Introduction: A 38 mm spindle bore decides which bar and tube diameters can travel straight through a benchtop lathe's headstock, and how much stock you can leave hanging behind it.
Anyone comparing small lathes eventually runs into a spindle bore number, and it is one of the few specs that quietly changes the kind of work the machine can take on. A lathe with a solid spindle forces you to cut long bar into short pieces before you can hold it. A lathe with a 38 mm through-hole lets the stock keep going out the back of the headstock, so you can machine one end while the rest of the bar sits out of the way. What follows is a practical read on what actually fits, how the chuck narrows that opening, and how support behind the headstock keeps long stock under control.
What the 38 mm Spindle Bore Measures
The spindle bore is the hollow passage running through the middle of the headstock spindle, the rotating tube that carries the chuck. On the NU210LSE that passage is 38 mm across at its narrowest point, so the number describes a diameter, not a length and not a cutting capacity. Anything you want to feed through the headstock has to fit inside that circle along the whole path. It is worth knowing that the bore is measured where it is tightest. Many spindles are tapered at the nose to hold a center or a collet, and the rear end sometimes necks down or carries a thread for an outboard support. A bar that slides in easily for the first 200 mm can still jam further along if it is bowed. Spec sheets from different lathe machine manufacturers do not always state where the measurement is taken, which is worth remembering when you compare two machines side by side. The habit that settles the argument is simple: test with an offcut before committing a full-length bar. Why this matters on a benchtop lathe comes down to length. With a solid spindle, the longest workpiece you can hold is limited by the space between the chuck and the tailstock, and long parts need support at both ends. A through-hole changes the job completely. You grip a short section in the chuck, let the remainder travel through the headstock, and turn the exposed end. The stock stays one continuous piece, which keeps it rigid and saves you from splicing shorter lengths together. The usual shop scene is straightforward: the operator slides the bar through by hand, checks that the jaws are biting on a clean, straight section, confirms the outboard end is supported or short enough, and only then starts the spindle.
Passing Bar Stock Through the Spindle
Fitting is only half the question. Once stock clears the bore, the chuck and the unsupported length decide whether the setup will actually cut cleanly.
1. Maximum Bar Diameter Depends on Bore and Chuck Clearance
A 38 mm bore points to a working range a little under that figure. Bar is rarely perfectly round or perfectly clean, and the practical sweet spot for through-hole work sits around 30 to 36 mm for most shop stock. Cold-drawn bar usually runs slightly under its nominal size, but hot-rolled stock, welded tube, and any bar with saw burrs or light rust can measure over. Straightness matters just as much, since a bowed bar loses effective clearance as it rotates. The chuck adds its own limit: the jaws close on the outside of the bar, but the bar also has to pass through the chuck body and whatever sits behind it. On a two-chuck-head lathe, the front chuck does the gripping while the rear chuck guides the bar, which helps alignment but means the stock has to clear both. A 36 mm bar that slides through smoothly is a better working choice than a 38 mm bar that needs force.
2. Long Stock Needs Support Beyond the Chuck
Anything hanging out of the back of the spindle is spinning mass. A modest overhang is harmless, while a long whipping tail is not, and the risk climbs with speed and with any bend in the bar. The common fixes are a support bushing or spider at the rear of the spindle, a shorter rear overhang, and a lower spindle speed until the setup proves itself. Inside the machine the same logic applies in reverse, because the length left protruding past the jaws is the part under cutting load, so it should be as short as the cut allows. Where that length has to grow, a steady rest takes over the supporting job. Chip control and guarding belong in the same conversation, since long stock produces long stringy chips that need somewhere safe to go.
Through-Hole Setup for Long Tube Stock and Shafts
Tube stock brings a different problem to a through-hole setup. The outside diameter has to clear the bore, but a thin wall does not enjoy being squeezed by three hard jaws. Thin-wall tube tends to deform into a triangle under clamping pressure, which throws the bore out of round before the tool even touches it. The useful habits are a lighter clamping force, a wider spread of contact such as a six-jaw chuck or soft jaws, and an internal plug or mandrel near the end being machined. Welded tube also deserves a reality check, because ovality and weld flash can push a nominal 38 mm tube past the bore. Shafts show the through-hole idea at its best. Instead of cutting a bar into short blanks and chucking each one, you feed a full-length bar through the spindle, machine the exposed end, part it off, then advance the bar and repeat. The chuck holds the same short section every time, which keeps runout consistent from part to part, and the bar itself provides the rigidity that a short stub cannot. This is the classic way small shops run a batch of pins, spacers, or threaded studs from one length of stock, and it is also how a single long shaft gets machined in stages without ever becoming too long for the machine. Two details keep the work honest. The machined overhang past the jaws stays short and picks up steady-rest support when it grows, and the bore fit is checked against the actual stock rather than a nominal size on paper. Clearing the bore settles the fit; clean turning also depends on jaw contact, wall stiffness, and how much stock is left unsupported at each end. The NU210LSE pairs its 38 mm bore with a two-chuck-head layout and an 1100W brushless motor, which suits steady cutting on medium-diameter bar and tube.
Conclusion
A 38 mm spindle bore is a fit dimension that opens up a specific kind of work: long bar and tube that passes through the headstock while the chuck grips the section being turned. Stock in roughly the 30 to 36 mm range moves through comfortably, the chuck and its jaws set the real ceiling, and support behind the spindle is what keeps a long tail from turning into a whip. Understand those three things and the bore number stops being a line on a spec sheet and starts telling you whether a machine suits the parts you plan to make.
FAQ
Q：What diameter of bar stock can pass through a 38 mm spindle bore?
A：A 38 mm bore passes bar just under 38 mm across, but most shops work with roughly 30 to 36 mm stock for a comfortable fit. Cold-drawn bar usually measures slightly under nominal, while hot-rolled bar, welded tube, saw burrs, light rust, or a bend can push the effective diameter over the opening. The chuck body and jaws narrow the clear path further, so the practical ceiling sits a little below the bore size.
Q：How does a 38 mm spindle bore affect workholding for long tube stock?
A：The bore lets tube extend out the back of the headstock so the chuck only grips the end being machined, which is what makes long tube and shaft work practical on a small lathe. Wall stiffness is the limiting factor, since thin-wall tube deforms under three-jaw clamping, so lighter clamping force, wider jaw contact, or an internal plug protects the tube. Ovality and weld flash also eat into the available clearance, so the outside diameter is checked before the tube goes in.
Q：Why is through-hole setup useful when turning a long shaft on a benchtop lathe?
A：A long shaft fed through the spindle stays rigid because it remains one continuous piece of stock, with the chuck holding a short section close to the cut. You avoid cutting the shaft short, joining pieces, or leaving it unsupported across a long span. Bar work also becomes efficient for batches, since the bar is machined at the end, parted off, advanced, and cut again while the chuck keeps gripping the same section.
Sources / References
Research at Purdue ME - Mechanical Engineering - Purdue University
Provision and Use of Work Equipment Regulations 1998 (PUWER) - HSE
Using emery cloth on metalworking lathes - HSE
Related Examples
NUMOBAMS NU210LSE 1000mm Auto Left&Right Threading Mini CNC Metal Lathe with Two Chuck Head
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