We are often asked if it is possible to fit a DRO to small lathes, and in short it is possible to retrofit a DRO system to the majority of lathes and milling machines often found within the model engineers workshop. To go about demonstrating this, we acquired a 1954 Myford ML7. This being one of the more common machines we encounter and set about preparation for installing the Digital Readout.
Step 1: Linear Scale Selection
Traverse the cross slide to its maximum outer position (photo 2). Be aware that on our ML7, we can wind the lead screw right out of its nut. Measure between the two marks to give the cross slide travel. We then repeat this for the carriage travel along the bed. Armed with these measurements we started the linear scale selection process.
We had a choice of three ranges of glass linear scale. These include the GS300 standard with reading lengths of up to 1 metre, the GS500 slim for limited space installations, for example our Myfords cross slide and the GS600 long length with reading lengths between 1 metre and 3 metres. All the linear scales are categorised by range and reading length, so the GS300-420 standard scale will measure 420mm (16 1/2) of travel and its total length will be 526mm (20 1/2).
Our Myford has the standard 4 tee slotted cross slide found on the ML7. The ML7S has the longer 5 tee slotted cross slide fitted as standard, which can be retrofitted as a common upgrade to the ML7. We measured 145mm of cross slide travel on our Myford, which gives us two possible scales, these being the GS300-170 Standard or the GS500-170 Slim. Generally we would always suggest the slim scale as this gives more options when coming to fit.
Our ML7 has 510mm (20) between centres and we had measured 450mm (17 ¾) of carriage travel. Based on these two measurements we opted for the GS300-520 standard linear scale. The standard scale can be fitted, as we don't have the same restriction with space on the bed as encountered on the cross slide.
Looking at the linear scales there are two mounting blocks fitted, one at each end of the scale body, both of which are slotted for adjustment. The reading head has two mounting holes with a grub screw in each corner to act as a jacking screws for mounting to a stepped or uneven surface. A selection of mounting bolts of either M3 or M4 depending upon the scale type are part of the fixing kit.
Step 2: Cross Slide Linear Scale
With the cross slide and linear scale set to the middle of their travel, we set about offering the scale around various locations on the cross slide (photo 4). In doing this it became apparent the cross slide was physically too short and it would be necessary to fabricate a bracket or extension plate. This is because the overall scale length was about 60mm longer than the cross slide itself. We started making notes and discussing some of the solutions found.
Some of our solutions were favourable and some not so. The number of possible positions found surprised us. From our sketches and notes we drew up an order of preference. We took into account the fixed obstacles encountered and in some cases the modifications they would need to fit the scale (photo 5). These included the saddle-locking bolt, which we thought could be reduced in height. This could accommodate mounting the scale above the bolt head or relocating the saddle locking bolt altogether. The saddle also had a number of lubrication nipples, again not an impossible task to reposition if necessary.
Our first position was on the left hand side of the cross slide when standing in front of the lathe. We found that it could restrict the travel around the chuck and reduce the lathe swing. In addition the scale would be potentially more exposed to the swarf and coolant. When we offered the scale around the left of the cross slide we found the gib strip screws would need to be changed to flush mounting screws, ideally with a nylon locking insert. The scale mounted in this position would also affect the tailstock. The scale would reduce the amount the tailstock could slide between the lathe carriage, not greatly so, but worth noting.
When we had exhausted the possible solutions for mounting the scale around the central area on the cross slide. We started a new page in our pad with the intent of fitting the scale so that it projected out to the front or rear of the cross slide. This would involve the fabrication of various extension plates and brackets. By having the scale away from the space restrictions and obstacles mentioned previously, the process of mounting would be made easier and less restricting to the lathe.
Offering the scale so that it projected out towards the front of the cross slide, would allow a bracket to be fabricated to avoid the obstacles such as the lubrication nipples and locking screws (photo 6). Mounting the scale in this position did encroach on the feed wheel. However, one thought we did have was to design the bracket so that a protective screen could be fitted. This would protect the machinists hands and feed wheel from the swarf chips and coolant.
