Why didn't I know this?

[Captain Jerry]

I finally got around to mounting a caliper based DRO on the cross slide of my lathe. The first thing I did was to compare it with the dial. One full turn = 50 divisions and the caliper reads .049". I thought my mounting was at fault, so I fiddled with it for a while. New test, same result. More fiddling, no change. I began to doubt the scale so I set up a dial indicator and it agreed with the caliper. I changed the caliper to the mm scale and one full turn = 0.125 mm! The dials are NOT inches. The lathe is a 10 year old HF 9x20 that I got second hand with no documentation and for some reason I thought the divisions were .001".

I downloaded an operators manual for the Grizzly 9x19 and there it is

"One division = approx .001"

If you are sneaking up on a finished diameter, using the dials is just a crap shoot. I spent the afternoon turning a piece of CRS to target dimensions. .500", .475", .450" ... using just one mic check per target and sneaking up using the DRO scale. After a few misses, I hit all of the targets within .001". I had never been able to do that using the markings on the dial.

I guess that's what approx means.

Jerry

 

[Sparticusrye]

Jerry,

I have a Craftex 10x18 that is very similar to the HF model. I have the same issue. Good to be aware of, I'll probably make the same modification and add the caliper DRO to my cross slide. Though I never trust the dial exclusively and always check the last two cuts to be sure of the ammount to remove. If I save .030 for the last two cuts I will take .020 measure see how close I am then finish off at a slightly higher speed with what ends up being a ~.008 cut most of the time. Old habit from work since the lathe I used there was old and neglected.

James

 

[Catminer]

Very interesting that you posted this, just today I was looking at how I could add a caliper readout to my 12x36 it has the very same problem. It is an oriental import sold by King tools, a Canadian importer of machine tools.

Peter

 

[Herbiev]

Just fitted a DRO to the X axis of my RF25 mill. I noticed the discrepancy and wondered where the error lay. Now i know. Thanx for the info

 

[Ken I]

Sounds to me like the machines have 1.25mm metric pitch screws (0.04921") which they have (via the dials) approximated to 0.05"

Ken

 

[Diy89]

Ken's right. Metric screws with "close" imperial markings. Not paying enough attention to this has scrapped more parts than i want to admit to.

 

[Deanofid]

"One division = approx .001"

Division of "approximately .001" on a lathe dial is like saying you got killed and lived to tell about it.
Doesn't make sense. It's supposed to be a precision tool, for cryin' out loud.

Glad you got it figured out, Jerry. Would have had me very frustrated.

 

[Maryak]

Both my lathe and mill have metric lead and feed screws

I made up this chart which keeps me under control :

Hope you find it useful.

Best Regards
Bob

View attachment In_mm.xls

 

[Cliff]

I thought that is what they made files and emery paper for. I never ran a lathe or mill that would machine that close. Cliff.

 

[Sparticusrye]

Thanks for the charts Bob, they'll come in handy. Laminating them and hanging them behind my lathe.

James

 

[Captain Jerry]

I am a long standing beginner. Almost everything I do in this hobby is a new "discovery" for me. Most discoveries are a joy, the light goes on, you sit back and say "that's cool." This discovery was different. The light went on, I jumped up and said "WTF", followed by "SOBs".

Dean

You are absolutely right, precision machine "really?" I paid top dollar (less than $500 including the stand) and I expect precision, no, I expect perfection! And if I end up in hell, I will be expecting ice water. My precision lathe has been made precise by the addition of a $15 digital caliper!

The reality is that without cheap imported machines, I would be spending my time doing something less creative and less enjoyable with my time.

Bob

Thanks for the chart. I'm nowhere near organized enough to be able to find it when I need it so I have decided to memorize it. That way I'll have it available whenever I need it. Even though on this dial, 40 divisions = 1 mm and the dial has 50 divisions. But I guess that is still better than my mill with the imperial thread and a dial with 62.5 divisions!

Jerry

 

[Ken I]

Jerry,
My metric lathe has 1.5mm screws - which makes a bit more sense than 1.25 so it's entirely possible that they only use the 1.25 on "imperial" lathes - in which case they chose the expediency of using metric threads over accuracy - in which case they should be slow roasted over a hot fire.

I started this reply suggesting that perhaps you buy the metric handwheels - but from the above - they might simply not be available - without changing the screws - which is the other way you could address the problem - replace you screws with "propper 20 TPI.

Obviously your caliper DRO solves the problem anyway. Just a thought....

Ken

 

[Tin Falcon]

From what I understand it was pretty common practice to use metric screws with inch dials. That is why companies like Micromark advertise true inch dials and screws. IIRC MY MM mill is true inch kind of mute because the dials ar in a box and Mach 3 moves things now. Never had a problem with the 7 x 10 . but I do cut measure cu anyway .
Tin

 

[Captain Jerry]

Tin

Electrons rule! Even in cheap caliper! I haven't looked at the 62.5 dials on my mill since I installed the Grizzly (IGAGING) electronic scales.

Measure, cut, measure, cut,measure, cut, measure, adjust tolerance.

Jerry

 

[gunboatbay]

Some time ago, I made up a chart similiar to Bob's, but with drill sizes and tap-drill info. I use it almost every time I'm in the shop. I've attached a PDF copy, but have the original excel format if anyone would like it.

View attachment Drill chart.pdf

 

[Chazz]

Hmmmm, I just came up from the shop to relax and enjoy the fact that my lathe is running again! and to think about what next?

Now the reason my lathe was out of commish was due to the fact I buggered-up the stock cross-slide nut while making (now made) a new 7\16" x 20 TPI cross-slide screw which also uses the 50 grad index from the stock screw assembly. woohoo1

i guess I'll do a few test cuts tonight and see how well the screw works. ;D

Cheers,
Chazz

 

[tornitore45]

This I will never understand, there are a lot of tools and gagets made in China with Imperial screws all over.
China produces Imperial taps and dies of all kind.

Yet they cheat with a cross-slide screw 1.25mm lead instead of making an accurate 20TPI.

The G4000 and similar 9/20 has a 16TPI lead screw, If they just made the screw with the same machine they sell it will come out perfect.