# Metric vs Imperial lead screws



## DavidR8

Hi all, in my continued search for a mill, I wonder how much the pitch type (I don't think that the right term) of the lead screw matters.
I know that some of the small mills have metric lead screws but dials in thousands with some weird fraction left over.
I would put a DRO on any mill I buy so I would think that it wouldn't matter whether the lead screws were metric or imperial.
Or am I missing something completely obvious?


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## MontanaLon

I would suspect it matters not a bit if you plan to put a DRO on it. It isn't like a lathe where you need the lead screw that drives the carriage to cut threads. Unless maybe you are planning to CNC it in the future to go all high tech and stuff. Even then I doubt it would matter as the software would be able to compensate.


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## darkzero

I work in imperial so I wanted an imperial lathe. Mill doesn't matter much if you have a DRO but I would still rather have imperial leadscrews.

Many beginners will buy a DRO right off the bat but IMO you should know how to you use your machines without a DRO. Should be familiar with how to use your dials, how accurate they are, edge finding/locating, dealing with backlash etc. I purchased my machines without DROs & installed them later. What happenes if your DRO system ever goes down?

I'm completely lost when it comes to metric measurements. I mean when I wrenched on cars for a living I worked mostly with metric so I can tell a bolt head size by looking at it. Fasteners I prefer to use metric. I can picture how much 1 thou or 100 thou of an inch is but if you ask me 1 tenth or 1 hundreth of a mm I'm clueless. Even though I have DROs I still use my dials for DOC & reference often.


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## DavidR8

Thanks gents, agreed on the need to learn to use the machine before diving into a DRO. I have since learned that the dials are actually metric so that takes away the insanity of imperial dials on a metric lead screw.


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## darkzero

Many people have learned to use both which is great. I have been using imperial measurements all my life, long before I even knew what a lathe was so that's what I'm used to. All my measuring tools are imperial. I do work with metric measurements but I always convert to imperial when making a part. It's just easier for me. I'd like to get familiar with metric measurements as far as machining but to be honest I doubt I ever will.

So in reference to ToT, I will always say "dead nuts" instead of "spot on" for the metric guys.


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## mksj

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. With this mill it forced me to buy a DRO which at the time was around $800, so wasn't happy about that. If your DRO ever goes out or you want to verify it to the dials, it is a lost cause. Also the Z axis for the head did not have a scale, and the dial was absolutely useless for measuring height adjustment. On a lathe it is a bigger issue because the lead screw pitch dictates the threading dial alignment, so you want the lead screw pitch to match the predominate measurement system you plan to use. In addition the tailstock and tool post dials you would also want to follow the predominate measurement system.  I do quite a bit of work in metric, so one area where the DRO's come in real handy. Now that they have come down in price worth considering, I find that they significantly decrease my work time and help reduce mistakes. 

Also from a resale point of view, it could have an impact.


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## stupoty




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## DavidR8

The lead screws are on a mill not a lathe


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## stupoty

DavidR8 said:


> The lead screws are on a mill not a lathe



I think I have lathe on the brain today  

opps


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## darkzero

stupoty said:


> I think I have lathe on the brain today
> 
> opps



But yes, to answer what you were talking about, it would just be vice versa. Lathe with metric leadscrew can use the half nuts to disengage/engage but not with imperial threads. Well you can if you keep track of the dial when disengaging/engaging. That's how I thread metric on my imperial lathe.


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## Bi11Hudson

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.









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## stupoty

Bi11Hudson said:


> To put a DRO on such a low end mill would be a waste.



They are getting so cheep now , the extra speed they offer for drilling a pattern of holes or doing bolt hole circles is a very tempting and time saving feature 

Stu


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## RJSakowski

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.


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## bill70j

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*.



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..


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## RJSakowski

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..


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.


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## ttabbal

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.


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## bill70j

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.


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.

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. 
Zeroed the DRO
Moved the table a little over 4" and positioned a 4" gage block against the 123 block
Then, without moving the DTI, moved the table to re-zero the DTI against the gage block.
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!!


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## RJSakowski

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.
> 
> 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.
> Zeroed the DRO
> Moved the table a little over 4" and positioned a 4" gage block against the 123 block
> Then, without moving the DTI, moved the table to re-zero the DTI against the gage block.
> 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!!


That's a strange one.  Your calibration check appears to be sound.  What are the scales on your dials?


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## bill70j

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.



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









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


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## RJSakowski

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


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".


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## Harley80915

stupoty said:


> I think I have lathe on the brain today
> 
> opps





DavidR8 said:


> Hi all, in my continued search for a mill, I wonder how much the pitch type (I don't think that the right term) of the lead screw matters.
> I know that some of the small mills have metric lead screws but dials in thousands with some weird fraction left over.
> I would put a DRO on any mill I buy so I would think that it wouldn't matter whether the lead screws were metric or imperial.
> Or am I missing something completely obvious?


Easy to do on the x, y and Z (verticle) lead screws.  Just measure the travel of the lead screw one full crank on the handle and
that will tell you what your lead screws are.  For example, if your table moves 0.1 inch cranking handle one turn, then your lead screw
is 10 tpi or  10 turns per inch.  Many cheap Chinese ones have metric leads so measure those with metric units.  I have a Precision Mathews and they have better 10 tpi lead screws.  Those are made in Taiwan, better products than China.


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## projectnut

DavidR8 said:


> Thanks gents, agreed on the need to learn to use the machine before diving into a DRO. I have since learned that the dials are actually metric so that takes away the insanity of imperial dials on a metric lead screw.



I don't know if you're looking for a new or used mill.  If you're considering a used one Bridgeport did make Series I mills with both imperial and metric dials.  I have a 1972 Series I machine that came out of a local high school.  It was ordered from the manufacturer with both sets of dials. 

Before I installed a DRO about 8 years ago I did use either the metric or imperial dials depending on the job at hand.  The metric dials were used far less than the imperials, but I never had a problem with poor measurements when I did use them.

As a side note when I brought the machine home the original invoice was still in the storage space in the column.  The machine was purchased with the dual dial option, a 6 inch vise, and a set of Hardinge collets by 1/16".  The total price was just a tad over $1,400.00.  I paid a little more than that when the machine was 30 years old.  Not to worry though, it came with several end mills.  Over the years students had apparently damaged a few and hid in the column.


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## macardoso

I just did a DRO install on my lathe and found the carriage handwheel to accumulate 12 thou of additional travel per every 0.7" revolution (yes, it has a graduated handwheel). I couldn't figure out why my parts were coming out wrong until I saw that.


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