Metric vs Imperial lead screws

I think I recognise the initial question... I think. Some time back I bought a small mill to use as a drill press for my model making. The speed control was what I was after. And the price. I think it came from Harbor Fright. The table screws are 16 TPI, or maybe 1.6 mm. Don't know, don't care.

In any case, the handwheel guages are calibrated in thousandths, 62 and a half. 1/16th in thou. To use it as a mill would be a PITA, having to calculate half thous every revolution. I don't use it as a mill so it's a non-issue to me.

To put a DRO on such a low end mill would be a waste. On a BP, sure, but not some Chinese mini-drill press. Besides, the bottom end for me is to run everything manually. I worked on electronics for industry for pushing 50 years. I don't trust it beyond how far I can throw it. At one time I had true NC control on my stuff. Before CNC existed at reasonable prices a hobbyist could pay, everything was analogue. Took it off, I'm not a production shop and it took the fun out of screwing up.

I did see an advert, LMS(LittleMachineShop) I think, for replacement screws having 20 TPI, 0.050 per revolution. Been a while, I don't know if they are still available. But the obvious answer to the question. It was a complete kit, screws, followers, handwheel dials, etc. Every thing needed to make the conversion.

littlemachineshop.com

Machinist Tools and Supplies | Machine Shop Tools for Sale

Cutting Tools, DROs, Kits, Lathe, Lathe Chucks, Mill, Precision, Threading Tools, Tooling, Videos, Books and Seminars at LittleMachineShop.com
littlemachineshop.com littlemachineshop.com

.
 
Last edited:
I worked by the dials for many years. I have DRO's on all my machines now and I wouldn't go back. Not having to deal with miscounting turns , remembering to account for backlash, and errors due to wear on a tired old machine make machining much more efficient and pleasurable.

I still do a lot of lathe work without the DRO, largely from habit, for non critical work. My G0602 lathe has metric cross feed, compound, and tailstock screws but inch dials. A .001" advance by the dial is slightly less (.00098") so there is a bit of cleanup left which is OK. I will advance the feed and measure the work and make a correcting cut. It's better than overshooting the advance and ruining the work. But when it comes to being able to make accurate parts, the DRO comes into play.

As to the reliability of the DRO, I have had a DRO on my mill/drill for fifteen years and it is never shut down. I have yet to have a malfunction. A 3 axis DRO can be added to a machine for a little over $200. Less than $150, if you went with the iGaging 35-700 series.
 
mksj said:
I had a mill with imperial dials and supposedly imperial lead screws, but were in fact metric. Nothing worked-out correctly and it was very confusing until I figured out the problem. This does occur on some of the cheaper machines and the dials are an approximation in inches.
Click to expand...

I just discovered this when testing my new DRO for accuracy using a 4" gauge block on the longitudinal axis.

I found that the DRO was dead-on vs the gauge block -- 4.0000"vs. 4.0000". But I also noticed that the dial reading was way off, reading about 4.114", or off by about 2.86%. This is close to RJ's experience of 2% (A .001" advance by the dial is slightly less (.00098") )

I had no idea the dials would be off by that much, but triple checked and kept getting the same result. The attached PDF shows the readings I took across a 20" span, and the correlation showing the 2.86% difference.

I suppose it's no big deal, but still, 114 thousandths over 4 inches or nearly 600 thousandths over 20 inches was a surprise to me..
 

Attachments

  • Dial vs DRO - Longitudinal Axis.pdf
    212 KB · Views: 133
bill70j said:
I just discovered this when testing my new DRO for accuracy using a 4" gauge block on the longitudinal axis.

I found that the DRO was dead-on vs the gauge block -- 4.0000"vs. 4.0000". But I also noticed that the dial reading was way off, reading about 4.114", or off by about 2.86%. This is close to RJ's experience of 2% (A .001" advance by the dial is slightly less (.00098") )

I had no idea the dials would be off by that much, but triple checked and kept getting the same result. The attached PDF shows the readings I took across a 20" span, and the correlation showing the 2.86% difference.

I suppose it's no big deal, but still, 114 thousandths over 4 inches or nearly 600 thousandths over 20 inches was a surprise to me..
Click to expand...
I am surprised that your error is as large as it is. Screw pitches are some nice round number, 8, 10, 12,16 tpi, etc for inch and 1mm, 1.5mm, etc. for metric. Your error of 2.86% doesn't conform to the expected error of (4.000-3.937)/3.937 or 1.6%. If your lead screw dial is marked .100" per revolution, a metric lead screw would probably have a 2.5mm pitch which would be .0984"/ revolution. It would take 40.64 turns to reach 4.000 on the DRO. Your linear error curve precludes dial slippage as well as lead screw wear.

