Benchtop Flat Enough?

[cpowell]

I sanded my benchtop. 80 grit followed by 120. I used a flat reference then marked the high areas and worked to blend them in. I also cleaned up the center glue line/offset.

The bench is roughly 21.75W x 2.75thk x 84L. I got a few sticks of extruded aluminum 60mm square tubing (85 inches long) that is quite flat according to a tight piece of monofilament. I laid the aluminum stock lengthwise and diagonally on the top. I am seeing about .016 - 0.025 inches gap longitudinally at the center of the bench - the center of the top is slightly lower than the ends. I measured at all parts across the bench lengthwise, parallel to the edges and diagonally.

Measuring crossways, the bench is fairly flat (0.016 or less) except at one end, which is dished about .03 inches or less. The dished area is about two feet in length and only a few inches wide. This was the result of a glueup issue and a slight difference between stock thicknesses. :eusa_doh:

The bottom is likely flat enough to do it's job.

I would never accept this tolerance for frame pieces on fine furniture but am wondering if it's close enough for a workbench. I will always know which area is a little bit off, right? My concern is that in further pursuit of perfection I'll screw it up worse than it is! :lol: :lol:

I could probably rig a base for a router flattening jig using the aluminum rails, and that's the reason I borrowed them, but now that I've looked it over and measured the problem areas I am wondering if it's worth it or not. Is there a potential problem I could run into in the future with this condition that I'm not aware of or is it good enough?

The top is no longer very portable. I can roll it from side-to-side but cannot carry it safely.

If the router sled idea will improve flatness and is doable with some planning I may try it. It will involve drilling holes through the assembly table to fasten down the frame and maybe a few hours constructing a sled. I have read up the details of this technique and printed out some pics. Maybe I just need a kickstart? If I use the router trick, how much cleanup will be required?

Opinions are appreciated.

Chuck

 

[erasmussen]

If it was my bench it would be flat enough, because I would soon mess it up anyway, but its your bench and you know what kind of work you want to do on it, so is it flat enough for you?:eusa_thin

 

[Deacon Shuster]

Chuck,

I don't know that the router sled is the way to go. I can only give you my opinion based on my machining background. In order to get a rotating cutter to cut a surface acceptably flat the cutter must be dialed in to rotate in a flat plane relative to the surface that the work is being held on and the mechanism that holds the cutter. On a bridgeport type milling machine you have to dial the rotating shaft that holds the cutter in a plane that is flat with the table/vise that holds the work. If you don't get this right you end up with ridges/hollows in the workpiece. One method to minimize this is to use a cutter with a substantially larger diameter than the workpiece. I don't see that happening with a router on a workbench top.

Then again, set up a test piece and see if it works and you are happy with the results, in which case I hope that I am wrong or over thinking your question.

I'm sure that others will have better answers for solving your problem and I look forward to seeing the results. The bench looks great so far and I'm sure it will give you years of satisfaction when its completed the way you want it.

Take care,
Doug

 

[Travis Porter]

You are off less than a 1/32? That is tough to get better. I don't know if I would do the router jig. I would try to find a cabinet shop with a wide belt first. My .02

 

[striker]

Chuck,

I'm by no means an expert on this issue but I was in your shoes 4 years ago when I built my bench. I did as your doing working it down with a sanding board (psa discs stuck to my 4ft level). I spent a good deal of time on this as I felt any dips or humps would translate into the boards I attempted to hand plane on the bench. Maybe my concerns are unfounded but all the sanding was good for at least 10 lbs....

fyi... I believe what Doug is refering to is "tramming the head" ..indicating the head perpendicular to the table on a knee mill.

Stephen

 

[cpowell]

Thanks for all the feedback.

I got back out to the shop around 9PM and rigged a sled with the rails and ply then mounted a dial indicator just for kicks and giggles. I measured the thickness variation across the entire bench, moving thre inches each pass, and the range overall (highest reading minus lowest reading) was around 0.10 inch or so. I kinda ran short on time so I didn't write the numbers down so I could generate a 3D surface graph in excel (I probably tend to over-analyze too). What bugged me wasn't the overall variation but there were several areas where the thickness would change by .030 inches over a few inch movement (where the table is dished in the center).

Points well taken on router flattening. The mathematician in me thinks about how perfectly the bit would need to be aligned to the frame structure in order to prevent grooves. It would be a lot of tedious work. I have a 3/4 straight bit so even if it were perfectly parallel to the top surface plane of the frame it would take a whole lotta passes to cover 84 inches. Still, the devil in me wants to maybe give it a go! :lol: :slap:

I will check around with cab shops on the wide belt sander possibility.

Chuck

 

[Travis Porter]

Depending on how wide your slab is and how much you can take off of it, you might consider a 24" planer if you know anyone who has one.

 

[LeftyTom]

It is close enough to flat for me, but I am left-handed....:eusa_whis

 

[JimmyC]

You'd better make sure that you get that bench perfectly flat, so that when you make chairs or tables there is no wobbling. Well at least not until you put it on the floor that's crooked in the house that's settled:lol::lol::lol:. For me, your bench is close enough. For you it's probably a different story.

Good Luck.

Jimmy

 

[Travis Porter]

For those that don't remember, Chuck is the guy that returned 2 bandsaws before he was satisfied with one.

On a different note, Chuck, you remember your comment about your drill press coming with a wrench for the table? It turns out you were right, I found mine last night.:BangHead:

 

[cpowell]

For those that don't remember, Chuck is the guy that returned 2 bandsaws before he was satisfied with one.

