Sunday, December 19, 2021

Weekly Update on the 1 by 1 - part 19

This week, I increased the bore of the cylinder from just over 9/16" last week to 0.950", or close to that.  Since the target for the bore is 1.000", I can enlarge that pretty easily, but I wanted to check for any bad taper in the cylinder.  

Taper in a part cut on a lathe can come from a handful of places.  The most common are that the lathe can be twisted over its length (usually called being out of level) or from the center of headstock not being exactly on line with the tailstock.  It can come from wear over time, errors in manufacturing, or could come from errors installing it in your shop.  In a discussion I was reading, one guy made reference to a video on YouTube channel BlondiHacks, which I watched and got some good ideas from. 

What got me thinking about this is that the lathe started shaking while taking off the corners of the square bar to get where I ended up last week.  I noticed the two handwheels (which I wasn't using or touching) were vibrating and moving off their settings.  I don't know if the cutter was really moving, or would have moved given enough shaking, but I came up with a cheap and dirty fix. 

That's right.  Blue painter's tape.  It certainly kept the knobs from moving, I just don't really know if it did any good for me.

The tape seemed to work but I wanted to try to measure taper in the cylinder.  I've been stuck here a few days.  To be fair, the fact that it seemed to be the week of "peak busy" enters into the lack of progress, too. 

To measure the internal diameter, I use a tool called a telescoping gauge.  It's a clever device that's a Tee-shaped handle with a spring-loaded, cylinder on one side of its head and a fixed cylinder on the other side.  The two ends have a slight dome to them that helps center the tool across a diameter while removing errors from a corner of one of those cylinders touching the wall you're trying to measure.  Here's a picture of one in use measuring the ID of a ball bearing set. 

Photo credit

Not visible in this picture is that at the end in the user's hand is small screw that locks the movable cylinder smaller than the diameter you want to measure.  In use, you squeeze the Tee head to make it small enough to fit in the cylinder, lock that screw down, then push it down into the cylinder to the spot where you're going to measure the diameter, then unscrew that locking screw.  That snaps the Tee to full size, contacting the diameter.  It seems to help to jiggle it around to ensure the Tee is fully expanded and not hung up in any way.  Then you tighten the screw tightly, tilt the handle slightly and pull it out of the cylinder.  Once out of the cylinder, you measure across the Tee and that tells you the inside diameter  In a measurement like I'm trying to make, I'll use a micrometer, but I know of no places where it's easier to measure an inside dimension than an outside one.

I'm not convinced I'm measuring the far end of the cylinder properly.  I ran 20 measurements on both ends, and the standard deviation for the far end is three to four times bigger than the near end.  I think that's telling me that I'm not getting the gauge properly set up when it's 2" deep in the cylinder. 


  1. I've used a telescoping gauge on a number of occasions. Never did like them as they are extremely sensitive to alignment. I much prefer a 3 point inside micrometer. I understand the cost difference factor, but if you are doing critical work where high accuracy is critical the inside micrometer is the way to go.

  2. They seem to be tricky tools. I usually slide the gauge in and rock it from one side to the other, so the handle is close to bumping the near sides of the bore, then bring the handle back to center and lock it. Then tip the handle sideways again to add clearance and remove it and measure. Then again at 90 degrees to check the other radial distance.
    The goal of course is to have the measuring head pass directly through the bore center, at 90 degrees to the bore axis. I think the rocking of the tool helps it to find center, across the bore-AKA the widest part of the cylinder. Then making sure the handle is in alignment with the bore axis ensure the the measurement is truly at 90 degrees to the cylinder bore.
    This is one place a really well made tool with smooth operation stands out- one can feel the gauge spring get tight as the tool is moved.

    All this is moot of course, if the bore is not smooth. If you are picking up a ridge on one measurement and a valley on the next, no way.

  3. I am not very good at using telescoping gages either. My readings always seem to fluctuate over a 3-4 thou range.

    Have you watched the video by Abom79 on using telescoping gages?

  4. I love using gage pins to measure hole ID’s. Unfortunately my largest gage pin is .750”.

    If I had to bore a 1.000” hole, I would probably make my own diy multi-diameter gage pin. I would turn a series of diameters in the same piece of round stock, for example .995, .996, etc. thru maybe 1.003. This way you could sneak up on your desired ID and have a pretty good idea of your progress as the hole is bored out.

  5. I have one of those gages too, it worked for a VW motor. Idea-make a ring, like a piston ring, with a small gap. Push it into the cylinder with a plug to square it and measure the gap when the ring is at different locations. BH also has an episode on boring:

  6. Once you are convinced you are getting consistent measurements of the bore at the same point, rotate the head 90 degrees to check for concentricity. Do this at several points of the bore length.
    Any idea what sort of tolerances are required for this cylinder?
    Taper is not good, but "ovalness" can be worse, for piston and ring setups.