Making a Slitting Saw Mandrel

Contents

• The Idea
• The Parts
• Using the Saw

The slitting saw assembled

When I found the Harbor Freight 2" diameter saw, I wondered if they would make a useful slitting saw. They're sold in packs of 3 for under $10 for use in their mini cut-off saw, and it is a high-speed steel saw with a very thin blade. I bought a set, and found that the blade is 0.5mm thick, and it has a 1cm hole. The obvious thing to do was to make a mandrel that would fit it.

Close-up of the business end

I made the mandrel to fit the Taig lathe spindle taper, with a fairly long nose that fits snugly in the spindle for stability. It mounts with 1/4-20 drawbar. The drawbar closer is bored from a 1/2" bolt that is turned to fit into the hole in the end of the headstock pulley and bear on the end of the spindle.

The photos here show the assembly resting on a promotional mint tin that holds all the parts but the drawbar and closer. The size of the Taig lathe is such that small tins are frequently the right size to store accessories.

The Parts

The parts, neatly packed

I turned the mandrel from a junk 1/2" bolt. I began with the mandrel clamped between the Taig 3-jaw chuck and the tailstock dead center. Using this setup, I turned the nose of the mandrel for a close fit in the Taig spindle and then I used my taper attachment to cut the taper.

I then pulled back the tailstock and mounted a drill chuck, using that to drill and tap the mandrel for the 1/4-20 drawbar. I drilled twice before tapping, once with the tap dril, going in about 1.5", and then with a 1/4" clearance drill, going in about 0.5" That means that the drawbar slides into the clearance hole before grabbing any threads, and then it screws in about 3/4" deep, more than enough to add stability to the connection.

The parts, lined up

With the back-end of the mandrel turned, I removed the chuck from the lathe spindle and mounted the mandrel in the spindle taper with a drawbar in order to turn the business end. I began by turning a clean cylinder, skimming off the zinc plating and texture left by forging the bolt (the precise diameter doesn't really matter). Then I turned the 1cm diameter section, turning it until it was a snug slide-fit in the saw blade.

There are two competing demands on slitting saws: Rigidity and reach. I opted to allow a bit of both by cutting a long mandrel and an assortment of sleeves so I could mount the saw closer to the headstock for rigidity or out farther to reach over what I was cutting.

I cut my sleeves from the same stock as the mandrel, boring them for a tight sliding fit on the mandrel. I also cut two washers to grip the saw blade at a larger diameter, from 1" bar stock, and two end caps for the assembly, one to take a cap screw, one with a taper to fit a flat-head screw. The latter offers a chance to cut up closer to an obstruction, should the need arise.

Using the Saw

Using a Slitting Saw

Here's a photo of the saw in use to slit my drill press depth stop. For that job, I cut a very narrow kerf, just one blade wide. I could have stacked two saws on my mandrel to make a kerf twice as wide. To make an even wider cut, I think I'd have used two blades with a spacer between them. I've never tried that, but I seen no reason it wouldn't work.

I have read questions on-line about whether these Harbor Freight saws are any good for cutting steel. My answer is yes, based on this experiment, they cut just fine. I think those who said no were running them at high RPM (the saws have "Max. RPM 13,000" imprinted on them). High RPM might be correct for cutting basswood or even brass, but to cut steel, go slow.

I made the slit through 5/16" thick steel plate in just one pass, running the saw at about 525 rpm, advancing the saw slowly and keeping the blade wet with oil. I started with the blade in the hole intended to fit the quill and cut toward the rear by moving the cross slide toward the front.

In the photo, you can see a pile of very fine swarf ahead of the blade as it cuts. The oil made this pile rather stable and it simply grew as the cut advanced. Another pile of very oily swarf accumulated on the cross-slide next to the edge of the support slug, scraped off the sides of the blade as it re-entered the cut from below. At no point did I experience any symptoms of binding.

This is the first job I've done on this lathe where I really could have used a cutting fluid reclamation and circulation system.

In making my dial indicator snug, I used the same saw blade to make at least ten more cuts in the same 5/16" (8mm) thick steel stock, both cutting around the outside of rectangular blanks and making several clamping slits. The blade still shows no sign of wear. All of my cuts have been through the full thickness of the plate in one pass, and I have seen no problems with the cut wandering over cut lengths a bit longer than one inch.

One change I made is to use climbing cuts with this saw. This saves oil (thereby reducing the mess). Why? Because with the blade entering the cut from the top, the workpiece scrapes oil off the blade and keeps it in a puddle on top, were it can easily recirculate. In contrast, a normal (non climbing) cut creates an oil puddle on the underside of the workpiece where most of the oil drips down onto the cross slide and carriage.

With climbing cuts (regardles of whether sawing, fly-cutting or milling) you need to avoid all backlash! On a small lathe like my Taig, I do this by using finger pressure on the cross slide. It really doesn't matter whether you press against the direction of motion or with the direction of motion, so long as you don't allow any slack in the cross-slide position. If you do allow it, the saw stops instantly, jamming, the same symptom you get if you feed too fast. This risks breaking the blade, although I have had several saw jams with no ill consequences.

The slitting saw mandrel has occasionally been useful for other purposes. For example, when I got an old Sherline chuck for my Taig lathe, I had to bore the back of the chuck, and I did that by clamping the chuck on the mandrel with its back exposed to the toolpost.