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Finally aligned my Dewalt 7491 table saw

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Forum topic by AJPeacock posted 05-16-2019 12:54 PM 3362 views 1 time favorited 11 replies Add to Favorites Watch
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AJPeacock

46 posts in 1547 days


05-16-2019 12:54 PM

Topic tags/keywords: dewalt table saw alignment 7491 7491rs

Hi all,

Thought I’d share my alignment success story with the forum.

The only other table saw I’ve been around is a big Grizzly cabinet saw with huge wings/outfeed tables. That baby is easy, loosen a couple bolts, tap it with a dead blow and voila! Not so easy with these little contractor/portable saws.

I bought the Dewalt 7491rs about a month ago and other than adjusting the riving knife position (it was visibly too far right), I started using the saw exactly as it came out of the box. I’ll be starting a couple projects soon that require a bit of precision so I thought I’d tune it up.

BTW: This table saw will be used with a sled to cut parts for hutches/cabinets/shelves … I use a Porter Cable circular saw and a shop built guide to break down my sheet goods.

During the alignment, I used a dial indicator attached to my miter gauge for all measurements (miter gauge shimmed with painters tape).

The front to back of the blade was out of alignment with the miter slots by more than 30 thousandths!

There are 4 bolts that hold the motor/blade assembly to the table, so I loosened 3 of them and barely loosened the fourth. I then realized the bevel lever needed to be loosened to allow free movement, but I needed to hold the bevel for measurement. I clamped a support between the frame and the lower right edge of the blade cover under the saw. I then loosened the zero degree stop, the motor/blade was free to move (or so I thought). After trying to align it several times with no progress, I realized the fabric that encloses the area for dust collection was too tight, holding it from moving freely. So I loosened the 4th bolt and slid the entire assembly to the right about 1/10” to give me some room to adjust. I positioned the assembly in the center of the slots front/rear and slightly tightened the 4 bolts. I then measured the alignment and was 100 thou out, but within about 10 minutes I had it down to .003 and then another 20 minutes I had it tightened and under .001 over the span of the blade.

To measure the alignment over a longer span, I replaced the blade with a narrow piece of 3/4” plywood. I was able to measure over an 11” span and got it aligned to around .0012”, which I was really pleased with. Measuring the blade, it is under .001 front to back! The miter slots are not perfectly parallel, but pretty darn close for a portable saw (about .002” from parallel). The left slot is a touch wider than the right slot. I used the left slot to align the blade.

The fence was .007” wider at the back than the front. I adjusted it to be .001” wider at the rear.

The fence/miter wander in/out about .003” over the entire length of the fence, but amazingly are less than .001” out over the span of the blade.

So far I’ve only used the stock 24T blade that came with the saw, but I have a Forrest Woodworker II TK 40T on order that I’ll use with the sled I’m putting together.

Overall I’m super pleased with the alignment, it took me 3 hours start to finish. I think I could do another one in an hour now that I know what to loosen, how to hold the motor still …

Here is the procedure I’d use if I had to do this again.
0) Unplug saw and make dial indicator jig (just short piece of board with 1/4” hole to attach dial indicator), then clamp that board to miter gauge. Shim miter gauge with painters tape to limit slop.
1) raise blade fully up, then back off 1/2 turn. (set bevel at zero degrees)
2) remove blade and riving knife
3) Install 10×2x .75” plywood arm in place of blade (use for measurements). (5/8” hole a couple inches from one end)
4) Make initial measurements and write them down. Use a mark on the board to assure you are measuring from exactly same spot on board front and back. This keeps any warp in the board from affecting your readings.
5) remove 3 machine screws that hold the right hand side fabric to the table.
6) clamp a piece of ply between the frame and the lower blade gaurd, then loosen the bevel lever.
7) remove crank from front of saw (makes access to zero adjust screw much much easier), don’t drop the roll pin/washer. Loosen zero degree bevel stop, so it doesn’t hinder movement.
8) Loosen the 4 bolts that hold the blade/motor assembly to the table (loosen just enough to be able to make the adjustments).
9) position motor/blade assembly in center of slots front/rear (you do this while laying under the saw).
10) Measure and adjust by tapping the mounting brackets with a long punch and dead blow hammer.
11) Tighten and re-assemble. (re-attach dust collection fabric under saw)
12) re-install blade
13) re-install crank.
14) Set bevel zero stop and 45 stop. (remove block between frame and motor assembly first)
15) Lower blade.
16) using same dial indicator setup, slide fence over saw blade and measure fence front/back distance from left miter slot.
17) Loosen slightly the 2 bolts holding the fence gear at rear of saw. tap the fence rail lightly with a small mallet until fence is aligned. Tighten the fence gear bolts.

