View Full Version : How to Measure Bore While Line Boring

Paul Alciatore
02-17-2008, 03:18 AM
I am making a part that needs a 25mm (0.984") bore three inches long. It needs to be parallel to three sides of the part so I felt that a line boring set up in the lathe was the best bet. I'm making an eleven inch long boring bar with 3/4" steel rod. This leaves less than 1/8" between the finished hole and the boring bar.

My question is how can I measure the size of the bore without taking the boring bar out after each cut? This may make adjusting for the next pass difficult. I definitely do not want to disturb the part once it is set up for the several parallel conditions. I suspect I may have to make some kind of a Go/No Go gauge. But what would it look like? Perhaps I can remove some of the boring bar at the tailstock end to make a bit more room. A couple of flats? Or what?

Ian B
02-17-2008, 04:25 AM

Can you machine an accurate OD on the boring bar and then use something like an 1/8" go / no-go gauge between that and the bore?


Forrest Addy
02-17-2008, 05:09 AM
I bored any number of split case pumps when I was working for a living. We used a variety of methods to measure bores in progress. I take it in this situation that removing or withdrawing the bar is out of the question.

You have a continuous round bore like your which does not separate on a neat axial case joint. The clearance between bar and wall is so small that you are forced to some expedients requiring some care and modifying som standard tooling. Inside calpers are cheap and easily modified to fit in awkward places. I suggest you grab your least favorite inside spring caliper and bend, bow, and dress the tips to fit between the bar and the bore. Once you get the right configuration you can use the nut to dial the caliper in and out to where the tips just graze the bore. This requires the most sensitive touch and a number of trials until you get the knack of its operation. Then mike the caliper tips. With great care you can get within 0.0005" or the true size for some distance down the bore.

Yes you will have to sacrifice an inside caliper but so what? They make them cheap so you won't hesitate to modify them and they make the tips big and clunky so you'll have material to modify. I have a dozen pairs of bitchered calipers; one pair has a Last Word fitting welded to it. It worked great on Aurora and Crane pumps but there the bores were a couple of inches affording room for even a dial test indicator. You might find a propane torch, a ball peen hammer, and a hunk-o'-arn anvil is the best start for doctoring a pair of dividers. Beat them out skinny and bend to suit.

This is among the most finicky of machinist's measurements but it has a history of wide use whereever line boring is routinely performed.

Hey, if it was easy anyone could be a machinist.

02-17-2008, 11:11 AM
I've only line bored once so I'm not an expert but I removed the boring bar for each pass. It was easier to set the depth of cut this way and also allowed measuring with the normal tools; I favor the T shaped tool for this.

I reasoned that the cutter is at the midpoint of the bar so any difference in position on the chuck end and/or the tailstock would be halved at the cutter; I also marked the bar so it could be oriented the same with respect to the jaws each time and I always tightened with the favored hole (the one that is most repeatable). By moving the work against the chuck I was able to retract the ram to get the bar out (my tailstock doesn't repeat well when it is released from the ways). The work had two parallel bores for ways and the result worked out well - the block moves nicely. See: http://www.gadgetbuilder.com/Brooks%20Grinder.html#Basic_Frame

I used the simple J shaped tool suggested by Gingery to set the cutter depth and this worked out well, allowing accurate adjustment without difficulty.


Edit: name of link changed

02-17-2008, 11:26 AM
Piccies of the part and setup please as you're going along?



02-17-2008, 12:14 PM
One easy way is to run your boring bar between centers and drive with a dog . Makes it easy to take out to measure bore and put back in the same place . I usually try to do it that way if possible.

02-17-2008, 12:31 PM
Paul, your boring bar will be most accurate if it is run between centers. With it between centers you can carefully remove it after each pass and measure the bore. Don't try to make the job hard by keeping the bar in the hole. With only 1/8" clearance between bar and bore it will be near imposible to measure with the bar in the bore.

02-17-2008, 12:34 PM
I thought that turning between centres was the only way to line bore.
Then the cutter is removed from the item to be bored and the cutter bit moved and the new distance measured with a mic then remove half the boring bar diameter then double it to give the bore.
Never done it myself but that is the way i've seen it in books.


Paul Alciatore
02-17-2008, 12:39 PM
Thanks guys! Several ideas I hadn't thought of here. I like Forrest's use of a modified caliper. I have an old inside caliper but the only problem is it was my father's and I spent some time rescueing it from rust so I don't plan to modify it. I will check the catalogs for another one or perhaps several if they are cheap enough.

The other idea of making a known diameter on the bar also sounds good. I could easily do an inch or so near the tailstock end. Perhaps some number sized drills for gauges. I may try that while waiting for the calipers: it would let me get started. Wow, two good ideas. I knew I couldn't be the first one to face this problem.

John's comment is also good. I don't know if I trust my tailstock to repeat even after just retracting the ram, much less repositioning. I would really like to leave the set-up alone once I start.

The bore is for a clamp on a 25mm bar and size is not the most critical spec. It is a replacement part for my Unimat: the bracket that holds the Unimat headstock on the vertical column for drilling and milling there. The new block is a brick shape and has mounting holes for the headstock on three sides so I want all three of them parallel to the hole. That is my primary spec.

