By Kyle Crane
- Material Selection
- Outside turning
- Spanner Slots
- Finish Work
One of the main reasons I purchased my Grizzly 10×22 lathe was to assist in the project to convert my milling machine to CNC control and it has worked out fantastically for that purpose. I have made many parts already using it and performed follow-up work on it from milled parts as well. However when it came time to work on the long, slender, ball screws needed for the X and Z axis it became clear that I needed a little rear support. My first attempt at cutting the Z axis screw ends resulted in a wobbling screw in part due to the rear of that long screw whipping around behind the lathe I believe.
Before I made any more attempts at taking on long shafts I wanted a way to really dial them in to the lathe rotation axis. A rear spider is used by many gunsmiths to do just this but they are often either bulky or installed on the work and butted against the spindle meaning that moving the work involves also moving the spider. I started thinking about what I wanted to do and how I wanted mine to work.
My design goals were:
- I wanted it to be attached, something that was just an extension of the lathe.
- As short as possible. I wanted it to be able to grab any bar that could reach the rear of the spindle bore and to not protrude much from the rear cover.
- The rear cover needed to be able to be opened and closed with the spider in place but not in use.
- I wanted a decent appearance, something that looked like it belonged there.
I settled on a design that would replace one of the spanner nuts on the rear spindle of the lathe and provide a short extension of the spindle to add the 4 set screws used in the spider. The extension would be just a bit over ½” with a heavy chamfer to make it less obtrusive and keep the rear cover from contacting it when opened and closed. I proceeded to take a few measurements to get started on the 3D model.
I determined that the thread used on the rear spindle was M39-1.5 which is not what is listed in the parts diagram or the replacement parts site on Grizzly. I have no idea if that is due to changes over time or just a misprint so you may want to confirm that against your own spindle before cutting any threads or finishing the boring work.
The inside bore of the lathe is just a tad over 1 inch so I went with 1.060” for the bore of my spider to match that and go a little over it. The hole in the rear cover was around 2 inches wide but poorly centered on the spindle. You can adjust it to some extent but it would not center well on my lathe so I decided on the outside diameter of the spider portion of the part being 1.750” to give plenty of clearance for both opening the door and so it doesn’t rub on the off-center hole.
The O.D. of the nut section of the part was chosen to be the same as the existing nut at around 2.230”. Not much of this is critical so it can be altered to suit the user or material at hand.
2.) Material Selection
There is not a lot to say here really. I wanted the nut to be durable and hold up to being yanked around with a spanner wrench and getting smacked now and again by hard metals. What I had on hand was a hunk of 2.5” diameter 1144 steel that I had snagged from the scrap bin. It also happened to be short enough to not need to be cut down much to get to final dimensions. I think AL could be used but durability would suffer while ease of machining would increase. Also if you could find a 2.5” tube with a 0.750” wall thickness you could cut down a lot of time spent boring but the cost would likely be high for the material.
3.) Outside Turning
This is pretty straight forward. Since perfect concentricity is not required I simply used the 3 jaw for all the operations. First I faced each side of the stock to clean up the rough ends. The Spider-Nut needs two external diameters cut; the 2.230” external diameter of the nut section and the 1.750” section for the spider section. The total length of the part is 1.180”. In my case the raw stock was short enough that I could not cut both ODs at the same time. If you use a longer piece of stock then consideration will need to be given to cutting away the excess, as 2.25 to 2.5” of steel is no simple thing to part off and facing off inches of material is also not fun.
I used standard carbide tools to turn the majority of the stock away. 750RPM to 1200RPM and 0.050” DOC with a heavy feed was showering the area in nice blue chips. Then when it was time to get a nice smooth finish I switched to the CCGT inserts normally used in aluminum but they also do very nice work with small DOC in steel. They are sharp enough to remove small amounts of material where normal molded inserts will only rub and make a mess. 1200RPM, 0.005” DOC and low feed provided a nice finish.
Then I faced each end down to the proper length but on my part I left the spider section about 0.500” long for now so I had some extra to grab onto for boring and threading work.
Since I began with solid round stock there was a LOT of material to remove for this step. I began with drilling a center in the back of the stock (the nut side) and then used progressive drills to step up through the sizes from 3/16” on up to 5/8”. I stepped up by roughly 1/8” with each drill but I have found the 5/8” drill cuts better going from 3/8” than it does from 1/2” so in that case I stepped up by 1/4″.