Voila!

Sorry for the poor writing skills, I’m much better with numbers than words.

AJ


11 replies so far

View Johnnyg123's profile

Johnnyg123

1 post in 425 days


#1 posted 07-28-2019 08:42 PM

Thanks for the great tips. Just got the 7491 and was about to pull my remaining hair out tuning it until I saw your solution. The motor and assembly on my 7491 seem to have some play when you grab the left end of the motor and jiggle horizontally. Diving knife assembly moves with it. Also, the entire assembly can be flexed up and down a little but I suspect this is due to the plastic parts that are used. Does your 7491 have play or flexing as I described?

View Sean_NZ's profile

Sean_NZ

3 posts in 327 days


#2 posted 11-04-2019 12:57 AM

I have just got a 7491 table saw, and the riving knife is not very well aligned (it is leaning to the left). From your experience aligning yours, i was hoping you could confirm a) what driver/bit do you use to loosen the adjustment screws, and b) turning those screws clockwise – does that tilt the knife to the left, or to the right?

thanks very much for your help!
Sean

View AJPeacock's profile

AJPeacock

46 posts in 1547 days


#3 posted 11-04-2019 01:02 AM



I have just got a 7491 table saw, and the riving knife is not very well aligned (it is leaning to the left). From your experience aligning yours, i was hoping you could confirm a) what driver/bit do you use to loosen the adjustment screws, and b) turning those screws clockwise – does that tilt the knife to the left, or to the right?

thanks very much for your help!
Sean

- Sean_NZ

Honestly I can’t remember which ones turned which way. I do remember that they were opposite how I thought they should be!

AJ

View Sean_NZ's profile

Sean_NZ

3 posts in 327 days


#4 posted 11-04-2019 01:05 AM

Thank you! That is helpful in itself. :) I don’t suppose you recall whether the adjustments were done with hex keys or with a screw driver?

Honestly I can t remember which ones turned which way. I do remember that they were opposite how I thought they should be!

AJ

- AJPeacock


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AJPeacock

46 posts in 1547 days


#5 posted 11-04-2019 01:11 AM



Thank you! That is helpful in itself. :) I don t suppose you recall whether the adjustments were done with hex keys or with a screw driver?

Honestly I can t remember which ones turned which way. I do remember that they were opposite how I thought they should be!

AJ

- AJPeacock

- Sean_NZ

I recall a hex key, I think you loosen one and the set the angle/distance with 2 others??

AJ

View Sean_NZ's profile

Sean_NZ

3 posts in 327 days


#6 posted 11-04-2019 01:15 AM

thanks!

Yes, the manual has fairly vague instructions on how to do it, (loosen one, adjust the others, tighten the loosened one), but just not with what, or in which direction. I tried to find a hex key to fit, but was missing a few in my set, which didn’t help :)

I really appreciate your assistance
cheers
Sean

Thank you! That is helpful in itself. :) I don t suppose you recall whether the adjustments were done with hex keys or with a screw driver?

Honestly I can t remember which ones turned which way. I do remember that they were opposite how I thought they should be!

AJ

- AJPeacock

- Sean_NZ

I recall a hex key, I think you loosen one and the set the angle/distance with 2 others??

AJ

- AJPeacock


View Travis's profile

Travis

471 posts in 619 days


#7 posted 11-06-2019 05:03 PM

I have the 7480 and use it basically out of the box. It’s been incredibly difficult to find tuning instructions. I might try some of your tips, assuming a basic similarity between the two models. Then again, this gives me a nice excuse for my poor woodworking results ;)

-- The plan is wrong; my finished piece is right.

