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Measuring precision length while part is on the lathe

John TV

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#1
Very new, just a handful of hours on my Logan 10 inch lathe. No dro and so far just doing non critical parts with huge tolerance forgiveness. But I started thinking about different ways to precisely measure and mark work for length while it is in the lathe. So far just using sharpie and scratch method using cutting tool or calipers. what if you really need something within a few thou, what are people doing for layout? I can't say I have a good memory but I don't remember seeing this on YouTube. If someone can point me to written or video info that would be great.




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bill stupak

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#2
One method would be to mount an indicator to the bed or ways (by clamp or magnet) and have it bear against the carriage to measure movement.
 

Ed ke6bnl

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#3
20170713_172909.jpg Might want to check into a magnet mount or even hard mounted and movable 3-4 in dial indicator preloaded against the carriage or plate attached to the carriage. I put a mag back on a 3 " dial indicator. I just up graded to the igageing type scales and added a blue tooth touch dro to read out to my tablet. See Randy Richards youtube video. I consider myself a newbe hobbiest.
 
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RJSakowski

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#4
It depends upon what you are trying to measure, its size, and the required accuracy. Diameters are easily measured, turned lengths, not so much. If I were necking down a piece to a shoulder (like a top hat), I would be inclined to set up a carriage stop corresponding to the top of the brim of the top hat. I would then place a spacer of the exact length of the neck between the stop and the carriage. Now the carriage will stop at the top of the neck. This is particularly useful if making multiple parts as there isn't a need for a lot of measurement checks.

If I was facing a bar to length, I would use a spindle stop. Place thwe spindle stop in the bore of the spindle. Set the compound so its travel is parallel to the spindle axis. Make a trial oversized cut with the work tight to the stop and the carriage locked down.. Pull the work out of the lathe and measure, determine a corrective cut and adjust the compound accordingly. For more accuracy, the compound can be set at an angle of 84.3º to the spindle axis so each inch of compound travel move .1" in the direction parallel to the spindle axis.

It is also possible to face a bar to length by cutting it slightly oversized, removing and measuring, and then touching off the cutting tool on the previously cut face and adjust the tool position as required.

Tapers can be a real bugger to measure. The two cylinder method works. A short cylinder with appropriate slightly different internal diameters is used. A dial indicator is set up to measure the position of the tailstock e4nd of the cylinder when the cylinder is fitted to the taper. The indicator position is recorded and the cylinder is reversed and its position measured with the indicator. The difference in the diameter, coupled with the difference in the indicator readings and a little trigonometry will give you the taper. Measuring the diameter of the large end of a taper is a piece of cake, the small end, not so easy. If possible, I will turn a cylinder on the small end of the taper with a diameter equal to the small diameter of the taper. I will then cut off the cylinder at the intersection with the taper.

Internal bores can be measured with gage pins, bore gages, or telescoping gages. Internal tapers can be measured with ball bearings. Having a mating part to the taper is also useful in determining correct machining. And of course, depth gages for bore depths.
 

John TV

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#5
Thanks guys a good place to start. I just have a hf cheap cheap dial ind and mag base now, the list keeps growing. Longer indicator-check. Also just started thinking of modifying cheap digital caliper, that might work too.


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darkzero

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#6
Before I had a DRO I used 1" & 2" dial indicators with mag backs mounted on the way like most eveyone commonly does. Well I still use a DI for threading.

Even though I had a 2" I found myself using the 1" more. Slower having to measure & move the indicator for lengths longer than the DI's travel but it did the job well.

Another option is a Trav-A-Dial but you have to be very patient to find one for a good price.
 

BGHansen

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#7
Not adding too much here, but +1/2/3 on the 2" travel dial indicator on a magnetic base. Bought mine from CDCO for probably under $30. I face both ends of the part and record the overall length. Then touch the tool bit to the end of part sticking out of the chuck and lock the carriage. Make sure the compound rest is turned into the work also. Then set the indicator on the lathe bed and zero the scale. Naturally, the indicator should be as parallel to the bed as possible.

Bruce
 

John TV

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#9
It depends upon what you are trying to measure, its size, and the required accuracy. Diameters are easily measured, turned lengths, not so much. If I were necking down a piece to a shoulder (like a top hat), I would be inclined to set up a carriage stop corresponding to the top of the brim of the top hat. I would then place a spacer of the exact length of the neck between the stop and the carriage. Now the carriage will stop at the top of the neck. This is particularly useful if making multiple parts as there isn't a need for a lot of measurement checks.

If I was facing a bar to length, I would use a spindle stop. Place thwe spindle stop in the bore of the spindle. Set the compound so its travel is parallel to the spindle axis. Make a trial oversized cut with the work tight to the stop and the carriage locked down.. Pull the work out of the lathe and measure, determine a corrective cut and adjust the compound accordingly. For more accuracy, the compound can be set at an angle of 84.3º to the spindle axis so each inch of compound travel move .1" in the direction parallel to the spindle axis.

It is also possible to face a bar to length by cutting it slightly oversized, removing and measuring, and then touching off the cutting tool on the previously cut face and adjust the tool position as required.

Tapers can be a real bugger to measure. The two cylinder method works. A short cylinder with appropriate slightly different internal diameters is used. A dial indicator is set up to measure the position of the tailstock e4nd of the cylinder when the cylinder is fitted to the taper. The indicator position is recorded and the cylinder is reversed and its position measured with the indicator. The difference in the diameter, coupled with the difference in the indicator readings and a little trigonometry will give you the taper. Measuring the diameter of the large end of a taper is a piece of cake, the small end, not so easy. If possible, I will turn a cylinder on the small end of the taper with a diameter equal to the small diameter of the taper. I will then cut off the cylinder at the intersection with the taper.

