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  1. #1
    Join Date
    Jan 2006

    Boring out an XL pulley.

    I am looking to put together a rack and pinion machine and have been playing around HubbardCNC and McMaster. I can't seem to find a 15tooth drive pulley and 72tooth pulley with the same bore. Can an XL 15t pulley with a 1/4" bore be enlarged to the 3/8" size found on the 72t pulley? If not, what size combination is out there that I'm missing to not require any drilling?

    Thanks and Merry Christmas!

  2. #2
    Join Date
    Jan 2006
    Well, boring out a pinion to be more precise.

  3. #3
    Community Moderator Al_The_Man's Avatar
    Join Date
    Dec 2003
    Either pinion or pulley, the manufacturer spec should give the max allowed above the stock bore.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  4. #4
    Join Date
    Dec 2006
    I just got done boring out an 24T XL series steel timing pulley from 0.25 to 0.987 I believe it was. One problem you may encounter is the bose (or Z dimensions as McMaser labels it) with the set screws may become too thin or be eliminated for that matter. In my case, I did not need the collar with the set screws.

    I believe the 15T is 0.75 so there should be enough material for the set screws if needed. I would only go with the steel version as the Plastic/Aluminum version may not work well when bored too much.

    Best Regards.


  5. #5
    Join Date
    Jan 2006


    Thanks for that. I have much more confidence in ordering my gears now.

  6. #6
    Join Date
    Apr 2007

    Shaft attachment question

    Hi, I hope you don't mind if I tag on a related question on mounting a pulley / pinion. (questions are at the bottom of the description )

    Similar to groomend, I am interested in mounting a pulley onto a shaft, and it will need to be bored out. There are some details of this seemingly simple excercise that still escape me so perhaps I am over analyzing this.

    General Description
    - nominal 0.75 or 1.00 inch dia. shaft
    - Will hold 3 timing pulleys (AT 10 x 25mm wide x 50mm dia) and at least 2 bearings
    - Approx 2 ft long

    The shaft diameter was chosen to keep deflection down to the sub 0.010 inch level assuming a 40 Kg / 100 lb load. 0.5 in shaft calculated to a large deflection.

    Shaft Selection
    - Thompson hardened shaft - seems like a great idea for rotary bearing mounting, but also troublesome for a good hold for the set screw.

    - Drill rod - McMaster sells unhardened, but is this ok for bearing mount use ? Straightness is surprisingly less than Thompson shaft.

    - Hollow steel shaft - nominal 1 in dia x 0.5 in nominal bore. Sold as a "shim" Seems like a good idea for mass reduction, but the inertial mass would still be similar, and the tolerances are much worse.

    - Shaft keyed the entire length - seems like a good idea, but still not all of the pulleys would be able to line in on the keyway and this does not allow for alignment adjustments.

    Mounting Methods

    - Shrink fit - I need to be able to make adjustments so this is difficult.

    - Quick mount pulley devices - basically a collet that snugs up around the shaft inside of the pulley. It seems like a good idea, but at nearly $ 50 / each it needs to be really important, and it is not recommended for highly finished shafts.

    - Bearings - 1/2 in bearings are cheap, but 1 in dia bearings are not - ball bearings are at least $ 20 / ea, and commonly 2-3 X this. I am starting to think about using a plain bearing like a bronze bushing / oil lite or Frelon type. I am wondering if this will give more of a stick / slip or not ?

    Sorry for the long detail, but now the questions

    a) Shaft - Please suggest how hard of a shaft I need for a setup like this. (hobby router use - nominal 5 x 10 ft, 3 hp router)

    b) Pulley Mounting - Am I being overly sensitive to the mounting method or are set screws just fine for nema 34 size steppers ?

    c) Shaft - Am I correct that a hardened, ground shaft is difficult to attach to or just missing the general idea ?



  7. #7
    Join Date
    Jan 2006


    as far as an answer to part b. of your question -- one guy swears by just using loctite to mount pulleys to shafts. Its on a thread around here someplace....

    I see his point too, if its already a fairly snug fit, loctite could make for a pretty solid bond that's easily reposition-able with a little heat if need be.

    everything else, i'm still learning. I personally just plan on finding a cold-rolled shaft at Home Depot/Lowe's and rolling it on the floor to make sure its fairly true, then taking my new prized possession home with me.

  8. #8
    Join Date
    Apr 2007
    If you are making a rack and pinion system or a screw based system, the fine tuning of the pulley / pinion position is not such a big deal. I am thinking that belt drives require a bit more sophistication in this area.

    Imagine that you have 2 each toothed belts driving an axis. (not a fixed belt drive, but a recirculating belt system) This would be "normal" to reduce racking effects.

    Both of these belts will be looped around timing pulleys on two shafts - one on each end of the table.

    At the stage where you tighten the first belt (at the joint), the pulleys for that belt must be free to rotate relative to the shafts, or it will only tighten one side of the belt, not the "back". After all of this, the pulleys on that belt can be fixed to the shaft and prevent rotation.

    Now the second belt tightening process. It is almost certain that the gantry or whatever will be out of square and have a loose belt at this stage. To trim it up, the belt will need tightening and the timing pulleys for that belt will need to rotate slightly relative to their shaft position. Once this is complete, then the timing pulley position can be "locked", at least until something changes.

    It is not clear that loctite mounting will work for this.

    I suppose you could use a timing pulley with dual set screws or perhaps some method with threaded rod and nuts.

    The cold rolled shaft might work as that is a hardness that is viable for mounting.

    What shaft dia. are you thinking about and bearings ?

  9. #9
    Gold Member
    Join Date
    Nov 2005
    I would certinly go for a keyway, tight fit and some spotting where the 2 set screws go ( at least .090 to .125 deep) .750 shafting will more than suffice for 24" length, and you can order that with a keyway cut in it.

    I cut the centers to match the shaft in a tight slip fit, you can use diesel fuel as a lubricant while slipping the pully's on ( no I'm not crazy, diesel fuel is real slick, try it sometime)and broach the appropriate key way. I have had sets with small and large pulleys on the same shaft,wherein the small pully is to small for the shaft key way size, so just make a conversion keyway ,one side the shaft key size, the other side smaller, matching the pully keyway size.

    Locktight is good, but from experiance I use only after tested and final finish assembly at the set screws..Do not use red Locker on Alum,you will pull threads. Some of the set screws they use are just too small to drive with a allen, so I use a reqular allen bolt in place of the stupid small set screw, you gain a least one or two allen wrench sizes and you can get some real torque on it.

    Hopes this helps.

    "Precision fit and finish will make life a lot more pleasant and lasting fun".

    Adobe (old as dirt)

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