With the starter hole punched through I could use my 1/2” boring bar for my best chance at boring the hole without chatter. The 1.680” length of the bore would be right at 3.4X extension on the boring bar which is approaching the limit on a 1/2” steel boring bar and my carbide inserts do not make things better at long extensions for chatter. Still I like to use them where possible because removing a lot of metal with HSS on a steel part is like watching paint dry.
I used 1200RPM and 0.020” DOC and an aggressive feed on the rough boring work. Since the through bore was not critical I simply used the roughing insert all the way through. As the hole opened up the finish improved a bit but I had to step down to 720RPM near the end to quell some chatter. Once the through bore ID reached 1.060” I stopped and reset for the next operation.
Next step was to bore the hole for what would soon be the threaded nut side. The minor diameter I came up with by measuring the inside of one of the stock spanner nuts was 1.480” or about 37.6mm. I stayed on the loose side as it was more important that it fit easily than perfectly snug. It’s a jam nut not a rocket nozzle. This is a much shorter boring job so I adjusted the boring bar to just 0.700” of overhang and using a magnetic dial indicator against the carriage I measured out the 0.680” bore depth from the face of the part and set my carriage stop for that position. I could rough using the carbide inserts much more aggressively now.
After roughing out the majority of the bore I switched to my AR Warner HSS inserts for my boring bar and slowed the spindle way down into the low hundreds. These sharp HSS inserts will take smaller cuts and leave a nicer finish behind but they are remarkably slow to use. I cut by 0.005” plus a spring-pass till I reached my target diameter on the nose.
Lathe threading is a topic covered all over the web in great detail. I am not going to completely cover that topic here but there are a few gotcha’s if you have not done an internal thread nor threaded on the Grizzly 10×22 before. If this is the first time you have threaded, stop here. You need to practice it a bit with setups where it is harder to crash. A flub here wastes a lot of time spent up to now.
Since this is a metric thread you need to be aware that you cannot use the thread dial to engage and disengage the half-nuts. They MUST remain closed through the entire threading operation. This means that you will have to run the lathe in reverse to back the tool up out of the bore for the next pass. As this is an internal bore, the compound position will need to be the mirror image of the normal position, being swung either 29° to the left-front or 29° to the right rear (see picture above). The position on the compound angle dial will not be 29° either. The 10×22 has its zero angle mark set where the compound is parallel to the axis of the spindle instead of perpendicular like most lathes. Instead on the 10×22 you set the compound to 61° (90° – 29°). Clear as mud right?
To do this job you need a 60° thread cutting tool of some kind that can thread inside a bore.
That tool must be sharply pointed enough to cut the root of the thread to the right depth before it cuts the sides of the thread too wide. You need a tool made for this or you need to grind one that will work which is a subject all to itself. I went with a carbide insert tool with a lay-down configuration. It looks a lot like a boring bar, but with a bit different geometry. An insert tool means I can do more work on projects and less work grinding tools.
Before I began, I made a copy of the spindle’s rear thread on some stock I had laying around. I cut the thread so that the rear spanner nut from the machine would thread on freely to male thread I was making. This would then give me a gauge to use when cutting the female threads for the nut and should mean that if this gauge threads in that the spider-nut would certainly thread onto the rear of the spindle. Time well spent; because if you remove the work from the chuck you are in a world of hurt trying to pick up the thread again if it does not fit. It may still be prudent to mark the chuck jaw and part together so it can be replaced in the chuck in the same position if need be.
Before you begin to cut your thread use the tool to make a relief groove at the back of the bore. This is a shallow groove about 0.050” deep and perhaps 0.100” wide at the back of the bore for the threads. This allows the tool to clear from the material at the end of each threading pass. So you don’t have the tool running into a mound of burr at the end of the thread.
Setup a reference of some kind to show you where the end of the bore is so you don’t crash the tool trying to eyeball it. You can use tape, dye, or something else. I use my carriage stop for this but you still have to turn off the spindle at the right moment to allow it to coast up to the stop since it will not stop the power feed if the spindle is running. Now, find your cross-slide zero by just scratching the inside of the bore. Finally with the spindle off pick up a number you want to use on the thread dial and engage the half-nuts at that position. I chose position ‘1’. (remember you must not disengage them again till the thread is finished)
For each pass I followed the below procedure:
- Set cross slide to your zero position.
- Dial the desired depth of cut for this pass on the compound.
- Turn the spindle on forward and make the pass.