I know that you said the DRO was accurate compared to gage blocks but are you aware the DRO scales need to be calibrated? If it were my system, I would want to resolve the mystery.
 
Wheels not matching the leadscrew would be awful without DRO. They are cheap enough that if you have one like that, install a cheap DRO.
 
RJSakowski said:
I know that you said the DRO was accurate compared to gage blocks but are you aware the DRO scales need to be calibrated? If it were my system, I would want to resolve the mystery.
Click to expand...
RJ:
The DRO I bought has both a linear and non-linear compensation function. But I assumed that since the DRO reading was exactly the same as the physical movement of the table, as measured by a gage block, that I didn't need to use the calibration function. Is that not the case?

Here's the method I used to compare the DRO to the table movement.
  1. Clamped a 123 block square to the table and zeroed a 0.0005"DTI -- that was on a mag base affixed to the bed ways -- against the 123 block.
  2. Zeroed the DRO
  3. Moved the table a little over 4" and positioned a 4" gage block against the 123 block
  4. Then, without moving the DTI, moved the table to re-zero the DTI against the gage block.
  5. Observed the DRO reading of 4.0000"
My assumption therefore, was that the DRO is working properly and doesn't need calibration, but the mystery of dial readings being off by 114 thousandths remains. (BTW, the crossfeed dials are way off also.)

Please let me know what you think. Thanks!!
 
bill70j said:
RJ:
The DRO I bought has both a linear and non-linear compensation function. But I assumed that since the DRO reading was exactly the same as the physical movement of the table, as measured by a gage block, that I didn't need to use the calibration function. Is that not the case?

Here's the method I used to compare the DRO to the table movement.
  1. Clamped a 123 block square to the table and zeroed a 0.0005"DTI -- that was on a mag base affixed to the bed ways -- against the 123 block.
  2. Zeroed the DRO
  3. Moved the table a little over 4" and positioned a 4" gage block against the 123 block
  4. Then, without moving the DTI, moved the table to re-zero the DTI against the gage block.
  5. Observed the DRO reading of 4.0000"
My assumption therefore, was that the DRO is working properly and doesn't need calibration, but the mystery of dial readings being off by 114 thousandths remains. (BTW, the crossfeed dials are way off also.)

Please let me know what you think. Thanks!!
Click to expand...
That's a strange one. Your calibration check appears to be sound. What are the scales on your dials?
 
RJSakowski said:
I know that you said the DRO was accurate compared to gage blocks but are you aware the DRO scales need to be calibrated? If it were my system, I would want to resolve the mystery.
Click to expand...

RJ:

Thanks for the challenge. I believe the mystery is solved.

This lathe has graduated dials on the both the carriage handwheel and on the handwheel attached to the end of the leadscrew - both graduated in 0.001". The leadscrew is 10TPI
Carriage Hand Wheel.jpg
Leadscrew Hand Wheel.jpg


I had used the graduated dial on the carriage handwheel -- which is geared to the rack -- to check for DRO accuracy. I found the difference between the dial readings and the DRO to be off by 0.114" over 4" of travel.

But when I engage the half nut and advance the carriage with the leadscrew handwheel, the difference between the dial readings and the DRO is off by 0.001" over 4" of travel.

Lesson learned.

Thanks again, Bill
 
bill70j said:
RJ:

Thanks for the challenge. I believe the mystery is solved.

This lathe has graduated dials on the both the carriage handwheel and on the handwheel attached to the end of the leadscrew - both graduated in 0.001". The leadscrew is 10TPI
View attachment 313906 View attachment 313907

I had used the graduated dial on the carriage handwheel -- which is geared to the rack -- to check for DRO accuracy. I found the difference between the dial readings and the DRO to be off by 0.114" over 4" of travel.

But when I engage the half nut and advance the carriage with the leadscrew handwheel, the difference between the dial readings and the DRO is off by 0.001" over 4" of travel.

Lesson learned.

Thanks again, Bill
Click to expand...
My lathe doesn't have a dial on the rack pinion so I have no problem there. If it did have a dial with 100 graduations, each full turn of the crank moves the carriage .950" so I would have an error of .05"/" if I assumed that each graduation was 1/100".
 
You must log in or register to reply here.
Back
Top