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Ski grinder project - Page 6

post #151 of 205

I see a potential problem, you want the ski fed into the wheel at a consistent angle and pressure, I think. Anyone else with more experience on this?

post #152 of 205

Lighter,  simple, (cheap)  and likely just as effective as rollers would be a pair of delrin half rounds attached at the infeed and outfeed locations.  Or full rounds that could be turned to offer fresh contact surfaces if the metal edges started to chew them.  They will be getting sharp after all! ;-)


Rollers would look good and do a fine job to help stabilize the ski during the pass over the revolving stones.  Just putting out what might be acceptable alternates for your consideration.


In direct opposed response to  bttocs's question regarding speed and pressure,  Skis have been pushed over belt style ski tuning machines for decades,  by hand!

The human machine is a truly wonderful thing.

post #153 of 205

I get it now, I did not explain myself well in my last post. I am not questioning feeding them by hand, what I am questioning is the rollers before and after the wheel are much lower that the wheel. The ski can approach the wheel (grinding stone) at a variable angle. It all depends on if the person has the ski touching the front roller. If so, the ski will not touch the back roller and will have a changing amount of weight on the grind point from the ski as you feed the length. It might be hard to control the angle when grinding the last couple inches of the tail.


My solution is to make the rollers very close to the horizontal tangency point of the grinding stone. The rollers should be level with the top of the stone and aligned with the stone. Or just below the stone by a properly sized amount (like maybe a 1/8 or 1/4"). 


I think with the roller very low, the section of the ski that has gone past the stone will start to bend and effect the contact point of the stone.  I am assuming the stone does not move vertically of course.

post #154 of 205

i thought we should use these as a feed and outfeed height guide: http://m.ikea.com/ca/en/catalog/products/art/60138759/

From what Cedrico tells me; i understand that conveyer rollers are pretty easy to source, and easy to mount.
post #155 of 205
Thread Starter 

If rolling surface is not necessary, ikea handles might be the easier option, but if I had to guess I'd say rolling surfaces would give better feel to the operator for what they're doing.

post #156 of 205

Rolling guides always seem to be the better solution,  right up until the moment the rolling element hangs up or the rolling surface developes unevenness.


Great material can be had in the kitchen wares department of any hardware store.  One of those "plastic" cutting boards is made from some durable and slippery stuff (UHMW would be ideal).  The boards can be cut with common wood working tools.


Again,  don't let me talk you out of anything you want to do.  Just trying to supply "design options".  ;-)

post #157 of 205
Thread Starter 

Your design suggestions are muchos appreciated.

What about teflon? 3/4" teflon rod across the front and back edges would cost 20 bucks and it'd be as slippery as it gets for something with no moving parts.

post #158 of 205

teflon,  though slippery,  wears rapidly, and cuts (gouges)  easily.


I bet you can get UHMW for about the same money.  And those cutting boards are less yet!

post #159 of 205
Thread Starter 

What about stainless steel? like those cabinet handles from ikea.

post #160 of 205

stainless steel will gall when rubbed against SS, probably will do the same when rubbed against steel. I would use brass or plastic. If roller, no issue.  I am thinking about the ski edges sliding over the rod

post #161 of 205
Originally Posted by bttocs View Post

stainless steel will gall when rubbed against SS, probably will do the same when rubbed against steel. I would use brass or plastic. If roller, no issue.  I am thinking about the ski edges sliding over the rod

Just make sure to deburr the base edges first no matter what your choice.  :popcorn

post #162 of 205
Thread Starter 

Plastic makes sense, how about polyethylene rod? Probably the cheapest option as well.


Got the first side machined. It took forever to do those threads because it turns out that I should've measured the major diameter of the drill press threads more carefully and it's a 11/16" rather than 3/4" thread (we think - nothing metric makes sense). It's the weirdest thread spec I've ever seen: 20 threads per inch (weird), 11/16" (weird), and left hand...

Anyhow, it's progress. Next side will be Monday.


post #163 of 205

UHMW   Ultra High Molecular weight ...Poly........... "plastic" ;-)


The best wear resistant option out there,  and CHEAP!


