Centerless Ground Shafts
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  1. #1
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    Default Centerless Ground Shafts

    I am considering a new product that will use three linear shafts. They will be ~1.00 diameter x 12" long and will have an assembly that rides on them using bushings (linear bearings are pretty expensive for this product). The clearances and tolerances will need to be determined through trial and error... in other words I have no idea. I'll tap and chamfer both ends on my turning center. Probably a couple of thousand per year.

    For material, I'm leaning towards one of the hardenable stainless steels.

    Face, chamfer, drill and tap both ends on my turning center.
    Send out for heat treat.
    Centerless grind to dimension (???). Or cylindrical grind... or send it out for grinding. My only grinding experience involves a bench grinder

    anybody done parts like this?

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    Made some from Thomson rod. Pre-hard and ground.

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    Quote Originally Posted by Vancbiker View Post
    Made some from Thomson rod. Pre-hard and ground.
    +1 Made Thousands of exactly what the OP is describing. Thompson rod is gummy in middle, so depending on the depth and diameter, the Screw hole can be sensitive. Usually just Rolled the Threads.

    R

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    Thompson shaft is really expensive, and a pain to cut lengths from (yes, I've done it). Buying pre-cut just adds to the cost. If the OP is using polymer bushings and it's not an abrasive or really dirty environment, then a regular stainless, mild or "eh" steel is likely fine. Either use a centerless grinder in your area to size the bars, or ship if price warrants it.

    More important is what form the three bars will take. If the three are arraigned in a co-linear fashion (at for instance, 120 deg.), then binding of the moving structure must be considered. In that case, having one or two cylindrical bushings and one rest pad or racetrack shaped may be needed.

    Fit of shaft to bushing matters, of course, but you also want to consider swelling due to moisture or oil absorption, temp changes, etc.

    Unless it's an "eyes only" situation, a sketch of the design will help.

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    Quote Originally Posted by Milland View Post
    Thompson shaft is really expensive, and a pain to cut lengths from (yes, I've done it). Buying pre-cut just adds to the cost. .....
    But it'll be cheaper than the process the OP is proposing assuming he is looking for a hard (58-60RC) surface. A hot saw makes quick (and dirty) work of cutting to length. Chamfer and facing cut ends is pretty easy and since he is doing end work anyway, not much added cost.

    Less expensive alternatives would be materials commonly used on higher end hydraulic cylinders....

    IHCP rod http://www.pmtsco.com/1045-high-strength-h-cpo.php
    HT ground and polished rod http://www.pmtsco.com/acralloy-ht-srd-gp.php
    1144 ground and polished rod http://www.pmtsco.com/stressproof-gp.php

    Depending on hardness and strength requirements one of the above might make a better, less costly choice than the Thomson rod.

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    Sure, there's harder options. If he's using bronze bushings he'll want a hard surface, but I did weasel my suggestion by asking if polymer bushings and the conditions of use.

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    The cheapest way to get centerless ground rods is to get oversize round bar stock and drop ship to your local centerless grinder. I get 3/8 and 3/4 ground for a fraction of the price I can buy it for.

    On the other hand if you want it hard then getting the oversize rod carburized and hardened and keeping it straight is going to be difficult, especially as any heat treater is going to quench it flat (more than likely) which doesn't help straightness.

    If it was me I'd the shaft + 1/8", harden and quench then grind to size. Leaving the ends long you can machine the ends and get under the hard layer, then drill and tap. Hopefully it stays straight

    Alternatively cut +1/32" long, get the heat treater to paint the ends so the ends don't get carburised, then harden and quench. Then finish ends

    I've used plenty of Thomson 60case shaft, it's not much fun cutting to length, then drill and tapping the ends, i'd avoid it if I could.

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    Quote Originally Posted by Vancbiker View Post
    But it'll be cheaper than the process the OP is proposing assuming he is looking for a hard (58-60RC) surface. A hot saw makes quick (and dirty) work of cutting to length. Chamfer and facing cut ends is pretty easy and since he is doing end work anyway, not much added cost.

    Less expensive alternatives would be materials commonly used on higher end hydraulic cylinders....

    IHCP rod http://www.pmtsco.com/1045-high-strength-h-cpo.php
    HT ground and polished rod http://www.pmtsco.com/acralloy-ht-srd-gp.php
    1144 ground and polished rod http://www.pmtsco.com/stressproof-gp.php

    Depending on hardness and strength requirements one of the above might make a better, less costly choice than the Thomson rod.
    Thank you for the wealth of information. I will have to look into the TG&P Stressproof. I love 1144 and since this will be an inside product, corrosion may not be as big of an issue. I was intending to use a bronze bushing, but for the PV loads it will see, I could easily get away with a Igus style polymer.

    As far as a sketch, it is basically a three post press with a moving carriage. So I will need to consider any sort of binding that could occur.