On a different note, Chuck, you remember your comment about your drill press coming with a wrench for the table? It turns out you were right, I found mine last night.:BangHead:

Now wait a minute!! This coming from a guy who almost returned his mammoth Minimax BS which he now raves about??? 8-O :lol: :lol:

The drill press wrench thing is too funny!!!! Have you aslready built a jig which renders the DP tilt unnecessary?

PLEASE tell me you have done so. :rolf: :rolf:


Chuck

 

[Travis Porter]

Yes, I built a jig which renders the DP tilt unnecessary. I admit it........

 

[Travis Porter]

I didn't get to respond in detail before (had 2 kids to put to bed). Yes, I built a perpendicular table/jig for the drill press. After buying a set of metric wrenches trying to adjust the table, getting frustrated with the mechanism, I took Monty's advice and built a jig. Then 2 days ago I find a wrench that I wasn't familiar with, and it fits the drill press......:eusa_doh:

Me complain about my bandsaw? Never....... Well, for a little while I did.:rolf:

 

[Gofor]

Is it flat enough for a work bench?

1: The dishout will probably not cause any problems unless you are working a small piece in that area and are using the top as a reference.
2.: What is moisture/humidity going to change after you are done?
3: What are you going to use it for? If hand planing and you are going to use the front edge to reference squaring a board, you can always adjust the plane iron to allow for a discrepancy.

Bottom line, if you are using it for a truly flat reference, you need granite. You also need to mount it to the base permanently and install the vise hardware before making any adjustments as these will stress and possibly cause variations in the surface. Not to mention any floor variations in the base that will eventually (several months) work their way to the top.

Looks to me like you've got a d**n fine bench top. :icon_thum:icon_thum Quit stalling and get-r-dun!!:eusa_clap:eusa_clap

Go

 

[cpowell]

I set up a router sled and flattened my top last night.

I first ran the sled with a dial indicator across the entire top and located the lowest point. I then set the router bit accordingly, made a pass and verified the stting with the dial indicator.

The top has a series of lands/grooves but they are VERY thin. I made a pass with ROS 120 grit and it improved things significantly. I laid my straight edges (85 inch extruded aluminum and 24 inch level) along the bench and trust me - it's now flat. I can't see the gaps between straight edge and wood that were evident before. I can literally lay the straight edge anywhere on the top and it is spot on. Both of my straight edges were verified with a length of monofilament stretched tight.

The visual improvement is incredible. It looks like a bowling lane surface. I believe I now have it flat within +/- 0.004 inches. I realize the top will move when subject to vise loading and perhaps with ambient moisture but it feels good to know it's flat at this point. :eusa_danc

I will make an additional sanding pass tonight then start on the front apron and end caps.

Pics of the sled are attached below.

(BTW I have read the post on using attachments but for some reason my connection to the site from home is agonizingly SLOW. I'm writing this from work but don't want to spend time setting up a gallery from here.)

Chuck

 

[Travis Porter]

Awesome Chuck. How long did it take you to do it with the router?

 

[cpowell]

Awesome Chuck. How long did it take you to do it with the router?

I probably spent 30 minutes flattening and assembling the router sled and aligning the rails, another 20 minutes with the dial indicator (DI) finding the low spot. Probably 90 minutes to rout the top but I went very cautiously at first - cut a pass, measure with DI, cut a pass, measure with DI. I wanted to be certain that the depth was consistent as I went along.

After about 5 passes, I felt confident the setup would work. From there on I would cut 1/4 of the top length then vacuum chips. The rails were not long enough to complete the top without relocation and it took maybe 10 minutes to relocate and verify depth was consistent.

In retrospect I don't know whether a hobby use wide belt sander would have a sufficient bed length to produce a flat reference plane as well the router/sled/rails. However, now that I have a flat reference plane, a wide belt sander would be nice to finish the flattening. If I had a wide belt sander, I could flip the top over, flatten the bottom side, flip it once more and run a single pass and clean up the top.

This top glueup exercise would have been much easier with a 12 inch jointer. Once the glueup sections got wider than my 8 incj jointer I relied on the planer to flatten. It is very difficult to run 12 inch wide x 3 inch thick slabs through the benchtop planer without getting "waves". My benchtop planer is just not set up to perform an accurate "jointing" operation on 70 lb slabs.

Chuck

 

[Big Mike]

Chuck, take it to Builder's Supply and have them run the top through their heavy duty wide belt sander. It will be flat and smooth after a couple of passes. Might cost you a few buck but it will save you time and you won't be worrying about how flat it is....

 

[Deacon Shuster]

Chuck,

I'm glad to hear the sled worked out. I think the reason it worked so well is because you took the time and set it up right, in fact I'm sure that's why. Good to hear that you're satisfied with your top and are another step closer to using that bench!!

Doug

 

[cpowell]

Chuck,

I'm glad to hear the sled worked out. I think the reason it worked so well is because you took the time and set it up right, in fact I'm sure that's why. Good to hear that you're satisfied with your top and are another step closer to using that bench!!

Doug

Truth be told it took longer to setup/align the jig than to rout the top.

I was aware of the technique but was a little tentative...then I revisited PeteM's post. http://www.ncwoodworker.net/forums/showthread.php?t=8893 I figured if he had the guts to use it on that beautiful claro walnut I'd be a wimp not to try it on a workbench!! :lol: :lol:

Chuck