View Dadan's profile

Dadan

1 post in 136 days


#8 posted 05-13-2020 11:43 AM

Yeah thx. In having the same problem with mine. I have mine hooked up in my home shop so it’s not easy to get to. I don’t want to unbolt it because I have it hooked up to an external fence and dust collector. I’ve tried the two bolts in the back with no luck. Just stayed the same. I’ve had the saw for about ten years so it’s definitely an older model but what you’re describing sounds the same. Hoping that releasing the miter will do the trick. Waiting till this weekend to pull the saw out if I couldn’t fix it. Can’t really see what’s going on. My math is way better than my writing too. Haha

View JetLaggedChef's profile

JetLaggedChef

1 post in 53 days


#9 posted 08-03-2020 07:10 PM

I made a video about how to align the riving knife since the manual that came with the saw is wrong and I couldn’t find a solution online.

https://www.youtube.com/watch?v=RcEoti_b3To

View LJRay's profile

LJRay

106 posts in 2358 days


#10 posted 08-04-2020 03:33 AM

I just purchased a 7491RS and had a bear of a time getting the riving knife aligned. I almost stripped out one of the lock screws just trying to loosen it. I was only successful at doing so after loosing the other lock screw. The instructions say to only loosen the lock screws and then adjust the set screws as needed. They don’t mention that the lock screws do have an affect on the knife alignment. Only after loosening both lock screws considerably was I able to get the set screws adjusted appropriately.

FYI.
The diagram on page 7 showed one of the three set screw as being on top, which is incorrect. The top is a pin – the third set screw is on the bottom.
I used a 4mm hex key for the lock screws and 2.5mm for the set screw. (at least i think the very small writing on the latter said 2.5)
I also removed three of the star bit screws holding the the fabric dust shield so I could see better and reach in through that way to assist in aligning the knife

-- Ray

View RomanS's profile

RomanS

1 post in 37 days


#11 posted 08-19-2020 04:24 PM

I got my riving knife misaligned as well. JLRay thank you for your note, it helped me solve the problem. I just wanted to expand on it to help others after spending 2 hours on this process.

So here is how I did it:

1. Unscrew two or three screws that hold the vertical black plastic plate covering the right side of the blade below the surface. That allows you to conveniently access to adjustment screws later
2. As JLRay says, use star head to unscrew 3 screws to that hold the black fabric.
3. Instructions say you have to raise the blade all the way up first. This is not correct. When you raise it all the way up you can’t really get to the adjustment screw. So, lower the blade to about 1/2 or all the way in.
4. Always loosen the two big screws first. I think it is hex 5/32. They are very tight from factory, I used my electric wrench to loosen them and then did by hand. Small ones are hex 3/32 I think.

About the screws. The way they work is this: big screws push the plate that keeps the knife into the table (if you are standing in front of the saw (normal working position) they push the plate and the knife to the left). The small screws act as stoppers, sort of allowing you to control how much to the left the plate will go until small screw touches the left side and stops the plate (and thus the knife). I.e. if you screw in the small screw it will increase the gap and will not let the plate go too close to the left (i.e. you use clockwise to make the knife go right). If you unscrew the small screw then it will allow the plate to go further to the left. But there are two small screws on the same level! In addition to being general distance stoppers they allow you to correct situation when your knife is not fully parallel to the blade (mine was not).

The bottom small screw works for vertical alignment. If you screw it all way in, it will make the top of the knife go left. If you unscrew it a bit, it will allow the top of the knife to become vertical and then go right.

So you continuously play with small screws and the large ones. Takes multiple cycles, because once you think you got small ones perfectly, when you tighten larges ones it shifts things again. So you loosen large ones, adjust small ones in anticipation of the move by the large ones, and try again. When doing all this, you have to periodically raise and lower the blade to make sure it is well aligned left/right and vertically on all depths. :)

So again, bottom small screw is for making the top of the knife be vertical (mine was leaning to the right), and the other two small screws are for controlling that the knife is fully in line with the blade, and that it is sitting strictly in the middle of the blade.

Good luck folks.

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