Odd 6 2/3 TPI elevation screw for Sanford MG surface grinder?

[ljwillis]

Does anyone know of a source for 3/4" x 6 2/3 TPI acme or square thread leadscrew and nut? I'm working on a Sanford MG surface grinder that had a very hard life and the elevation leadscrew needs to be replaced. I nearly bought a 7TPI but then looked more closely at the measurement and realized this wasn't going to be as easy as I had hoped.

I guess they used this oddball thread to get 0.050"/rev of the handwheel with their particular ratio on the bevel gears. I'm not enthusiastic about re-engineering the entire downfeed mechanism, nor am I excited about sourcing a potentially expensive custom screw for a very worn out grinder, so I'm hoping someone knows of a source for these. If I'm not mistaken, Sanford was not the only manufacturer that used this pitch.
Worst case I guess I'll be putting in a 7TPI and living with meaningless handwheel graduations until I get around to a different solution.

Thanks for any suggestions!

 

[ErichKeane]

Yikes! 6 2/3 TPI is a super oddball. Even my lathe (which has a bunch of odd imperial threads) only does 6 1/2 (and it does 1 5/8 ). Presumably someone with some change gears could cut you something (20 TPI /3 is 6 2/3, so messing with the ratio 3:1 would work). My change-gears are massive and generally not easy to obtain, otherwise I'd offer.

Are you sure it isn't something more sane? Can you try counting teeth on the bevel gears to make sure?

 

[projectnut]

A 6-2/3 tpi lead screw does strange, but it shouldn't be all that difficult to change it to a 7 tpi or other more standard pitch. Keep in mind if the lead screw is badly worn the feed nut is probably equally worn. To get the same travel per rotation you would only need a new matching lead screw and nut and a different set of miter gears.

When I purchased my Sanford MG machine I noticed it had oil ports over the miter gears and the lead screw. In conversations with other owners I learned these ports were not standard and had apparently been added by the previous owner. I didn't think much of the oil ports, but after setting up the machine I found I had a problem hitting the target dimensions. On close inspection I found someone had changed out the original miter gears that had a 3 to 1 ratio to a pair that had a 4 to 1 ratio. The result was that for every .005" movement on the hand wheel scale the spindle was only being moved .0041".

I'm guessing that the original gears had worn due to lack of lubrication and grinding dust getting between the mating teeth. Since the lead screw and nut also appear to be new I believe they were also replaced as part of the repair.

I was originally going to replace the miter gears with a pair with a 3 to 1 ratio. After some consideration I decided to leave the system as it is. The main reason being I haven't measured the TPI of the current lead screw and nut, and I'm not sure that they are the same pitch as the originals. In my case I couldn't see spending the money (about $200.00) for the minor amount of math it would take to compensate for the difference between the handwheel readings and the actual movement of the spindle.

In your case it might be worth the expense since you do have a badly worn lead screw that needs replacement

 

[ljwillis]

Thank you both for the input. It's definitely 6 2/3 TPI and the bevel gear ratio is 3:1 (48 teeth and 16 teeth), which makes the math work out for 0.050"/rev on the handwheel. From what I've read, K.O. Lee did the same on some of their grinders. Everything on the machine looks to be original other than the motor, though.


projectnut, this one had an oil cup over the gears and one in the spindle casting for the leadnut, but nothing for the screw. Someone was using grease instead of oil on everything, though, which surely contributed to the destruction of the screw and nut. Here's what I found under the cover before cleaning:


I think it seems reasonable to swap out for 4:1 and maybe use a dual-start 5tpi screw or 2:1 and a regular 10tpi. I just didn't want to have to machine a replacement for the "gear box" casting, hopefully I can find some gears that will drop in with only having to change the length of the handwheel shaft.
Is there any evidence of modification to the elevation mechanism on your machine other than the gears? I haven't looked for off the shelf replacement gears yet but it just seems improbable to be able to change the ratio and have everything fit back together otherwise why would they have gone with these in the first place? If that was the case, then that's highly encouraging.

 

[ErichKeane]

Yikes! 6 2/3 is definitely an oddball! IMO, I would think tossing some 4140 in your lathe and single-pointing shouldn't be that difficult if you can get a lathe gear set that'll cut that.

You might consider posting in the "can you make this for me" thread to see if someone can just turn one out. It IS an oddball TPI, but it is 1/3 of 20, which makes the math not terrible for someone who has a gearbox + change gears.

 

[Lo-Fi]

Using something readily available and fitting a travel indicator seems like a good option? Probably more accurate than the handwheel regardless.

 

[ljwillis]

Using something readily available and fitting a travel indicator seems like a good option? Probably more accurate than the handwheel regardless.

Absolutely, I'm in the habit of using an indicator when it counts, anyway. Having a handwheel that roughly corresponds to reality helps to avoid bonehead mistakes, though!
I'm certainly leaning towards this as least in the short term.

 

[ljwillis]

Yikes! 6 2/3 is definitely an oddball! IMO, I would think tossing some 4140 in your lathe and single-pointing shouldn't be that difficult if you can get a lathe gear set that'll cut that.

You might consider posting in the "can you make this for me" thread to see if someone can just turn one out. It IS an oddball TPI, but it is 1/3 of 20, which makes the math not terrible for someone who has a gearbox + change gears.

New to the forum, so I hadn't found "can you make this for me" yet, I may give that a try, thanks.
I won't kid myself into thinking that I can cut a serviceable leadscrew on my import 9 x 20 lathe at the moment.

I hadn't thought about making one, but I have access to an HLVH with an electronic "servo threading" attachment at work in the R&D/prototyping shop. No follow rest, though, and it's a 16" long screw.

 

[ErichKeane]

New to the forum, so I hadn't found "can you make this for me" yet, I may give that a try, thanks.
I won't kid myself into thinking that I can cut a serviceable leadscrew on my import 9 x 20 lathe at the moment.

I hadn't thought about making one, but I have access to an HLVH with an electronic "servo threading" attachment at work in the R&D/prototyping shop. No follow rest, though, and it's a 16" long screw.

A 9x20 definitely wouldn't cut that (simply due to the bed length), but at 3/4" diameter a follow-rest isn't particularly necessary as long as you take a couple of spring passes at the end and have tailstock support.

That said, some people hang out here (https://www.hobby-machinist.com/forums/can-you-make-something-for-me.452/) and ask for people to do 1 offs like this that might be able to help. 6-2/3 TPI is super oddball, but hopefully someone with change gears can volunteer and we can help them with the math

The extra interesting part to me, is how to grind the tool! Most ACME grinding gauges only use single TPI, so I'd suspect the person would have to just 'guess'.

 

[JRaut]

Second/third what everyone is saying about it being super oddball.

But any old lathe with change gears ought to be able to cut it. I've got an excel calculator I use for my old Logan when I need to cut an oddball or metric thread.

My calculator says you can do it with all the stock gears: 16 stud -- 72 idler -- 18/54 gearset -- 40 screw gear -- 8TPI lead screw. Voila!

(Whether those gears physically fit together is another question. I can take a look this evening if'n you're interested.)