Adding Variable speed feed to your mini metal lathe

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More4dan

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Using the change gears in a mini lathe, the finest feed you can get is 256 threads per inch. This doesn‘t leave a very smooth finish. So, I set out to slow down the feed rate. I’ve combined a couple of ideas from others that have done a similar modification. Here is what I came up with, how I did it, the parts used, and the results.


I am driving the feed screw with a DC geared motor from the right side of the lathe. I’ve also added a variable speed drive that uses Pulsed Width Modulations (PWM). This essentially turns the motor on and off at set intervals (pulses) but still at the same DC voltage preserving the motors torque at lower speeds. The motor is 24 volts and 45 rpm unloaded. I used a 24 volt power supply, laptop power brick and cord. I added a double throw switch between of the variable speed drive and motor so I can reverse the voltage to the motor and reverse the direction. This allows me to feed in both directions with variable speeds.

I had to modify the drive screw to be able to connect the motor. I removed the drive screw, drilled into the right end on the lathe and tapped for a M6 x 1.0 thread. I made a small shaft to thread into the drive screw, one end matching the motor shaft diameter and the other threaded M6 x 1.0. I used Blue LocTite when attacking it to the feed screw so it wouldn’t back out when turning in reverse. I crossed drilled it with a 3/32” drill to take a 3/32“ pin. I then cross drilled the motor shaft and added a 3/32” pin. To connect the two I used some 1/2” brass rod drilled slightly larger that the motor shaft. Using a cutoff wheel on my Dremel, I cut slits on each end to engage the pins. The one on the motor side was cut deep enough to allow you to pull back the coupler and disconnect from the feed screw.

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The motor is mounted using a piece of sheet steel screwed to the right side of the lathe bed. I started with a cardboard template to get the rough shape then cut out of the steel sheet. I attached the motor to the sheet, aligned the motor shaft to the feed screw using the brass coupler, and clamped the steel sheet to the end of the lathe bed. Make sure the alignment allows the coupler to easily slide back and forth. I then drilled two holes through the sheet and into the lathe bed then tapped the holes for mounting.

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The variable speed controller comes with the potentiometer and an on off switch wired in. I installed it along with the switch for reversing in an electronic project box. Wired everything up, soldered the connections, and covered the connections with heat shrink tubing.

Danny
 
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More4dan

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PART 2

Here are the parts I used.
variable speed drive:

Power Supply:

Project Box:

Motor and Gear Box and switch

The motor and gear box mounts with #10 32tpi screws. the top left screw has to be flush with the steel sheet because it sandwiches between the sheet and lathe bed. I countersunk the hole and used a flat head screw. For the others I used Allen cap screws.

Danny
 
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More4dan

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PART 3 Testing

Here are the results of some quick testing. I started with a piece of Acrylic turned between centers at different feed speeds and feed directions. Here is at full feed speed with the spindle speed at approximately 2000 rpm.

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This one is at 50% feed speed cutting toward the head stock.

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Here is the same speeds however feeding toward the tailstock with much better results.

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Here is the final result with NO sanding nor polishing.


I tried at 25% feed speed and the temperature got too hot leaving a rough finish. Here is the result on a piece of aluminum.

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I’m very happy with the results. The acrylic was turned with a $2 carbide insert using the tool holder I made for metal lathes. Slowing down the feed rate made a big difference in surface finish. This design allows you to slide back the coupler, and reinstall the changes gears for threading. Let me know if you have questions, suggestions, or comments.

Danny
 

FGarbrecht

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Very cool Danny. I'd be curious to see how ebonite comes out with this setup. Did you use a triangular carbide insert or the round carbide insert for the acrylic? I'm def going to have to give this a try. Thanks for the detailed explanation and parts list / sources.
 

More4dan

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I used a 12mm x 30 degree carbide round insert designed for wood turning. The aluminum was turned with a Korloy 9mm round carbide insert designed for aluminum.

I will give some ebonite a spin and post.

Danny


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magpens

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Thanks, Danny !!
I'll probably have some questions after I "study" what you have done.
Your $2 carbide insert .... I assume that is a round insert.
 

