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I have an early 2000 PRT with an Ascension box (similar to the 4G). After some years my X and Y pinions are getting visibly worn. I have 20 tooth now and I'm having second thoughts about replacing with same. In perusing the forum I see that Brady suggests using 25 tooth pinions because they wear better. I think I originally had 25 two gears and went to 20 in hopes of increased power and accuracy. So now I would like any suggestions about which way I should go, 20 or 25. TIA
Don,
The reason the 20T wear out more quickly than the 25T is for 2 reasons: 1) The motor mounts do not keep the pinion perfectly square to the rack - this is especially true for the early PRT angle iron motor mounts. Making new mounts out of 3/8" or thicker AL or 1/4" steel & adding another bolt so you can get rid of the spring & push it into the rack like the PRS, will make a big difference. The spring setup is to make up for slip-shod manufacturing tolerances. 2) There are more teeth engaged in the rack on a 25T vs a 20T, so the load is spread across more teeth. However, if you address #1, then #2 is less of an issue. And of course there is C) People don't clean and lubricate the racks and steel on steel is bad...I've seen new tools with no grease wipe out the 20T pinions in only 2 or 3 weeks of use.
I know the prospect of making new mounts may seem daunting...but it isn't without merit. Once you make a set and drill the gantry for an extra bolt hole each, the tool should both run more smoothly and pinion life should increase. They don't have to be fancy. They can be square edged cut from a piece of AL bar with some holes drilled into them. This hardest part is the big center hole for the bump out on the stepper near the shaft.
-B
Thanks for the reply, Brady. hardware modifications are no problem and I will make them. My secondary question (though not necessarily second in importance) is power/accuracy/speed issue vs what I understand to be a little less power and accuracy and increased speed with the 25 tooth pinions. Are there really any significant differences in performance between the two that should concern me. Most of my work is prototype/one-off and the like. What prompted me to inquire about the two is to get some additional rapid speed. Of course, right now I am limited because if I get above 125 or so, the x axis occasionally just stops as if it found a piece of metal between the teeth, which I concluded may well have been from the obviously warn pinions. TIA
And, I've got to give you some strokes. Your willingness to spend the time to spread your expertise is phenomenal. Thank you so much.
Donn,
No sweat - you're welcome. I believe that the sound you heard is most likely linked to a communication bottleneck, rather than anything mechanically grinding. The stepper 'grinds and loads up' then just STOPS...If you're still running off the parallel port, that's most likely the issue. I haven't messed with Mach in a very long time but I think you can upgrade to a USB or maybe even ethernet communication setup (PMDX maybe?). It depends how fast & how much resolution you want your tool to have. Things have progressed quite a bit in the past 12 years in CNC land...
Now to answer your question regarding 20 vs 25s, it's simple math. The 25 will give you 20% more the top speed at the expense of 80% the resolution, in comparison to the 20T. This is obviously the cheapest 'solution', but it doesn't give you more horsepower. I don't know about you - I don't want to give up ANY resolution...I want to have BOTH! ...and it's completely possible to do so these days with modern controls.
-B