JessEm Clear-Cut TS™ Stock Guides

[jsburger]

I think you guys are totally over thinking all of this. Jessem certainly calculated the forces of a kick back. They supply aluminum T track adapters to attach their product in some cases. I see nothing wrong with SS T nuts like RFGuy. Someone tell me why a slight tilt makes the slightest difference. The rollers are infinitely adjustable in height on the stock.

This thing is supposed to prevent kick backs. If you get a kick back with this product that is bad/strong enough to rip it off the top of the fence then I don't want to be in the same room.

[reible]

I had to go back and look at my "solution" on the other thread to see what I had to say.

I personally like the flat rather then tilted mounting. When I first put this together I didn't have the washers and I looked for very simple solution to get it operational ie flat. The washer idea is a common solution in that most people have some laying around the shop.

The reason I have only two t-nuts is due to the nuts I had to use. They don't fit the countersinks so a mid location then takes some of the flexibility out of mounting locations of the arms. I've thought about looking for better fasteners but I still haven't done that. With the correct fasteners there is no reason not to add a third t-nut.

I wouldn't be surprised if the whole shopsmith could be lifted off this mounting. I have no fear about any kickback being a problem with this. It is very rare I have even seen kickback and with this and other systems I employ now at this stage of my woodworking experience I don't think I will ever see any again.

This thing is way over engineered. Who ever designed it was overly conservative. Funny story, well to me at least. One of my first designs I did at Bell Labs was a mounting for miniature coax jacks. I did the design and my mentor had me take it to the lead engineer. I had used .060" aluminum and done my due diligent in the design.

They said go back and do it in .120" thick. So I did and then when the prototype came back I unpacked it and stood on it, yes it was way over designed. Took it to my mentor and we took it to the lead, I stood on it there too. Good thing I had saved my first design.

And BYW I am a considered conservative designer.....

Ed

[RFGuy]

I do agree that we may be splitting hairs here. As I said in my last post I think all of the mounting solutions that I have seen so far on the forum for the Jessem guides look good enough for normal operation (IMHO). Since the Jessem guide is a safety device, you want it to work in the worst case scenario. Given this, let's split some more hairs out of academic curiosity. The Jessem guides and Jessem T track are overbuilt, on purpose. That is why I focused on what I perceive as the weakest point which is the mounting hardware, i.e. screws. Of the installs shown so far on the forum, one might argue that mine is the weakest design (and it very well may be) due to the very small tilt of the T track. This combined with the right side gap could result in an off axis stress to the mounting screws causing the screw heads to shear off in a worst case kickback scenario. I argued in my last post that I thought I had an advantage over Ed's since I have 3 Shopsmith T-nuts instead of his 2 T-nuts. I still think this is true with regard to hold down strength, but with regard to this worst case scenario I describe here, Ed's probably wins as a better design. I believe the cleanest looking install is claimdude's. It is perfectly centered, his machined aluminum runners engage the entire length of the 520 fence AND he anchors it on both sides. The only negative I can possibly see is with regard to thread pullout of aluminum (vs. steel) in his design. One final comment on Jessem's engineering...The Jessem T track has 4 screw holes per side, so 8 total screws are intended to anchor it to a Biesemeyer™ style TS fence. This is what Jessem engineered it for. So, if we only use 2 or 3 screws to anchor it when 8 were intended, what do you think might happen in a worst case, max load scenario?

For anyone new getting the Jessem guides, I still think the best (most robust) solution is to drill and tap new holes in 6 Shopsmith T-nuts so you can mount the guides level and centered on top of the 520 fence. Another option is to mill some aluminum or steel bar stock as claimdude has done. No matter how you decide to adapt the Jessem guides to a Shopsmith fence, I recommend trying to get as many anchor points as possible to the fence. I intend to go back and redo mine so that both sides are attached...

split_hair.jpg (173.34 KiB) Viewed 8379 times

[reible]

We suffer for a great number of unknowns in life. How ever here is an example of some things we do or can know.

The fasteners can be either a number 10 or a 1/4". Since the track length is longer then the fence we know that the most of either that can be used is 6.

I have no idea how thick the material is on a typical fence is so lets forget that detail. We do know that the shopsmith fence is pretty hefty. And we also know that the screws might be going into aluminum on that other fence and if we use the t-nuts on the shopsmith they are going into steel.

So lets first look at the #10 screws. A typical #10 fastener will have an area of .0175 square inches. We can use 6 of these. So a total area of about .105.

Now lets say that we chose to use the 1/4" option. The area of a 1/4" fastener is .0318 sq inches. If we decided on 3 of these then the total area would be about .0954. If we managed to get 6 of them to work then we have an area of .1908.

Given typical 60000 psi the value is then in the area of 6300 for the six number 10's, 5724 for the three 1/4" screws and 11448 for the six of them.

