FrankyLetters
Plastic
- Joined
- Nov 6, 2019
I finally decided to put a proper cross slide chip guard on my circa 1959 South Bend 13. I bought an aftermarket chip guard off of ebay and it has mounting holes that are 21/64ths. I tried a couple of 5/16 bolts in the mounting holes on the cross slide. 5/16 seems to be the correct diameter, but 5/16-18 doesn't fit and neither does 5/15-24. I looked up in the Machinery's Handbook (MH) and there is a 5/16-20. I used a set of calipers to confirm the minor id of the mounting holes on the cross slide and it looks correct to be 5/16-20. Online, I discovered that 5/16-20 are not available and everyone uses M8-1.25, so a tried one and it starts but I only get about 1 thread in. I looked up the specs on M8-1.25 in MH and the major and minor diameters are really not all that close to 5/16-20 so I decided to make some screws on the lathe.
After 3 tries, I still can't get a screw that works. Admittedly, I've never single point threaded a part that small, I've made some large 2" + threaded pieces, but for sub 1 inch I've always used dies. The best screw, photo attached please excuse the banged-up knurling but I think the threads look OK, I have been able to make gets 2 threads in and stops. I've checked the major diameter and it's right on with MH. I've checked the TPI with a thread gauge and it's correct. I don't have a test piece because the only threaded part I have is the cross slide itself. I don't have any way to measure the minor ID of my part.
So, can someone please confirm that 5/16-20 is the correct size for a cross slide chip guard mounting hole on a circa 1959 South Bend 13 before I soldier on to make some more screws?
After 3 tries, I still can't get a screw that works. Admittedly, I've never single point threaded a part that small, I've made some large 2" + threaded pieces, but for sub 1 inch I've always used dies. The best screw, photo attached please excuse the banged-up knurling but I think the threads look OK, I have been able to make gets 2 threads in and stops. I've checked the major diameter and it's right on with MH. I've checked the TPI with a thread gauge and it's correct. I don't have a test piece because the only threaded part I have is the cross slide itself. I don't have any way to measure the minor ID of my part.
So, can someone please confirm that 5/16-20 is the correct size for a cross slide chip guard mounting hole on a circa 1959 South Bend 13 before I soldier on to make some more screws?