y-Axis through it’s travel while watching the indicator. While your goal is less than .004 from one end to the other, the closer to zero you can get it, the better your DRO will work (see below).. Note that we are primarily interested in the areas around the mounting screws. BOTH should be within tolerance. sometimes, and in this case, there is a slight bow in the mounting rail towards the middle. This will not affect the scale. In this case, the middle was about .003″ further in than the mounting points. Keep checking and adding/subtracting shims until you get it as parallel as humanly possible. THEN check the other axis. Again we are looking for less (much less) than .004″. Adjust the rail in this axis by gently tapping the rear up or down. Once it is “level”, start tightening the screws. Tighten/check, Tighten more/check. It may take a while to get the screws properly tightened while maintaining the correct position. You will also have to check the other axis from time to time, to ensure it is still parallel – hence my dual indicators. This is a PROCESS, take your time, get it right, and the scale will work right. Now, why is this so important?

It is important for accuracy, but more important, repeatablity. Inside the scale is a strip of glass with finely inscribed/etched markings along it. The DRO is simply a counter. As the read head moves, it counts how many of the etched marks it has crossed. That translates into how many thousands, or tenths, it has moved, which is what is displayed. These marks are VERY close together (measured in microns). The slightest tilt in the sensor will cause it to sometimes read two or more marks as one. This will naturally throw the count off. What you then have is “backlash” in the DRO. When you return to a previous position, it may be wrong, because it has “mis-counted”. So, go for as close as possible, and likewise, make the installation as tight and unshakable as possible.

Incidentally, a good way to check an existing scale is to move the axis near one end of it’s travel. zero the DRO and the mechanical dial scale, and proceed to rapidly move the axis about 2-3 inches away. If it is going to mis-count, this is when it will. Sometimes the scale is so badly misaligned (or bad) you will actually see it skip counts when moving fast. NOW try to go back to zero. When you get back to zero on the DRO, where is your dial scale (remember to allow for lead screw backlash). This will tell you pretty much how badly it miscounted. A few thousandths within zero, and all is pretty well. Any more, either the DRO is broke, or else someone installed the scale crooked.

Now, where were we?

Ah. Once you have the mounting rail tightened down, and “perfectly” positioned, it is time for some assembly. Remove the brackets from the read head – the one you carefully set aside a while back. Remoe your dial indicators. Then, mount the scale to the bracket making sure it is seated properly. Tighten the mounting screws securely, but remember it is plastic they are holding. Now, assemble the bracketry and bolt it together. BUT DO NOT attach the read head to the bracket yet. We are now going to do a little “fudging”. Fudging in this highly accurate install? Yep, but just a very little, carefully calculated, and because there is no other way. At this point, ensure the read head is properly secured by those red clips all the way up, and centered. (Push it – gently – up towards the scale to make sure they are seated.

We now have to make the position of the mounting bracket exactly touch the read head – but no more. This is very hard to describe, easier to do… But, I will try. Tighten the saddle (L-Bracket) bolts in approximately the correct position, but with the read head mounting slightly below where it belongs. Snug up the bolts between the two brackets, and then move the flat bracket until the read head mounting bolts will go into the read head. Do not tighten them! just make sure it is aligned, then start tightening the bolts between the two brackets while keeping this alignment. Another “juggling act”. Once this is close, remove the read head bolts again, slightly loosen the saddle mounting bolts and, using a hammer, lightly tap the entire bracket up until it is just touching the bottom of the read head – on BOTH sides. Now start tightening the saddle bracket bolts while keeping this relationship. Tighten these fairly tight, but continue to ensure that the read head is just touching the flat bracket. You may also have to readjust the “between bracket” bolts. Just keep juggling all of this until everything is tight, and the read head will bolt to the plate without moving (say .001-.002″ max). There are no “vernier” adjustments, just a bit of “fussy, fidgity, fudging”.

Once everything is properly positioned. Re-install the read head screws, check everything, make sure the entire travel looks OK, and sit back and admire your work. Really, the “hard part” is done.”

In part two, we will mount the X-Axis scale, and finish up the installation.

 Posted by at 6:22 am