We have no idea of where or how a failure would/could happen but if you were really worried about the fasteners failing I don't think that is were it is at.

The three 1/4 fasteners give you 90% of the strength of the six #10.

If I had to guess, and this is only a guess, a catastrophic failure is unlikely and if it did happen I would expect a twisted mess and if any thing the screws pulling out of the typical fence that I'd expect to be quite thin.

Anyway I should go and try and figure out what length fasteners I need one of these days and add a third t-nut.

Ed

[jsburger]

Thank you Ed. Like I said above, if that thing comes apart on a SS because of a kick back I don't want to be anywhere near it.

One question for you. Aren't the #10 screws used on Biesemeyer style fences self taping? How does the strength of those threads compare to a conventionally taped thread?

BTW, I just ordered one on sale from Incra. It was $5 cheaper than the Jessem sale. I plan on mounting the Jessem T track on my Powermatic PM2000. Then see what I can come up with to mount just the arms on the SS fence T track.

[RFGuy]

Thanks Ed. Appreciate your insight and expertise. For my application at present, I have 3 1/4-20 screws holding the T track. Assuming 60,000psi material, max tensile strength should be 5,724lbs correct? As a rule of thumb, isn't shear strength about half of the tensile strength? If so, then shear is something like 2,872lbs maybe? Given the angle of mine not being level and assuming a board during kickback gets lifted up and then thrown towards operator, I was more worried about a screwhead getting sheared off, i.e. force normal to the axis of the screw. I've never seen anyone identify what a "typical" force might be for tablesaw kickback. I think I saw somewhere on a forum once that it was thought to be on the order of a few hundred pounds of force. That is probably an estimate for the force directed at the operator and not the force directed up to the ceiling as the board first lifts. Just curious if anyone has any idea of what kind of forces a kickback might produce? However, based on the estimates here it looks like there is nothing to worry about. Thanks for your patience and for indulging me on this thread.

[JPG]

I have stayed out of this fray because I have neither a 520 nor a Jessem.

However if I did and wanted to mount to a 520 fence I would do the following.

Get a 3/16" oir 1/4" thick plate sized to cover the entire top of the fence.

I would tap 1/4"NC or NF threads into the plate for attaching the jessem - the more screws the better.*

I would add through holes for bolts to secure the plate to the fence by way of ss t nuts - the more the better.*

* there is a point where more of each will interfere with the other.

Now where is the weak link/ I think it would be the 520 fence t slots. Nothing we can do about that other than use as many ss t-nuts as possible.

*The more t-nuts, the longer putting it on/off takes.

t t t hats all folks.

[reible]

Yes if the bolts were in shear then you can half the value, but if the fence is attached and the stock guides are forced up then the vectors will be divided between horizontal and vertical, what that leads to is pretty much unknown unless someone what to do a study.... as in not me.

Also unknown to me is the force. From descriptions I have read where people get hit it is pretty high but no where near thousands of pounds I wouldn't think.

Now lets look at the plastic Feather Boards that shopsmith and others sell. I would think they are no where near as safe as the stock guides but yet I have used them for a lot of years and never experienced a kick back while using them. Could it happen?

If you keep the lifting from happening by the downward pressure things are a lot safer. No guard or other way of holding the stock down, well then things can happen.

Since I know a lot of people don't like and don't use saw guards, those are the people I worry about. The anti kickback paws are your friend when it come to staying safe from kick back.

BTW you can also go to say grade 8 fasteners. They are in the area of 20% stronger if memory serves me.

As a side note I have been retired for 19 years as of June first, so a lot of this stuff is slipping away, I will forgo how long ago I was in school..........

Ed

[jsburger]

I have a set of these on my router table and they are absolutely great.

https://jessemtoolcompanycanada.myshopi ... cohy9d2rci

JessEm has the table saw product on sale for Fathers Day.

https://jessemtoolcompanycanada.myshopi ... cohy9d2rci

Have any of you adapted them to the SS 520 fence? They will live on my Powermatic 2000 cabinet saw most of the time. I am wondering if the guides them selves are easily adaptable to the 520 fence with the SS T nuts. It looks like they would without the JessEm T track. Just SS T nuts and probably replacement knobs. The only issue would be the hole size in the guide body for the T nuts.

John,

If you mean the table saw version, it was discussed already here:

https://www.shopsmith.com/ss_forum/wood ... 22171.html

Thanks for the link. I was wondering if the guides them selves could be mounted to the 520 fence without the JessEm track that will be mounted permanently to my PM2000 table saw fence.

It seems like that is possible based on the response by Mike in this post. #255878 by Mike907 in your quoted thread above.

I guess I could also buy an additional T track from JessEm for the SS.

I finally got my Jessem TS Guides and they mount fine on the 520 fence just as Mike showed. To The Jessem T track can stay on the Powermatic TS and the guides can easily be mounted on the 520 fence by themselves if needed.