No index wheel for calculated fraction?

Hi Everyone,

I'm building a cross slide and thinking about indexing the feed dial I've gotten a bit stuck.

My cross-slide lead screw is M8x1.25mm. If I wanted to have a dial which represents 0.01mm per division I'd obviously need 125 divisions around the circumference of the dial. (which is 50mm diameter).

My rotary table ratio is 90:1 so if I take 90 / 125 , the lowest common denominator is 5 and gives me the indexing fraction of 18 / 25 but the three indexing wheels for the rotary table do not include a set of 25 holes.

1. Do I need to find the next highest denominator above 5 which matches a set of holes I do have?

2. Is it better to have a dial which represents removing material from both sides of the cut, or is this just personal preference?

3. Should I just get a lead screw with 1mm pitch to make things easier?

Thankyou,
Ed.

Have you got a DRO with PCD function? If so its easy to make a 25 hole plate.

There is also a man who frequents this site that sells plates that I think include 25 divs.

J

Edited By JasonB on 20/04/2014 19:01:46

Hi Jason, no I'm afraid not. I have some basic DRO's for X and Y axes. Although I could probably make a drawing and extract coordinates in CorelDraw to use with those.

Hi Ed,

The only prime factor of 125 is 5, so you need 25, 50, 75 or 100 holes.

You could use a 50T changewheel to index a new plate or even a temporary one from MDF.

Neil

When I marked out the dials on my Dore Westbury mill I used a saw blade (100 teeth) mounting the dial and blade together in the lathe with a stop on the teeth. Used a tool and the saddle to scribe lines down the dial at the prescribe divisions.

Bob

The reduction factor of the worm - 90:1 - improves the accuracy of your index late by that factor. So you can glue a piece of cardboard to one of the existing plates and roughly mark that into 25 and it will be good enough. If you can't find a kids plastic protractor remember a clock face is divisible by five to make some initial marks, then divide those divisions by eye.

Q2. The diameter radius thing is personal choice. Sometimes you get posts saying how daft one or the other is msotly because the writer has been brought up on a particular variant.. In my mind it is less confusing to have the dial show the actual movement to match the topslide. You are just as often wanting to make a cut that is say .1 deep regardless of diameter as to be aiming for a particular diameter.

My index dials are marked in 0.025mm or "0.001" divisions. For putting on a cut they are fine for imperial and I generally do this without error.

If I have to remove 0.81 of a mm, my brain turns to jelly and I'm likely to mess up more often than not.

Solution, set calipers or micrometer to metric target dimension, zero it and work in thous!

Neil

Some great ideas there, thankyou everyone. I think I might just have a 50 or 100 tooth gear as a starter.