RangeRat

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Can’t argue with those results! That’s an amazing finish right off the turning tool. I really like your idea of using the carbide insert on the metal lathe. Definitely one I’m gonna have to copy.
 

More4dan

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Here is a test using black ebonite.

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Turned at approximately 1500 rpm with the feed set at 50% with a 0.005” depth of cut. As turned with no polish or sanding. Same carbide cutter as the acrylic above.

Danny
 

magpens

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That's an amazingly nice cut on the Ebonite, Danny !! .

Especially for an 0.005" depth of cut, which is fairly deep when you are approaching the last cut on acrylics and other materials like Ebonite.
With hand-cranking, I usually do my last acrylic cuts at 0.001" or 0.0005" (if I can), and I can get close to what you show but not quite as good.
And I have to sand a little at 320, 400, 600 and 800 before the Novus (there are always some slight visual blemishes with hand-cranking as I do it).

It looks to me that you might be able to proceed to Novus 3 (and then Novus 2) without any prior sanding. . Would you say that's the case ?
 
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magpens

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@More4dan
Danny,

Not sure what you mean by saying "50% feed speed" in this statement:

"This one is at 50% feed speed cutting toward the head stock. "

And then you say :

"I tried at 25% feed speed and the temperature got too hot leaving a rough finish. " ... ( is the 25% possibly a typo and maybe sh be 75% ? )

I thought there would be an improved surface as you lower the speed. . What am I missing ? . Hope you can clarify please.

You do say ( not sure if this is for aluminum ) that :

"Slowing down the feed rate made a big difference in surface finish. "

[BTW, I do see your control box has a readout which is %, and I do understand the concept of % of top speed (or, perhaps, width) for a PWM.]
 
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More4dan

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The speed control display is in percent of the motor speed. 100% is about 40-45 rpm, 50% would be have that speed., about 20 rpm. With the motor speed turned down too low, the cutter moves very slowly across the blank. The friction caused the acrylic to get too hot.

The surface finish was definitely smoother using the reduced feed rate for plastic, ebonite, and aluminum.

Danny


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More4dan

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The speed control display is in percent of the motor speed. 100% is about 40-45 rpm, 50% would be have that speed., about 20 rpm. With the motor speed turned down too low, the cutter moves very slowly across the blank. The friction caused the acrylic to get too hot.

The surface finish was definitely smoother using the reduced feed rate for plastic, ebonite, and aluminum.

Danny


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randyrls

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An "Electronic Lead Screw" (ELS) is an over-the-top way to control feed rates. It eliminates all the change gears. Check out Youtube Clough42 channel. If you are at all interested in machining he is very clear and concise. His explanation of threading is the best I've ever seen. It isn't a "put this gear here". He really shows the math and why it works.
 

More4dan

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An "Electronic Lead Screw" (ELS) is an over-the-top way to control feed rates. It eliminates all the change gears. Check out Youtube Clough42 channel. If you are at all interested in machining he is very clear and concise. His explanation of threading is the best I've ever seen. It isn't a "put this gear here". He really shows the math and why it works.
Randy, thanks for sharing. Something I will definitely consider if/when I upgrade to a larger lathe. First step in converting to a CNC lathe.

Meanwhile, I rarely turn threads using the lathe anymore. Taps and dies are so much more convenient especially for the sizes in making pens.

Danny


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More4dan

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Someone asked for a drawing of the motor mounting plate. Here you go with measurements to 3 decimal places.

The definition of an Engineer? Calculates to 3 decimal points, measures with a yard stick, and cuts with a chainsaw.
 

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More4dan

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This motor mounts with #10 32 tpi screws. I used the same for the lathe bed mounting screws. I used a #20 drill bit to drill the lathe bed. It took slow speeds, cutting fluid, and lots of patience. You can use what ever size is convenient to tap/screws/bits you have on hand.

Danny


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Curly

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As a former quality inspector you would have gotten a lot of curses 🤬 from us for chain dimensioning and dimensioning features from multiple locations without datums.:rolleyes: No tolerance either! Engineers. Sheesh!;)
 

More4dan

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And, some more testing with a piece of Bethlehem Olive wood. Again no sanding. First time I’ve seen ribbons while turning wood.

