# I need a sanity check on osing a DRO on a rotary table for gears.



## fratzog (Jul 18, 2013)

*I need a sanity check on using a DRO on a rotary table for gears.*

Hey guys. I don't have any experience on cutting gears, so I need some advice as to the feasibility of my plan. 

I have a Grizzly H7527 rotary table with indexing plates. It's a 90 to 1 head. I want to cut a 127 tooth change gear for my lathe. I have the proper involute cutter and I'm ready to set this up.

I know that 127 is a prime number, therefore indexing plates will not give the right "resolution" and besides that, my plates only go to 100.

I'm not ready to go CNC yet, mainly due to financial constraints, so I found a possible DRO solution. I found a company that makes a rotary angle encoder and display kit for less than sixty bucks. It has a resolution of .1 degrees.



I'm not very good at math, but here's what I came up with: On the 90-1 table, 360 degrees of travel on the crank gives you 4 degrees on the table. Using that logic I divide 360 by 127 to get the number of degrees per tooth on the table. I get 2.834 rounded (I'm only rounding here, not on my calc). Taking that number times 90, I get about 255.118, so I need that number of degrees on the crank to advance one tooth. Using the DRO, I can go to 255.1 on the crank. Dividing that by 90 and multiplying that by 127 I get an accuracy of 359.97444444... degrees over the entire gear.

First question is, am I figuring this correctly? and if so, is that close enough resolution to cut this gear without too much error?

I'm hoping this works, as I don't see any other way to do this without putting me in the poor house.

I would gladly accept any opinions and advice on this. Thanks.


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## iron man (Jul 18, 2013)

The number of holes in your plates dont make any differance it is the right number of holes and how many holes you advance each time. I am lost like you when I worked in a machine shop we had a chart that went with the dividing head it was slick you looked up the number of spaces or teeth to cut and it give you the right plate and the number of advances the problem was I got so use to it I forgot how to find the answer otherwise. Maybe someone out there can help.. Ray


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## tekfab (Jul 18, 2013)

If your 127 tooth gear is to enable you to cut metric thread pitch on an imperial leadscrew machine then there is a possibly much easier solution.

Mike


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## fratzog (Jul 18, 2013)

"The number of holes in your plates dont make any differance it is the right number of holes and how many holes you advance each time". *Yes, I understand that. My chart only specifies up to 100 divisions. I should have made that clearer.*


"You would have to make up a spread sheet before you start. Could you please post pix before you make the first cut." *I'm not sure what you mean by this.
*


"If your 127 tooth gear is to enable you to cut metric thread pitch on an imperial leadscrew machine then there is a possibly much easier solution." *Yes it is. Please tell me more.*


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## tekfab (Jul 18, 2013)

Harold Hall's website is very informative. I have used his changewheel tables to cut metric threads on my imperial (4tpi leadscrew) lathe with great success.

http://www.homews.co.uk/page30.html

The bottom paragraph being the most relevant one !

Mike Young


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## fratzog (Jul 18, 2013)

Ok, it looks like I have to solve a different problem before I can resolve my original problem. 

It seems that I am not able to see some of the posted replies here. In other words, I have received several replies to this thread in my email box but some of them are not showing up when I load this thread page. 

Admins, do you have any idea why this is happening?


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## Bill Gruby (Jul 18, 2013)

If they were from me I posted a couple but deleted them after rereading your original again. I was in error. On your set up, will the DRO be reading table movement or handle movement? I have the same rotary table.

 "Billy G"


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## fratzog (Jul 18, 2013)

Well I'm not sure what's going on, but I'll answer the last questions I see.

The DRO is mounted to the input shaft, that the crank handle/Vernier scale normally mounts to. It does not directly measure the table. In other words it provides a digital scale in degrees, and replaces the vernier scale. That makes it subject to the same 90 to 1 ratio that the vernier is. That means that the DRO will indicate 360 degrees for every revolution of the crank. It takes 90 cranks to make 360 degrees on the table. Or 4 degrees on the table for every single crank revolution. 360*90 = 32400 degrees indicated on the DRO for one full table revolution. So if I put in one degree on the crank, I get .01111111111 etc on the table. Right? As I said before, each tooth on the gear equals 2.83464 degrees rounded. That divided by the .011111111111 gives 255.118 degrees on the crank/DRO. If anybody sees an error in that, please let me know, but it looks right to me. Mind you, I don't have the DRO yet. That's why I was asking if the resolution was good enough. I don't want to buy this thing, set it up, and then find that it won't work. As I said before, I can only do up to 100 divisions on my indexing plates, so I am trying to take them out of the equation entirely.




Regarding Harold Hall's website. I tried to make sense of that, but couldn't. I think the problem there is that I don't understand what he means by drive gears and driven gears, and there's three of each. Which is which, and where do they go?



I would like to thank all of you for your effort in helping me so far. Please be patient with me though, as I am not the best at conveying my ideas on this subject.


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## Bill Gruby (Jul 18, 2013)

You have a PM.

 "Billy G"


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## tekfab (Jul 19, 2013)

If you let me know what thread you want to cut and what TPI your leadscrew is i can do you a sketch showing how to set up the gears. It can seem daunting but i'm pretty sure if someone explains/shows you it'll all fall into place.
If i can help i will !

