Mini Lathe Operations: Boring

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Introduction

Boring is the process of cutting cylindrical holes concentric with a workpiece. The result is similar to drilling holes, but with several important differences:

  • Bored holes are typically more perfectly symmetrical than drilled holes.
  • Bored holes can be finished to any desired internal diameter greater than about 1/4″ rather than being constrained by specific drill bit diameters.
  • Bored holes can be made to diameters much larger than is practical for drilled holes.
  • Bored holes can easily be formed with a flat bottom rather than the cone-shaped bottom typical of drilled holes.

 

Commercially Made Boring Tools

Distinct from turning, which finishes the outside diameter of a workpiece, boring finishes the inside diameter of a hole concentric with the workpiece center. As such, a cutting tool for boring must reach into the hole and remove material from the inside diameter of the hole.

The boring tool is clamped in the toolpost so that the tool shaft is parallel to the length of the workpiece and is advanced into the workpiece using the handwheel crank.

As with most lathe cutting tools, you have a choice of grinding your own or using any of a variety of commercially made tools. Several styles of commercial tools are available such as these from LittleMachineShop.com:

3/8″ CCMT insert bar, LMS 1779

 

3/8″ HSS insert boring bar, LMS 1720

 

For mini-lathe hobbyists, a good tool set to start with is one of the inexpensive imported sets with cemented carbide cutting tips attached to a round shaft. These typically come in sets of 9 or 12 with a variety of lengths and diameters for around $15 in the usual sale flyers.

The shank end is finished to a specific diameter such as 3/8″ and, unlike end mill sets, all of the tools in the set share the same shank diameter. The 3/8″ diameter shank size is about right for the mini-lathe.

Because the cutting tip is supported at the end of a relatively long shaft, strength and rigidity are important qualities in a boring tool. If the shaft is too limber it will flex and vibrate causing chatter, and resulting in a poor finish. In general, you should use the shortest shank possible consistent with the depth of the hole you are boring.

 

In the following photos, observe that the cutting tip of the tool forms a corner at less than a 90º angle. This provides side clearance and allows the tool to work all the way to the blind corner of a bored hole. In the right-hand photo you can see the side clearance formed by the angle of the cutting bit.

If the hole you are attempting to bore is tool small in diameter for the tool, the curvature of the side of the bored hole will cause the side of the tool to rub against the side of the hole, making it difficult to advance the tool into the work and producing a poor quality finish.

The smallest of these tools are fine for holes about 3/4″ or more in diameter; for smaller holes you will need a different style of tool. With some practice you can easily grind you own boring tools to meet these special requirements.

 

Grinding Your Own Boring Tools

As with other lathe operations, learning to grind your own tools from High Speed Steel (HSS) blanks can be a great advantage as you will soon discover situations where commercially available tools are incapable of meeting the requirement (or are not readily at hand when you need them).

A good example would be boring a hole of less than 3/4″ diameter, which is just too small for the typical import boring tools to handle. Crafting your own tools on the grindstone is another aspect of shop work which can provide much enjoyment once you get the hang of it.

Here’s a variety of boring tools I have ground for various purposes:

 

The three tools on the left are ground from 5/16″ blanks, the fourth one from a 1/4″ blank and the two on the right from 3/16″ blanks. I have taken to using 3/16″ tools for most of my work on the mini-lathe.

They are large and stong enough to handle most work, but are much easier to grind since much less metal needs to be removed to form the desired tool shape. Boring tools are about as bad as it gets in terms of the amount of metal you must grind away, so the 3/16 bits are especially advantageous when making boring tools.

The following series of photos shows the process of shaping a boring tool from a 5/16 HSS tool blank.

 

Making a Boring Tool Holder

If you decide to use the import carbide boring tools, you will need to make some type of holder to mount them to the tool post. There are quite a few ways to do this, and you may come up with some ideas of your own.

For example, Varmint Al made an insert for the standard tool holder. LittleMachineShop.com sells a similar one for 1/2″ shank tools, ready made. Soon after I bought my first mini-lathe, I made the simple holder shown below.

Even though I have made some others since then, it continues to serve me well. It is easy to make and requires only the lathe, a drill press or hand drill, a hacksaw and a 1/4-20 tap to make it.

