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



## ljwillis (Sep 22, 2020)

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!


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## ErichKeane (Sep 22, 2020)

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?


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## projectnut (Sep 22, 2020)

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


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## ljwillis (Sep 22, 2020)

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.


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## ErichKeane (Sep 22, 2020)

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.


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## Lo-Fi (Sep 22, 2020)

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


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## ljwillis (Sep 22, 2020)

Lo-Fi said:


> 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.


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## ljwillis (Sep 22, 2020)

ErichKeane said:


> 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.


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## ErichKeane (Sep 22, 2020)

ljwillis said:


> 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'.


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## JRaut (Sep 22, 2020)

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.)


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## projectnut (Sep 24, 2020)

When I was home yesterday I forgot to look at the pitch of the elevating screw on my Sanford grinder.  I thought I had a set of acme thread pitch gauges but couldn't find them.  Today I looked at several sets sold by McMaster and MSC.  None have a gauge for 6-2/3 TPI pitch threads.  I also looked for go/no go thread gauges and again couldn't find any with the 6 2/3 TPI pitch.

I'm out of town again, but when I return I'll make it a point to measure the threads.  I find it difficult to believe Sanford would go to the trouble of having a special lead screw made for their machines.  Almost all the other parts other than the castings are off the shelf parts.


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## RJSakowski (Sep 24, 2020)

I looked at my thread spreadsheet for the Grizzly 602. I would need a 54t. gear to cut a 6-2/3 tpi thread.  Someone with a Clough 42 ELS modification should be able to cut that thread with a modified thread table as described by ian999 in https://www.hobby-machinist.com/threads/electronic-lead-screw.76101/page-16#post-778321 post # 458.


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## ErichKeane (Sep 24, 2020)

RJSakowski said:


> I looked at my thread spreadsheet for the Grizzly 602. I would need a 54t. gear to cut a 6-2/3 tpi thread.  Someone with a Clough 42 ELS modification should be able to cut that thread with a modified thread table as described by ian999 in https://www.hobby-machinist.com/threads/electronic-lead-screw.76101/page-16#post-778321 post # 458.


Perhaps you just need to ask someone to make a 54 tooth gear for you  We could form a bit of a 'can you make this for me' tree  

That said, this says you should have a 54 tooth gear: https://cdn0.grizzly.com/manuals/g0824_m.pdf


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## RJSakowski (Sep 24, 2020)

ErichKeane said:


> Perhaps you just need to ask someone to make a 54 tooth gear for you  We could form a bit of a 'can you make this for me' tree
> 
> That said, this says you should have a 54 tooth gear: https://cdn0.grizzly.com/manuals/g0824_m.pdf


The lathe is G0602 and no 54 tooth gear.  I have the ELS installed on my lathe so no gears any more.  It would require an editing of the firmware which would entail pulling the Launchpad controller and reprogramming it with my office computer.  Aside from that, the 602 has (barely) enough travel to cut 16" of thread plus whatever addition features are required for the end of the shaft.  The lathe's lead screw having 12 tpi, it may even work out that the thread dial would work for a 6-2/3 tpi thread.

However, I have been retired for seven years and don't take on paying jobs any more.  The wife strongly disapproves of it when there are honey do jobs in the que (and it's a long que).   There is a growing list of individuals on this forum who have retrofitted the Clough ELS to their lathes. Additionally, the Rocketronics ELS should be able to cut this thread and several members have their ELS installed.


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## projectnut (Oct 9, 2020)

i finally got down to the shop for a few minutes yesterday afternoon.  About the only things I did were to clean up a few tools laying around and measure the pitch on the elevation screw on the Sanford Grinder.  In my case the screw is a 3/4-8 Acme thread.  To be certain of the pitch I used an Acme thread pitch gauge like this:









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If you choose to buy one there are a few listed on eBay.  The one from HHINDT is listed for less than 1/2 the price this vendor is asking.


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## ljwillis (Oct 9, 2020)

projectnut said:


> i finally got down to the shop for a few minutes yesterday afternoon.  About the only things I did were to clean up a few tools laying around and measure the pitch on the elevation screw on the Sanford Grinder.  In my case the screw is a 3/4-8 Acme thread.



Interesting!  Are your bevel gears still 3:1 or have they been changed?  With 3:1 you would only be getting 0.0416"/rev, no?  

Thanks for checking!  

I think I'm going to be changing the bevel gears.  At this point I've changed the spindle bearings, replaced motor bearings and balanced the pulleys, replaced the crossfeed screw, and replaced the rack and pinion and I'm pretty happy with the surface finish and flatness that I'm getting. It's at least as good as my old worn out surface plate haha. I need to do something, though, as the worn out elevation screw has it so that I must be very careful with adjustments, as there's backlash and stick-slip that can make for somewhaterratic downfeeding.  That led to one unfortunate gouge when I was dusting the chuck, for instance, so I had to re-do that.  I'm a little concerned that the stick-slip in the vertical ways may be the bigger problem than the worn out screw, though, in which case maybe I need a chunk of iron to hang off the motor moreso than a new screw (until I get a couple hundred free hours for some scraping.)

Thanks again


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## projectnut (Oct 9, 2020)

I mentioned in post #6 on this thread I thought the gear ratio had been changed from 3:1 to 4:1

Looking for Sanford MG-612 Surface Grinder owners 

I didn't offer the tooth count at that time, and I'm thinking now I miscounted.  That same post does mention that rather than raising or lowering .005"  according to the handwheel the spindle only moves .0041.  That number would coincide with a gear ratio of 3:1 and an 8 TPI lead screw.

I'm not sure now whether the gears were replaced with an identical set or they are the originals.  I'm inclined to believe they are direct replacements.  They seem to be in similar condition as the leadscrew and nut.  All look to be in excellent shape.  I would think after 66 years of use the originals would show moderate to serious signs of wear.  

The leadscrew shows slightly less than .001" backlash.  The same size "ultra precision" leadscrews from major industrial vendors show a spec of +/- .0006" from the factory.


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