John, I do plan to take and post pictures. I am presently making the boring bar and I am actually using the partially complete Unimat mounting block to mount my Unimat head on the SB to do the milling and drilling on the bar. The 25mm bore is the last feature to be machined so the rest is functional. When I realized that the block I was making was a perfect way to mount the Unimat head on the SB for milling and drilling I added a single 1/2" hole through it for a stud to hold it on the compound. Now it is assisting in it's own manufacturer.

We are having an ice storm here now but I plan to go out to the shop shortly. I will post pictures later today. Look for a thread titled "Another Way for Milling in the Lathe".

02-17-2008, 01:06 PM
The bore is for a clamp on a 25mm bar and size is not the most critical spec. It is a replacement part for my Unimat: the bracket that holds the Unimat headstock on the vertical column for drilling and milling there. The new block is a brick shape and has mounting holes for the headstock on three sides so I want all three of them parallel to the hole. That is my primary spec.

I know you're probably already done with the job by now, but reading this made me wonder.

Is it easier to mill the mounting pads after the hole is bored? In that case, the accuracy issue is one of aligning the pads versus the bore rather than vice versa.

The other thing is I would be tempted to do is a couple of test runs to investigate the repeatability between centers. It's something it would be useful to know for your lathe anyway.



02-17-2008, 01:15 PM
Setup the bar,rough out the bore using inside spring legs to measure within .010" of the finish pass.Set a DTI on the carrage with a pad tip to read 0.00 on the tip of the cutter.Remove the tailstock and crank the setup back,measure the bore with your method of choice,reset the tailstock and use the DTI to advance your tool to the finished diameter pass.

Another method I have used is to make a boring bar with a built in telescope gauge which can be locked with a setscrew set at an angle.

Paul Alciatore
02-17-2008, 01:33 PM

Well, it is a brick shape. Totally simple - six flat sides all at right angles. Simplicity in design, I guess. The sides ARE the mounting pads. Anyway, the sides are milled already (the stock was quite rough on most sides so I had to) and I have even done some lapping on the critical three sides to get them flatter and parallel. Perhaps you are right and this is my first attempt at a part like this. It seemed simple.

I do figure I will have to do some final work on the sides to get them as parallel as possible. I had hoped it would just be lapping, not more milling.

02-17-2008, 02:30 PM
Use an Indical.

I have two of these I use regularly. They can take a bit to get used to setting, but after you get used to them they are the greatest measuring item for bores and ID grooves ever.

I set with a gauge block set up - setting for ID.


02-17-2008, 03:52 PM
Just thinking out loud. Make a nice boring bar. Bore a piece of scrap until you get the size you want. Call this the FINISH BORING BAR. Set it aside. Make another boring bar, not so nice. Use it to get close to final ID. Switch to the pre-set FINISH BORING BAR for the last pass. If you can't get repeatable results when you R & R work between centers, you have a problem that I would want to solve before doing much else.

Norman Atkinson
02-17-2008, 04:13 PM
Although we Brits get 'Poo Poo'd', the late George Thomas designed a boring bar in which the tool was micrometer advanced. The first requisite is a boring bar accurately centred and of a known diameter ie 0.750"
You then have a known advance using a clock gauge tween centres to add the tool advance.

I bored my two Quorn bedbars 1.0005 and 3.500 parallel to each other.

Paul Alciatore
03-04-2008, 09:43 AM
I thought some of you might like to see the results. I actually used two methods to check the diameter of the bore while working. As Ian suggested, I machined an accurate diameter on the end of the boring bar and used several drills as a Go/NoGo gauge. I selected a diameter to allow several dirlls with only slight differences in their diameter to be used so the method was quite accurate. It also allowed me to use several smaller sized drills to check progress as I approached the final size. I also used Forrest's suggestion of bending up an inside spring caliper. I only had one so I ordered several cheapies from Enco and one of them worked just fine after several minutes at the vise and some sand paper.

Here's my set up:


In use, the two methods did correspond within one or two thousanths. I attribute the differences to my relative lack of experience using inside calipers in this manner and to my haste in using them when I saw that the drills were also working well. With more experience and a bit more time, the calipers would be just as accurate, I am sure.

The flats on each side of the tool allowed me to position two 1-2-3 blocks on it and use a depth mike to set precise tool depths. This worked fairly well.

And here is a shot illustrating the use of the drills to check the bore diameter. I checked on two opposite sides (top and bottom) to insure no errors crept in. I acutally did this with the tailstock center in place, so this shot is just for illustration.


And a final check with the mating part. Perfect fit!


Thanks to all for the help, it really helps. You guys are great.

kap pullen
03-04-2008, 11:01 AM
In the past I have set a boring head up in the spindle.
Most boring heads have a tapered shank avalible as an option.

Set a center in the boring head. You may have to make a special center up.

Use a dog as a driver, and set the cutting tool in line with the boring head slide.

You can now dial the head to offset the tool for the final cuts.

Assuming the tool hole is in the center of the bars length, the heads adjustment will be halved.

Still have to take the bar out to check your hole.


you must be really good to hold .0005 with a cheapo caliper!

Good luck.


03-04-2008, 11:25 AM
Nice idea kap! I assume you could make a similar arrangement by holding the boring bar in a 4-jaw with a center and dog instead of the 3-jaw and fine adjust using the 4-jaw.

Paul, do give us final closure with pics of what it is you've built?

Thanks for an interesting thread.