Mike Young


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## fratzog (Jul 19, 2013)

tekfab said:


> If you let me know what thread you want to cut and what TPI your leadscrew is i can do you a sketch showing how to set up the gears. It can seem daunting but i'm pretty sure if someone explains/shows you it'll all fall into place.
> If i can help i will !
> 
> Mike Young




OK. I think I understand how the chart works. Unfortunately I don't think it will work for this lathe. 

You see, It's a clone of the Jet/Enco 1024. This lathe has an 8TPI leadscrew, but also uses a quick change gear box, and only three change gear positions. I bought this lathe used, without the optional metric change gears, therefore it only has the stock inch change gears in it. This consists of a 40 tooth drive gear, 70 tooth intermediate gear and a 40 tooth driven gear on the leadscrew gearbox. The intermediate gear I have is a single pattern, but the optional gearset for this position is actually a compound gear with a 120 and 127 tooth gears in it. I have attached a photo of the gear set/quadrant to illustrate better. Page 4, figure 2 shows B, C and D as "tumbler" gears. Those are the change gears.
	

		
			
		

		
	




Since I don't have ANY extra gears for this, I can not use the system you linked for me. My only choice appears to make the compound gear set.


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## fratzog (Jul 19, 2013)

Well, never mind. I just invalidated the whole project.



I just checked the gear spacing and found that a 127 tooth gear with a DP of 16 measures out to 8.06 inches. I made a cardboard mockup, and it won't fit. Not even close. The quadrant is too small, with not enough adjustment/travel to get enough space between the gears, or even the hubs. I must have got some misinformation somewhere, as I was told a 127 tooth gear would fit in this machine. I should not have assumed that to be correct. 

Now I have to re-figure the whole project, and come up with a combination that will work.  

The good news is that I don't need the DRO anymore.


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## iron man (Jul 19, 2013)

I use to buy brass and steel change gears from browning at a local bearing supply shop they had a catalog with a large range of change gears. Ray


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## Bill Gruby (Jul 19, 2013)

FWIW -- You have done well so far. To clarify, you went thru all the math. You used all the suggestions. You made a dummy part from cardboard.  You proved it will not work without wasting any time or material. R&D at its best. Congratulations are in order.

 "Billy G"


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## tekfab (Jul 19, 2013)

Your 3 speed gearbox is handy, it means that when you set up your gear train for a chosen tpi you'll have another 2 to select as well ! Gears can be got quite cheap and if you read that article HH tells you what gears you need to give you a comprehensive gear cutting range, just build up slowly starting with the ones you need for this project.

Mike


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## fratzog (Jul 19, 2013)

So I guess my priorities have changed on this. 

I had been trying to do this with very limited information about this lathe, but I finally found the missing manual for it today. It describes the following optional change gears by tooth count: 25, 26, 43, 46, 47 and 60. Odd to say the least. I feel that I should point out that this unit does indeed have a full quick change gear box, with the typical A,B,C,D, /1,2,3,4,5,6,7 selector configuration. That means that while I don't have to change any gears for normal inch cutting, I have to jump through several gear combinations as well as use the quick gear selectors to cut metric threads with this thing. Certainly not like any modern lathe I have used. This leaves me to do the search for these gears with a DP of 16 and 20 degree PA. So far I have not come up with an affordable solution, but I will keep looking using others suggestions. I really don't feel like trying to cut all those gears right now. especially with a busy summer coming up. 

I would like to thank all of you for your helpful suggestions. Even though this may have seemed like an exercise in futility, it served as a learning tool nonetheless. Now to find those gears.......


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## fratzog (Jul 19, 2013)

PS. Extra Thanks to Bill Gruby for the encouragement.


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## tekfab (Jul 20, 2013)

I second Bill's comment, you have done well. What thread pitch are you trying to cut ?

Mike Young


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## fratzog (Jul 20, 2013)

tekfab said:


> I second Bill's comment, you have done well. What thread pitch are you trying to cut ?
> 
> Mike Young




Thanks. 

As many as possible. I am restoring this machine, so I want it to have the full capability it had when it left the factory. I have another lathe, but this one is smoother, more stable and more powerful. Originally I was planning to sell it, But I think I have talked myself into keeping this one, and selling the other one. The only thing stopping me is the ability to cut metric threads. I do a lot of fab work for RC cars, planes and helicopters. It's ALL metric, so I have no choice.


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## benmychree (Jul 20, 2013)

For all that desire it, I have an article from American Machinist 40 years or so ago that details all that needs to be done to cut metric threads on a lathe with inch lead screw; so far as I know, nobody sells stock gears with 127 teeth.  I made mine by differential indexing, first as a pitch that fits my automatic gear cutter, then in the pitch that I needed for my lathe, which was coarser and of course larger in diameter than the dividing head would accomodate without blocking up for height.  This being done, I am able to cut 127 tooth gears without having to index teeth, just start it and walk away, come back when it is done and no chance to make indexing errors, which is all too easy, especially with diferential indexing where the dividing plate moves while you turn the crank ---


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## fratzog (Jul 28, 2013)

I have sold this machine, so I will be closing this thread shortly. Thanks to everybody for all the help.


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