South Bend 13 chip guard bolt size?
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  1. #1
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    Default South Bend 13 chip guard bolt size?

    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?

    img_3237.jpg

  2. #2
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    How old is your machine? I’ve got a 13” CL8145B that is definitely 5/16-18 bolts on the chip guard. I’d try running a tap to clean out the holes. The likelihood of them being 5/16-20 is negligible.

    Edit: I see the age now. Our machines aren’t that different in age (mine is a tad older).

    Edit again: is that a bolt you made? I can look at it and tell something is wrong here. Look how wide the radius in your root is, are you using full profile inserts or partial profile? I’d make sure if you’re using full profile inserts that they aren’t for something like 8tpi- if you ground your own tool get rid of the nose radius and take it to a sharp point then LIGHTLY hone on a radius. If you are using inserts, toss whatever you used and pick up some partial profile inserts that are made for a range of TPI’s. Either way whatever you used, your root is way too wide for 18 tpi.

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    Thanks, I'll try a tap, I didn't want to go that route if I wasn't sure of the actual thread. The bolt seemed really punky to me so I didn't want to be too aggressive. I'm used to working with aluminum components and I back off instantly if I meet any resistance while putting bolts in.

    I used a 60 Deg insert, I understand now the root/tip problem. I looked at indexed insert threading tools but couldn't figure out how to ensure I was getting partial profile inserts so I can do a range of threads with the inserts. Is there a naming or numbering convention?

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    They’re just called partial profile inserts, you can find them just as that. I believe (don’t quote me) the standard range is 8-48 TPI? But you can get them in other ranges as well. Just make sure to get the correct hand insert. Probably an 16ER-AG60 size insert. These are somewhat pricey and you can definitely find cheaper ones this is a good place to start looking though. These are “lay down” style inserts but you can use top notch or on edge style inserts depending on what you have tooling for.
    https://www.mscdirect.com/product/details/56952021

    Here’s a cheap solution if you don’t have the tooling either. Even though these look the same as triangular inserts they won’t work in the same holders.
    https://www.amazon.com/dp/B07FDWYHGC..._wx0WDbSCSWGZR

    We’re you trying to use a triangular insert tool for turning? The nose radius on those is way too big for threading and they may or may not have appropriate clearance for threading. Typically you don’t have issues with clearance angles with 60° inserts.


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