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Saturday, March 08, 2014

Lathe improvements - Tailstock DRO

Digital Read Out

The mini-lathe has been quite an asset for the past two years, and I have learned so much using it. As good as it has turned out to be, there are a few things that have been bugging me, one of them is the difficulty to drill to a precise depth.

The only way to do so on the stock machine is by referencing some hard to see graduations on the tailstock’s quill, an approximation at best. 

Since 99% of the times the holes I drill are through holes, this issue has not been a big problem, but two years into its use, I thought there might be some improvements I could make to rectify this minor infraction once and for all, and make my turning experience a tad more enjoyable.

In the past, whenever a hole of a precise depth was called for, I have used a 2” (5.1 cm) dial indicator attached to a magnet, positioned on the tailstock, and indicating off of an aluminum plate I machined, and attached to the quill. 

Dial gauge on a magnet indicating off of an aluminum plate attached to the quill

This setup is straight forward, but presents a number of inconveniences. For one thing you subtract the reading to obtain a measurement, since the dial indicator’s shaft extends with the quill reading backward. Another issue is placement, this setup is almost as big as the tailstock itself, and gets too close to the tailstock crank not to get bumped occasionally, throwing off the measurement sought for.

A few months ago, I bought a 4” (10.2 cm) digital caliper on sale for $9.99 at Harbor Freight, with a vague idea of using it on the lathe. 

Harbor Freight #47256

Well, the time has come to put this caliper to good use.

I started out by cutting off the “inside jaws” with my bandsaw, and removing the “depth probe”. 

Removing the inside jaws

Laying the mutilated calipers on the tailstock, I decided where to attach them in order to minimize interferences with other parts, and maximize travel. 

Figuring out where to place the caliper

I eventually ended up cutting a few inches off the bottom end of the caliper as well, so that it wouldn’t hit the crank when fully retracted.

With the location chosen, I drilled and tapped the tailstock, and the aluminum plate I had previously made, on my drill press.

Drill press comes in handy from time to time

Tapping for a small screw

A couple of fastener later, and I could already test the concept.

Initial testing of the assembly

One last internal modification remained to be made, and for that the tailstock had to be disassembled.

Exploded view of the tailstock

Releasing a chuck, or a dead-center (#143) from the tailstock requires the back of the chuck to push off of the left end of a threaded rod (#139). As the quill retracts (#142) the back of the chuck comes into contact with the rod and is expelled.

Unfortunately, since I attached a thick aluminum piece to the end of the quill, it is now unable to retract fully into the tailstock, and disengage the chuck.

My first thought was to manufacture a new longer rod, but it turned out that its thread was a lefty (clockwise to unscrew), in itself not a limitation, but it also presented a metric pitch. Since I do not have gears to allow my lathe to cut metric threads, and my dies are all right handed, this proved to be a no go.

In order to lengthen the rod, I decided to drill it, tap it (imperial), and insert a bolt in it. This is a pretty normal procedure on a lathe, but with my tailstock being the subject of the surgery, it could not be counted upon. I decided to use a T socket wrench to push on the quill by hand, until the hole was deep enough to tap it.

Pushing the quill by hand (note the missing crank wheel)

¼-20 socket head cap screw extending the range of the threaded rod

A view down the quill reveals the added ¼-20 screw

The tailstock DRO modification turned out wonderfully, and it was easy to do. I can now drill holes to any depth I choose with very high precision, The DRO it is out of my way for all normal operations, and I no longer have to fumble trying to set it up and work around its mass.

Completed tailstock DRO modification

Tailstock DRO in action

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