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Danny
 

More4dan

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As a former quality inspector you would have gotten a lot of curses 🤬 from us for chain dimensioning and dimensioning features from multiple locations without datums.:rolleyes: No tolerance either! Engineers. Sheesh!;)
LOL!

Probably because I spent 10 years as a draftsman and designer for homes and light commercial before becoming an Engineer. We tried to dimension how best to allow the framers to do their work in the field. But for engineering drawings, you are correct SIR.

I did give a tolerance, measure with a yardstick and cut with a chain saw (well a bandsaw).

Danny


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Curly

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Glad you took it as intended Danny. I was lucky in that Boeing, Bombardier, Cessna, Lockheed etc were very fussy about that sort of thing so when we as a subcontractor got the paperwork there were few errors.
 

rherrell

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Great job Danny! You can get an even better finish on plastic if you use HSS. Grinding your own cutters is not that difficult and it's WAY cheaper than using carbide. You can buy a 1/4" HSS blank for less than $1.00 and it will last you a lifetime.
 

magpens

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@rherrell

You can get an even better finish on plastic if you use HSS. Grinding your own cutters ....
I am really quite amazed to hear that switching from a carbide cutter to a HSS cutter improves the finish on plastic.

Do you know the reason for that, Rick ? . Perhaps there is some detailed physics/chemistry involved.

If you are grinding your own HSS cutter, I assume that you would grind the angle before parting the cutter off the stock.
Is there a preferred angle ?
 

More4dan

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Great job Danny! You can get an even better finish on plastic if you use HSS. Grinding your own cutters is not that difficult and it's WAY cheaper than using carbide. You can buy a 1/4" HSS blank for less than $1.00 and it will last you a lifetime.
I have HSS bits I’ve ground for both metal turning and wood/plastics. I wanted to come up with something to improve the surface finish by slowing down the feed rate. But your right, if you can get this kind of finish with carbide, imagine what one could get with a finely honed HSS bit. I was actually surprised with how nice a finish can be had with carbide on wood, plastic, and aluminum.

Mal, the grain size of the carbide is larger than that of HSS. Therefore HSS can be ground to a much finer edge. The advantage of carbide is its high hardness that will hold the edge longer. As in most things in life it’s a trade off between perfection and ease of use.

HSS has another advantage when rough turning and making interrupted cuts, rounding blanks. It’s much tougher and able to take impact loads without chipping.

Danny


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Curly

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Mal as Danny said you can get a much sharper edge with HSS. It also lends itself to grinding custom shapes like cutting little coves, beads, round over shapes grooves etc.

You can get high speed inserts to use in place of a carbide insert but they will not last as long as the carbide inserts but for those without a grinder or wanting a sharp indexable cutter they have a place. https://www.arwarnerco.com/default.asp

You can also get a different sort of holder that uses HSS bits which because of only having one surface to grind simplifies sharpening. It is called a tangental holder and an Aussie company make them under the name Diamond Tool Holder. https://www.eccentricengineering.com.au They will work for most of what we encounter making pens. I have one. They also sell an alloy that is midway between HSS and carbide called Crobalt. It came along after HSS but were usurped by carbide. It has lots of the benefits of HSS and carbide at a cost. Tantung is another brand of similar material.

Using traditional HSS holders and learning how to grind the cutters unlocks the most versatility in your lathe.
 

randyrls

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Grinding your own cutters is not that difficult and it's WAY cheaper than using carbide. You can buy a 1/4" HSS blank for less than $1.00 and it will last you a lifetime.
Rick; I'm sure you know this, but posting for others... Look up "Vertical shear tool" for an EASY to make tool and a super finish. This tool is NOT for stock removal, only for finishing cuts (remove only .002" - .003").
 
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bmachin

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Here's a good series of articles on grinding your own HSS tools on a belt grinder. Note that it is Part 3; however it links back to parts 1 and 2. Worth a read IMHO.


If you're interested in grinding a tool for Alumilite (and possibly other plastics as well) take a look here:


Bill
 
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