Denker's Mini Lathe Page


Mini Lathe Information

I purchased the old version of the HOMIER 7" x 12" mini lathe. It comes with 4mm change gear keyways and a number of other differences from most other mini lathes. I don't think they make these anymore so unless you buy one of these used, you will get 3mm gear keyways with most of the parts the same between makers. Now the main differences are color, metric or inch cross-slide screw and compound rest screw and the length of the machine. The motor has plenty of power for this machine size but the drive gearing is a bit high for most threading and large diameter machining. Also the plastic drive gears may be a little light duty. As a result, I greatly modified my lathe.

 

Modification Contents:

 Machine Stop Addition

 Low Speed Addition

 Gear Plate Change (Updated)

 Multiple Gear Plate Addition

 New Gear Cover

 Tailstock Change

 Spindle Tachometer (New)

 Other Additions & Changes

Home


Machine Stop Addition

This addition is mainly used during threading operations to stop the machine at the end of the threads. I usually cut a groove where the threads end so there is room for some variation where the cutter stops. A set ring on a rod below the leadscrew is set so it is hit by an addition to the carriage assembly. This rod can move in ether direction and activates a switch inside the control cover to turn off the lathe. The carriage must be moved away from the stop before the motor can be turned on again. It can also be used as a safety stop. METRIC THREADING NOTE! When doing metric threads with a non-metric leadscrew, do NOT disengage the leadscrew because threading will not match again. This is a problem because when the switch is open the motor cannot be started. For metric treads I add a spacer (shown in picture) on the stop rod for each cutting pass. When the machine stops I remove it, (the switch closes) then I back off the cutter and reverse the lathe back to the start of the thread. This is quite a hassle but is the only way it can be done without changing the leadscrew to a metric one. (which I think would also be a hassle!) I don't do many metric threads so I'll keep using the inch one.

The next picture shows the switch with the controller removed.

Back to top

Home


Low Speed Addition

This addition lowers the spindle speed limit to about 600 RPM and almost doubles the torque. This is a major addition with many parts and I use it most of the time. It completely bypasses the internal plastic gears. (The speed selector must be in neutral) Low speed can be disabled to use the higher speeds when needed. The method I used reverses the direction of the spindle so a switch must be added to reverse the half speed function. This is a nice addition for the additional power needed when machining bigger parts and for the lower speeds needed when threading, grooving or cutting off parts.

This addition and others required big changes to the gear cover. Always have the gear cover on when operating the lathe!

Note: This cover has now been replaced. (see New Gear Cover)

Back to top

Home


Gear Plate Change

I greatly disliked the original change gear plate. It was hard to adjust and required a lot of adjustment every time a change was made. Adjustment was centered on the leadscrew shaft when it should be on the "A" shaft. So after much engineering, out with the old and in with the new! This .25" thick plate was laser cut at work and has many built in gear options. The gear plate mounting shaft and the "A" shaft mount had to be slightly modified too. With the addition of 21, 25 and 42-tooth gears, I can make just about any metric thread to within .00015" per inch. (For threads within the machine capability limits using plastic gears) For inch threads, I can get many threads by changing only the leadscrew shaft gear and adjusting the gear plate. Also this configuration gives me many more machining feed possibilities. (Down to .0018"/rev with the added 25T gear.) I use 8mm x 20mm long shoulder bolts with spacers to hold the gears to the plate. There is also a T-nut held in place with a thin spring steel bracket for gear adjustments. (No need to get a wrench behind this thing!)

ENGINEERING NOTE: Because this plate uses some machined in gear mounting locations, it does require good machining location on the "A" shaft and good quality gears! The gears center hole should be straight for minimum wobble and centered within .005". It may be necessary to design in additional length between gear locations if the gears are not of good quality. If you do not want to use machined in mounting locations, there is room for two T-nuts in the middle slot. That would give you gear adjustment for most gear placements. I have inch setup and metric setup charts using only the slot.

Now that the gear plate does not rotate on the left leadscrew mount, I can improve wear and performance by replacing it with a ball bearing mount.