Nice threads!  Cut to fit!


I cut similar "cut to fit" threads today as well,  but not as nice as those!

I was shortening a ceiling fan strut ;-)  1" X 14 tpi,  Or a near approximation ;-)

post #164 of 205
Thread Starter 

The shaft is all done! :)




Unfortunately there's a bit of bend in the shaft and the 1" surface isn't quite concentric with the bearing axis, so the stones will have to be dressed quite a bit at first, and I expect balancing to be required. If I had to remake the shaft, I'd start with 1 1/8" diameter, and machine it down to 1" to be perfectly concentric, that way the shaft doesn't have to have tight tolerances straight from the supplier.

post #165 of 205

Straighten it.

Small hydraulic press, two v blocks and a dial indicator.

Piece of cake.


post #166 of 205
Thread Starter 

Ah, should've done that before machining.

post #167 of 205
Originally Posted by cedrico View Post

Ah, should've done that before machining.

You still can.

Sometimes even machined shafts need to be straightened.

Almost always after heat treat.

You should be able to get the runout down to 0.005 or less.

If you had started with centerless ground drill rod you wouldn't have this problem......


Then there is this...

post #168 of 205

A bent shaft such as you have is trivial to straighten.  When strait,  it will run just fine.


On the other hand,  if the two bearing journals are not on the same axis,  the machine will NEVER be smooth.  Not only that,  but the shaft will "walk" it's self in or out of the bearing no matter what retention mechanism you imploy.  (baring pinning through the shaft.)


Shaft work is normally done between centers for just this reason.  The alternative is to do a careful job of indexing the work before the first cut and then again when swapping ends.  Jaw chucks are notorious for holding less than acceptable concentricity part to part unless machined in place "soft jaws" are used.


I would be interested in how much out of concentricity you are dealing with.

post #169 of 205
Thread Starter 

I think the 1" surface is around 5 thou off the bearing axis at one end and less at the other, but I think it'll be fine with dressing and balancing. We'll see what happens, I'm definitely learning lots! Seems like a simple machine but there's quite a bit to it, at least plenty for one course workload.

post #170 of 205

It's hard to learn machine design and machinist skills at the same time.  But the learning happens!


I celebrate your accomplishment!

post #171 of 205

:popcorn  Sweet.

post #172 of 205
If you don't straighten it you will induce a balance vibration in the stones as they are dressed as the outer grinding surface and inner bearing surface become missaligned.
post #173 of 205
Thread Starter 

Wouldn't dressing make the stone surface concentric with the rotation?

post #174 of 205

Yes but the steel and stone have a different density, so the center of balance has changes as the center of rotation is zeroed.

post #175 of 205

Yes but even when concentric the center of mass would not lie on the shaft centerline unless you add balancing weights.

post #176 of 205
Thread Starter 

Yup, so dressing and balancing.

post #177 of 205
Thread Starter 

Finished these stone compression plates today with a sliding fit on the shaft.



Here's the shaft assembly :)



The last steps are to remove the cardboard between stones, add 1 thou plastic shim on the shaft, and clamp it together for good.

Game plan for Monday is CNC oxy-acetylene cutting the frame plates from quarter inch steel.


Video of the completed shaft assembly:  https://youtu.be/tXzf2YgweWM

post #178 of 205

Are you meeting your college's diversity quota?  Why is one stone different color from the others?  My gut says you should put a thin piece of rubber sheet (inner tube thickness) between the compression plates and the stones. I forget if you have something between them or not to increase friction and absorb shock.


Also, you mentioned Nordlock washers. I have used them in my designs and they work well. You use them in pairs and they do what they are supposed to.


Looks good.

post #179 of 205
Thread Starter 

The stones were all shipped together but I think the pale one was made in a different batch; same part number but different identification number. There's cardboard glued to both sides of each stone. I'm going to remove them all except the outer cardboard of the outer stones.

Yeah, the nord-lock washers are a nifty concept.

post #180 of 205
Thread Starter 

There's going to be one of these guides on each side of the stones. 3D printers now days are getting impressive, it's hard to tell the part was printed when layer thickness is 0.1 mm.


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