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    As a thought, why not use silver steel?
    I think you guys call it drill rod.
    High carbon and tough and supplied ground to a 10 micron diameter by memory, and also 300 mm long.
    Bushing will run on it a treat from experience - bronze and oil light work well.
    I've never used any plastics though.
    But i wouldn't even bother hardening it if its just shafts for a press (low speed I'm assuming here).

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    Quote Originally Posted by barbter View Post
    As a thought, why not use silver steel?
    I think you guys call it drill rod.
    High carbon and tough and supplied ground to a 10 micron diameter by memory, and also 300 mm long.
    Bushing will run on it a treat from experience - bronze and oil light work well.
    I've never used any plastics though.
    But i wouldn't even bother hardening it if its just shafts for a press (low speed I'm assuming here).
    Seemed like the tolerances were pretty loose on the stuff I looked at, but the price is reasonable.

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    Quote Originally Posted by G00 Proto View Post
    ......I love 1144 and since this will be an inside product, corrosion may not be as big of an issue. .....
    You can get 1144TGP in chrome plated if corrosion is a concern. It is also "slicker".

    http://www.pmtsco.com/stressproof-s-cpo.php

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    Quote Originally Posted by G00 Proto View Post
    Seemed like the tolerances were pretty loose on the stuff I looked at, but the price is reasonable.
    Upto 25mm dia, is plus 0 minus 15 microns depending on manufacturer you may get plus 0 minus 10 microns.

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    How about chrome rod? No corrosion and you can get it with or without case hardening in a few different materials. I run a lot of case hardened 1045 and just part right through it, it's not that bad. Try scot industries for the material.

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    Quote Originally Posted by G00 Proto View Post
    I am considering a new product that will use three linear shafts. They will be ~1.00 diameter x 12" long and will have an assembly that rides on them using bushings (linear bearings are pretty expensive for this product). The clearances and tolerances will need to be determined through trial and error... in other words I have no idea. I'll tap and chamfer both ends on my turning center. Probably a couple of thousand per year.

    For material, I'm leaning towards one of the hardenable stainless steels.

    Face, chamfer, drill and tap both ends on my turning center.
    Send out for heat treat.
    Centerless grind to dimension (???). Or cylindrical grind... or send it out for grinding. My only grinding experience involves a bench grinder

    anybody done parts like this?
    thats a great plan if you are trying to make it as expensive as you possibly can.

    why stainless if it is "indoors" (plenty of corrosive environments indoors too)?
    why does it need to be hardened if you are considering a polymer bushing?
    why centerless ground if its for a sloppy longitudinal sliding fit?

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    Quote Originally Posted by cyanidekid View Post
    thats a great plan if you are trying to make it as expensive as you possibly can.

    why stainless if it is "indoors" (plenty of corrosive environments indoors too)?
    why does it need to be hardened if you are considering a polymer bushing?
    why centerless ground if its for a sloppy longitudinal sliding fit?

    Did you have anything to add or were you just being an asshole?

    Linear motion is not one of my areas of expertise, but I have had good luck with 4 series stainless rods that were done as above on many high volume products (100k plus units per year); however those were used outdoors hence the corrosion resistance. Since this is in a benign indoor environment, I am certainly considering TG&P 1144 or possibly even O1 drill rod... just concerned about the tolerance. On that topic, this will be a consumer product, so any perceived "sloop" in the mechanism will be detrimental (even if it is just perceived).

    On the downside, this will be a product that will be stretched for margin, hence the desire to make the shafts as economical as possible without compromising the functionality of the assembly.

    PS this will all be manufactured in a home built router with HSS tooling held in a drill chuck

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    Quote Originally Posted by G00 Proto View Post
    PS this will all be manufactured in a home built router with HSS tooling held in a drill chuck
    That router better be made from billet plywood, or it'll never be accurate!

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    You may want to look at misumi for a configurable shaft. I took a crack at what you described and ended up at ~$50 each for higher quantities of finished parts.
    enter this part number:
    U-SSFJW1.00-L12-T0.5-U0.5
    good luck with your product development
    Mike

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    Heat treated 416 is considered the stainless equivalent of stressproof. All kinds of variations in heat treat and chemistry available. we used to buy bearing quality "BQ" from Carpenter. Hardened extra hard when oil quenched or vacuum quenched. The BQ bars can still be machined when oil quenched and tempered at 300F.
    Not sure if they are still available but Pacific Bearing sent me a sample linear bearing that may be low cost. This was years ago but they may still be available. The bearing was called Simplicity. The phone number on the sample is 1 800 962 8979.

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    Hard to go wrong with case hardened 8620, centerless ground.

    Some, not all, Thomson shafts are hard chromed. Probably overkill for this application.

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    We did +1/32" Thompson shaft. So Face, Chamfer, Drill and Tap both ends, is about 5m per Shaft. About 15m on a Manual.

    R


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