Note: Because this plate can have more gear locations, it may not maintain a direction standard. Some gear setups may require using the reverse of the normal feed selection. This setup is for a M0.7 thread. (To be more accurate: M0.70009)

I have inch thread setup and metric thread setup charts for this plate. Many of my setups use additional gears. I have found getting gears and modifying them for this machine quit easy and inexpensive. (My machine uses 4mm keyways.) I have also designed a metric leadscrew version of this plate for metric threads but have not made one yet.

GEAR PLATE UPDATE:

After much use I have found that the gear plate could be improved. Here is what the new improved version looks like.

Notice the slot now has two adjustable mounting T-nuts and the plate has only three machined in gear locations.

(One hole is for mounting the multiple gear plate addition)

Back to top

Home


Multiple Gear Plate Addition

OK - on this one I got a little carried away! With this addition I can get 3 different machining feeds by changing a gear location. The 35-tooth gear mounted on the plate of the next picture uses a spring and ball indent so it can be moved. I can get most standard inch and many other threading feeds by changing only the leadscrew gear and the position of the gear plates. I don't think it's going to get much easier on this machine. I did have to buy and modify more gears. I recommend using metal gears whenever possible and I have switched over to all metal gears. I have also added a 12mm ID x 21mm OD x 5mm ball bearing to the "A" shaft in its mount and a bronze bushing in the aluminum belt cover for the 7mm end of the shaft. (Required modifying the "A" shaft ,its mount and the belt cover.) I made a steel 20-tooth 3 deep (.95" long) gear from a purchased gear blank to use on that shaft. For machining, the 3-gear cluster is made up of 25, 20 and 40-tooth gears with the 20 or 40-tooth used to drive the next gear. These 3 gears are connected by a key on the modified brass 25-tooth gear. It looks like a lot of gears, but everything works well. I use this addition most of the time but I do remove it for metric threads. Using other gearing possibilities, this addition gives me a very wide range of machining feeds. NOTE: This addition also required modifying the gear cover for gear clearance.

The next picture shows the setup for machining feeds. The 20-tooth gear of the cluster is used to drive the middle 80-tooth gear.

The selection knob below the gear plate is used to setup for different machining ranges so changes can be made easier and faster. Adjustment screws are used to preset the location of the gear plate for a 20, 25 or 30-tooth gear on the inside of the middle 80-tooth gear. This gives me 9 machining selections from .0020"/rev to .0059"/rev. I have removed the middle 80-tooth gear on the next picture to show it better. The knob is turned to select one of the three adjustment screws or none at all. The knob position is held by a ball and spring indent.

 This gear plate can also be used for threading. I am now able to thread 28 inch sizes from 7 to 104TPI using one plate setup by changing the location of the movable 35-tooth gear and the location of the leadscrew gear. I can do that using 4 threading groups. The 3-gear cluster is now made up of a 40-tooth, a gear spacer and a 20-tooth gear keyed together on a bushing. The clusters 40-tooth then drives a 20-tooth keyed to a 40-tooth and the clusters 20-tooth drives a 40-tooth on its own bushing and not keyed to the next 40-tooth. The inside 40-tooth gear turns at 1/4th the speed of the outside 40-tooth gear. One of these gears is then used to drive the leadscrew gear. The next picture shows one of these setups. The 14 to 26TPI group is shown with the 50-tooth gear (20TPI) on the leadscrew shaft using the outside position. If the 50-tooth gear was on the inside, it would be setup for 80TPI of the 56 to 104TPI group.

The setup in the next picture is for the 28 to 48TPI group with the 45-tooth gear (36TPI) on the leadscrew shaft using the inside position. If the 45-tooth gear was on the outside, it would be setup for 9TPI of the 7 to 13TPI group. I have a drawing showing these inch thread group setups for this plate addition. I do not recommend threading under 14TPI on this machine without the low speed addition and the addition of ball bearings on the "A" shaft and the leadscrew shaft. I would also recommend converting to metal gears. For the overall capabilities of this lathe, the gear plate additions and the low speed addition are big improvements and greatly increases the value of this machine to me. Even though it required a lot of work, I am very happy with both!

Back to top

Home


New Gear Cover

As of Dec. 2008 I replaced the old gear cover with a new one made just to fit the low speed and gearing changes. The original cover has always been slow and awkward to take off and put on. Especially with the low speed addition. This new one is much easier and faster to open and gives much more room for the additional gears of the gearing changes. It has a hinged door so the rest of the cover can be better attached and has a magnetic latch. The next 2 pictures show it on the lathe. It was made using .050" and .062" think aluminum. Next I am going to order blue paint so it will match the rest of the lathe.