Internal bores can be measured with gage pins, bore gages, or telescoping gages. Internal tapers can be measured with ball bearings. Having a mating part to the taper is also useful in determining correct machining. And of course, depth gages for bore depths.
Great information RJ. I think I get what you are talking about with all but the two cylinder taper method. I get the concept but it's a bit past my knowledge base. Time to up the ginseng and work out the old brain cells. Constant learning, Another cool part about this hobby.


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RJSakowski

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#10
Great information RJ. I think I get what you are talking about with all but the two cylinder taper method. I get the concept but it's a bit past my knowledge base. Time to up the ginseng and work out the old brain cells. Constant learning, Another cool part about this hobby.


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A taper angle can be defined by the two diameters separated by a known distance. A cylinder of convenient length and two different bore diameters appropriate to the taper being measured is made. The small bore end of the cylinder is fitted to taper, using care to square it to the spindle axis, and a dial indicator is zeroed on the tailstock end of the cylinder. The cylinder is removed and reversed and another dial indicator reading of the tailstock end of the cylinder is made. The difference between the two indicator readings is recorded as "l2 -l1" . The taper in in./in. or mm/mm is (d2-d1)/2*(l2-l1). %Tom Lipton of Ox Tools has a good video on this measurement (as well as a lot of other good videos).
One caution would be since you are usually dealing with small differences, it is important that careful measurements be made. Of I were going this route, I would "calibrate" the difference in the cylinders with a known taper, say a Morse or Jacobs taper, rather than trying to accurately measure the diameters. The length of the cylinder doesn't enter into the calculation so as long as the ends are square and there is a sharp edge on the bore, you should have a good tool.

Taper Cylinder.JPG Taper  Measurement.JPG
 

John TV

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#11
Wow RJ, you just raised the bar for giving explanations to this old brain. I think I get it. Now I'm just a little jealous of your computer drawing and layout skills too. More to learn[emoji106]


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Ryan383

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#13
I have set the compound rest to parallel with the bed and used the dial on the compound, when I didn't have my mag base. Just face off and set your zero from there. I can only get about 2" of travel this way however.
 

f350ca

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#14
I have set the compound rest to parallel with the bed and used the dial on the compound, when I didn't have my mag base. Just face off and set your zero from there. I can only get about 2" of travel this way however.
Won't help John without a DRO but I do the same with a scale on the carriage and one on the compound. The readout sums the two, or will read either. The carriage on my large lathe probably weighs 5 or 600 pounds, tough to nudge it that last thou, so I get it close and use the compound for the final setting. Had a similar setup from DRO Pros on a Colchester that didn't have graduations on the carriage wheel.

Greg
 

John TV

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#15
Thanks Ryan and Greg, both good thoughts and I will try to put these and other suggestions learned from this thread to use. Hope this old brain remembers theses tips when I need them. Also, I need to get better at the search function. I didn't even know some carriage wheels had graduations. Humm seems I have scale envy!


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benmychree

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#16
The one that I have been using for many years is the "Travadial" it consists of a wheel that rolls against the front vertical surface of the lathe bed; this wheel is crowned and textured so that it does not slip, and by fine tuning the angle that the wheel contacts the way or other surface, it can be fine tuned for accuracy; the display is a dial indicator face with 100 grads of .001 each, and there is also a separate dial that is zero resettable (as is the dial indicator) and marks one inch distances up to 5"subdivided into .100" graduations. They are made by Southwest Industries Incorporated (SWI). They seem to be quite accurate and repeat nicely and of course can be used on any length lathe bed or any other machine that has a surface parallel to the travel to bear against. I also use step gages subdivided in 1/16" steps from 1/8 to 1"; when used against the carriage saddle wing and a micrometer adjustable stop and a 1-2-3 block is used, the combination has a significant range for a lot of jobs.
 

atunguyd

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#17
Another tip from me (although I am sure others will have thought of this). If you lathe has a DRO on the the carriage then you also have a DRO on the tailstock. When drilling with the tailstock bring your drill up to touch on your part, now advance your tool holder on the carriage up to the drill chuck until it comes to rest against any flat reference surface on the chuck and zero your dro. Now advance your carriage towards the workpiece until the intended drill depth is displayed on your dro and lock the carriage. The tool holder will now form a stop on the advancing drill chuck at the required depth. Beauty is you can retract the tail stock ram and relocate the tailstock and this stop will still be correct provided you do not movement the drill in the chuck.

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benmychree

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#18
Another tip from me (although I am sure others will have thought of this). If you lathe has a DRO on the the carriage then you also have a DRO on the tailstock. When drilling with the tailstock bring your drill up to touch on your part, now advance your tool holder on the carriage up to the drill chuck until it comes to rest against any flat reference surface on the chuck and zero your dro. Now advance your carriage towards the workpiece until the intended drill depth is displayed on your dro and lock the carriage. The tool holder will now form a stop on the advancing drill chuck at the required depth. Beauty is you can retract the tail stock ram and relocate the tailstock and this stop will still be correct provided you do not movement the drill in the chuck.

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Having Aloris tool holders for my 19" Regal lathe, I use the Morse taper holder and do virtually all of my drilling with the carriage, using power feed.
 
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