Measuring length on a lathe

[Shopgeezer]

A friend has asked me to make him up some aluminum bench clamps for a project he is doing in his wood shop. These are simple cylinders of 0.785 barrel diameter with a 0.886 bevelled rim at the top. They have an M8 thread in the bottom. Simple turning task and ideal for us newbie lathe owners. He wants about 40 of them so great practice. I am working on hitting those diameters as accurately as I can. Some success so far. What I am having trouble with is getting the length right. He wants them 1.33" long. I have tried scribing with calipers, measuring and scribing, marking with a sharpie and can get sort of close to the mark but not consistent. My lengths are all over the place. This is entirely unimportant for this part, but in future for IC engines it could be critical. It occurs to me that while a lathe has very accurate capabilities for measuring diameters, measuring the length of the part is not built in. I have seen set ups using a dial indicator mounted on the lathe to measure the carriage travel but that would be hard for distances over an inch. You would have to count a lot of dial rotations. So what are the tricks to get my lengths more accurate? Doesn't matter much for bench dogs but might for pistons.

 

[rklopp]

I usually leave parts over-length, un-chuck, measure, determine how much to take off, re-chuck, and take that amount off by touching off axially and moving over by the determined amount. The right amount is determined by DRO or, before I had DRO, by a long-travel dial indicator mounted to the lathe bed and touching the saddle. If doing 40, I'd set a length stop in the chuck somehow. The nature of the stop depends on whether you have a jaw chuck or collets, drawbar in the way, etc.

 

[kwoodhands]

You can convert a digital caliper for the Z axis on the lathe.
I am sure a you tube video will explain it.
I am going to do the same when I get back to machining. I had an Igauge piece of crap digital scale and removed it Saturday morning.
There is another thread I posted telling what i thought of Igauge junk. Not having done this yet I do not know enough to assist.
I understand that the modification is straight forward and not difficult at all.
mike

 

[Vietti]

If you can find one reasonably priced and willing to make a mount, a Travadial will change your life. A DRO may be easier and cheaper. As mentioned above a long stem DRO is the quickest and easiest.

 

[Shopgeezer]

Ah the magic letters, “DRO”. The stuff of dreams. My hobby budget is subject to reduction due to the need for food, rent and other such nonsense. It has been said, many times, that they won the war with manual lathes so if a supervisor told a machinist that a Spitfire wing bevel pin had to be 8.157” long, that’s what you produced. I wonder how they did that?

 

[ninefinger]

Well, if your lathe is capable of threading, you could always add a hand wheel to the lead screw and with a graduated dial (suitable to the lead of the screw) you could move the carriage a known distance (if you can keep count). I had a 3-in-1 machine that operated like that, mostly to be able to do milling but works for lathe travel too. Was able to disengage it when power feeding or threading...

 

[Captain777]

If you don’t need to remove too much length, then set your top slide to 90 degrees ( i.e. parallel to the lathe axis). Now measure your piece’s length and determine how much to cut off. Touch off your tool, set zero on the top slide and move it the required cut off distance.
Dan

 

[timo_gross]

bevel pin had to be 8.157” long, that’s what you produced. I wonder how they did that?

They probably made them approx. 207.2 mm long. Grouped them by length, and inspected at the required temperature.
Looking at some of the internet offerings cheap DROs do not cost an arm and a leg anymore. You might get away with stalling the hair cutting for few months.

Find a sponsor! You will safe lots of time in the Workshop and could do the dishes. (do not tell anyone, you will do the dishes, keep it strictly hypothetical).
DRO will not make you "magically" hit dimensions, but is a time saver.
Spot on, cool down -> undersize... .

 

[HMEL]

A friend has asked me to make him up some aluminum bench clamps for a project he is doing in his wood shop. These are simple cylinders of 0.785 barrel diameter with a 0.886 bevelled rim at the top. They have an M8 thread in the bottom. Simple turning task and ideal for us newbie lathe owners. He wants about 40 of them so great practice. I am working on hitting those diameters as accurately as I can. Some success so far. What I am having trouble with is getting the length right. He wants them 1.33" long. I have tried scribing with calipers, measuring and scribing, marking with a sharpie and can get sort of close to the mark but not consistent. My lengths are all over the place. This is entirely unimportant for this part, but in future for IC engines it could be critical. It occurs to me that while a lathe has very accurate capabilities for measuring diameters, measuring the length of the part is not built in. I have seen set ups using a dial indicator mounted on the lathe to measure the carriage travel but that would be hard for distances over an inch. You would have to count a lot of dial rotations. So what are the tricks to get my lengths more accurate? Doesn't matter much for bench dogs but might for pistons.

I favor the use of jigs. The simpler the better. Sometimes I can make them so the work stays in place so removing it from a chuck is not necessary. In your example I would probably make a sleeve to fit over the part. Reduces the chance of a measurement error considerably. The disadvantage is it takes a little thought to make a good one that is effective and usable.

 

[timo_gross]

I favor the use of jigs. The simpler the better. Sometimes I can make them so the work stays in place so removing it from a chuck is not necessary. In your example I would probably make a sleeve to fit over the part. Reduces the chance of a measurement error considerably. The disadvantage is it takes a little thought to make a good one that is effective and usable.

Now as you mention it. I vaguely remember someone fitted a stop to the parting tool holder. (will work for short parts) Move carriage in until the stop hits, part of. Maybe even integrate small parts catcher.

