Cutting square shaped spiral

Running a gear train to cut a long spiral on a lathe requires a quite fast speed on the lead screw, as RJS points out above, this can create quite a strain on the feed gear train and risks breaking something. Spirals of that sort are best left to CNC or universal milling machines, especially if multiple starts are part of the equation.
 
RJSakowski said:
I cut spirals like that when I move the carriage rapidly while not retracting the cutting tool. The trick is cutting a precision spiral. In order to do that, the lead screw has to somehow be synchronized with the spindle rotation. For threads, this is done through the gear train or with an electronic lead screw. The problem in cutting a fast , as stated earlier, is the pitch of the lead screw will require the lead screw to turn at a significantly higher rpm than than the spindle. This would be hard on the bearings in the gear box and for the lead screw. Obtaining a 8:1 or better increase in rpm would be difficult as well. For a lathe with a QCGB, it would have to be done with the external gear train. The largest pitch that your lathe can cut is 4 tpi so if you wanted to cut a 1 tpi helix, you would need to have an external 4:1 gear ratio between the the spindele and the input shaft to the gear box.

Assuming you were to do this in two stages and could split it more or less evenly, That would mean 2:1 on each stage. Your spindle gear has 48 teeth and if you drove the 120t gear, that would be a 3:1 reduction in speed. The second stage would need to provide a further 1.33 reduction or mor to be able to achieve thew overall 4:1 ratio. A further consideration would be the physical clearance. The sum of the number of teeth in the second stage will have to exceed 120 plus some in order to accomplish this. With the OEM gear set, you would probably have to use the 127t gear on the input shaft and one of the other gears on the banjo. You can the do fine adjustments using your QCGB.
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What if I drove the carriage hand wheel with a stepper motor? Kept the doc very light. It would be tough to start and stop at the same place. How deep of a spiral do you think the tool you modeled could cut? What were you saying about the ball screw? Replace the leadscrew with it.

I also wanted to cross drill spiral holes in stock. What’s the best way to adapt my boring bar holder to hold a drill chuck. Do you know of any cross drilling holders for a bxa pm1236 type qctp?
 
benmychree said:
Running a gear train to cut a long spiral on a lathe requires a quite fast speed on the lead screw, as RJS points out above, this can create quite a strain on the feed gear train and risks breaking something. Spirals of that sort are best left to CNC or universal milling machines, especially if multiple starts are part of the equation.
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I don’t have a mill yet, unfortunately.
 
rwm said:
I am confused by the question, and I think others are as well? Do you intend to cut round or square bar stock? When you say "spiral" a standard thread comes to mind. Do you mean something else? The example you showed is a shallow groove in round stock. I would guess the pitch to be about 2TPI. If you want something like this example, the cutting force would not be very high. If this is a one-of part,, and the exact pitch is not critical (i.e.it is decorative) you could drive the lead screw with an independent motor and get something useful. Perhaps a geared hand drill? The ratio of the spindle rpm to the hand drill rpm would relate to the pitch.
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I like that idea. I didn’t think about this many options. It’s helping me understand my lathe even better.
 
WesPete66 said:
https://www.hobby-machinist.com/gallery/albums/axa-cross-drilling-fixture.499/

Here is a link to 'one' typical holder for cross-drilling on a lathe. (compliments of Ulma Doctor) Search the forum for "cross-drill".
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The part I don’t understand is whether I would want mt2 chuck, what dimension jt33 is, and what would be the best options to buy and use with my bxa qctp. Should I make it tapered, or should it just be a straight sleeve with r8 bearings? eBay used to have them for sale for good prices, but now they don’t offer them unless you buy a 400 dollar aloris brand.
 
FTlatheworks said:
What if I drove the carriage hand wheel with a stepper motor? Kept the doc very light. It would be tough to start and stop at the same place. How deep of a spiral do you think the tool you modeled could cut? What were you saying about the ball screw? Replace the leadscrew with it.

I also wanted to cross drill spiral holes in stock. What’s the best way to adapt my boring bar holder to hold a drill chuck. Do you know of any cross drilling holders for a bxa pm1236 type qctp?
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you can get live spindles for making a diy grinding fixture but it wouldn't be impossible to make a bearing block and a shaft with a reamed hole for an end mill(grub screw for the end mills with a flat) , maybe driven with a hand drill or similar small motor.

Stu
 
stupoty said:
you can get live spindles for making a diy grinding fixture but it wouldn't be impossible to make a bearing block and a shaft with a reamed hole for an end mill(grub screw for the end mills with a flat) , maybe driven with a hand drill or similar small motor.

Stu
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I used an ER20 extended collet chuck to make a spindle.
It's about 7" long (excluding the chuck part) and 20mm diameter for CNC use.
It does mean you can use standard metric bearings plus change collets to hold various diameters.
Mounting a toothed pully on end was pretty simple plus, having a complete through hole makes it useful for longer material and not just cutters.
The locking collets shown in hints and tips post as a mandrel could also be very useful now I've seen them, look very similar to what I'm used to in motorcycle engines holding the valves in
 
Yep ok. I misunderstood you entirely. What you’re wanting just looks like an oil groove. Sorry.
 
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