ballen
Diamond
- Joined
- Sep 25, 2011
- Location
- Garbsen, Germany
Dear Metrology group,
I have a question about mounting a vertical axis scale/encoder on my surface grinder.
I'm adding a digital readout (DRO) to the vertical axis of my 1986 Jones and Shipman 540APR surface grinder. I'm the second owner of the machine, which was apparently only used for a few months by the original owners, and then shoved in a corner and forgotten for the following three decades. The ways on the machine are pristine, and I've mapped the grinding footprint (150 x 450mm) comparing it to a slightly larger surface plate. Within this rectangle the total deviation from planar is at most 2-3 microns (and might be better, since that's comparable to the accuracy of the surface plate). I've had the machine for several years and really like it. The only shortcoming of the machine is that it doesn't have a DRO.
So I'm adding a three axis DRO to the machine. The long and cross axes are easy enough. But for the vertical axis I'm trying hard to do it as well as possible. I'm more interested in repeatability and resolution than in absolute accuracy. Total range of vertical motion is 280mm. Total cross section available is about 12 x 60mm, which is not enough for a sealed encoder.
I'm using Renishaw parts for an open non-contact system: an RGH25U optical read head, an RGB25Y interpolater, and RGS20-S scale tape. I've used this system in the past for the long axis of my cylindrical grinder, and it works very well. The tape is 6x0.2mm, with a 20 micron period, accurate to +- 3 microns/meter and within +-0.75 microns in any 60mm section. The interpolator provides 10 counts per micron, so a 0.1 micron resolution. That makes sense here: the machine hand wheel scales have 1 micron divisions, and with care, it is possible to work to micron precision. So another order of magnitude in the resolution is justified.
I'm mounting the scale inside the machine, almost directly over the vertical lead screw, and close to halfway between the two vertical guide rails. The measuring tape is attached to a strip of annealed ground O2 steel (thermal coefficient ~11.2e-6/K) 50 x 10 x 360mm, which in turn is mounted to the cast-iron carriage (thermal coefficient ~10.7e-6/K) that carries the grinding spindle and motor. The 50x10x360 strip is attached only at the top and bottom ends, not along its length or in the middle. The optical read head is mounted to the cast-iron machine chassis with steel blocks, bolted in place over an epoxy "shim".
Because of this mounting location, and the slow change in machine temperature in that area, I expect that the O2 carrier strip and RGS20-S measuring tape will be in good thermal contact, and will have a temperature similar to the cast-iron body of the machine. The RGS20-S tape is "mastered" to the O2 strip (glued down at both ends) so will expand and contract with that strip. However there will be some differential expansion between the O2 strip and the cast-iron spindle/motor carriage. Probably this will be less than 5 microns but might be as much as 20 microns if the carriage and scale differ by 5C in temperature.
My question concerns the attachment of the O2 strip to the cast-iron carriage. I can either fix the strip (secure datum point) at one end, and leave the other end free to "float", or I can fix the strip at both ends. I am about 90% sure that the first choice would be best, in part because I have no good way to attach a 10 x 50 mm cross section strip well enough to ensure that the strip rather than the fasteners would deform. My question, how to decide between fixing the strip at the top or at the bottom? Is there a better choice? Mechanically, either is OK. The top of the strip is the region that is in use when the grinding head is near to the grinder table. This might argue for the top being the secure datum point, and the bottom of the strip being allowed to float. But I'm not sure.
I can provide some photos if that would help.
Cheers,
Bruce
I have a question about mounting a vertical axis scale/encoder on my surface grinder.
I'm adding a digital readout (DRO) to the vertical axis of my 1986 Jones and Shipman 540APR surface grinder. I'm the second owner of the machine, which was apparently only used for a few months by the original owners, and then shoved in a corner and forgotten for the following three decades. The ways on the machine are pristine, and I've mapped the grinding footprint (150 x 450mm) comparing it to a slightly larger surface plate. Within this rectangle the total deviation from planar is at most 2-3 microns (and might be better, since that's comparable to the accuracy of the surface plate). I've had the machine for several years and really like it. The only shortcoming of the machine is that it doesn't have a DRO.
So I'm adding a three axis DRO to the machine. The long and cross axes are easy enough. But for the vertical axis I'm trying hard to do it as well as possible. I'm more interested in repeatability and resolution than in absolute accuracy. Total range of vertical motion is 280mm. Total cross section available is about 12 x 60mm, which is not enough for a sealed encoder.
I'm using Renishaw parts for an open non-contact system: an RGH25U optical read head, an RGB25Y interpolater, and RGS20-S scale tape. I've used this system in the past for the long axis of my cylindrical grinder, and it works very well. The tape is 6x0.2mm, with a 20 micron period, accurate to +- 3 microns/meter and within +-0.75 microns in any 60mm section. The interpolator provides 10 counts per micron, so a 0.1 micron resolution. That makes sense here: the machine hand wheel scales have 1 micron divisions, and with care, it is possible to work to micron precision. So another order of magnitude in the resolution is justified.
I'm mounting the scale inside the machine, almost directly over the vertical lead screw, and close to halfway between the two vertical guide rails. The measuring tape is attached to a strip of annealed ground O2 steel (thermal coefficient ~11.2e-6/K) 50 x 10 x 360mm, which in turn is mounted to the cast-iron carriage (thermal coefficient ~10.7e-6/K) that carries the grinding spindle and motor. The 50x10x360 strip is attached only at the top and bottom ends, not along its length or in the middle. The optical read head is mounted to the cast-iron machine chassis with steel blocks, bolted in place over an epoxy "shim".
Because of this mounting location, and the slow change in machine temperature in that area, I expect that the O2 carrier strip and RGS20-S measuring tape will be in good thermal contact, and will have a temperature similar to the cast-iron body of the machine. The RGS20-S tape is "mastered" to the O2 strip (glued down at both ends) so will expand and contract with that strip. However there will be some differential expansion between the O2 strip and the cast-iron spindle/motor carriage. Probably this will be less than 5 microns but might be as much as 20 microns if the carriage and scale differ by 5C in temperature.
My question concerns the attachment of the O2 strip to the cast-iron carriage. I can either fix the strip (secure datum point) at one end, and leave the other end free to "float", or I can fix the strip at both ends. I am about 90% sure that the first choice would be best, in part because I have no good way to attach a 10 x 50 mm cross section strip well enough to ensure that the strip rather than the fasteners would deform. My question, how to decide between fixing the strip at the top or at the bottom? Is there a better choice? Mechanically, either is OK. The top of the strip is the region that is in use when the grinding head is near to the grinder table. This might argue for the top being the secure datum point, and the bottom of the strip being allowed to float. But I'm not sure.
I can provide some photos if that would help.
Cheers,
Bruce
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