- 1/8” or better from your reference mark turn the spindle off.
- Turn the spindle by hand all the way up to the mark if needed.
- Push the cross-slide forward by 0.045” to 0.055” and
- Turn the spindle on in reverse till you are clear of the bore.
- Stop the spindle.
- Back to step 1.
I continued that process until I had cut in by 0.041” of compound in-feed (Use that as a guide, your mileage may vary depending on the tool you are using). At this point the thread gauge I had made would just thread on. I gave it an extra 0.001” of compound feed and that made for a nice snug but free spinning fit. Threading was now complete.
6.) Spanner Slots and Screw Holes
The next step is to add the spanner slots in the outer body of the spider nut. There are multiple ways to do this that depend on the equipment you have at hand. The position is not critical so by-eye alignment is fine but too much slop and it would start to look bad. I happened to have a shop-made indexer I made for just these kinds of needs. I loaded the Spider-Nut into the indexer’s chuck and set up the indexer to allow me to stop at 90° positions and locked the indexer into my mill’s vice.
After centering the part on my Y axis, I used a 0.250” solid carbide end mill at 3000 RPM and 0.020” DOC and 10 IPM to make the slots in the X axis direction. I cut to a final depth of 0.125” for each slot. Once complete I reset the Z axis and indexed the part another 90 degrees and repeated this process. Lock your indexer down well when cutting. This is steel and the cutting forces can get pretty high. My failure to lock it down well enough cost me an end mill and left some decorative knurling around the rear of the Spider-Nut.
The slots could also be done with a shaper, or a broach of some kind. The main thing is you want the slots to allow for tightening and removing the nut with a spanner wrench just like the existing jam nuts on the lathe. A mill and indexer are but one way to skin this cat. Once finished cutting I used a file to clean up the edges and knock down the sharp corners.
Next I drilled and tapped my holes for the spider screws using the same index positions as the slots. I had positioned the part such that I could drill between the jaws of the indexer’s 4 jaw chuck at each index position. Before I drilled I plunged a 3/16” end mill a few thousandths to cut a small flat into the outside of the part to give the tap-drill a place to start without wandering too badly. I tapped using my mill and a spiral-point 1/4-20 HSS tap. I tapped using power in low gear and at less than 100 RPM just letting the tap pull the quill down with it.
After that was complete I hit the inside of the bore with a round file to knock off the drilling burrs. I also used a 90 degree counter sink to do the same for the outside of each hole around the part.
7.) Finish Work
Now it’s back to the lathe to put the finishing touches on the part. This is mostly all cosmetic and can be adjusted to suit the taste of the person making it. I loaded the part up and faced away the excess I left on the spider screw side of the Spider-Nut. I left the length at 0.5” to allow for the 1/4-20 set screws and the heavy chamfer I planned for the spider side of the nut.
Next it’s time to rotate the compound to make the 45 degree chamfers. I used 1/16” chamfers on the inside edges of the bores, flipping the part in the 3 jaw to get to both of them with the compound on the same setting. I then reset the compound to do the 45 degree outside chamfers and did both sides of the spanner half of the part. I now again flipped the part back around and re-chucked. Last step was to cut the large 0.200” chamfer on the outside of the Spider half of the part. Finally I used a little sand paper to clean up the appearance of the part. The chamfers are all cosmetic so feel free to adjust those to your own taste.
Now I removed it from the chuck and threaded it onto the rear of the spindle to replace one of the spanner nuts. It fit perfectly and extends only slightly from the rear of the G0602’s rear cover. DO NOT OPERATE THE LATHE WITH THE REAR COVER CLOSED AND THE SPIDER SCREWS PROTRUDING FROM THE SPIDER-NUT! Doing so may allow them to snag the cover while rotating and bad stuff is likely to result. When using the Spider-Nut to support long work make sure the rear cover is open and propped out of the way. When not in use, removing the screws completely would be prudent.
This part fulfilled my goals for the project. A rear spider that allowed me to support long limber work in the spindle that did not need be removed to operate the lathe normally nor be installed when needed with this design the spider is always ready to use when I need it and looks right at home on the lathe. The rear cover is not interfered with when operating without the spider in use. This not a difficult project but neither is it simple. Without a mill and indexer a little ingenuity will be required to add the non-lathe features.
Here’s the link to the plans:
Spider_Nut_for_the_G0602.pdf (55.4 KiB, 595 downloads)
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By Kyle Crane About the author