 

[Whittler]

As HMEL suggested, a sleeve will work well in this case. I would call it a ‘top hat’ type, where the rim of the hat is pushed up against the chuck jaws. The top hat should be a blind hole so as to act as a depth stop to your part. Make the tube wall thickness thin enough, and saw slits (2 or 3) down the sides, so the top hat can work as a collet. Make the ‘hat’ depth a little less than your overall part length.

Cut your parts a little oversize. Place one part in the top hat collet and push up against chuck jaws and secure. Lock the lathe saddle and Face off to obtain correct length, note that in this case you can remove and replace the part for measuring, as the top hat will return the part to the same place. Control stock removal with top slide dial.

Once you have the correct length on one part, leave all settings alone. Place new part in top hat and face off. The part willl be the same length as the first, without any measuring. Sometimes if the blank lengths are too much to face off in one pass, I will either set the top slide dial to zero, then back off and take several facing passes to come back to the zero, or leave all settings alone, set a saddle stop, wind the saddle away a little and make facing cuts advancing the saddle small amounts until you reach the stop.

obviously one end of all the blanks should be faced square as a first operation. Make sure there are no centre pips remaining.

 

[Charles Lamont]

Putting it another way, don't expect to part off to length, or flat, or without a pip. A final facing cut to length is needed for anything but rough work.

Another form of 'back stop' fits in the spindle taper and has an adjustable stop stick out of it, either sliding or screwed.

For thin discs I use soft chuck jaws.

 

[PMarkey]

How much travel does your top slide have ? I touch on the front face with a parting tool and advance the top slide the length you want plus the width of the parting blade then part off, if you want to face the parted off end just add a facing allowance turn the part in the chuck and face off the allowance.

Paul

 

[Ken I]

I do the same as Paul for short parts or as long as my slide movement permits.
Otherwise I guess it slightly long against a Vernier backing away from the chuck to take up backlash for the final adjustment (lock the saddle) - take a partial cut with the PO and measure. Dial in the necessary adjustment et voila.
For really accurate lengths, leave on a few thou and turn it around to second-op the parted off end - sometimes twice - one cut to get a good finish to accurately measure and then a final to size.
For second op on short parts use soft jaws as per Charles Lamont - or have a look at how I do it in a collet chuck :-
Holding Short Parts In A Collet Chuck
If you are "mass producing" these bits, part them all off with a bit to spare and then set up a backstop in the spindle bore and batch process the entire lot to finished length.
Regards, Ken

 

[Bentwings]

A friend has asked me to make him up some aluminum bench clamps for a project he is doing in his wood shop. These are simple cylinders of 0.785 barrel diameter with a 0.886 bevelled rim at the top. They have an M8 thread in the bottom. Simple turning task and ideal for us newbie lathe owners. He wants about 40 of them so great practice. I am working on hitting those diameters as accurately as I can. Some success so far. What I am having trouble with is getting the length right. He wants them 1.33" long. I have tried scribing with calipers, measuring and scribing, marking with a sharpie and can get sort of close to the mark but not consistent. My lengths are all over the place. This is entirely unimportant for this part, but in future for IC engines it could be critical. It occurs to me that while a lathe has very accurate capabilities for measuring diameters, measuring the length of the part is not built in. I have seen set ups using a dial indicator mounted on the lathe to measure the carriage travel but that would be hard for distances over an inch. You would have to count a lot of dial rotations. So what are the tricks to get my lengths more accurate? Doesn't matter much for bench dogs but might for pistons.

we do have a dro on our small lathe but I’ve seen digital calipers mounted on the lathe there are relatively inexpensive ones available
You could make a carriage stop if your machine does not have one add an adjusting screw and you can dial in very close many lathes have these as optional .

 

[clockworkcheval]

Three methods I use 1) use topside to set carriage stops; this works for about 100 mm plus minus 0,01 mm on my lathe 2) use precision gauge blocks between carriage stop and carriage to set the carriage stops; this is practical till about 150 mm and is very precise 3) the best: install DRO; the cheapest costs less than 30 euro and is easy to install.

 

[fcheslop]

Part roughly to length and use a back stop.
http://www.homews.co.uk/page93.html

 

[awake]

I have seen set ups using a dial indicator mounted on the lathe to measure the carriage travel but that would be hard for distances over an inch.

I use a 1" dial indicator on a mount that lets me position it and lock it in place as needed. While it would be convenient to have longer travel (2" or even longer), I have not had any situation where I could not achieve what I needed with the 1" travel. One simply has to be creative in positioning the dial indicator. In particular, it turns out that one doesn't actually need to measure 1.33" of travel each time; one measures the part, sees how much needs to be cut off, and measures only that much travel. Alternately, one measures 1" of travel, then repositions the dial indicator to measure the last .33". Or so on ...

 

[mattb.351]

What spec are you trying to hit on length? I know you say you want to get them as accurate as you can, but you should really get an idea of what you will be happy with. I'm NOT trying to be funny here, you should really decide what you will accept for this job. When you say your lengths are all over the place, how big an error are you measuring?

 

[Gordon]

I put a 36" digital linear scale on my lathe several years ago and it works great. I have only had to replace batteries. It takes a little bit of work to get it mounted but well worth it. Cost is about $100. Similar to:

https://smile.amazon.com/Digital-Re...7&sprefix=linear+scale+digital,aps,110&sr=8-4