Next we looked at the possibility of mounting the scale so that it projected out towards the rear of the cross slide (photo 7). This immediately seemed an ideal mounting location. As the ML7 has its motor and drive pulley assembly mounted on the rear of the headstock this meant the scale would protrude into the dead space to the rear. Again we have the advantage of avoiding lubrication nipples and locking screws. For our mounting beam we intended using an off cut piece of aluminium bar, we had put aside for this project.
We marked the bar out for the two tapped mounting holes for the scale and two clearance holes for mounting the bar to the cross slide (photo 8). Adjacent to the two clearance holes, we added four tapped holes for jacking screws. These jacking screws offered very fine adjustment to make the beam parallel and square to the cross slide travel. Lock nuts should be fitted to the grub screws, so that they do not adjust over time.
We decided that a support strut should be fitted, attached to the far end of the mounting beam and saddle for extra rigidity. With our scale in position a bracket for mounting the head was sketched out. In this case the main scale body is fixed and the bracket is fitted to the moving cross slide to drive the reading head along. The other option is to fit a bracket to mount the scale body to the moving part of the cross slide and fit a fixed bracket to the reading head.
We followed the same procedure as before by offering the scale around various positions. Starting at the rear of the lathe bed, we were both instantly drawn towards the machined area for the taper turning attachment (photo 9). This is a large parallel area along the entire length of the bed, with a series of tapped holes, which would be used to fix the attachment. This opened a debate as to whether we had a regular need for using the taper attachment.
Did we ever use it or were likely to in the near future and if we were to fit the scale in this area how much of a pain would it be to remove and reinstate the attachment. Our idea was to mount the back plate using two of the existing tapped holes for the attachment, which were an imperial thread. We counterbored the two fixing holes for flush
mounting of the bolt heads.
Next to the counterbored fixings, two grub screws tapped into the backing plate would act as jacking screws (photo 10). These grub screws can be adjusted to bring the scale parallel to the bed. With the scale mounted (photo 11) and located by the ridge along the back beam we needed to fabricate a suitable bracket setup to drive the reading head.
Looking at the standard bracket kit supplied, the long L-shaped bracket could be used with a packing piece (photo 12). The packer would need to space the bracket away from the saddle. Fitting the packer to the rear of the saddle is complicated by the un-machined surface on our machine. Our solution was to use jacking screws drilled and tapped into the packing piece. This would allow us to adjust the packer to the correct thickness and parallel with the scale.
The packer can be tapped with the appropriate fixings to use a pair of slots on the bracket. This will also give us height adjustment.
The reading head is attached to the bracket via the two tapped holes on the underside. Again the bracket has two slots, which allow for adjustment to centralise the reading head with the scale body. The combination of jacking screws and slotted brackets provide the means for simple adjustments, making fitting the scales much easier.
While mounting the scale along the taper turning area does have its advantages, it will of course prevent the use of the taper attachment. We offered the scale up below the taper attachment. This area is unmachined and very curved and tapered. The backing plate of the b-type cover could be drilled and tapped with jacking screws. When marking out the jacking screw positions on a backing plate or bracket the fixing bolt should be drilled adjacent. This will reduce possible distortion and bending.
Another option that found its way on to our sketchpad, avoiding the taper attachment and un-machined area on the rear of the ML7 bed, would involve a length of angle being fitted between the rear lathe feet pads (photo 13), so that it is clamped between the feet pad and lathe bench, with an equal thickness packing piece under the front to maintain the level. The scale can be mounted to the angle, adjusted parallel and brackets made for the reading head.
Using this method will reduce the fixing holes drilled into the lathe bed. We also thought that this method could be adapted to fit the scale on the front of the lathe bed (photo 14). This is particularly useful when access to the rear of the machine is restricted.
Step 4: Avoiding Drilling and Tapping
The cross slide on the Myford has a series of tee slots that could be used to attach the scale. We manufactured two tee nuts from steel angle. The tee nut profile was machined along one side with a tapped hole for a grub screw (photo 15). On the other part of the steel angle we drilled and tapped a hole for mounting the plate. The two tee nuts and the plate are slotted into position and secured with the grub screw (photo 16). The scale would need to be fitted so that it is parallel with the travel. This method would also allow us to quickly remove the scale.