Rolle's Dad's Method

Rolle's Dad's Method (RDM) is a brilliant way to accurately measure the alignment of a spindle to the bed/column, and is extremely useful for aligning the head on the mini mill (with the column removed from the base) and the headstock on the mini lathe. It's great because it takes run-out completely out of the equation and only measures the alignment. However, I haven't found a description which I felt practically explained the method to me, and therefore it took me a little while to figure out what it meant. Here's how I do it:

1. Chuck as straight and smooth of a rod in the spindle as you can. Using RDM the smoothness and straightness of the rod doesn't actually matter, but it does make doing the measurements easier.
2. On a lathe mount a DTI on the carriage so it can indicate the rod. On a mini mill I'll move the head to the bottom of the column and then mount the DTI on the column right below the head so it can indicate the rod. You check the X and Y alignment separately, determined by whether you're indicating the side or the top of the rod.
3. You want to indicate two parts of the rod as far apart as possible. On the lathe this means indicating it with the carriage at the chuck, and then as far to the tailstock as possible. On the mini mill it means indicating with the head all the way down and then all the way up. Generally I can get about 7-8" of separation. I'll mark the two points with a Sharpie so I can easily hit the same spots.
4. With the DTI at the head, I turn the spindle by hand and observe the DTI dial, and adjust the dial so the needle is traveling the exact same amount above and below zero; I call this "average zero".
5. Then, without touching the dial, I go to the opposite end of the rod. With the lathe this means moving carriage and with the mini mill the head.
6. Again, I turn the spindle by hand and observe the DTI. The needle will usually move a lot more, but that's ok. I mentally determine where the new average zero would be. The difference between the old average zero and your new average zero is how far out of the alignment you are. Again, it doesn't matter how much the needle moves, all the matters is where your average zeros are.

For example, I'm going to check the horizontal headstock alignment on my mini lathe. I chuck the rod and mount my DTI so it's indicating the side of the rod and move the carriage with the DTI on it all the way to the chuck. I turn the spindle and see the needle is moving a total of .004", and I turn the dial until the needle is traveling exactly 0.002" above zero and 0.002" below zero. I now have my average zero.

Average zero set at the headstock. You can see the Sharpie on the rod marking where I measured.

Without touching the indicator, I move the carriage to the end of the bed. I again turn the spindle by hand and observe the DTI. It's now traveling 0.004" below zero and 0.008" above zero. Mentally I calculate the new average zero is +0.002" on the dial. That means the headstock is 0.002" out of alignment with the bed. If the needle had actually traveled 0.006" below zero and 0.006" above zero it would have meant my headstock was perfectly aligned with the bed.

The needle at average zero at the far end of the bed.
It's reading +0.002", so my headstock is 0.002" out of alignment with the bed. 

Remember, the key is the difference between your average zeroes.

Lathe: Power Cable

I really dislike how the power cable goes through the motor cover and lathe bed to get to the control box. It means whenever I want to remove the motor cover I also need to remove the control box to disconnect the power cable. It makes much more sense to me to simply connect the power cable directly to the control box.

Power cable in place and bolted to the control box.

With the chip tray removed there was plenty of room to route the power cable under the lathe and to the bottom of the control box. I drilled a 1/2" hole and made a bracket out of aluminum which was riveted into place. The old cable hold down bracket was then used to bolt the cable to it.

Lathe: Chip Tray

I hate the chip guard on my lathe. It's not shallow enough, and the chuck can easily grab the chips and throw them around. I've ended up with chips in my hair too many times because of it. So I removed it. The motor cover has vent holes on its end, so I formed a piece of aluminum to cover it and deflect chips.

Aluminum chip deflector riveted in place.

While I was at it I also removed the chip tray. It serves absolutely no propose and just made things harder to clean up. It was fairly easy to remove the tray and reinstall the legs themselves.

Both chip guard and chip tray removed.

It hasn't been any harder to clean up, and it definitely makes it easier to work with and to clean up afterward. The only downside is I'm still looking for a good place to put the DRO control box.

Lathe: QCTP Rotation

I have a 0XA QCTP installed on my mini lathe. Instead of using the included nut to bolt it to my carriage I used the handle off the stock tool post. This allows me to quickly rotate the QCTP into a new position, an option which I use a lot to either provide clearance or give the tool bit a better angle.

Tool post in rotated position.

Lathe: Gear Reduction Pulleys

I realized I never used the high speed on my mini lathe, and I really needed a lower speed on there than what I had for parting and knurling. In addition, the mini lathe belt is a proprietary part which I'm not a fan of.

There are a lot of sellers on eBay selling ratio reduction pulleys for the mini lathe. They'll drop the low speed to 700 RPM and high speed to 1800 RPM with an increase in torque and lower low speed.

I bought the cheapest one I could find expecting it to need modification to be usable, and it did. I needed to turn the large pulley down to 10mm wide to fit. I then needed to cut a keyway in the motor pulley. However, once I was done they fit nicely and use a standard xl (163xl037) belt which is easily available on Amazon.

Pulleys and belt in place.

In low speed it's now usable down to about 150 RPM which is a really nice change from before, and should really help parting and knurling. I may eventually experiment with the ratio and go up a tooth on the motor.

PWM Control Notes

Peak Voltage:
The KB PWM control I use produces a peak voltage of 160VDC which is very rapidly switched on and off to produce a lower average voltage. I was worried about the 110VDC motor being hit with 160VDC, but after research I found if the frequency is high enough, then it doesn't really matter (assuming the peak voltage isn't absurdly high).

"As long as the PWM frequency is fast enough, it's average voltage is what counts. No, the average PWM voltage should not exceed the motor's rated voltage, at least not for long. This is no different that applying a DC voltage to the motor.

Using a high voltage supply and then less than 100% PWM to compensate is a perfectly legitimate way to run a motor, again, as long as the PWM frequency is fast enough. In effect you are creating a switching power supply that converts the high voltage to the lower one used to drive the motor. It may not look that way because the induction of the motor windings are a integral part of this power supply."

- Olin Lathrop

The things which kill a motor are overloaded bearings, bearing run too fast, brush arcing, and damage from overheating. None of those is really a result of voltage, and a high frequency PWM drive will actually allow the motor to run cooler while producing more torque.


Adjusting Max Voltage:
The high frequency a PWM motor control outputs to the motor can leave a digital multimeter confused or inaccurate if you're trying to measure the max volts, while I've found using an analog multimeter will respond accurately to the PWM output.

If you have a digital multimeter which can read duty cycle and you know the peak voltage (160VDC in case of the KBWS using 115VAC), you can multiply the duty cycle by the peak voltage to get the average voltage, which is what the motor should see. 

To set the max volts to 110VDC for the mini lathe and mini mill's motors I first used a analog multimeter to set the max output to 110VDC. Then to double check I calculated the motor RPMs from the spindle RPMs since I knew the gear ratio; the lathe has a 5000 RPM motor while the mill has a 6000 RPM motor.

Lathe: Headstock Bolt Torque

The mini lathe's headstock is a very poor design. Tightening it to the bed puts a lot of force on the channel in the headstock's bottom which mates to the bed's prism, and the outer side of the channel is not well supported. Even worse, a square relief was cut in the channel which concentrates the stress at the corners of the relief. Ultimately, if you tighten the headstock down tight, you run a good chance of cracking it.

After some research by Grizzly, a recommended torque of 96-144 inch pounds is recommended for the bolts, and I would stay toward the bottom end of that. I torque them to 102 inch pounds, or 8.5 foot pounds.

Crack originating from inside corner.