MrWhoopee
Super User
When I need a tool like that, I grab an old tap and grind away all but one "tooth". Spares me having to grind the profile and it as built in top rake. The tap just has to be a coarser pitch than the thread you're cutting.
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Identifying an odd thread.
- Thread starter Susquatch
- Start date
When I need a tool like that, I grab an old tap and grind away all but one "tooth". Spares me having to grind the profile and it as built in top rake. The tap just has to be a coarser pitch than the thread you're cutting.
Good to remember that. Thanks for the reminder. It wouldn't have mattered in this case because I don't have any old 20 tpi taps laying around.
ChazzC
Ultra Member
@Susquatch
Check out this on the Southern Forum:
https://www.hobby-machinist.com/thr...ou-make-in-your-shop-today.67833/post-1210429
Seems like there are a lot of metric/inch threads out there!
Check out this on the Southern Forum:
https://www.hobby-machinist.com/thr...ou-make-in-your-shop-today.67833/post-1210429
Seems like there are a lot of metric/inch threads out there!
Ya, it isn't the first one I found. But it is the most annoying one! Mostly because it was buried and hard to measure.
Another aspect that really pissed me off was my bad conclusion that it was an M20-1.25. That cost me valuable time and the cost of a tap I'll never use again!
Worse, it soured me of any desire to document the process for the benefit of others. How can you recommend a process that doesn't work!
Anyway, the job is done now. I finished the rest of the assembly today. I'm glad to have it behind me.
When I started installing my knee power feed, It was pretty late on a weekend night. So there may have been a beverage or two involved. Since this thread keeps popping up, it has jogged the fuzzy memory. I got an adapter with the Align power feed kit. I didn't fit. I measured the thread that I needed at 20tpi vs the 16tpi adapter they sent. You can't measure the knee shaft dia while its in its housing, so I assumed 3/4-20tpi, and made an adapter shaft out of an old axle shaft. It didn't fit either. I'm pretty sure I pulled the shaft out of the mill, and it was also 20mm-20tpi, like yours. So I machined it to fit the adapter that came with the Align kit because once I had it out to get a proper measurement on it, it was easier to machine the 20mm-20tpi portion to 3/4 16tpi to fit the supplied shaft extension/adapter.
Just hoping that this saves someone a few beers if they run into this.
It would have saved me a lot of trouble that's for sure! But I prolly would have still cut the adapter instead of the shaft just so I could restore it to stock if I ever wanted to.
A related question for you. How did you deal with the key slot? On mine, I had to remove the original key to screw on the adapter. But then what keys the adapter to the old shaft? There doesn't seem to be a provision for that!
Lifting is tightening the adapter so that shouldn't be an issue (until you want to remove it that is), but lowering the knee could simply unscrew the adapter.
There is nothing in the manual about it. I considered drilling a hole through the assembly for a drive pin, but that would make the alignment permanent. Instead I drilled and tapped the adapter for 4 tiny little 3mm grub screws. There is only about a 16th of meat on the adapter for the screws so I also had to shorten them. I suppose I could have countersunk the shaft for them, but I offset them axially to avoid weakening the adapter excessively so alignment would have been permanent. So I didn't do any countersinking. Besides, you can't really put a lot of torque on a 3mm set screw.
Anyway, I don't like it. What did you do?
@Darren - a few more questions. My install instructions say to use shims to set the bevel gear engagement. That's fine. I see how that works. But how the heck do you know where you are? I'm thinking of making a gauge of some kind. But even if you know where you are, how much play is appropriate?
I don't know why they couldn't have gauged all that for you ahead of time and simply said "install x shims" when new, and remove one after every x hours of accumulated usage.
And what about those dumb limit switches. I'm planning to tie them up and not use them. I'll prolly never use the lift at its extremes. And for those few times that I do, I can crank by hand.
Last, but not least, what about clocking the unit? As is, it won't be straight. The screw holes are not in the right place. I can drill and tap new holes, but I'm not sure I want to. I think there might be some merit to leaving the unit clocked clockwise a bit for better access to the controls. Especially for old bones that don't bend well. What did you end up doing there?
I don't know why they couldn't have gauged all that for you ahead of time and simply said "install x shims" when new, and remove one after every x hours of accumulated usage.
And what about those dumb limit switches. I'm planning to tie them up and not use them. I'll prolly never use the lift at its extremes. And for those few times that I do, I can crank by hand.
Last, but not least, what about clocking the unit? As is, it won't be straight. The screw holes are not in the right place. I can drill and tap new holes, but I'm not sure I want to. I think there might be some merit to leaving the unit clocked clockwise a bit for better access to the controls. Especially for old bones that don't bend well. What did you end up doing there?
On mine, the instructions say to drill and roll pin. I used red locktite.
On mine, there was only one option for clocking it. It only fit one way.
For shimming, you can feel when it's too tight and too loose. It definitely takes some playing around because the shaft isn't free to rotate so you need to go by sound to get it just right.
I never installed the limit switches. I probably should though.
On mine, there was only one option for clocking it. It only fit one way.
For shimming, you can feel when it's too tight and too loose. It definitely takes some playing around because the shaft isn't free to rotate so you need to go by sound to get it just right.
I never installed the limit switches. I probably should though.
Thanks Darren. A roll pin makes good sense. Didn't come with one though.
Setting the gear by ear gives me the heevie jeevies.
Mine too. But drilling and tapping some new screw holes at a better clock location, is a really simple job. Just a matter of deciding if I want to.
Setting the gear by ear gives me the heevie jeevies.
Mine too. But drilling and tapping some new screw holes at a better clock location, is a really simple job. Just a matter of deciding if I want to.
I use a similar method to set the thread gearing on my lathe. So I have been noodling similar methods for the knee lift system. It is a bevel gear drive though. It is quite different from the spur gears on my lathe.
Actually, I thought your earlier suggestion was fantastic. You suggested using paper and let dirt and oil on the existing threads make marks on the paper. It left nice clean marks that I used to gauge against taps, thread pitch gauges, and other screws. The problem is that 1.25mm and 20 tpi are just thousandths apart. Unfortunately, I let the 20mm diameter lead me to the wrong conclusion. My bad.
In future, I'll keep in mind that mixed standard threads do exist. But I really liked your paper idea and I honestly don't think bluing or other compounds would really improve on it. It's only real downfall is that it isn't fool proof......
WAIT - I think I misunderstood and you are talking about the gear engagement now! I'll have to think about that a bit before I react. It might be a good idea.
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ChazzC
Ultra Member
Kind of like analyzing wear on a car’s differential?
I actually worked on this a bit today with very unsatisfactory results.
So I think I am going to setup a fixture to evaluate this a bit more. It should be possible to determine what the maximum engagement is and then dial in a relief of some appropriate amount. That could even be correlated to your gear marking compound.
I certainly don't like working blind.
Right now, I don't think even maximum engagement is anywhere near optimum. Something is holding the gear back. But I don't know what it is just yet. I may need to relieve the adapter a bit. Seems really odd though cuz normally you would add shims to increase the clearance. Is there such a thing as a negative shim? Yup, it's called a lathe......
More to come......