So I decided to undertake a non-destructive conversion of my King 10x36 Gear head. Not replacing the acme screws with lead-screws means that the mechanical conversion is pretty easy. Backlash and precision are not going to be great, but they may be good enough for the application. Time will tell.
Here is the saddle with the compound and the cross slide removed.
The lead screw is 5/8 x 10tpi acme. This has always been the weak point of this machine. It seems impossible to torque the acme nut to the compound and have the acme nut correctly adjusted to eliminate unreasonable backlash. Fixing that would likely require replacing the bearing, mount and probably also the nut and screw. More trouble that I think it is worth going to at this stage.
A shot of the existing screw and fixed bearing assembly. The double nuts at the end of the journal bearing are a very poor solution as counter-tightening the second nut tends to dramatically alter the load on the bearing.
I'll be using 5mm HTD timing belts and pulleys from SDP/SI (who have a very useful pulley designer on their site) rather than direct drive for this build. The X axis is the tricky one as the pulley plus the belt and the cover have to fit under the cross-slide. I was trying for around 2:1 advantage, but couldn't quite get there on the X. The configuration for the X is 13 double flange : 22 no flange with a 60 tooth belt. The Z is 19 df : 38 nf with a 77 tooth belt. Three of the four hubs all had to be re-bored to the correct shaft size. The 13 tooth was a total pita.
A 6A25M022NF1508
A 6R25M060150
A 6A25M013DF1506
A 6A25M038NF1510
A 6R25M077150
A 6A25M019DF1508
The disassembled fixed bearing and, at the bottom of the shot, the journal extension with a 0.3745" stub, a section of M10 x 1.5 thread and a 0.5000" shaft to receive the drive pulley.
Here, the existing threads have been cut off the end of the lead screw journal and I'm just about to centre drill, drill out and ream a 0.3750" hole that will receive stub of the new journal extension.
A little loctite and a coffee break later and the new journal is done.
All finished except for a cover (which turned out to be a little hard than I thought and after a couple of false starts isn't done yet). Note the brass nut replacing the double M10 nuts. This can be tightened to put just the right load on the bearing and then locked in place with a set screw. I dropped a little brass slug into the set screw hole before torquing it down to avoid damaging the screws on the journal.
The stepper (an Oriental Motor PK296DAA I think) and the mounts just fit inside the splash guard. A couple of final thoughts... There is an alternative to this mounting position as the King has an excellent gear box on the saddle to power either the Z or the X axis. Mounting the motor on the drive shaft of the gear box instead of directly to the end of the X lead screw would be very easy. While taking the X axis fixed bearing apart and reassembling it a few times I realized that, in addition to it being the least well-built part of this machine, it also isn't really intended to handle radial loads. While the motor and timing belt aren't particularly large loads, they still shouldn't really be on this bearing. If this proves problematic, I can either attempt to replace the whole bearing assembly with something better off-the-shelf, make a housing for the existing bearing that will handle the load or move the whole shebang to the gear box drive shaft. I decided not to go this route at the start because the gear box will induce considerably more backlash.
Here is the saddle with the compound and the cross slide removed.
The lead screw is 5/8 x 10tpi acme. This has always been the weak point of this machine. It seems impossible to torque the acme nut to the compound and have the acme nut correctly adjusted to eliminate unreasonable backlash. Fixing that would likely require replacing the bearing, mount and probably also the nut and screw. More trouble that I think it is worth going to at this stage.
A shot of the existing screw and fixed bearing assembly. The double nuts at the end of the journal bearing are a very poor solution as counter-tightening the second nut tends to dramatically alter the load on the bearing.
I'll be using 5mm HTD timing belts and pulleys from SDP/SI (who have a very useful pulley designer on their site) rather than direct drive for this build. The X axis is the tricky one as the pulley plus the belt and the cover have to fit under the cross-slide. I was trying for around 2:1 advantage, but couldn't quite get there on the X. The configuration for the X is 13 double flange : 22 no flange with a 60 tooth belt. The Z is 19 df : 38 nf with a 77 tooth belt. Three of the four hubs all had to be re-bored to the correct shaft size. The 13 tooth was a total pita.
A 6A25M022NF1508
A 6R25M060150
A 6A25M013DF1506
A 6A25M038NF1510
A 6R25M077150
A 6A25M019DF1508
The disassembled fixed bearing and, at the bottom of the shot, the journal extension with a 0.3745" stub, a section of M10 x 1.5 thread and a 0.5000" shaft to receive the drive pulley.
Here, the existing threads have been cut off the end of the lead screw journal and I'm just about to centre drill, drill out and ream a 0.3750" hole that will receive stub of the new journal extension.
A little loctite and a coffee break later and the new journal is done.
All finished except for a cover (which turned out to be a little hard than I thought and after a couple of false starts isn't done yet). Note the brass nut replacing the double M10 nuts. This can be tightened to put just the right load on the bearing and then locked in place with a set screw. I dropped a little brass slug into the set screw hole before torquing it down to avoid damaging the screws on the journal.
The stepper (an Oriental Motor PK296DAA I think) and the mounts just fit inside the splash guard. A couple of final thoughts... There is an alternative to this mounting position as the King has an excellent gear box on the saddle to power either the Z or the X axis. Mounting the motor on the drive shaft of the gear box instead of directly to the end of the X lead screw would be very easy. While taking the X axis fixed bearing apart and reassembling it a few times I realized that, in addition to it being the least well-built part of this machine, it also isn't really intended to handle radial loads. While the motor and timing belt aren't particularly large loads, they still shouldn't really be on this bearing. If this proves problematic, I can either attempt to replace the whole bearing assembly with something better off-the-shelf, make a housing for the existing bearing that will handle the load or move the whole shebang to the gear box drive shaft. I decided not to go this route at the start because the gear box will induce considerably more backlash.
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