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

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fratzog

Fratzog
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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.
 
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
 
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
 
"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.
 
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
 
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?
 
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"
 
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.
 
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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