 

Here’s how to make one for yourself:

  • Cut a block of aluminum to about 1x2x2″ in size
  • Clean up the surfaces with a file or belt sander as needed
  • Drill out the hole for toolpost stud, 9/32 diameter using a hand drill or drill press
  • Clamp the block to the compound using the toolpost clamp
  • Chuck a 3/8″ drill in the 3-jaw chuck and drill out the hole for the boring bar shank – use the carriage handwheel to advance the block into the drill
  • Remove the block and cut the slot with a hacksaw or metal cutting bandsaw
  • Drill a hole for the 1/4″ bolt using a 1/4-20 tap drill () – drill past slot about 1/2″
  • Drill out upper portion of bolt hole to 1/4″, stopping at the slot
  • Tap the lower portion of the bolt hole 1/4-20

Because the 3/8″ hole is drilled with the block mounted on the compound, the hole is centered on the lathe centerline so that the boring tool will be at the right height.

 

Basic Boring Operations

Because boring tools are not designed to do plunge cutting directly into the faced end of a workpiece, boring operations nearly always begin by first drilling a pilot hole into which the boring tool can be inserted.

Therefore, you will first need to face the end of your workpiece and drill it out to the desired depth of the bored hole using a drill bit that is big enough to enable the boring tool to get a start (typically 3/8″ minimum for the import boring tools, but could be smaller for custom-ground boring tools.)

If you intend to bore all the way through the workpiece from end to end, drill your starter hole all the way through as well. Otherwise, drill the starter hole to a depth such that the tip of the drill stops at the desired depth of the bored hole. Refer to the Drilling Operations page for tips on how to measure the depth of the drilled hole.

For our first exercise, let’s bore a hole 5/8″ (.6125) diameter by 1/4″ (.250) deep in the end of a piece of 3/4″ diameter 6061 aluminum bar stock. Chuck the stock in the 3-jaw chuck with about 3/4″ extending. Now, face and center drill the end of the stock, then drill out with a 3/8″ drill to a depth of 1/4″.

 

Since we are boring a shallow hole, select a boring tool with a short, stiff shank to minimize chatter.

Mount it in the boring tool holder and adjust the angle of the holder so that the the tool has clearance both on the end and the side. Make sure that both the tool and the tool holder are firmly clamped in place.

 

Now position the tip of the tool so that it is just overlaps the edge of the pilot hole. Turn the lathe on and adjust the speed to about 400 RPM, then use the carriage handwheel to advance the tip of the tool towards the headstock until it starts cutting along the edge of the hole.

 

Continue advancing the tool until it hits the bottom of the pilot hole. One of the challenges with boring operations is that you often cannot see the cutting tool, so you soon learn to judge what the tool is doing from sound and feel.

You will be able to tell when the tool reaches the bottom of the hole because you will a distinct resistance to further motion and will probably feel and hear some vibration and chatter.

Crank the tool back out of the hole, rotate the cross feed handwheel counterclockwise about 1/2 turn to draw the tool towards you about .020″ and then repeat the cutting process. Do this a few more times enlarging the hole on each pass.

 

As the hole gets larger it is easier to see what the tool is doing. After a while you can clearly see the cone-shaped depression left by the drill tip. To remove this depression, position the left edge of the tool tip so that is centered in the hole, then advance the carriage until the tool is positioned just inside the depression.

Now lock the carriage in place and use the cross-feed handwheel to withdraw the tool towards you. The result is basically a facing operation, but starting at the center and working out.

You will be able to tell when the tool contacts the outer edge of the hole as you will hear and feel some chatter. Repeat this process until the drill depression is removed as completely as possible.

 

Check the diameter using the inside tips of your dial caliper. Measuring inside diameters takes more care since it is difficult to determine when you are actually measuring across the full diameter.

Advance the caliper outward until it is contacting the sides of the hole, then while maintaining gentle outward pressure on the caliper thumbwheel, rotate the caliper tips slowly from side to side.

You will probably observe and feel that the tips move outward a little bit until they finally settle on the true diameter of the hole. Lock the reading with the thumbscrew and carefully withdraw the caliper tips so as not to change the reading.

 

As you approach the final desired diameter, you can use the graduations on the cross-feed dial to make the final measurements. Keep in mind that, just as with turning, boring removes twice the amount of metal indicated by the dial.

For example, in the measurement above, the caliper reading is 0.602. The final desired reading is 0.625, so we want to increase the diameter by .023″. Half of this amount is 0.0125 so we will withdraw the tool first by 10 divisions on the cross feed dial, then take another reading with the caliper before making the final finishing cut.

As with turning and facing, finishing cuts for boring should be made at relatively high speed, say 1500 RPM and with very light cuts, say .002″ or so. If the other end of the work is not finished and the depth is a little deeper than desired, you can bring the bored hole to the desired depth by facing off a small amount from the lip of the bored hole.

Finally, you may wish to champfer the enge of the hole to leave a nice clean finish. Turn the boring tool holder to about a 45º angle and gently advance the edge of the tool into the lip of the hole using either the carriage handwheel or the cross-feed handwheel.

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