 

Back to top

Home


Tailstock Change

The tailstock had 2 things that needed changing. The first thing is the nut to bolt it down. Itís a pain in the neck. There are a few kits available to fix this but I went with a lift and set adjusting nut. The inside bed surface of my machine is not very parallel with the top so some of the available methods would not work well. I couldn't find an adjustable lift handle that fit in the tailstock cavity so I made this one. It is easy to adjust and can be moved out of the way in ether direction when done.

The second change I wanted was a gauge dial on the tailstock screw shaft. This was not possible with the metric threads being used. I needed something with 20 TPI or a M1.0 thread. I drilled out the old quill thread with a .390" diameter drill and tapped it 7/16 - 20 left hand. After making a new tailstock screw shaft, buying an inch style 50-division compound rest dial, compound rest retainer and making the other necessary modifications, I was almost done. My lathe did not come with depth markings on the quill so I added them too.

Back to top

Home


Spindle Tachometer

I wanted to add a tachometer to the lathe but with the changes made to the gearing, there was no place to put an interrupter wheel or sensor. The only place left was the chuck side of the gear housing. At that location an interrupter wheel was out so I was going to have to bounce a light beam off the spindle itself. I had some old infrared LED's and sensors from an old computer card reader, so I made a sensor bracket out of .020" brass shim stock and soldered these on it. After some testing I was reasonably sure it would work so I designed the rest of the project and got started. The only good place for a sensor was inside the plastic bearing cover but I could not get the cover off unless I took the spindle off! As a result I decided to cut the cover in two. I used a hot knife to cut it at the top mounting hole and at the bottom. After cleaning up the hot knife cuts, it now comes off and mounts back on easily and it did not look bad. I milled a spot for the sensor on the back side of the cover and ground a notch in the gear housing to run the wire out the back of the lathe. I did not want the sensor wire anywhere where metal chips could get at it. This will plug into the display housing using a 3.5mm stereo audio plug. The next pictures show the finished sensor assembled on the bearing cover.

Next I painted 6 marks about .20" wide using flat black paint evenly spaced on the spindle. For 6 marks, the display has to have a 1 second update rate. I also have the option of using 12 marks at a half second update rate, but 1 second seems to work nicely.

The next pictures shows the finished display board and housing which was made of .050 aluminum and painted.

And here it is assembled and running on the lathe.

This project turned out quit nice. It displays RPM to the nearest 10 +or-10. The power supply is a 5.5V regulated wall plug unit so no power supply parts were needed in the box.

Back to top

Home


Other Lathe Additions & Changes:

 With the addition of the low speed modification, I found it harder to pull out and move the feed engagement and direction lever. So I cut off the gear shifter handle to about .50" long and pressed on a .875" diameter brass ring to it. Now it is much easier.

 

 This lathe does not come with a carriage lock. My lathe has a different carriage design then most others and that made it easy to add one. This is the lock I am using.

 

 It is also a good idea to add some chip shields. The next picture shows what I am using. It has an extension and moves out of the way if necessary.

 

 I added a chip pan on the carriage and a shield that clamps to it. This one is made from .050" SST. I have several chip pans of different sizes for different chucks. This helps keep most of the metal chips off the wear surfaces and the leadscrew. It also adds a place to attach a chip guard.

 

 To increase the range of the cross-slide and compound rest, I added spacers to both. The above picture shows one of them. (.35" thick) The picture before that shows the other. (.50" thick)

 I added a dial to the right side of the leadscrew and a crank that can be attached to it on the left side. (I don't use this much anymore)

 I added an extension to the spindle so metal chips would not fall into the gears. (See the Low Speed Addition picture) Note: The gear end cover should always be on when operating the lathe.

 It is also a must to widen the spacing of the feet on the bottom of the lathe for better stability. This is easy to do by adding an easy to make bar.

Back to top

Home


All pictures and content Copyright © 2007, 2008, 2012 David Denker. All Rights Reserved.

Last Revised: October 2012